OXYGEN ENRICHMENT USING MEMBRANE TECHNOLOGY
Thesis for admission to the degree of
Doctor of Philosophy
in The School of Chemical Engineering and
Industrial Chemistry at The University of New South Wales
April 1991
Xiang Zhou Jiang UNIVERSITY OF N.S.W.
1 2 AUG 1392 LIBRARIES CERTIFICATE
I hereby declare that this submission is my own work and that, to the best of my knowledge and belief, it contains no materials previously published or written by another person, nor material which to a substantial extent has been accepted for the award of any degree or diploma of a university or other institute of higher learning, except where due acknowledgement is made in the text.
\
Xiang Zhou Jiang
i ACKNOWLEDGEMENT
I wish to thank my supervisors, Professor Anthony G. Fane and Professor Christopher J. D. Fell for their supervision during my study in the School towards this thesis.
I would also like to give special thanks to Dr. Hans J. Griesser, CSIRO Division of Chemicals and Polymers in Melbourne and Mr. Michael Rooney, CSIRO Division of Food Processing in Sydney for their kindness, advice and help. I thank the CSIRO for providing the plasma treatment and photochemical reaction equipment and chemicals.
I am grateful to Commonwealth Industrial Gases Limited for supporting the programme in the form of a scholarship.
ii PUBLICATIONS AND REPORTS ARISING FROM THIS WORK Oxygen Enrichment from Air Using Immobilized Liquid Membrane and Silicone/Complex Micelle Membranes-- Interim Report to C. I. G. Limited, by X. Z. Jiang, A. G. Fane and C. J. D. Fell. The School of Chemical Engineering and Industrial Chemistry. The University of New South Wales. Sydney, November 15, 1986. Novel Membranes for Gas Separations. by M. S. Brennan, X. Z. Jiang, A. G. Fane and C. J. D. Fell. 'Membrane Science and Technology', Abstracts of Boden Conference, Australia Academy of Science. Threbo Alpine Willage, February 11-13, 1987. Oxygen/Nitrogen Separation with Photosensitive Membranes, by X. Z. Jiang*, M. L. Rooney**, A. G. Fane* and C. J. D. Fell* Proceedings of IMTEC '88, Sydney, November 15-17, 1988. *The University of New South Wales **CSIRO Division of Food Processing OXYGEN ENRICHMENT USING MEMBRANE PROCESS — The final report to CIG Limited, by X. Z. Jiang*, M. L. Rooney**, A. G. Fane* and C. J. D. Fell* Sydney, October 1989. *The University of New South Wales **CSIRO Division of Food Processing
iii ABSTRACT
Oxygen enrichment using membrane technology has been studied. These studies have covered three processes namely, membranes with immobilized liquid, membranes with plasma treatment and membranes utilising photochemically-driven facilitated transport.
Immobilized liquid membranes imbibed with a perfluorocarbon FC-43 and silicone oil into commercial microporous filters are less selective to oxygen over nitrogen than silicone rubber which is the most permeable polymeric membrane for oxygen/nitrogen separation. The former has a separation factor of 1.6 and the latter of 2.0.
Silicone rubber membranes can be modified by plasma treatment and the separation performance is improved. With little affect on the permeation rate, the separation factor of the coated layer reaches 2.96 for the membrane treated with a mixture of methane and pentafluorostyrene. The experimental studies have also shown the significant influences of conditions on plasma treatments, such as temperature and the arrangement of the electrodes.
Facilitated transport membranes based on the photochemical reactions of 1,4-dimethoxy-9,10-diphenylanthracene and 1,4-dimethoxy-9,10-bis(4'-bromophenyl)anthracene have been investigated in order to enhance the oxygen transport which depends not only upon the stability of the carrier but also the reaction rate of forward and reverse reaction. Besides this, the solubility of both free and oxygenized forms of the carrier in a solvent plays a critical role in this process. A hypothetical process with total light blockage and complete forward and reverse reactions has been modelled and the calculated result is significant if this
iv process can be conducted successfully. In principle, the permeation of oxygen could be increased to 3.49 times that of the immobilized liquid membrane with a potential separation factor of 8.9 when 0.04 molar of oxygen carrier is used.
In a separate series of experiments, the concentration of oxygen in absorption/desorption tests using 1,4-dimethoxy- 9,10-diphenylanthracene as oxygen carrier and dibutyldigol as solvent reached 60%. This carrier has more potential as an oxygen-storage material than as a membrane.
v CONTENTS
ABSTRACT.
CHAPTER I. INTRODUCTION.
CHAPTER II. LITERATURE REVIEW.
2.1. INTRODUCTION.
2.2. CONFIGURATION OF MEMBRANE MODULES.
2.3. PERMEABILITY AND SEPARATION FACTOR OF A MEMBRANE.
2.3.1. MICROPOROUS MEMBRANES.
2.3.2. HOMOGENEOUS MEMBRANES.
(!)• STEADY STATE PERMEATION. (2). TRANSIENT PERMEATION - DETERMINING SOLUBILITY AND DIFFUSIVITY IN A POLYMER.
2.3.3. DIFFUSIVITY OF GASES IN POLYMERS.
2.3.4. SOLUBILITY OF GASES IN POLYMERS.
2.4. ASYMMETRIC MEMBRANES.
2.5. IMMOBILIZED LIQUID MEMBRANES.
2.6. FACILITATED TRANSPORT IN MEMBRANE PROCESSES.
2.7. PHOTOSENSITIVE MEMBRANES.
2.8. MEMBRANE MATERIALS.
2.8.1. HYDROCARBON POLYMERS. to • • to 00 • FLUORINE CONTAINING POLYMERS.
2.8.3. SILICONE CONTAINING POLYMERS. CM 00
• COMPOSITE POLYMER MEMBRANES.
2.8.5. TERTIARY CRYSTAL MEMBRANES. to • 00 • cn • FIXED CARRIER MEMBRANES. CM 00 r" IMMOBILIZED LIQUID MEMBRANES. 00 CM 00 • • PLASMA TREATED MEMBRANES.
2.8.9. SUMMARY OF MEMBRANE MATERIALS.
vi 2.9. OXYGEN CARRIERS.
2.9.1. NATURAL OXYGEN CARRIERS.
2.9.2. SYNTHETIC OXYGEN CARRIERS.
2.9.2.1. INORGANIC OXYGEN CARRIERS.
(1) . IRON COMPLEX CARRIERS.
(2) . COBALT COMPLEX CARRIERS.
(3) . COPPER COMPLEX CARRIERS.
2.9.2.2. ORGANIC OXYGEN CARRIERS.
(1) . PERFLUOROCARBONS.
(2) . SILICONE OILS.
2.9.3. SUMMARY OF OXYGEN CARRIERS.
2.10. COMPARISON OF DIFFERENT MEMBRANE PROCESSES FOR OXYGEN ENRICHMENT.
CHAPTER III. IMMOBILIZED LIQUID MEMBRANES.
3.1. INTRODUCTION.
3.2. EQUIPMENT.
3.3. MATERIALS USED IN THE PERMEATION MEASUREMENT.
3.4. FACTORS INFLUENCING THE EXPERIMENTAL RESULTS.
3.5. MEMBRANE PREPARATION.
3.6. GENERAL OBSERVATIONS AND DISCUSSION.
3.7. COMPARISON BETWEEN EXPERIMENTAL AND CALCULATED RESULTS.
3.7.1. DIFFUSIVITY OF 02 AND N2 IN LIQUID PHASE FC-43.
3.7.2. SOLUBILITY OF 02 AND N2 IN LIQUID PHASE FT-43.
3.8. MEMBRANE STABILITY.
3.9. CONCLUSIONS ON IMMOBILIZED MEMBRANES.
CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES.
4.1. INTRODUCTION.
4.2. CHARACTERISTIC OF PLASMA TREATMENT.
4.3. EQUIPMENT AND MATERIALS.
vii 4.3.1. EQUIPMENT.
4.3.2. MATERIALS USED IN THE EXPERIMENT. 4.4. CONSIDERATION OF EXPERIMENTAL CONDITIONS. 4.4.1. CHOICE OF SUBSTRATES. 4.4.2. POSITIONS OF THE ELECTRODES AND SUBSTRATES. 4.4.3. CHOICE OF OPERATING CONDITIONS. 4.5. GENERAL OBSERVATIONS AND DISCUSSIONS. 4.5.1. SILICONE RUBBER MEMBRANES. 4.5.2. PROPOSED MECHANISM OF REACTION UNDER PLASMA.
4.5.3. MICROPOROUS MEMBRANES. 4.6. INFRARED SPECTRUM INTERPRETATION. 4.7. SURFACE ENERGY ANALYSIS. 4.8. MORPHOLOGY OF THE PLASMA-POLYMERIZED MEMBRANES. 4.9. CONCLUSIONS ON THE PLASMA-TREATED MEMBRANES. CHAPTER V. PHOTOSENSITIVE MEMBRANES. 5.1. INTRODUCTION. 5.2. MATHEMATICAL ANALYSIS OF FACILITATED TRANSPORT. 5.3. CHEMICAL ASPECTS OF THE CARRIERS. 5.4. SYNTHESIS OF ANTHRACENE DERIVATIVES. 5.4.1. MATERIALS USED IN THE SYNTHESIS. 5.4.2. SYNTHETIC PROCEDURES.
5.5. SPECTROPHOTOMETRIC STUDIES. 5.5.1. OXYGENATION AND DECOMPOSITION EXPERIMENTS BY SPECTROPHOTOMETRIC STUDIES. 5.5.2. SUMMARY OF THE SPECTROPHOTOMETRIC STUDIES.
5.6. PERMEATION TEST FACILITY. 5.7. GENERAL OBSERVATIONS AND DISCUSSION. 5.7.1. IMMOBILIZED LIQUID MEMBRANES SUPPORTED IN POROUS MEMBRANES. 5.7.1.1. ONE LAYER MILLIPORE MEMBRANE.
viii 5.7.1.2. ONE LAYER DYED MILLIPORE HVHP. 5.7.1.3. MULTI-LAYER SANDWICH MEMBRANE CONFIGURATION. 5.7.1.4. THREE LAYERS METAL FILTER CONFIGURATION. 5.7.2. DENSE MEMBRANES WITH FIXED CARRIERS. 5.7.3. DENSE MEMBRANES SWOLLEN WITH CARRIER SOLUTION. 5.7.4. CARRIER STABILITY.
5.7.5. CONTROL OF LIGHT PENETRATION.
5.8. ABSORPTION AND DESORPTION EXPERIMENTS. 5.9. KINETICS OF PHOTOCHEMICAL REACTION. 5.10. CONCLUSIONS ON PHOTOSENSITIVE MEMBRANES. CHAPTER VI. COMPARISON OF DIFFERENT MEMBRANES. 6.1. EFFECTIVENESS OF DIFFERENT MEMBRANES FOR GAS SEPARATION.
6.2. FEASIBILITY OF MANUFACTURE OF DIFFERENT MEMBRANES. CHAPTER VII. GENERAL CONCLUSIONS AND RECOMMENDATIONS. 7.1. CONCLUSIONS. 7.2. RECOMMENDATIONS. APPENDIX I. THE PROPERTIES OF OXYGEN AND NITROGEN APPENDIX II. PERMEATION PROGRAM IN COMPUTER LANGUAGE C. APPENDIX III. SURFACE ENERGY AND CONTACT ANGLE.
APPENDIX IV. SILICONE RUBBER/COPPER COMPLEX MICELLE MEMBRANES.
APPENDIX V. CALCULATIONS OF HENRY'S LAW PERMEATION AND FACILITATED TRANSPORT UNDER IDEAL CONDITIONS. NOMENCLATURE. REFERENCES.
ix CHAPTER I. INTRODUCTION.
CHAPTER I. INTRODUCTION.
The object of this study is to develop a simple, low energy oxygen enrichment process based on membranes. As an example reported recently, a considerable amount of energy can be saved if oxygen-enriched air is used in a furnace (Figure 1.1). In this situation, even 30% to 40% oxygen-enriched air can have a substantial effect. The question is whether a large amount of oxygen can be produced at low cost.
A comparison of oxygen-enriched air for different methods was made by Matson et al [1986] in Table 1.1 and Imafuku [1985] in Figure 1.2. It can be seen that the membrane method is attractive, especially for the relatively low concentration and small quantity of oxygen produced.
Table 1.1. Specific energy requirement for oxygen production from air.
Type of process Product Energy required kW-hr/ton EPOa
Reversible, isothermal 100% 0^(gas)at 1 atm 31
Cryogenic 99.5% 02(gas)at 1 atm 275 - 375b
Cryogenic (large) 99.5% 02(gas)at 450psig -750
Cryogenic (large) Liquid oxygen 950-1100
Cryogenic (large) 90% 02(gas) at 1 atm -250
Pressure-swing absorption 90% 0^(gas) at 1 atm 400
Single-stage permeatorC 30-35% 0^(gas)at 1 atm 460-1030
Single-stage permeator 43% 0^(gas)at 1 atm 260
0 Equivalent pure oxygen. Depends on size of plant, with smaller plant requiring more energy. Silicone-polycarbonate copolymer with separation factor 2.2. Poly(phenylene oxide) with separation factor 4.8.
-1- CHAPTER I. INTRODUCTION.
*1927 C (3600 F)
371°C (2500°F)
Concentration of oxygen used. Z Figure 1.1. The effect of oxygen concentration on gas savings in furnaces (reproduced from Kimura and Browall [ 1986 ]) .
Cryogenic method
PSfl method 1
Mentorane method »—» v . . i • . I
CO o (J
10 [0 '0 30Z oxygen enriched air (Nm3/hr) Figure 1.2. Oxygen costs produced by different methods (reproduced from Imafuku [1985]).
-2- CHAPTER I. INTRODUCTION.
Stage cut 1/20
30Z oxygen
Pressure ratio 1:10
Separation factor.
Figure 1.3. Relative power requirement vs separation factor (reproduced from Kimura and Browall [1986]).
StoQe out 1/20
30Z oxygen
Preseure ratio 1 MO
Separation factor
Figure 1.4. Concentration of oxygen vs separation factor (reproduced from Kimura and Browall [1986])
-3- CHAPTER I. INTRODUCTION.
The cost of oxygen-enriched air produced by a membrane process depends on the permeability and selectivity of the membranes. The relative power requirements for membranes of different selectivity are given by Kimura and Browall [ 1986 ] in Figure 1.3. It can be seen that increased selectivity can dramatically decrease the cost of production, especially as the separation factor changes from 2 to 6. Figure 1.4 shows the possible concentration of oxygen in the product in one stage as the separation factor increases.
Some gas-separation membrane processes have already been used on a commercial scale [Schell, 1985], The Monsanto Company has designed the PRISM™ membrane system to recover 90 percent of hydrogen from synthesis loop purge gas [Maclean et al, 1979]. Simonet [1985] reported the recovery of methane from landfill gas, Stern et al [1965] the recovery of helium with a membrane permeator, and Gardner et al [1977] the use of an hollow fibre permeator in an ammonia plant. A recent review of membranes applied to gas separations was made by Rigby and Jones [1990].
As interest grows, the market could increase sharply over the next few years. As estimated by Weber and Bowman [1986], the market for membrane systems in gas separation is expected to grow from $25 million in 1985 to about $500 million in 1995. The largest market consumption would go to hydrogen recovery, which is estimated to reach about $200 million in 1995. Many companies, such as Du Pont, Monsanto, Dow and others have already developed and tested their equipment in the laboratory and in the field. The second largest application of membrane processes in gas separations would be carbon dioxide separation from natural gas, which could reach about $150 million in 1995. The third application could be the production of oxygen- enriched air with a total value of more than $100 million
-4- CHAPTER I. INTRODUCTION.
in 1995.
A process giving oxygen enriched air can be considered as a separation process of oxygen from nitrogen because they are the main components of air. Therefore, we usually discuss the separation of oxygen from nitrogen instead of the production of oxygen enriched air. The normal composition of air and the properties of oxygen and nitrogen are given in Appendix I.
Generally speaking, most of the effort in membrane research has been directed towards the search for a semipermeable material with high permeability and high selectivity. This is also the main purpose of this study. Although membrane processes have been known for a long time, they are still in their infancy and the application of membrane processes to gas separation is very limited, simply because of the lack of suitable semipermeable membranes.
Several alternative membrane techniques for the separation of oxygen and nitrogen have been examined in this study. The thesis describes the development, characterization and comparison of different types of membrane concept. The thesis is divided into Seven Chapters
Chapter I is the introduction.
Chapter II is a review of literature. It is the author's intention to give a general picture of the situation of the development of membrane science and technology with emphasis on oxygen separation.
Chapter III reports the immobilized liquid membrane studied in the earliest stage of the experimental work. Liquid perfluorocarbon and silicone oil membranes were examined in this period. In addition a modified silicone rubber
-5- CHAPTER I. INTRODUCTION.
membrane was examined for comparison with the liquid membranes.
Chapter IV describes an investigation of plasma-polymerized membranes. A silicone rubber was used as substrate and different fluorine and silicon containing compounds as monomer.
Chapter V presents the photosensitive membrane studies. The concept in based upon the reversible oxygen binding properties of certain aromatic species. Anthracene and its derivatives were used in our experimental investigations.
Comparisons between immobilized, plasma-polymerized and photochemical membranes are made in chapter VI and general conclusions are drawn in chapter VII.
In addition, several appendices are included that detail certain procedures and calculations.
The International System (SI) Units are used in this thesis. Some conventional units such as permeability, activation energy, etc. are also used in order to compare the data obtained with other works. Some empirical or semiempirical equations are used as they are in their conventional units. Details of the SI units used are shown in the NOMENCLATURE of the thesis. Other units, used in some empirical or semiempirical formula, are given accordingly.
-6- CHAPTER II. LITERATURE REVIEW
CHAPTER II. LITERATURE REVIEW.
2.1. INTRODUCTION.
Membrane science and technology has become a fast developing area in recent years. It would be a big effort to cover every aspect as well as the important historic events. This review is focussed on the more recent developments in membrane science and applications as they apply to gas separation, and air separation in particular. More general reviews can be found elsewhere [Barrer and Chio, 1965; Lonsdale, 1982; Matson et al, 1983; Park, 1986; Stannett, 1978; Stannett et al, 1979; Stern and Frisch, 1981; Strathmann, 1981; Strathmann, 1986].
The first part of the review gives general descriptions of membrane gas separations --membrane modules in Section (2.2) and two important parameters, permeability and separation factor in Section (2.3) and then the factors which affect these two parameters: diffusivity and solubility of gases in liquids or polymers. After that come the alternative ways to improve the separation performance of a membrane: asymmetric membranes in Section (2.4), immobilized liquid membranes in Section (2.5) and facilitated transport in Section (2.6). Photosensitive membranes are considered in a separate section, Section (2.7), because of their particular interest in this study although they belong to the immobilized liquid membrane category. Section (2.8) describes recent developments in membrane materials including special polymers and modifications. Oxygen carriers as additives into liquid membranes to achieve facilitated transport used in oxygen enrichment come in Section (2.9). Finally, a comparison of the different processes for oxygen enrichment is given in Section (2.10).
-7- CHAPTER II. LITERATURE REVIEW
2.2. CONFIGURATION OF MEMBRANE MODULES.
The arrangement of separation elements into an effective separation plant requires careful design of membrane modules. Considerable development has been made in relation to practical separation modules.
The plate-and-frame arrangement is the most basic module and is still used in laboratories because of its simplicity due to its simple structure, flow patterns and the ease of data treatment. However, more effective modules such as hollow fibre (Figure 2.1) and spiral-wound (Figure 2.2) devices, have become popular in industrial practice. They provide more permeation area than the flat sheet module in the same volume of equipment.
The arrangement of modules has also been developed in this period. Firstly, a process with a single stage was studied for different flow patterns: perfect mixing, cross flow, countercurrent and cocurrent flow [Blaisdell and Kammermeyer, 1973; McCandless, 1984(a), 1984(b) and 1985; Pan and Habgood, 1974; Stern and Wang, 1978; Walawender and Stern, 1972]. Except for the first case, it was assumed that there was no mixing in the processes. Following this the concept of recycling was introduced [Paul, 1971] and a multistage system was developed in order to improve the efficiency of the separators [McCandless, 1985; Thorman et al, 1975]. Inspired by the concept of distillation towers, Hwang et al developed the membrane column system [Chen et al, 1986; Hwang et al, 1980; Hwang and Thorman, 1980]. The advantage of this is that the high concentration for both the most permeable and the least permeable components can be reached in the column system at the same time. The disadvantage is that productivity would be decreased for the same area of the separator.
-8- CHAPTER II. LITERATURE REVIEW
Filtrate Feed solution Shell tube Concentrate
Hollow fiber
Epoxy resin
Figure 2.1. Schematic diagram of a hollow fibre module, (from Strathmann, 1981)
Cover leaf
Feed solution
Spacer screen
Filtrate Permeate i flow path
p orous Membrane support
Membrane
Figure 2.2. Schematic diagram of a spiral-wound module (from Strathmann, 1981).
-9- CHAPTER II. LITERATURE REVIEW
A multistage system is suitable for some precious materials. However, any recycling will increase the membrane area and power consumption. It would be less attractive for oxygen enrichment to use the recycled system because the raw material is abundant [Majumdar et al, 1987 ] .
2.3. PERMEABILITY AND SEPARATION FACTOR OF A MEMBRANE.
In general, gas permeation through a membrane is a complex process. Firstly, there are many different types of membrane - homogeneously dense, microporous, asymmetric and immobilized liquid membranes. Different models have therefore been developed to describe the different permeation mechanisms. However, the permeation coefficient or permeability which is employed to describe the mass transfer process across the membrane in a steady state is fixed by a definition which is independent of the module or membrane used. The gas flux, in terms of cubic metres of the penetrant per square metre per second, is given by,
J =PCT ------(m3/m2 -sec) (2.1) L where P -The permeation coefficient or permeability, which is expressed in the units of m3 (STP)-m/m2-sec-Pa (The units are usually expressed as cm3 (STP)-cm/cm2-sec-cmHg in order to compare with different sources of data); p and p - The partial pressure of penetrant gas in h 1 the upstream and downstream respectively, Pa; L - The thickness of the membrane, m.
The Arrhenius form of permeability is given as [Barrer, 1939a and 1939b],
-10- CHAPTER II. LITERATURE REVIEW
P = P exp(-E /RT) (2.2) SI O p where P - Pre-exponent factor of permeation; o E - Activation energy of permeation, J/mol; p R - Gas constant, 8.31439 (J/mol-K); T - Absolute temperature, °K.
This equation shows the temperature dependence of the permeability. The activation energy of permeation E is p always positive. Therefore, the permeability always increases with a temperature rise.
The gas separation is based upon the different flow rates for the different gas species. For a binary system separated by a membrane if x represents the molar percentage of penetrant at the permeate side of a membrane and y at the retentate side of the same membrane, then the separation factor can be given as a ratio of permeabilities of gas species 1 to that of gas species 2,
X(1 - y) (PSI)X (2.3) “ " y(l - X) (psl>2
2.3.1. MICROPOROUS MEMBRANES.
When a gas or gas mixture passes through membrane pores, various types of flow may occur: (a) dissolution/diffusion flow, (b) slip flow, (c) surface flow [Kamide et al, 1980; Kamide et al, 1982], (d) viscous flow and (e) free molecular flow. The dissolution/diffusion flow only takes place in a dense part of the membrane matrix. The slip and surface flow occur for absorbed species of the penetrant. The first three flows are usually not significant in a membrane process. Therefore, one only needs to consider the viscous flow and free molecular flows.
-11- CHAPTER II. LITERATURE REVIEW
Knudsen first studied the free molecular flow and pointed out that if the mean free path of molecules is larger than the diameter of the pores, the gas flow rate in the case of no slip and surface flows is given by the equation [Bradley and Baker, 1971],
4rf - 2RT -| 0.5 p,-p, F . ------(mol/m2 .sec) (2.4) D 3 r - ttM J LRT where M - Molecular weight; r - Radius of pores, m; f - Porosity; r - Tortuosity of the membrane matrix normally greater than 1 [Shindo et al, 1985].
Suppose that two gases with molecular weight M and permeate through a membrane by Knudsen diffusion. Combining equation (2.3) and (2.4), the separation factor can be expressed by the ratio of the molecular weights as,
(2.5)
Some data calculated from equation (2.5) are given in Table 2.1. It is clear that the separation factors based on Knudsen flow are very low, except for the separation of hydrogen or helium from other gases [Agrawal and Sourirajan, 1970; Ash et al, 1970]. In particular, the separation factor for 0 /N is only 0.94 based on Knudsen 2 2 flow (or separation of 1.06 for /O^ ) . Therefore, the separation of interest usually cannot be achieved with microporous membranes, despite their high permeability in comparison with homogeneous dense membranes.
-12- CHAPTER II. LITERATURE REVIEW
Table 2.1. Separation factors based on the Knudsen flow.
System Separation factor System Separation factor
H /CO 4.67 He/Ar 3.16 2 2 H /0 3.98 He/O 2.83 2 2 2 H /N 3.73 He/N 2.65 2 2 2 H2 /CO 3.73 N /0 1.06 2 2 H /CH 2.82 N /Ar 1.19 2 4 2 H /He 1.41 0 /Ar 1.12 2 2
When the mean free path of the molecules is smaller than the diameter of the pores, viscous flow (Poiseuille flow) will dominate the permeation process. The gas flow rate for the Poiseuille flow is given by,
2 2 F . Ph~Pi (2.6) 3 8/iTT LRT where n - Viscosity of the gas mixture, N-sec/m2 .
It can be seen from the above equation that all gas species will move with the same average velocity, and separation based on this type of flow is impossible. It should be noticed that these two processes often occur in one membrane with different intensity depending upon the pressure for a particular system. Bradley and Baker [1971] and Huckins and Kammermeyer [1953] measured permeation of porous glass and polymers and found the variability from the Knudsen flow was caused by the combination of free molecular and viscous flow. The contribution of Knudsen and Poiseuille flow is given in Figure 2.3.
-13- CHAPTER II. LITERATURE REVIEW
Total flow
2 0.8 Poiseuille flow
q) 0.4
Knudsen flow
Ratio of radius to mean free path
Figure 2.3. Knudsen, Poiseuille and total flows (reproduced from Bradley and Baker [1971]).
It is important to notice that other transfer mechanisms could occur in microporous media as well as Knudsen and Poiseuille flow. Surface flow, for instance, plays an important role in gas separation processes using microporous media [Ash et al, 1976; Hwang and Kammermeyer, 1966; Kammermeyer and Wyrick, 1958; Tamon et al, 1985] to separate gas species with similar molecular weight.
It is interesting to notice that analysis of the extent of Knudsen and Poiseuille flows could lead to a method of characterising the pore structure, such as pore distributions [Altena et al, 1983; Choji et al, 1985; Yasuda and Tsai, 1974].
-14- CHAPTER II. LITERATURE REVIEW
2.3.2. HOMOGENEOUS MEMBRANES.
Homogeneous dense membranes are usually made of polymers. Silicone rubber and polycarbonate, for instance, are common membrane materials. In comparison with other rigid materials, polymers are often soft because of the flexible long chain structure in a polymer. A polymer in this state would be similar to a liquid to some extent. The permeability of a polymer will be determined by the flexibility of the chains [Park, 1986]. In a "rubbery" state, one can imagine that the polymer would be similar to rubber, the polymer could have more flexible chains in which "holes" or free volume may exist. The free volume provides a way for the permeant molecules to jump from one place to another. It is not necessary that the holes or free volume exist permanently. It is a dynamic picture: a hole is activated and formed to accommodate a molecule when a penetrant molecule jumps in and the hole disappears after the molecule jumps out. The theory developed to interpret permeation this way is called free volume theory [Kumins and Kwei, 1968].
A homogeneous dense membrane can change from its amorphous soft state to its "glassy" state if the temperature decreases to a certain degree, which is often referred to as the "glass transition temperature". Poly(bis-phenol A carbonate) and polydimethylsiloxane, for example, have glass transition temperatures of 149° C and - 123 °C respectively [Brydson, 1982].
(1). STEADY STATE PERMEATION.
If a polymer membrane is used at a temperature higher than its "glass transition temperature", it will be in the rubbery state. The permeation of gases in this case can be described by the solution/diffusion model, where Fick's
-15- CHAPTER II. LITERATURE REVIEW
first law of diffusion and Henry's phase equilibrium law are employed. In this model, the permeation process involves the steps of (1) diffusion of a gas from the bulk stream to the interface; (2) absorption into the membrane matrix; (3) diffusion through the membrane; (4) desorption from the membrane matrix; and (5) diffusion through the boundary layer to the downstream.
Fick's first law of diffusion describes the diffusion flow rate, which is the amount of substance diffusing across unit area in unit time in the x - direction, as [Crank, 1987],
F = - D (ac /ax) (mol/m2-sec) (2.7) j j j where C - The concentration of j species, mol/m3
Dj - The diffusion coefficient or diffusivity of j
species, m2/sec.
The equation shows that the flow rate is proportional to the concentration gradient. The minus sign implies that the diffusion occurs down the concentration gradient. Normally, the diffusivity is a function of temperature, concentration and direction. In this study, the diffusivity is considered to be a constant at all times except where mentioned otherwise. The subscript j may be omitted if only one component is diffusing.
Diffusion can occur in all directions. For an element of volume in Cartesian co-ordinates with differential sides of dx, dy and dz in its x, y and z directions, the diffusion can bring about concentration changes in the element and the quantitative description of mass transfer can be given by Fick's second law with constant diffusivity [Crank, 1987; Crank and Park, 1968], i.e.,
-16- CHAPTER II. LITERATURE REVIEW
ac ( a2c a2c a2c ^
— ——+ ——+ •—— at = ° 1{ ax ay az J } <2-8)
However, the diffusivity can be a function of concentration and direction. In this case, Fick's second law becomes,
ac a r dC i a r aci a r ac n + n + n at ax - ax - ay L 3yJ a z - a z
In many cases, the diffusion is only significant in one direction in a process whereas those in the other directions are negligible or do not occur. When the diffusion takes place across a flat sheet or a slab, it can only take place perpendicularly to the surface. The simplified second Fick's law becomes,
ac a2c ---- = D --- j- (2.10) at ax where D is the diffusion coefficient or diffusivity. Usually, the diffusion coefficient in gas phases is about a thousand times higher than that in a polymer phase. So steps (1) and (5) are often a thousand times faster than step (3). Furthermore the absorption and desorption processes are assumed to obey Henry's law [Stannett, 1978], i.e. ,
C = Sp (2.11) where S - Solubility of the gas in the polymer film, mol/m3-Pa.
Then step (3) becomes the limiting step and the resistances of (1), (2)/ (4) and (5) can be neglected, as mentioned
-17- CHAPTER II. LITERATURE REVIEW
above, and the diffusion in the membrane matrix will dominate the whole transfer process. If D and S are independent of the pressure, which is particularly true for oxygen and other inert gases but is invalid for most organic gaseous species such as methane and ethane and so on, equation (2.10) can be integrated between the limits p=p (x=0) and p=p (x=L). For steady state permeation, i.e. 1 h 3C/at = 0, the gas flux can be expressed as,
ph-Pi F SD (mo1/m2-sec) (2.12) L or,
F = (2.13)
Therefore, for most gases at a low pressure, the permeability can be expressed as a product of solubility and diffusivity,
P = SD (mol-m/m2-sec-Pa) (2.14) m
The permeation data can be converted to the data under standard conditions P and T by, o o
J(m3/m2-sec-Pa) = 0.0224F (mol-m/m2-sec-Pa) (2.15) and the permeability can be converted to the conventional units by a factor,
P = 2.99x105P (cm3(STP)-cm/cm2-sec-cmHg) (2.16) m or SI units,
P = 0.0224P (m3(STP)-m/m2-sec-Pa) (2.17)
-18- CHAPTER II. LITERATURE REVIEW
It can be seen from equation (2.14) that gas permeation through a membrane is governed by interaction between the gas and polymer film. More precisely, it depends upon an ability for the gas to be dissolved into the membrane domain, i.e. solubility, and a mobility of the gas inside the membrane domain, i.e. diffusivity. In fact, these parameters which determine gas flow rate can become very important when selecting a suitable material for a particular separation process.
The constant value of diffusivity and solubility and then of the permeation coefficient or permeability is valid for gases well above their critical point (e.g. , He, 0^ , around room temperature). In most cases, the diffusion coefficient D and solubility S change with pressure, especially for more condensable gases such as carbon dioxide, hydrogen disulfide, and organic vapour at a high partial pressure. For a certain pair of gas and polymer, a particular temperature called the critical temperature T c exists. The diffusion and permeation coefficients show a strong concentration dependence even if Henry's law is obeyed [Stern and Frisch, 1981] when the temperature is below the critical temperature. Therefore, the general form of flux equation should be,
F (2.18)
and the mean permeation coefficient will be,
_fph DSdp (2.19) ph-PiJPi
The concentration or pressure dependence of solubility and diffusivity usually needs to be considered if organic
-19- CHAPTER II. LITERATURE REVIEW
vapours are involved in a permeation process [Fujita, 1968] .
As the temperature decreases, the chains of polymer molecules continuously lose their mobility. If the temperature is lower than the "glass transition temperature", the polymer becomes hard and brittle. There will be microvoids permanently existing in the membrane polymer matrix. In this case, the polymer membranes will be more or less similar to other microporous media. This sorption model, known as the dual-sorption theory, was discussed by Meares [1954 and 1986] and has been comprehensively reviewed by Chern et al [1983], Vieth et al [1976], Stannett et al [1979], Petropoulos [1984] and Schell and Houston [1983]. The earliest version of dual mode theory says that the sorption will involve two concurrent processes: (i) a Henry's law sorption component which is the mobile part of the sorption contributing to the diffusion process, and (ii) an immobilized Langmuirian adsorption component which takes place at a fixed number of sites that has no contribution to the diffusion at all. However, this theory was modified following a series of experimental studies [Paul and Koros, 1976]. It was accepted that the Langmuirian component also makes its contribution to the transport. The quantitative expression of the solubility of a gas in a membrane matrix in dual mode theory is given as [Stannett et al, 1979],
CHbP C=CD+C = k p+ —22--- (2.20) 1+bp where C - The solubility, mol/m3 ; CD“ Henry's law contribution to the solubility, mol/m3; C - Langmuirian contribution to the solubility, mol/m3; kj> ” T^e Henry's law dissolution constant, mol/m3-Pa;
-20- CHAPTER II. LITERATURE REVIEW
C' - Hole saturation constant, mol/m3 ; H b - Hole affinity constant, Pa'1 .
The hole affinity and saturation constants represent the characteristics of Langmuirian adsorption.
If these three constants are independent of pressure, the mean permeation coefficient can be expressed as,
CHDHb P =k_.D_ + (2.21) m D D 1+bp or,
KR n P =kD (2.22) m D D 1+bp J and,
K = CH'b/kD (2.23)
R = VDD (2.24) where D is the diffusivity of Henry's law absorption D component, is the diffusivity of adsorbed component, and K is characteristic of the relative amounts of gas absorbed by the two mechanisms and R is the characteristic of immobilization for Langmuirian sorption. The value of R changes from zero to one, i.e. R=1 for total mobility and R=0 for total immobilization of the Langmuirian absorption.
It is important to recognise the implication of dual-mode adsorption in the membrane separation practice. The transport of interest may be significantly reduced when switching from pure gas to a multicomponent mixture because of competitive absorption [Chern et al, 1983] (i.e. ' may be reduced by a second gas species).
-21- CHAPTER II. LITERATURE REVIEW
(2). TRANSIENT PERMEATION - DETERMINING SOLUBILITY AND DIFFUSIVITY IN A POLYMER.
In the oxygen and nitrogen separation process the diffusion coefficients for both gases are quite similar because of the similar molecular weight and size of oxygen and nitrogen. This means that the separation is mainly based on the difference in solubility of the two gases. Many experimental studies have contributed to measure these two parameters individually in a single experiment, the so- called time-lag experiment.
The solubility measurement of various gases in polymers has been a target of interest for a long time, and there are many useful results available now. It seems that an amorphous elastic polymer possesses a property similar to an organic liquid: a gas with polar molecules is likely to be dissolved in a polymer with a polar repeating unit and a gas with non-polar molecules in a polymer without a polar repeating unit [van Amerongen, 1946].
There are numbers of measurement methods to determine the solubilities and diffusivities of dense and microporous membranes because of their essential importance in membrane science and technology. The time-lag experiment, which was considered as a major breakthrough in this area, was developed by Daynes in the early 1920's [Daynes, 1920], and is still being used in many laboratories.
Considering Fick's second law, the equation can be integrated for a flat sheet (one dimensional transfer) under certain conditions. Daynes used the following boundary conditions and initial conditions:
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C = C for x = 0 and all t, (2.25) o C = 0 for x = 1 and all t, (2.26) C = 0 for t = 0 and all 0 < x < L, (2.27) a C/at = 0 at t = *. (2.28)
The solution is,
2C r—' 1 n?r x r n^r 2- C= --- \ --- Sin ----- exp I - (--- ) (2.29) 7r l~* n L L l -
The mass balance for diffusion at the boundary x=L into a chamber with a volume V and concentration C is given by,
dCv ac V =-D( ) (2.30) dt ax x=L
On the other hand, from equation (2.29), by differentiating with respect to x and putting x=L, the total amount of the permeation through the area A at the time t is,
Qt= ad J^- (ac/ax)x=Ldt (2.31)
The gas flows through the membrane into a low pressure (vacuum chamber). The concentration in the low pressure chamber can be obtained by combining equation (2.30) and (2.31) and integrating between t=0 to t=t,
r, 2 2+ -Dn n t (-1) exp (2.32)
If the time is long enough, the above equation can be reduced to,
-23- CHAPTER II. LITERATURE REVIEW
Dt 1 (2.33) LC 6
This is a linear equation (Q^ vs t). Extrapolation of this linear part to the time axis gives the time lag t=a, and the diffusion coefficient can be calculated accordingly, i.e. by,
a = L2/6D (2.34)
Therefore, it is possible to measure the permeability, the diffusivity and solubility via equation (2.1) and (2.34) in a single experiment. The term "time lag" has been applied since then. Barrer [1939(a) and 1939(b)] used the same method in a vacuum downstream system which was easy to carry out in a laboratory. A typical curve of downstream pressure against the time is given in Figure 2.4. A time- lag, a, can be obtained from this graph.
Pi
0 o Time, t
Figure 2.4. A typical curve of pressure versus time
-24- CHAPTER II. LITERATURE REVIEW
It has been shown by several investigators [Stannett, 1968; van Amerongen, 1946] that the solubility calculated from the permeation measurement is in agreement with the direct absorption and diffusion measurement.
Some modified versions of the time lag measurement have been developed because it is such an excellent method to determine the permeability and diffusivity. The method has been extended to some more complicated cases, such as the concentration dependent diffusivity made by Frisch [1957 and 1958]. Considering the dual mode sorption model, Paul [ 1969 ] and Paul and Kemp [ 1973] extended the time lag method to the system containing completely immobilized absorption and then modified it to the partially immobilized absorption [Petropoulos, 1984; Paul and Koros, 1976]. Although the earlier time lag expression was developed for flat sheet membranes, alternative time lag measurements for cylindrical and spherical systems have also been developed [Ash and Barrie, 1986]. Meldon et al [1985] summarized the time lag method applications if a chemical reaction takes place in a membrane process.
Another convenient expression using the same boundary conditions, except for dC/dt=0 (t=«), can be derived by using Holstein's solution [Crank, 1987],
2 2 DSC r D (2m+l) L F exp r- (2.35) 3x J x=L 7T t- 4Dt
For a small t, Rogers et al [1956a and 1956b] took only the leading terms and obtained,
ln(t*F) = ln[2C (D/tt)*5] - --- (2.36) ° 4Dt
The diffusivity can be determined by using the graph (t J)
-25- CHAPTER II. LITERATURE REVIEW
against (1/t).
2.3.3. DIFFUSIVITY OF GASES IN POLYMERS.
The diffusion coefficient for polymer membranes was first given by Barrer [1939(a) and 1939(b)] in the Arrhenius form of activation energy and was called activated diffusion as,
D = D exp(-E /RT) (2.37) o d
The significance of this equation is that the permeation is temperature dependent, i.e. the activation energy which may refer to the energy acquired by the system to make way for movement of the molecules is positive. A quantitative expression was experimentally formed by van Amerongen [1946] for 0 N and CO in elastomers as, L 2,2 2
1 1 D = 6.3xl0“5exp [-E(--- — ---- )] (cm2/s) (2.38) RT 870
Gas or vapour diffusion through a polymer is considered to be a complicated process. The process has still not been fully understood even though many contributions have been made over half a century. However, the situation seems to be less complicated with gases than vapour. Firstly, gases are usually closer to the ideal state especially not far from atmospheric pressure. Secondly, the solubility of gases in a polymer tends to obey Henry's law. In other words, the diffusion of gases is independent of the concentration [Stannett, 1968]. Thirdly, gases, especially permanent gases such as oxygen, nitrogen, argon etc, do not cause swelling of a polymer. All this may lead to a simpler treatment of the permeation process.
Permeation expressed by Fick's first law can be used for 02 in developing the relation between the driving force and
-26- CHAPTER II. LITERATURE REVIEW
the diffusion coefficient.
Firstly, considering the chemical potential /i , mass transport can only occur if there is gradient d^ /dx. For a molecule, the driving force can be expressed as [Park, 1986] ,
x. = - (l/N)dM./dx (2.39) D 3 where N is Avogadro's number and the flux can be expressed as,
F. = - (1/Nf. )C. (dM./dx) (2.40) where f is a velocity-dependent frictional resistance and j C is the concentration of the j species. Comparing the j chemical potential and Fick's first law, the following expression is given for an ideal solution,
D = RT/Nf (2.41) j j and for a non ideal solution,
D = (RT/Nf )(dlna /dlnC ) (2.42) j j j j where a is the activity of the species j. The diffusion j coefficient D is called the "thermodynamic" diffusion j coefficient and the group (RT/Nf ), the "self" diffusion coefficient, can be given by utilizing Stoke's law for a rigid spherical molecule with radius r in a low-molecular weight viscous liquid, i.e. the resistance,
f = (2.43) j where n is the viscosity of the medium. Therefore,
-27- CHAPTER II. LITERATURE REVIEW
D = RT/67r/iNr (2.44)
This is called the Stokes-Einstein equation. The application of this equation is very limited because the model does not represent most physically realistic conditions. The difficulty in expressing the permeation property of polymers is due to the special property of polymers: they are not as mobile as a gas or a liquid which permit a diffusant to move around relatively freely and not as rigid as typical crystals which make the permeation of the diffusant, especially of the molecules larger than the crystal lattice, nearly impossible. However, the similarity between liquids and elastomers makes it possible to extend some of the diffusion law of liquids to elastomers. No theory or model, of course, can give a satisfactory explanation to all permeation phenomena. The most acceptable theory of permeation in polymers may be the free volume theory which has been developed for a long time and to which many investigators have made their contributions.
The free volume theory suggests that there are "holes" or free volumes which can accommodate penetrant molecules in the polymer lattice. The diffusing molecule jumps from one hole to another if the jump is successful. Whether the jump of a molecule makes successful mass transfer depends upon the energy of the molecule and the segment of polymer. Kumins and Kwei [1968] have given a comprehensive review of free volume theory.
2.3.4. SOLUBILITY OF GASES IN POLYMERS.
A membrane based oxygen separation from air involves transport of oxygen and nitrogen. The diffusion coefficients of oxygen and nitrogen should not be significantly different because of the similarity of the physical properties of these two molecules, i.e. the
-28- CHAPTER II. LITERATURE REVIEW
similar molecular weight and size. On the other hand, the solubilities may differ from each other. The solubility of 02 in water, for example, is twice as high as that of .
It seems reasonable to develop a theory of solubility of gases in polymers from thermodynamic theory because of the similarity between most elastomers and liquids. Therefore, the thermodynamic theory of liquids which has been developed for quite a long time can be modified to describe polymer systems.
The criterion for equilibrium is the equality of the chemical potentials for both phases, i.e. for phase equilibrium conditions, the following equation will stand,
A4 = M (2.45) g p
The absorption isotherm is usually linear if there are no fillers in the polymer. Henry's law is obeyed for both amorphous and even glassy polymers when the pressure is up to one atmosphere [Stannett, 1968]. The Arrhenius form of solubility is,
S = S exp(-AH /RT) (2.46) O 8 where aH is the heat of solution which can be expressed as B the molar heat of condensation, AH , and the partial c o n d molar heat of mixing, aH^ ,
AH =aH + AH (2.47) ■ c o n d 1
The first term in this equation is usually very small for gases in the atmosphere and the second term can be estimated according to solubility parameters,
AH = V (8 - 6 )2
-29- CHAPTER II. LITERATURE REVIEW
where 8and 8^ — the solubility parameter of the gas species and the polymer respectively (or solvent), MPa^;
The solubility of oxygen and nitrogen in a polymer can be calculated according to this equation.
The applicable forms of these equations (2.46) and (2.48) are given in Chapter III of this thesis. The modified forms of these equations can be made using the Flory-Huggins correction considering the molecular size and shown in Section (3.6) of this thesis for an analysis of the immobilized liquid membrane.
2.4. ASYMMETRIC MEMBRANES.
Porous membranes usually have high permeability and poor selectivity. Homogeneous membranes, on the other hand, have lower permeability, but some of them possess good selectivity. However, permeability data reported by Hwang et al [ 1974] show that not one of nearly a thousand membranes was good enough to compete with the cryogenic process in the production of oxygen enriched air. A summary of some permeation data and separation factors is given in Table 2.2.
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Table 2.2. Some permeation data and separation factors for 0 /N [Hwang et al, 1974] 2 2 Permeabi Separation Membrane barrier ° C lity of 0 factor 0 /N 2 2 2 1. Microporous Vicor Glass 25 56700 0.935 (No 7930) 2. Polydimethyl siloxane 25 281 2.15 3. Silastic 500-1 25 276 2.09 4. Silastic RTV-501 25 274 2.04 5. Nitrosorubber 25 29.8 2.76 6. Natural rubber (density 0.9689) 25 8.68 2.73 Gutta percha 25 6.47 2.67 7. Polystyrene 25 7.80 3.09 8. Butadiene rubber 25 6.45 2.96 9. Poly(butadiene-styrene) 25 6.32 2.71 10. Ethyl cellulose (Ethoxy 25 4.43 3.32 49.5%) 11. Polyphenylene oxide 25 3.81 4.41 12. Polyvinyl toluene 23 0.46 7.61 13. Polycarbonate (Lexan) 25 0.300 4.67 14. Polyformaldehyde (acetal) 30 0.17 17.3 15. Cellulose triacetate 25 0.17 5.88 (43% acetyl) 16. Nitrocellulose(Butanol 30%) 25 0.116 16.8 17. Polyvinyl fluoride 25 0.0042 4.67
* The permeability coefficient is expressed in units of [1x10" 0 (cm3 (STP)-cm/cm2-sec-cmHg)] called Barrer.
Both porous and dense membranes have advantages and disadvantages. So by proper combination of the two kinds of membrane, it may be possible to obtain the high selectivity of the dense membrane and the high permeability of the porous one. This leads to the idea of manufacturing a microporous membrane with a dense thin skin [Finken, 1985]. The microporous sublayer would be strong enough to withstand the high pressure difference and the very thin skin would serve as the separation barrier. The series resistance models were developed to quantitatively describe permeation through these membranes. The series model for a two layer composite membrane can be schematically illustrated as [Sirkar, 1977 and 1978],
-31- CHAPTER II. LITERATURE REVIEW
Figure 2.5. Series model from two layer membrane.
The overall permeability is given by,
FL P . P = --- l (2.49) Ap PiU-*) 6 + p ii where 6 is the fractional thickness of the dense layer and P and P are the permeability of the dense and porous i i i layers respectively. The expression for the separation factor for the gaseous species 1 and 2 will be,
Pil 1 + Pi2^1 9 )/Pii2* ------(2.50) Pi2 1 + Pil^1_*)/Piil*
Sometimes, more than two layers are needed in order to have better separation performance. For a multilayer membrane with 1^, 12 , . . .1 , ...1 having permeabilities P^ , P2 , . . .P , ...P , the series model gives the overall i n permeability P as,
(2.51)
These analytical expressions have been used effectively in this study to analyse the performance of plasma-polymerized membranes (Chapter IV).
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2.5. IMMOBILIZED LIQUID MEMBRANES.
A membrane separation process is based on the preferential absorption and diffusion of different gas species. The membrane in this process provides a thin separation barrier (of about 100/im or so) . There is an alternative way to use the concept of the preferential absorption and diffusion and at the same time to use the concept of a thin layer separation barrier, that is a new separation method known as the immobilized liquid membrane.
The general principle of immobilized liquid membranes is that an appropriate liquid material which has more affinity to one or a few species in the gas mixture is put into a microporous membrane. Therefore, the mass transfer mainly occurs through the liquid rather than the polymer domain because the diffusion coefficient in liquids is usually higher than that in solids.
The mass transfer in immobilized liquid membranes can be computed in a similar manner as a polymer membrane according to the sorption/diffusion/desorption model. It is important to notice that the diffusivity in a liquid may be several thousands times higher than that in a polymer. Therefore, a liquid membrane may have a flux thousands of times higher than a polymer membrane.
The main problem in this case is to find a solvent with high solubility and high diffusivity for a special gas species. Some organic liquids, for instance, have a higher solubility for gases than water. Therefore, a high permeability may be obtained if they are used to impregnate a porous membrane and this type of liquid membrane may offer a better separation factor than Knudsen diffusion. Carefully selecting a proper combination of the separation factor and permeability one may make a good separation
-33- CHAPTER II. LITERATURE REVIEW
barrier for a particular gas species.
2.6. FACILITATED TRANSPORT IN MEMBRANE PROCESSES.
It is possible to dissolve some oxygen-complexing compounds into a liquid. In this way, a facilitated transport membrane is constructed. A chemically reactive compound, for instance, can be put into membranes that selectively reacts with oxygen. The additives are often referred to as carriers. That means they can carry oxygen when travelling across the membrane. Thus, an additional mass transfer of oxygen will bring about a high oxygen permeation rate while the transfer of nitrogen and other species cannot be facilitated. This effect will increase the selectivity of the membranes. Pioneering work was demonstrated by Scholander [1960] who employed a haemoglobin solution and was able to promote the transport of oxygen.
The carrier must be chemically stable towards the permanent oxidation, and this means that the reaction must be reversible. The reversible conditions may be generated in different ways. The partial pressure difference, for example, could cause the forward or backward reaction.
Usually there are two types of oxygen carriers used for oxygen enrichment: mobile and fixed carriers. A mobile carrier is one in which a liquid with a dissolved reactive compound is put into the membrane pores to form an immobilized liquid membrane. The carrier molecules can travel freely within the boundary of the membrane. Schultz [1986], Smith et al [1977] and Way et al [1982] have given a general survey of liquid membrane transport. A fixed carrier is one in which the carrier, usually a metal complex which can reversibly react with oxygen molecules, is put into the membrane domain as a component of the polymer, or sometimes as a constitutional unit of
-34- CHAPTER II. LITERATURE REVIEW
copolymer. The penetrant molecules jump from one active site to another, but the carriers cannot move.
In facilitated transport, one oxygen molecule may bind one or two carrier molecules. The chemical reaction can be represented by one of the following equations,
A+0 =A*0 (2.52) 2 2 v or 2A+0 = A .0 (2.53) 2 2 2 where A is a molecule of the carrier. The whole process of mass transfer can be schematically represented by Figure 2.6. From the diagram it can be seen that two diffusion processes occur together. One is the normal Fick's law diffusion and the other the carrier-mediated transport. The latter can be several orders of magnitude higher than the former if a proper carrier material is selected. Therefore, the selectivity of interest in this case is higher than that in the unfacilitated diffusion. The concentration of carrier substance can be altered, so that permeability, and even the selectivity, can be controlled. The main problem in facilitated transport is to find a compound which can reversibly react with oxygen.
Many models have been developed to deal with different situations of the facilitated transport membrane system.
-35- CHAPTER II. LITERATURE REVIEW
Interfacial absorption
upstream downstream
t
Figure 2.6. Schematic concentration profile of facilitated transport of oxygen with a reversible reaction.
Ward III [1970] and earlier Olander [1960] have given the quantitative expressions for two simplified circumstances. In the first case, assuming the reaction(2.52) is a reversible chemical reaction, the general equation for mass transfer in steady state can be given as,
A*N.=R (2.54) where N is the component flux given by Fick's law and R is the local net rate of formation of the i species. Considering one-dimensional transport through a homogeneous medium, the differential mass balances are,
d2C, (2.55) klC0CA k-lCA0 0 ^
-36- CHAPTER II. LITERATURE REVIEW
(2.56) klC0CA k-lCAO
(2.57) k-lCAO " klC0CA where k - Forward reaction constant, m3/mol-sec; i k - Reverse reaction constant, sec"1; i D - Diffusivity of dissolved oxygen, m2/s; o D - Diffusivity of free carrier A, m2/s; A D - Diffusivity of oxygen-carrier complex A02 , m2 /s; A O c - Concentration of free oxygen, mol/m3; ( c - Concentration of free carrier, mol/m3; A c - Concentration of oxygen-carrier complex A02 , A O mol/m3.
If the reaction is sufficiently fast the reaction species will be in equilibrium, i.e.,
(2.58)
Using the boundary conditions with constant upstream and downstream concentrations, i.e.,
^0c =c^0h ( at x=0 ) (2.59) it ' o n ( at x=L ) (2.60) H- o o
The carrier is dissolved in the liquid which is imbibed into the microporous membrane. The total concentration of carrier in both oxygenized and free form is constant, i.e.,
-37- CHAPTER II. LITERATURE REVIEW
L Q(CA+ CA0>dx = M (2‘61)
In the boundary of the membrane, only unbound-oxygen can travel but not the bound one, therefore,
(2.62) x=0 x=L
The equations (2.60) to (2.62) can be solved in the form,
C0h~C01 COh C01. DAOKeqCT*COh C01^ F =Dq(1+G) (2.63) L<1+KeqC0h><1+KeqC01>
and,
KeqCT (2.64) ’0 <1+KeqCOh><1+KeqC01> where h and 1 represent the upstream (high pressure) and downstream respectively; C =C +C is total concentration T A AO of bound carrier and free carrier and G is the flux augmentation factor. The diffusion coefficients of A and AO are assumed to be similar, i.e. D = D , because carrier molecules are usually so big that the complexing does not change their physical properties much.
Schultz et al [ 1974 ] and Goddard et al [1970 and 1974] reviewed different reaction mechanisms in facilitated membrane transport. Schultz [1986] divided the facilitated transport into two categories: simple "free carriers" and "immobilized carriers" transport and each category is further divided into different types. After mathematical analysis, he concluded that the analytical solution of facilitated transport is only available for very restricted conditions. For a non-equilibrium chemical reaction or multicomponent reaction a numerical solution may be
-38- CHAPTER II. LITERATURE REVIEW
necessary. Schultz also pointed out that the ratio of reaction constant to diffusion coefficient plays an important role in developing an analytical solution for the mass transfer equation. The Damkohler number, defined as the ratio of reaction time constant to diffusion time constant, could be used to identify the reaction regimes. As shown in Figure 2.7, the analytical solution existed only in two extreme conditions.
This type of analysis has proved useful in the interpretation of the results obtained in this study for photosensitive membranes (Chapter V - Photosensitive membranes).
Diffusion Non-equilibrium Equilibrium * regime regime needs special regime mathematical technigues
k r C r Reaction time constant -.-1 D/L ~ l Diffusion time constant -I
Figure 2.7. The reaction regime versus Damkohler number (reproduced from Schultz [1986]).
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2.7. PHOTOSENSITIVE MEMBRANES.
One of the processes examined in this study is the use of photosensitive compounds. In this section a brief review is given of previous work on photosensitive membranes.
The process was described by Schultz [1986] as "coupling external energy sources to carrier-mediated membranes". To distinguish from normal facilitated transport, this process is initiated by an input light energy. Solar energy, for instance, can be used to carry on the mass transfer. On the other hand, this process is interesting for it can also be used to catch and store the sun's energy. However, in the following discussion attention is focussed on the possibility of air separation using such reactions (or other appropriate mass transfer system).
The oxygen molecule has naturally different states with different energy levels. The lowest energy state of the oxygen molecule, the ground-state or triplet oxygen designated as 3 s , is the most stable state for the oxygen g molecule. However, this state can be transformed to excited states called singlet oxygen designated as and :A with g g energies 156.9 KJ/mol and 94.3 KJ/mol [Ogryzlo, 1978] respectively above its ground state. Although singlet oxygen can be in both states, the term "singlet oxygen" usually refers to 1 a , simply because it apparently only g exists when chemical reactions related to singlet oxygen occur.
Singlet oxygen is more active than its ground state. It can, for instance, attack aromatic rings to form endoperoxides or nonconjugated olefins with allylic hydrogens [Gollnick, 1978] to form hydroperoxides. Some of these peroxides are not stable. They may return to their original state by giving off singlet oxygen. In other
-40- CHAPTER II. LITERATURE REVIEW
words, the reversible oxygenation is conducted by singlet oxygen.
The singlet oxygen can be generated by many methods. Illumination by light, for example, can generate singlet oxygen if there is a photo-sensitive material in a system. Several mechanisms have been suggested and different equations may be used to describe different systems. However, the following equation could be used to describe the overall process,
Illumination A + 0 AO , (2.65) 2 dark 2 where A represents an oxygen accepter (oxygen carrier), and A02 represents the endoperoxide formed in this process. The molecular oxygen here refers to the singlet state. The mass transfer in this process can schematically be represented by Figure 2.8.
-41- CHAPTER II. LITERATURE REVIEW
Interfacial absorption
upstream downstream
Figure 2.8. Schematic diagram for photo-mediated transfer of oxygen.
This process is complicated in that oxygen is first excited photochemically to the singlet state and this singlet oxygen reacts with the carrier to form the endoperoxide. Sometimes a photo-sensitizer may be involved to start the reaction.
Some endoperoxides can undergo a thermochemical dissociation reaction when they are heated. In some cases, this thermolysis occurs at room temperature to a large extent with a considerably high reaction rate.
At least two thermolysis modes need to be considered here, a loss of molecular oxygen and 0-0 bond cleavage. The former is the reverse process to give the original chemical while releasing oxygen. The latter produces a new chemical.
-42- CHAPTER II. LITERATURE REVIEW
Which process dominates will depend upon the nature of the endoperoxides. For example, the number of hydrogens substituted in aromatic rings and what groups are substituted onto the rings are two major factors influencing the thermolysis result of aromatic endoperoxides.
If a facilitated transport process is based on the reaction,
k (hu )
A + 0 i ■■ i-j—Tv AO (2.66) 2 (dark) 2
The differential equation for one dimension can be solved under the conditions:
(i) the reaction is fast compared with diffusion; (ii) the diffusion coefficient for both the carrier and its oxygenized form is the same (D =D ). A AO
The transport rate or flux has been given by Schultz [1977] as,
^A^T (2.67)
This equation is similar to equation (2.63) except that it allows for different equilibrium conditions on each side of the membrane (i.e. light-side, and dark-side, ). If the difference of these two equilibrium constants is significant, the transport of oxygen can be enhanced to a significant extent.
-43- CHAPTER II. LITERATURE REVIEW
This equation also shows that the total concentration of the carriers, C , plays an important role in the mass T transfer process.
Thus the challenges in this study are to select and prepare carriers likely to confer reversibility on the membrane system. This would require that a difference in the equilibrium constant on the two sides of the membrane and that the carrier solubility would be sufficient to give reasonable values of C . T
2.8. MEMBRANE MATERIALS.
In the past few decades, research workers have been searching for more efficient membranes and better liquid carriers. As a result, significant progress has been made. In this section of the review, emphasis is given to the recent developments of membrane materials.
The material science of polymers is a subject which attracts scientists in many ways. Both high and low permeability polymers have their applications in quite different areas. A thin sheet of polymer, for example, may be considered as a membrane in the mind of the scientist who works on a separating process or as a barrier in the mind of another scientist who works on a packaging process. As a result, different polymers with their particular characteristics have been developed according to their purposes. This review does not attempt to cover all the membrane materials developed to date but the focus of this review is always on those used for oxygen enrichment. However, there are a number of general review papers available [Chern et al, 1985; Henis and Tripodi, 1981? Nakagawa, 1985; Koros and Chern, 1987].
There are several theoretical studies which aim to
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correlate the solubility and diffusivity in a polymer with material structure [Stannett, 1978; Shimidzu, 1985; Nakagawa, 1985]. These may lead to correlation or even prediction of the permeation property with the membrane structure. However, it is still more reliable to experimentally determine permeabilities, especially for designing new polymers.
Generally speaking, the permeation rate decreases as the structural symmetry and cohesive energy increase [Rogers, 1985]. So does the apparent activation energy. Therefore, a polymer with polar repeating units such as polyvinylidene chloride can be expected to have less solubility, diffusivity and permeability due to the high cohesive energy of the polar groups. On the other hand, a less symmetrical polymer such as rubber has a high permeation rate because of the looser structure in the repeating chains.
2.8.1. HYDROCARBON POLYMERS.
It has been known for many years that natural rubber membranes are permeable to many gases. Studies in this area were contributed by Daynes [1920], Barrer [1939(a), 1939(b) and 1942] and Meares [1954]. The separation factors which varied for different rubbers were not very high despite their high permeabilities. Normally, the larger permeating molecules possessed larger solubilities and lower diffusivities than the small ones and this often resulted in similar permeabilities for a range of molecular weights [Matson et al, 1983]. Later studies dealing with silicone rubber drew a similar conclusion. Nevertheless, both natural and synthetic rubber are interesting polymeric materials for gas separation because they belong to the highly permeable material category.
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Besides rubber-like materials, earlier studies were also conducted to examine some commercially available polymers, such as polyethylene, ethyl cellulose and polystyrene [Brubaker and Kammermeyer, 1952; Weller and Steiner, 1949]. Some plasticized polymers were assessed [Brubaker and Kammermeyer, 1954]. Generally, the permeability was too low to be used in designing a separation system.
A series of polyurethane polymers was evaluated in terms of solubilities, diffusivities and separation factors [Tsuchida et al, 1988; Pegoraro and Penati, 1986] and it was suggested that they could be used for oxygen enrichment. However, the performance was not good enough for commercial exploitation.
The permeability of poly(4-methyl-pentene-l) was improved by blending it with siloxane [Lai and Wei, 1986; Lai and Wu, 1987]. It was reported that this polymer was grafted with vinylpyridine and the selectivity was even better, reaching 7.5 with an oxygen permeability of 35.6 Barrer. The reason for this enhancement is supposed to attribute to the affinity of siloxane with oxygen.
Haraya et al [1986] reported an encouraging result using a polyamide membrane. A separation factor for oxygen and nitrogen of 7.97 was achieved. The properties of this kind of polymer were studied in terms of their separating thermodynamic solubility and kinetic mobility contribution [Chern et al, 1985].
2.8.2. FLUORINE CONTAINING POLYMERS.
It is believed that fluorine introduced polymers have better selectivities than hydrocarbon polymers because fluorocarbons usually have more affinity with oxygen. Recently, many studies have been reported aiming to use
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fluoride groups to improve selectivities. Nagase et al [1988] synthesized siloxane copolymer containing alkyl, aromatic and fluorine that gave a better separation result than polydimethylsiloxane, as well as improving the mechanical strength. Kagawa et al [1989] introduced fluorine containing monomers to poly(vinylacetyl) . Terada et al [1985] used plasma-polymerized perfluorobenzene and Kajiyama et al [1985a] used fluorocarbon for liquid crystal membranes. In all cases the selectivity was improved.
2.8.3. SILICONE CONTAINING POLYMERS.
Silicone rubber has been studied intensively since the early days of membrane separation. In fact, the first commercial low-purity oxygen enricher was made of silicone rubber [Baker et al, 1987]. Barrer and Chio [1965] examined different gases passing through silicone rubbers. Kawakami et al [1985] and Kaiser et al [1985] carried out a series of syntheses of heteropolysiloxane utilizing different methods used for membrane applications.
Polydimethylsiloxane/polycarbonate copolymers have been studied intensively. Barrie et al [1984] studied polydisiloxane and poly(bisphenol-A carbonate) blend copolymer for organic vapour permeation. Since then, as the membrane separation processes have developed, scientists have started to modify the structure in order to improve the separation properties. The copolymers are either used to form polymer membranes or to modify the membrane surface [Kawakami et al, 1986 and 1987].
Poly(1-(trimethylsilyl)-1-propyne) (PMSP) was not a popular polymer in the scientist's mind until 1983 when this polymer was synthesized and reported to have a high permeability [Masuda et al, 1983]. In fact, it is the most permeable material among the polymers observed so far
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[Nakagawa et al, 1989]. It is in a glassy state at room temperature. The permeability of gases and organic vapour [Witchey-Lakshmann et al, 1990] to the polymer was measured as well as the solubility and diffusivity at 25°C (Table 2.3). For 0^ and the ratio of permeation (i.e. separation factor) is only 1.7, and this does not offer effective separation. Moreover, it was found that the permeability of this polymer declined with time for extended runs. An intensive study of the polymer has been carried out in order to improve the selectivity. Related polymers were also studied, but PMSP is still the best one in this series [Masuda et al, 1988; Hamano et al, 1988]. A highly selective membrane of the polymer can be formed by fluorinating, e.g. by exposing the polymer to fluorine gas. Some of the hydrogen atoms in the methyl group are replaced and some of unsaturated double bonds take up fluorine resulting in high selectivity membranes.
Table 2.3. The permeabilities of different gases to Poly- (trimethylsilylpropyne) [Witchey-Lakshmann, 1990]
Gas Permeability Solubility Diffusivity (Barrer)* cm3 (STP)/cm3-cmHg xlO3 cm2/sec xlO7
He 2200 2.0 1100
H 5200 2.9 1800 2 0 3000 14 220 2 N 1800 12 150 2 CO 19000 76 250 2 CH 4300 27 160 4
*Units: Barrer, 1x10'10 [cm3 (STP)-cm/cm2-sec-cmHg]
Both aliphatic and aromatic organosiloxane polymers have been examined in order to produce permselective membranes for oxygen enrichment [Kawakami et al, 1985]. It was found
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that poly(p-trimethylsilylmethylstyrene) has better selectivity than the others and that long chains of siloxane tend to be phase separated which diminishes the selectivity.
2.8.4. COMPOSITE POLYMER MEMBRANES.
A multilayer laminate membrane was investigated by Albrecht et al [1985]. An ultrathin silicone/polycarbonate film as thin as 0.015/i was made by Ward et al [ 1976]. It was found that the selectivity declined from 4.5 to 2.2 with an increase in the concentration of silicone rubber in the polymers and the permeability, as expected, showed the opposite direction which increased from 1 Barrer for the pure carbonate to 600 Barrers for the pure siloxane.
2.8.5. TERTIARY CRYSTAL MEMBRANES.
The different aggregate states of a polymer could have different permeation characteristics. The polymer/liquid crystal composite membrane, for instance, has a higher separation factor than the isotropic membranes. It is a dense membrane with liquid crystalline materials embedded in the membrane matrix. Cowling and Park [1979] studied crystalline 1,4-polybutadiene membranes and found that their selectivities are higher than that of amorphous membranes but the permeabilities are lower. Kajiyama et al [1982] and Washizu et al [1984] examined polycarbon/EBBA (ethoxybenzylidenebutylamine) membranes and found that the diffusive permeation coefficient of hydrocarbons increased 100-200 times when the temperature increased through the phase transition temperature. Moreover, Kajiyama et al [1985(a) and 1985(b)] employed a PVC/liquid crystal/ perfluorocarbon tertiary composite membrane to enrich oxygen in air with separation factors of 2 to 5. Both the permeability and selectivity increased because of the high
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solubility of oxygen in perfluorocarbons (Table 2.4).
Table 2.4. Oxygen and nitrogen permeability coefficients, PO and PN separation factors, PO /PN in the 2 2 2 2 PVC/EBBA/PFTA tertiary and PVC/EBBA binary composite membranes (summarized)
Membrane Temp,° K P P a 0 N (P /P ) Barrer Barrer 0 N
PVC/EBBA/PFTA Crystal* 283 0.101 0.0482 2.11 Nematic 307 10.2 2.00 5.10 Isotropic 348 85.7 25.1 3.72 PVC/EBBA Crystal 283 0.0794 0.0457 1.74 Nematic 307 5.75 1.95 2.95 Isotropic 348 51.6 20.6 2.52
* There are three aggregate states owing to the different temperatures.
2.8.6. FIXED CARRIER MEMBRANES.
Carrier-composite membranes have been developed recently. They are a combination of polymer membranes and oxygen carriers. There are two ways of joining carriers and membranes: copolymerization and coating. The former is a membrane containing a metal complex inside the polymer matrix as a fixed carrier. The latter is a multilayer membrane with one or more facilitated substrate containing a metal complex. Some successful examples of the use of carrier composite membranes were reviewed by Shimidzu [ 1984] .
Tsuchida et al [ 1982 ] made a cobalt(II)/poly(ethyleneimine) membrane and Nishide et al [ 1986 and 1989] employed a cobalt porphyrin complex [a',a',a',a'-meso-tetrakis (o- pivalamidophenyl)-porphynato]Co(II)-1-methylimidazole- (CoPIm) as the oxygen carrier which was homogeneously
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dispersed in the poly(butyl methacrylate) polymer domain and modified silicone rubber. They found that the sorption and transport process obeyed dual-mode transport theory [Nishide et al, 1987], The results of their experiment are shown in Table 2.5. Unfortunately, the permeability of oxygen decreased when the upstream pressure increased, whereas the permeability of nitrogen remained unchanged. As a result, the separation factor decreased dramatically to 1.5 when the oxygen pressure reached 70 cmHg. Barnes et al [1988] put CoSDPT, a cobalt ion complex [N,N'-bis- (salicylideneamino)di-n-propylamine]Cobalt(II), into polystyrene incorporating blanks containing nickel(II) complex. The best result they achieved was a separation factor of 2.52.
Table 2.5. Oxygen permeability coefficient (at 25°C) and permeability ratio (0 /N ) [Nishide et al, 1987].
CoPIm in membrane,wt% Permeability coeff* 0 /N 2 2 0 6.4 3.2 2.5 9.8 4.8 4.5 23 12
In units of Barrer at upstream pressure of 5.0mmHg.
2.8.7. IMMOBILIZED LIQUID MEMBRANES.
Immobilized liquid membranes have already been used to successfully separate oxygen from air. A limitation is believed to be the slow but irreversible oxidation of the carrier. One example is the production of oxygen-enriched air via carrier-facilitated transport developed by Bend Research, Inc. [Johnson et al, 1987]. In this process, oxygen of about 88% purity can be produced from air in one stage (Table 2.6). The oxygen carrier, the best one in this series, is CoSDPT in 1:1 DMSO and a-butyrolactone solvent with a lifetime of up to 3 months. The results obtained with this are the best reported to date.
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Table 2.6. Oxygen-enrichment performance of selected facilitated-transport membranes [Johnson et al, 1987].
Membrane Oxygen permeability* Selectivity Oxygen 0 /N content, % 2 2 1 1500 30 88 2 1200 14 78 3 750 25 87 4 750 16 79 5 700 22 85 6 700 21 84 7 600 20 84
* In units of Barrer.
2.8.8. PLASMA TREATED MEMBRANES.
Plasma polymerization or plasma treatment is used to improve membrane selectivities. In Chapter IV of this thesis, this technique is described and used to modify dense and microporous membranes. In this section, a brief survey is given to emphasize applications of this method conducted by many investigators with different monomers and substrates.
There are many ways to use plasma treatment in membrane manufacturing and membrane treatment. Skin layers formed by plasma polymerization should be good separation barriers because they are generally dense, highly branched, highly crosslinked and do not have a crystallization tendency. The property of the deposited layer varies depending upon the monomer used and polymerization conditions. Nevertheless, most of the experiments have achieved good results.
The plasma polymerized layer can be deposited onto a microporous membrane substrate or onto hollow fibre membranes. It can also be used to modify the surface of a dense membrane to change its surface properties. In oxygen enrichment, silicone rubber membranes are usually used as separation media because of their high permeation rate of
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oxygen, although the separation factors are not satisfactorily high. It would be desirable to have a selective layer on the surface of a silicone rubber membrane so that the separation factor would be increased, while keeping the high permeation rate.
A plasma polymerized polymer protective layer could not only be used on a metal surface [Cho et al, 1987] but also on another polymer surface [Inagaki et al, 1983]. The advantage of these kinds of layer are that the film is always thin and pinhole free. The plasma polymerized film also posseses high wet adhesion strength against peeling. The protective property is higher than normal polymers in terms of permeability.
Ultrafiltration membranes have been prepared with oxygen or nitrogen plasma [Vigo et al, 1988]. It has been found that hydrophilic groups, such as amine, hydroxilic and carboxylic groups, were responsible for the wettability improvement. On the other hand, polymer films treated with a plasma of oxygen or nitrogen containing compounds [Shimomura et al, 1984] have been reported to increase their hydrophilicity. The ammonia or nitrogen-hydrogen mixture under plasma conditions has been reported to play a similar role to amino compounds.
Reverse osmosis membranes have been prepared by applying nitrogen-containing compounds [Stancell and Spencer, 1972; Bell et al, 1975]. The membranes formed in this process have a very stable performance with less flux decline and salt rejection is still high after a long period of observation.
The plasma-polymerized coating has been applied on the internal surface of plastic tubings with an internal diameter of 0.3 mm to improve hydrophilicity [Matsuzawa and
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Yasuda, 1984]. Hollow fibre membranes made of polysulfone have also been reported to be treated with plasma [Nomura et al, 1984; Kramer and Yasuda, 1988] to improve their permselectivity.
The monomers used for preparing oxygen separation membranes are usually perfluorocarbons and silanes because of their affinity for oxygen [Inagaki, 1988; inagaki, 1981; Inagaki et al, 1984]. Sometimes, a mixture is necessary to produce a membrane possessing both high permeability and separation factor. However, other compounds could also be employed in the polymerization process in order to improve selectivities.
Several fluorocarbons, including perfluoro-2-butyltetra- hydrofuran, hexafluoropropylene, silicon tetrafluoride, pentafluorostyrene and perfluoro-1-methyldecalin, have been tried by Nomura et al [1984]. It was found that the deposition rates and separation factors reached the maximum when the power consumed by unit weight of monomer reached 1.25, regardless of what monomer was used. It was also concluded that the higher the monomer molecular weight, the higher the separation factor for the system of H^/CO^ .
Inagaki and Ohkubo [1986] have examined the plasma polymerization of perfluoropropene and methane mixture. It has been found that the ratio of perfluoropropene to methane plays a critical role for the formed membrane performance. The separation factor reached 2.8 when the above mentioned ratio reached three. It is of interest to note that the permeation tests with pure oxygen and pure nitrogen gave different permeabilities from that with an oxygen and nitrogen mixture and at the same time the permeabilities were independent of the partial pressure. The separation factor, therefore, changed to 3.5 with a permeation test gas mixture of 27.8 % oxygen.
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Yamamoto et al [1984] used glass fibres as substrates and hexamethyldisiloxane, tetrafluoroethylene and diallyl- methylsilane as monomers. They made a gradual compositional layer with a fluorine rich surface and a silicon rich interior that had a best separation factor of 3.8.
Perfluorobenzene as a monomer has been intensively studied by Terada et al [1983, 1984, 1985 and 1986]. An aliphatic perfluorocarbon structured polymer was found instead of an aromatic one. This indicated that the molecules were rearranged in the process of plasma polymerization. The aromatic rings were opened and the linear chains of perfluorocarbons were constructed. Their experimental results showed that a moderate power input would produce a smoother deposit layer than higher or lower power input. However, higher energy input resulted in higher crosslinking, and a higher separation factor which reached 4.5 -4.8 when the deposited layer was over 0.6 micrometres in thickness. The oxygen transmission rate of this membrane reached 1x10"5 [cm3 (STP)/cm2-sec-cmHg] (P^ consult Table 2.7 below).
The following table shows recent references to membranes produced by plasma polymerization and their performance. As a basis for comparison, silicone rubber, which is probably the most permeable membrane material, has a permeation for oxygen of 800x10’10 [cm3 (STP)-cm/cm2-sec-cmHg], and a separation factor of 2. Usually, this membrane has a 100 /im thickness, or an oxygen transmission rate, P , of 0.8x10"5 2 [cm3 (STP)/cm2-sec-cmHg]. The composite membrane, being generally thinner, will have a higher transmission rate and possibly better separation characteristic due to its greater degree of crosslinking.
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Table 2.7 Reported data on plasma polymerization membranes.
Membrane description substrate/active dense layer $** pi p2 VN2 Reference
Cellulose acetate/ethylene glycol with sebacyl chloride 0.5 0.786 4.7 JP 61 57,221
Polysulfone/polyperfluoro- tributylamine* 2.7 JP 60 255,106
Poly(acrylsulfone)/poly(per- fluoropropene)+trimethyl- vinylsilane 1.9 10.2 JP 60 255,110
Microporous membrane 10- Inagaki and /hexafluoropropene+methane 120 2.8 Ohkubo, 1986
Millipore PTFE/dimethylsilo- xane/4-methyl-1-pentene 1.2 3.5 US 4533369
Porous/dimethyIsiloxane 4.0 JP 60 257,806
UK-200/poly(trimethyl- silylpropyne) 0.1 3000 2.0 JP 60 110,304
Millipore/C F Terata et al, 6 6 0.2 200 4.5 1985
Silicone rubber 100 800 0.8 2.0
P - Permeability, xlO"10 [cm3 (STP)-cm/cm2-sec-cmHg] . P2 - Transmission rate, xlO"5 [cm3 (STP)/cm2-sec-cmHg], is defined as cubic centimeters of penetrant (STP) per square centimeter of membrane area per second. * - Hollow fibre substrate. ** - Thickness of active layer in micrometres.
2.8.9. SUMMARY OF MEMBRANE MATERIALS.
Membrane material studies are concentrated on the polymers listed below:
Polysulfone membranes; Silicone and natural rubber membranes;
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Fluorine containing polymer membranes; Bisphenol-A polycarbonate and copolymer membranes; Plasma polymerized membranes.
These materials have been studied intensively but a breakthrough is still awaited because separation based on these materials cannot so far compete with the cryogenic process in terms of economics.
From the first attempt to separate gases using membranes, many materials have been used with different degrees of success. Table 2.8 summarizes the materials used and some separation data for oxygen enrichment (See also Table 2.2).
Table 2.8. Polymer membranes for oxygen enrichment.
Permeability Separation Material of oxygen factor(02/N^
Ethylcellulose 14.7-9.6 3.3 Polydimethyl siloxane 600 2.0 Polyalkylsiloxane 200-300 2.1-2.3 Poly(4-methylbutine-l) 34 2.5 Cellulose acetate (Separex) 4.0-5.0 Polysulfone/polysiloxane(Monsanto) 5.4 Polystyrene/polydimethyl siloxane 400 2.3 Poly(4-benzene-l) 2.9 Polyimide composite 0.25* 8.0
* The active layer is 15/xm.
2.9. OXYGEN CARRIERS.
Oxygen carriers have been known for a long time by biomedical investigators. In fact, haemoglobin and myoglobin, which support vertebrate life by carrying and storing oxygen in blood, are the most important natural iron compounds that are known. Chemists have tried to make other chemicals, called synthetic or artificial oxygen carriers, which display the same function but which have different structures [Gubelmann and Williams, 1983;
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Mclenton and Martell, 1956; Tsuchida, 1979; Traylor and Traylor, 1982]. The natural and synthetic oxygen carriers will be reviewed separately.
2.9.1. NATURAL OXYGEN CARRIERS.
In the biological area, "natural oxygen carriers are of three main types: the haeme - containing proteins such as myoglobin(Mb) and haemoglobin(Hb); the haemerythrins; and the haemocyanins" [Jones et al, 1979].
Myoglobin and haemoglobin are iron containing proteins which can reversibly combine with molecular oxygen. This porphyrin structure is shown in Figure 2.9. The iron(II) will occupy the central position when a complex is formed while the dioxygen and the imidazole groups will be located on both sides of the porphyrin plane. Therefore, the iron is situated in a pseudooctahedral environment which stabilizes the complex. Every iron atom can bind one 02 as described above.
Figure 2.9. Porphyrin and iron(II) porphyrin (from Buchler, 1978).
Haemerythrins are also iron containing proteins but without porphyrin so the iron atoms are directly connected to proteins. A molecular oxygen is bound with two iron atoms. The haemocyanins are copper containing proteins, the structure of which is unkown. Oxygenation of haemocyanin gives oxyhaemocyanin. The dioxygen is bound between the two Cu(I) centres giving a copper binuclear complex.
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2.9.2. SYNTHETIC OXYGEN CARRIERS.
The synthetic oxygen carriers can be classified into two categories: inorganic and organic. The latter are mainly perfluorocarbons. The former contain many complexes which will be described separately according to the metal atoms- iron, cobalt and copper complexes.
2.9.2.1. INORGANIC OXYGEN CARRIERS.
(1). IRON COMPLEX CARRIERS.
The studies and syntheses of iron complexes are of great interest to both scientists and engineers because they are found in living creatures, including mankind. So a great deal of experimental and theoretical work has been done to elucidate the mechanism and kinetics of situations where oxygen, carbon monoxide and other gases react and dissociate with some iron complexes.
Porphyrin complexes are the direct logical extension of haemoglobin or myoglobin. In fact, chemists are trying to copy and modify these natural substances in many ways. The result of such efforts is that many similar coordination complexes, such as iron coordination compounds, coboglobin and other metalloglobins, have been synthesized [Niederhoffer et al, 1984].
The porphyrin molecule in Figure 2.9 is the essential substance of all porphyrins [Buchler, 1978], whereas the porphinatoiron molecule is the essential part of all haemes. It consists of four pyrrole rings linked together into a macrocycle by four methine bridges. It is a planar structure in which the main part is a porphyrin ligand with
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some side chains linked with a big molecule of globin protein. If this active site, the porphinatoiron complex, is isolated from the globin protein and exposed to oxygen, it immediately and irreversibly oxidizes and cannot act as an oxygen carrier. Therefore, equally important parts of this complex are the side chains, which prevent the irreversible oxidation of iron(II). The central metal iron(II) may not be so important because it can be substituted by metal ions of suitable size.
The main task for the synthetic porphyrin is to find a way to shelter the central metal from irreversible oxidation. Battersby et al [1976] and Battersby and Hamilton 1980] synthesized bridged porphyrin systems or strapped porphyrins and doubly-bridged porphyrins. Almog et al [1975(a) and 1975(b)] synthesized capped porphyrins. Collman et al [1975] and Collman [1977] made "picket-fence porphyrins" and pocket porphyrins [Collman et al, 1983]. Unfortunately, all of these are only stable in either the solid state or in an organic solvent to a certain extent. In other words, they will lose their ability to reversibly bind oxygen when used for a period of time.
Tsuchida [1985(a) and 1985(b)] reported that a quite stable and efficient oxygen carrier, which could compete with blood in carrying oxygen under physiological conditions, was synthesized. It is an amphiphilic iron-porphyrin complex embedded in a phospholipid bilayer. It is quite interesting that this chemical has a red colour and similar properties to haemoglobin.
Non-porphyrin iron complex systems have also been studied. Although a great number of these complexes exist, only two groups are significant oxygen carriers. These are Schiff bases and lacunar iron or dry cave complexes.
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Herron and Busch [1981] and Herron et al [1983] synthesized the lacunar iron complexes or "dry cave" ligands which were fully functional oxygen carriers. They were reported to reversibly react with oxygen under ambient conditions in dilute solution. However, stability data given by them showed that the most stable complex is still not good enough to meet gas separation requirements. The concentration of the most stable complex would be reduced to half of its original amount in one day. More work is needed to make totally functional iron complexes which will carry out the same function as haeme.
(2). COBALT COMPLEX CARRIERS.
The cobalt(II) complexes form another big family of oxygen carriers, even bigger than the iron(II) complexes. In fact, nearly a hundred cobalt complexes can be found in some review papers [Daul et al, 1979; Erskine and Field, 1976; Gubelmann and Williams, 1983; Jones et al, 1979; Mclenton and Martell, 1956; Niederhoffer et al, 1984]. However, only some of them are important in both chemical and biochemical processes, and a few of them have already been used in separation systems. The following part of this review is devoted to some typical cobalt complexes involved in the reversible binding of oxygen in practical and potential facilitated transfer processes.
Cobalt porphyrins were the first group of oxygen carriers to attract a great deal of attention because of the similarity of their structure with haeme [Basolo et al, 1975]. Collman et al [ 1978 ] and Beugelsdijk and Drago [1975] synthesized picket-fence cobalt porphyrin complexes, measured the thermodynamic constants and found that the oxygen affinities of these complexes were 10-100 times less than those of natural haemoglobins and myoglobins. Recently, a new bidentate complex of porphyrin with
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"sandwich geometry" was synthesized and an oxygenation mechanism was suggested [Uemori et al, 1985].
The Schiff base cobalt(II) chelates also comprise a large number of complexes. They are macrocyclic compounds formed by the Schiff base condensation reaction,
R R R \ \ / 2 C=0 + H NR -* C=N + H 0 (2.68) / 2 2 / H H where R^ and R^ are alkyl or aryl groups. Schiff base ligands are also planar structures with a cobalt atom which combines oxygen. This kind of carrier is generally a tetradentate ligand, but some can be pentadentate ligands. At least two of these four or five ligand atoms are nitrogen atoms, and the others may be nitrogen, oxygen, sulfur, or a combination of the three. Chen et al [1989] and Chen and Martell [1987] synthesized and examined Schiff base cobalt complexes. Johnson et al [1987] reviewed Schiff base cobalt complexes in relation to oxygen separation as membrane imbibing liquid additives. Two kinds of such complexes are important as oxygen carriers: five-coordinate and four-coordinate cobalt complexes. The first type is Co(II)[N,N'-bis(salicylidene)-n-propyldipropylenetriamine]- cobalt(II), referred to as Co(salPr). The second example is [N,N'-bis(3-methoxysalicylidene)tetramethylethylenedi- amine)]-cobalt(II), referred to as Co(3-MeOsaltmen). These two examples are depicted in Figure 2.10 (a) and (b). Interestingly, one of these chelates, [5,5 ' -(1,2-ethane- diyldinitrilo)-bis(2,2,7-trimethyl-3-octanonato]Co(II), has been used in chromatography as the stationary phase to produce a marked increase in the retention time of oxygen [Gillis et al, 1985].
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The next group of cobalt chelates of interest are the so called "dry cave" ligands, which can bind oxygen in a hydrophobic pocket created by a simple bridging group in the complex molecule [Stevens and Busch, 1980; Stevens et al, 1980] in a similar way to the iron complexes. These ligands are neutral (forming cationic complexes), nonplanar, and, while containing two unsaturated chelate rings, non-aromatic as shown in Figure 2.10(C). It has been shown that the oxygen affinities of these complexes are as large as coboglobins.
(A) Co(salPr) (B) Co(3-MeOsaltmen) (C) "Dry-cave"
Figure 2.10. Schiff base cobalt complexes and "Dry cave" cobalt complexes (from Johnson et al [1987]).
(3). COPPER COMPLEX CARRIER.
The copper complex is a natural oxygen carrier as mentioned above. The active site which carries oxygen in the copper complex is a haemocyanin, which is part of a large protein molecule containing about 300 copper atoms per protein unit. The actual structure and composition of this molecule and its subunits are still unknown. However, by studying and synthesizing small functional molecules which mimic the
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natural one, scientists can reveal the nature and real structure step by step, and, further, can synthesize the totally functional complexes in laboratories.
Copper Schiff base complexes, which are different from cobalt and iron complexes because they are binuclear complexes, were examined [Countryman et al, 1974]. Modifying the method used for synthesizing macrocyclic ligands [Black and Mclean, 1971], Bulkowski et al [1977] made bi-metallic copper complex [LCu ]2 + (L=C H NOS), 2 ' 2440224' 1,4-bis(l-oxa-4,10-dithia-7-azacyclododecan-7-ylmethyl)- benzene which was reported to react reversibly with carbon monoxide and molecular oxygen. Gagn et al [1977] synthesized and examined four-coordinate copper(I) complexes and five-coordinate copper(I) complexes in relation to their molecular structure and oxygen reactivity and found these complexes rapidly and irreversibly reacted with oxygen molecules. Later, they synthesized other bi metallic copper complexes, Cu2(TPEN)2+, which reversibly react with carbon monoxide [Gagn et al, 1982]. Simmons et al [1980] made the synthetic copper complex, [Cu(I)(imep)]+, which functionally mimiced haemocyanin in its capacity of reversibly binding oxygen under ambient conditions with a reversibility factor 80% per oxy/deoxy cycle. However, all the complexes mentioned above cannot be used until considerable improvement is made in the reversibility of their reaction with oxygen.
Kawakami et al [1982] demonstrated the permeation of oxygen facilitated by Cu(I)-tetraethylenepentamine. The complex takes oxygen in the ratio CurO^ = 1:1 in a dilute solution. The permeation coefficient increased several times and the separation factor reached about twenty at 313.2°K at first and then fell to about ten while the oxygen pressure continued increasing to lOcmHg. It is not surprising that this complex facilitates the transfer of carbon monoxide as
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well [Kawakami et al, 1984]. The complex suffered irreversible oxygenation at the first run, and then stablized. Although this complex only works in dilute solution, it could be a potential oxygen carrier in some cases.
2.9.2.2. ORGANIC OXYGEN CARRIERS.
(1). PERFLUOROCARBONS.
Usually, liquids that are less polar and have lower surface tension than water are better solvents for oxygen. The perfluorocarbons (PFCs), for instance, can generally dissolve approximately 40-50 vol% of oxygen and even larger amounts of carbon dioxide, whereas the solubility of oxygen in water is only 2.3 vol% at 37°C [Geyer, 1984] and 3.1 vol% at 25°C [Riess and Le Blanc, 1982].
Perfluorocarbons are basically hydrocarbon chemicals in which the hydrogen atoms have been replaced with fluorine atoms. Other related compounds containing atoms other than carbon and fluorine have also been developed, although strictly speaking, they are not perfluorocarbons. However, the term perfluorocarbon is used for both types of chemicals.
The PFCs are usually "colourless, apparently nontoxic, have low surface tensions, are insoluble in water, biologically very stable and have low viscosity" [Abel, 1982]. Separation scientists and biotechnologists are both interested in perfluorochemicals as oxygen carriers. Biomedical users, perhaps, are more interested in studying PFCs than separation scientists, for the PFCs have a potential to formulate a life-saving liquid, a so-called artificial blood. However, the two processes have opposite requirements. The PFCs and some additional substances need
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to be excreted from the body very quickly so biomedical users want highly volatile PFCs [Dagani, 1982]. On the other hand, the chemicals used in separation technology need to be kept inside the membrane as long as possible so a low volatility PFC certainly has an advantage. Sometimes, it is difficult or even impossible to use volatile chemicals as the separative barrier in membrane processes. Therefore, it is quite common for separation scientists to use chemicals with low vapour pressure.
Perflourotributylamine (transformer coolant FC-43) has attracted a great deal of attention because it is a commercially available product. It has a very low vapour pressure, high oxygen carrying ability and very low toxicity. This chemical emulsion is very interesting since it rapidly takes up oxygen, reaching its equilibrium concentration within 1/2 second. Therefore, this compound may make a good liquid membrane, despite being unsuitable as a blood substitute.
Riess and Le Blanc [1982] systematically studied PFCs and found that the solubility of gases in PFCs is generally as much as 20 times or more higher than that in water if expressed in volume percent (or as much as 200 times if expressed in molar fractions), but only three times higher than those in related hydrocarbons. The general trend of gas dissolving capacity decreases in the order:
CO >> 0 > CO > N > H > He. 2 2 2 2
Apparently, the solubility decreases with the molecular size of the solute, which is similar to most organic solvents and silicone rubber.
The solubility of oxygen and nitrogen varies linearly with gas partial pressure. In other words, it obeys Henry's law.
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The temperature dependence of solubilities of various gases in a given PFC for different gas species may follow different trends. This means that the solubilities may increase or decrease with increasing temperature.
Fifty different PFCs were studied by Yokoyama et al [1984] as blood substitutes. Wesseler et al [1977] also examined many perfluorocarbons for the same purpose. In Table 2.9 some gas solubility and vapour pressure data for PFCs have been collected [Lawson et al, 1978; Riess and Le Blanc, 1982; Tham et al, 1973; Wesseler et al, 1977]. It can be seen from the table that PFCs may also be good to separate carbon dioxide.
Table 2.9. Solubility of 0 , CO , N and vapour pressure of PFCs. 2 2 2
Vapour Solubility, vol%* pressure Code Name mm Hg 0 CO N 2 2 2 Water (38°C) 49.7 3.1 110 1.4 FO F-octane 52.1 FDD F-5,6-dihydro-5-dodecene 44.6 240.6 F-44E F-5,6-dihydro-5-decene 12.6 50.1 247 F-66E F-7,8-dihydro-7-tetra- decene 2.3 41 211 FOD F-octylbromide 52.7 FBC F-bicyclo[5,3,0]decane 13.5 FTN F-trimethyl[3,3,1]nonane 2.9 FDMA F-l,3-dimethyladamantane 39.4 FDC F-decalin 12.5 40.3 179.7 29.5 FMD F-methyldecalin 5.2 42.0 FHE F-dihexylether 11.8 FDPH F-l,4-diispropoxybutane 48.7 185.8 37.5 FPH F-l-isopropoxybutane 55.9 221.6 43.2 FC-75 F-2-butyltetrahydrofuran 58.0 71.4 179.3 36.8 FTPA F-tripropylamine 45.3 166 FC-43 F-tributylamine 1.1 38.9 152 28.4 FHQ F-N-mehtyldecahydro- quinoline 8.1 44 FMOQ F-4-methyloctahydro- quinolidizine 10.7 46 FCHP F-N-cyclohexylpyrrolidine 8.6 49
* Volume dissolved in 100 ml of PFCs at one atmosphere pure gas.
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A disadvantage of perfluorocarbons is that they dissolve both oxygen and nitrogen. This means that the separation factor may not be very high if PFCs are used as the separative barrier. In fact, none of the perfluorocarbons have a solubility ratio of oxygen to nitrogen higher than two in Table 2.9. Therefore, the separation factor between oxygen and nitrogen is likely to be less than two. From Table 2.9 it can be concluded that perfluorotributylamine is probably the best of the organic oxygen carriers among the known perfluorocarbons.
Some investigators have begun working on the second generation blood substitutes [Riess, 1984]. The first question to be answered is that of solubility and volatility. The general rule is that the highly volatile PFCs are cyclics and ones with low volatility are acyclics. From the viewpoint of liquid membrane applications, the latter are better. Lawson et al [1978] developed methods to estimate the volatility and solubility from the structure only. They concluded that the difference in solubility among PFCs was insignificant. In other words, there is little hope of finding a new chemical with significantly higher solubility of oxygen.
(2). SILICONE OILS.
Silicone oils, which are like silicone rubber but with a lower molecular weight and no cross-linking, have high oxygen affinity. They have been used in membrane tests [Johnson et al, 1987]. They, perhaps, have a similar disadvantage to PFCs, i.e. the high solubility for both oxygen and nitrogen. Therefore, high permeability and low selectivity for oxygen and nitrogen can be expected.
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2.9.3. SUMMARY OF OXYGEN CARRIERS.
Microporous membranes cannot separate oxygen from air efficiently despite their high permeability. Dense membranes, on the other hand, have low permeability and potentially high selectivity. The improvement of manufacturing techniques for both porous and dense membranes has increased the permeabilities and separation factors to some extent, but in fact they are still far from satisfactory.
Greater potential may lie in membrane separations based upon immobilized liquid membranes and fixed carrier membranes. Perfluorotributylamine, FC-43, is probably the best of the organic oxygen carriers, because it possesses both high solubility for oxygen and low volatility. However, organic oxygen carriers may have difficulty in reaching high separation performance because they usually have a high solubility for both oxygen and nitrogen. In spite of this perceived limitation the availability and simplicity of the PFC-based membrane is attractive. Part of this thesis (Chapter III) involves an assessment of this type of liquid membrane.
The inorganic oxygen carriers, on the other hand, could have a high selectivity because a chemical reaction occurs only between certain species of molecules. The chemistry of transition metal complexes, especially the group eight elements, is a very interesting subject. Many investigators have worked on both biotechnical and gas separation processes utilizing the same compounds. There may be a great breakthrough sometime in the future when a new compound of metal complex emerges which is similar to or even better than blood in terms of oxygen carrying.
In summary, from the view point of developing an oxygen
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specific membrane the metal complexes are very attractive. In particular they offer the potential to achieve high separation factors. This may be achievable without loss of permeability, providing the complexes have an adequate solubility in a proper solvent.
The concept of using membranes based on metal complexes has not been studied in this thesis. However an alternative approach using reversible chemical complexation, a membrane process incorporating a photosensitive organic species has been examined (Chapter V). In addition a brief study using a copper complex within a solid membrane is reported as an Appendix (IV).
2.10. COMPARISON OF DIFFERENT MEMBRANE PROCESSES FOR OXYGEN ENRICHMENT.
Many processes can be used to produce oxygen-enriched air and each process has advantages and disadvantages. Moreover, oxygen-enriched air may be applied to different purposes. In combustion applications, for example, the concentration of oxygen need not be very high but the quantity needs to be large. In clinical applications, as another example, the impurity of oxygen-enriched air needs to be considered carefully but the quantity would not be large. Therefore, it is difficult to make an absolute recommendation as to what membrane process should be used for a certain application. However, in Table 2.10 the potential membrane processes for 0^ /N separation are summarized as a general guide.
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Table 2.10. Comparison of different membrane processes for oxygen enrichment
Membrane type Description Advantage Disadvantage
Dense Homogeneous Simple Less polymer polymer Moderate selective membrane permeability Long life
Composite Microporous + Simple Moderately membrane dense skin High permeability selective Long life
Plasma Coated High permeability Complicated polymerized polymer Long life Moderately membrane selective
Immobilized Microporous + High permeability Complicated liquid organic Long life Moderately membrane liquid selective Short life
Immobilized Microporous + High permeability Complicated liquid metal High selectivity Short life membrane complex
Immobilized Microporous + High permeability Complicated liquid photosensitive High selectivity Short life membrane reagent
-71- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES.
CHAPTER III. IMMOBILIZED LIQUID MEMBRANES.
3.1. INTRODUCTION.
The initial focus of this research was to study immobilized liquid membranes as alternatives to conventional polymer membranes for 0^/N^ separation. There are two kinds of liquid membranes currently being studied for gas separation purposes: "passive" liquid membranes and carrier-mediated liquid membranes. The first type is a membrane using an organic liquid, and this is described and discussed in this chapter. The second type is a membrane in which an oxygen carrier is employed and this is described and discussed in Chapter V under the title of "photosensitive membranes".
Liquid membranes are usually more permeable than solid membranes to both oxygen and nitrogen. This is attributed to the diffusion coefficient in liquid which may be hundreds or even thousands of times higher than that of solid, polymeric materials. This is the first advantage of liquid membranes. The second advantage is the variety of liquid membrane materials available.
In principle, the microporous support for the liquid does not have any significant influence on the permeation properties. However, a microporous membrane with a small pore size would be preferable because it can withstand a higher pressure drop across the membrane. In addition, as a general rule, a hydrophobic microporous support may be better than a hydrophilic one, since the liquids used in immobilized membranes are usually organic solvents. The hydrophobic materials generally provide better compatibility with an organic solvent. In contrast, a hydrophilic membrane would be the first choice if an aqueous solution is used as an imbibing liquid. This is
-72- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES.
particularly the case when some metal complexes which are soluble in water are put into a membrane.
The selectivity is normally caused by preferential absorption of oxygen or nitrogen. Therefore, a solvent with a large difference in solubility of oxygen and nitrogen is desirable in order to bring about a high separation factor.
A test facility was designed to evaluate the immobilized liquid membranes. The experimental studies were carried out for three purposes: (i) to find suitable liquids to be imbibed into commercially available substrate membranes; (ii) to examine the validity of mass transfer theory for liquid membrane processes; (iii) to assess the feasibility of liquid membranes for practical separation of oxygen and nitrogen.
3.2. EQUIPMENT.
A permeation cell was designed to accommodate flat sheet membranes of 47 mm diameter, and this is shown in Figure 3.1. It consists of two parts between which a test membrane is clamped. A stainless steel screen is placed beneath the membrane to withstand the imposed pressure difference across the membrane. The effective permeation area measures 39 mm in diameter or 11.95 cm2 in area.
A flow diagram of the equipment used to test the permeabilities and the separation factors is shown in Figure 3.2. Compressed air as a feed gas from a cylinder (1) passes through the upstream side of the permeation cell (14). Its pressure is controlled with a regulator (2) and the flow rate is measured with a rotameter (5) and a burette (6). Helium from another cylinder (7) passes through the downstream side of the permeation cell (14) as a sweep gas and is then analysed with a chromatograph (15).
-73- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES.
The sweep gas pressure is controlled with a valve and maintained at a low pressure but 1-3 cm Hg higher than the ambient pressure to avoid any back diffusion from the vent. The pressure on the upstream side is kept 1-3 cm Hg higher than that on the downstream side because the permeation cell cannot withstand a reverse pressure. The flow rate is controlled with a valve and measured with a rotameter (12) and a burette (13). The pressure difference between the two sides of the membrane is measured precisely with a manometer (11) to b 0.5 mm Hg.
In order to increase the life span of the immobilized liquid membrane, two pre-saturating tubes were connected to feed and sweep gases to eliminate the evaporation of solvent from the wetted membrane. By doing this, the membrane could last 6-8 hours and still remained wet.
Gas compositions of the permeate and feed were analyzed by a Gow Mac 150 Chromatograph (GC). An integrator, Shimadzu: Chromatopac C-R 3A, was connected to the GC to obtain areas of the GC peaks. Two molecular sieve 13X columns were equipped to analyse oxygen and nitrogen. The compressed air and helium purchased from CIG Ltd were used directly without any purification.
-74- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES.
7 «
Sintered metal
777
Figure 3.1. The permeation cell.
-75- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES.
£ 3 . co •H C (1) o 3 H 00 -H 3 ,Q P D CO 03 0 P 0 (D 0 m co r- 0 o *h o -—i chromatograph Figure 3.2. The test equipment. -76- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. 3.3. MATERIALS USED IN THE PERMEATION MEASUREMENT. The membranes used in the experiment were Millipore microfiltration membranes. The thickness of these membranes is normally 100/xm. Their properties are given in Table 3.1. There was no pre-treatment before use. Table 3.1. The Millipore membrane specifications [The Millipore, 1987] Typical Mean Filter Hydro-lipo Mean pore porosity bubble point type Materials* behaviour size (/im) (%) kg/cm2 KPa GVHP Durapore Hydrophobic 0.22 75 1.20 118** HVLP Durapore Hydrophilic 0.45 70 1.55 152 GSTF MF-Millipore Hydrophilic 0.22 75 3.52 345 VSWP MF-Millipore Hydrophilic 0.025 70 21.1 2068 * Durapore-Polyvinylidene difluoride. MF-millipore-Mixed cellulose acetate and cellulose nitrate. ** The bubble point is the differential pressure required to force air through the pores of a methanol-wet filter. The liquids used in the test were perfluorotributylamine (FC-43) and silicone oil which have been reported to have high solubility for oxygen and low volatility at ambient conditions. 3.4. FACTORS INFLUENCING THE EXPERIMENTAL RESULTS. The permeability can be calculated by the modified equation (2.1). Usually, two gas fluxes need to be considered: a flux through the polymer matrix and a flux through the immobilized liquid. The former is negligible in this case because it is thousands of times less than the latter as mentioned before. Therefore, only the portion occupied by -77- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. liquid, i.e. the porosity f, contributes to the gas transport. Furthermore, the diffusion path for gas species must follow the tortuous track of pores, i.e. tortuosity r needs to be considered. Adding these two parameters into equation (2.1), the following equation can be derived, ^Ph~Pi) J =P (m3 /m2 -sec ) (3.1) r L or using the solubility and diffusivity, r P = (3.3) The permeation process in this case can be visualised as occurring across the whole membrane (including the polymer matrix) with a modified diffusivity, (D f/r). The separation factor can be calculated by the equation, Q (3.4) Calculations were performed by an IBM PC computer on the raw experimental data to give permeabilities and separation factors. The computer program for permeabilities and separation factors in the computer language "C" is given in Appendix II of this thesis. The flow pattern through the cell was considered because it affected the concentration along the membrane surface. The carefully designed permeation cell allows the gas mixture inside the cell to be well-mixed which makes uniform -78- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. concentrations in the upstream and downstream compartments of the cell. In the experimental set-up, the stage cut (fraction of permeate to feed) was very low (less than one percent). This meant that the flow rates of air and sweep gas were high in comparison with the permeation flow. Therefore, the composition of air in the inlet flow and outlet flow were only marginally different. In other words, the oxygen concentration on the feed side was that of air, whereas the concentration of oxygen on the permeate side was almost zero and both were kept constant. The assumed concentration profile across the membrane is illustrated in Figure 3.3. The flow rates on both sides of the membrane needed to be kept as high as possible. The flow rate on the feed side was usually more than one millilitre per second. The upper limit of the sweep flow rate depended on the sensitivity of the chromatograph. The lower limit depended on whether boundary layer effects were eliminated. Tests were done using different flow rates and it was found that the permeation was constant if the sweep rate was greater than 0.5 millilitre per second. -79- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. Concentration of oxygen t — Immobilized — Helium 4- Feed air — liquid membrane — sweep Retentate 21 vol% Permeate 0 vol% Distance upstream boundary downstream boundary Figure 3.3. The concentration profile across a membrane 3.5. MEMBRANE PREPARATION. An immobilized liquid membrane was made by immersing the Millipore membranes in corresponding liquids. This took only 1-2 minutes for the Durapore GVHP membrane which is hydrophobic. However for the hydrophilic membranes, wetting was more difficult and it is possible that they were not completely wetted even after 2 hours contact. After wetting the membranes were carefully loaded into the test cell. The liquid remained in the pores, provided the pressure drop across the membrane was not too large. The maximum applied pressure drop depended on the surface tension of the liquid, the contact angle and the size of pores. -80- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. 3.6. GENERAL OBSERVATIONS AND DISCUSSION. The general trend of gas flux versus partial pressure is plotted in Figure 3.4. It is a straight line over the range of pressures, passing through the origin. This figure shows that the absorption in this process is well represented by Henry's law and the diffusivity of oxygen through the membrane is constant. In this case, the pressure gradient occurs as shown in Figure 3.3 across the membrane matrix rather than in the stream bulk. When the pressure difference across the membrane exceeded a certain limit, the permeation increased rapidly. This indicated that the bubble point had been reached. In this -case, the absorption/diffusion model is no longer suitable to describe the permeation process and Knudsen or Poiseuille flow occurs. The pore size of the Durapore GVHP membrane has been calculated later in this chapter according to the maximum pressure drop attainable for this membrane. The permeabilities of FC-43 impregnated membranes are depicted in Figure 3.5. From this plot, it can be seen that the permeability of different membranes increased in the order of, VSWP < GVHP < HVLP < GSTF The reason for this is presumably related to pore size, porosity and the nature of wetting of the various membranes. This is discussed in more detail in Section (3.7). The separation factors of the FC-43 membranes are depicted in Figure 3.6. -81- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. The permeabilities of oxygen through silicone oil liquid membranes are given in Figure 3.7, which shows a similar trend to the FC-43 liquid membranes in the different membrane materials. That is, the permeability of the different membranes increases in the same order, i.e., VSWP < GVHP < HVLP < GSTF The separation factors of silicone oil membrane are depicted in Figure 3.8. -82- o GVHP with FC-4 3 Figure CHAPTER o 3.4. •D0S/ III. Permeate C UID IMMOBILIZED o '0;ej flux - 83 uox^eeuijad - vs LIQUID pressure. o MEMBRANES o oo O Partial pressure upstream, cmHg. Figure 1500 m O GSTF with FC-4 [6HUI3-09S- ro A HVLP with FC-< 3.5. ro C\J O O □ GVHP with FCH L <3 CHAPTER Permeability ...... ^ > •H X s T ro * ■P u En 3 WD/WO- °|0 Oi | i° 1 i 1 1 1 i \ | 1 O O III. CD o o (dliS) ’ ‘ IMMOBILIZED of £ uio] ___ Tr-- oxygen ___ 0 - T CO O O < 84 l i . J 3 O _0TX □ £ l r : - 3 4 V / P J di 3 j n □ □ through A^TXTqeeuuad □ LIQUID 1* j 1 -----*_ * CO o O * * FC-43 MEMBRANES. membranes. o oo o c o o CO o o o LO o o CO o \ j Pressure drop [cmHg] CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. [cmHg] drop Pressure joqoe:; uoxqejiedas Figure 3.6. Separation factors of FC-43 liquid membranes. -85- Pressure [cmHg] drop O oo membranes. d MEMBRANES. oil CO 0UIj:a l^ c LIQUID Aq.xxT - silicone CD 86 - _oix t through IMMOBILIZED uio] £ ) CD III. diS ( - uio / uio 2 Permeability CHAPTER - OJ s 3.7. -O0 < d * hu 0 [6 0091 Figure CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. O [cmHg] drop Pressure •H — LOO jogopij: uoxq-pjpdas Figure 3.8. Separation factors of silicone oil liquid membranes. -87- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. Table 3.2. Experimental result — Permeability and separation factor of immobilized liquid membranes. Maximum Membrane Liquid pressure Permeability Separation materials difference P, Barrer factor, a cmHg GSTF FC-43 28 950 1.60 HVLP FC-43 30 540 1.62 GVHP FC-43 58 540* 1.62# VSWP FC-43 28 350 1.60 GSTF Silicone oil 28 650 1.97 HVLP Silicone oil 30 420 2.01 GVHP Silicone oil 70 330 2.00 VSWP Silicone oil 35 240 2.03 * The standard deviation for the GVHP/FC-43 experiment was calculated according to the equation, 2 -i % where P is the average value of P, P^ is the individual value of P and n is the total number of data. The calculated result was a = 28. The relative precision was calculated by, r] — o/ P — 8.5% # The standard deviation, relative precision of the separation factor a is a = 0.064, rj = 3.2% respectively. Similar precision was achieved in the other experiments. -88- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. The membrane substrates played a less important role than the liquids in the permeation processes, as expected. The results showed that the separation factors were nearly the same value when the same liquid was used in different support membranes. This indicates that the permeation through an immobilized liquid membrane is mainly determined by the nature of the solvent. However, the influence of the membrane type on permeability was bigger than that on the separation factors. The average separation factor was 1.61 for FC-43 and 2.00 for silicone oil impregnated membranes. The properties of the substrate membranes, such as porosity and pore size and hydro-lipo behaviour, certainly had some influence on the permeation process. The stability, for instance, was considerably affected by the affinity between the membrane and liquid. When the GVHP membrane was impregnated with silicone oil, it became semi-transparent and the stable range increased to double that of the other membranes. The GVHP membrane and FC-43 combination showed a similar phenomenon and result. However, the stable range of both silicone oil and FC-43 liquid membranes was rather limited. The principle reason is that the surface tensions are relatively low, with that for FC-43, for instance, being less that one quarter that of water (its surface tension is 72 dynes/cm) - 16.8 dynes per centimeter (or 1.68x10”2 newton per metre) in ambient conditions. 3.7. COMPARISON BETWEEN EXPERIMENTAL AND CALCULATED RESULTS. One way to obtain permeability and separation factor is to carry out an experiment and another way is to carry out calculations according to the molecular theory of liquids and gases. Perfluorocarbons have been studied as an artificial blood for a long time and as a result the necessary physical property data, such as solubility parameter and molecular volume, are available. Therefore, -89- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. it is possible to predict the permeation characteristics by calculation. The following section has two purposes: to compare the experimental result with the calculated one and to examine the valid range of the calculation equations. 3.7.1. DIFFUSIVITY OF 0 AND N IN LIQUID PHASE FC-43. 2 2 The diffusivity of oxygen and nitrogen was calculated according to the Wilke-Chang modified Stokes-Einstein equation [Reid et al, 1986] -8 7.4x10 ) (cm /s) (3.5) 076 "1V2 where D Mutual diffusion coefficient of solute A at A B very low concentration in solvent B; Equation (3.5) is applied to the pure solvent. Considering the membrane porosity t; and tortuosity r , one can give the modified diffusion coefficient for an immobilized liquid membrane as, 0 D (3.6) Data which are used to calculate the diffusivities and solubilities are given in Table 3.3 for FC-43 and Table 3.4 for oxygen and nitrogen. -90- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. Table 3.3 Physical properties of Perfluorotributylamine. [Barton, 1983; Lawson et al, 1978; Wesseler et al, 1977] Molecular weight 671 Boiling point (bp) (at 760 torr), °C 174 Vapour pressure (at 37.5°C), cmHg 0.25 Density (25°C), gm/ml 1.900 Surface tension (25°C), dyne/cm 16.8 Viscosity (25°C), cs 2.52 (4.788 cp) Oxygen solubility (25°C), ml/100 ml 38.4 Solubility under air (25°C) 0 8.6 2 (cm3 /100 cm3 ) N 22.1 2 Solubility parameter, MPa^ 12.7 Enthalpy of evaporation, J/mol, 54400 molar volume, cm3/mol 360 Table 3.4 Physical properties of oxygen and nitrogen [Barton, 1983; Weast et al, 1989-1990] Oxygen Nitrogen Density at bp, g/cm3 1. 14 0.808 Molar volume at bp, V (cm3/mol) 28.07 34.65 A Solubility parameter, MPa^ 8.18 5.30 "Liquid volume", cm3/mol 33.0 32.4 3.7.2. SOLUBILITY OF O AND N IN LIQUID PHASE FC-43. 2 2 The solubilities of oxygen and nitrogen can be calculated -91- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. from the equation proposed by Prausnitz and Shiar [Prausnitz et al, 1986] using the regular solution theory. The volume of solution and enthalpy are assumed to have no change when they form a solution. It involves two steps to form a solution: firstly a gas is condensed to a hypothetical liquid and secondly the hypothetical liquid and solvent are mixed together to form a corresponding solution. The changes of free energy in the two steps are, Agj = RTlnf£ure /ff (3.7) AgTI= RTln72x2 (3.8) and then according to the regular solution theory, Ag = Agz + AgIZ = 0 (3.9) Therefore, the solubility can be given as the molar fraction by [Prausnitz et al, 1986], 1 V2< 61 - <2>M ' exp (3.10) x 2 RT where x2~ Molar fraction of the solute in the solution, %; f^ “ Fugacity of a gas in its hypothetical liquid state; f^> ~ Fugacity of the gas at temperature T, bar; - Molar volume of the gas in its hypothetical state, cm^ /mol; <5 2 - Solubility parameter of gas, ( MPa)^; 6^ - Solubility parameter of solvent, (MPa)^; 4> i ~ Volume fraction of solvent in solution; The fugacity of the hypothetical pure liquid is correlated in the form of a graph [Barton, 1983] in which the reduced temperature versus reduced pressure is plotted. -92- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. From the graph, the following data have been obtained, fL^ = 267.3 bar (oxygen); fL = 209.1 bar (nitrogen). The solubility of oxygen and nitrogen can be calculated using the molar volume of the solvent. 0.8 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.8 2.8 3.0 3.2 T/T C Figure 3.9 Fugacity of hypothetical pure liquid of gas. (reproduced from Barton [1983], where subscript c is referred to as critical conditions) Calculations were done for the Durapore GVHP membrane for the solvent FC-43. The calculated results shown in Table 3.6 were much smaller than the experimental ones. In fact, equation (3.10) is used to calculate the solubility of gas -93- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. in a solvent where the molecular size of the solvent is similar to the molecular size of the solute. However, the molecular sizes in this case are substantially different. In comparison with oxygen molecules, the molecules of perfluorotributylamine are much larger. Lawson et al [1978] suggested that if the size of the molecule was considerably different, the Flory-Huggins correction can lead to a better approximation. The correlation in this case is, v5(*r«2)2^i i V2 V2 - lnx. (3.11) -lnx2 + + ln (——) + ( l---—) RT V. where x^ is the ideal solubility of the solute and calculated from, (3.12) o where aH - Enthalpy of evaporation at the boiling point T , V o Joule/mol; - Molar volume of the solvent, cm3/mol. The data used in this equation are different from the former. The values of log x^, molar volume and solubility parameters for the equation are self-consistent constants which are given in Table 3.5 below. -94- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. Table 3.5 Data for the calculation of solubility of 0 and N [Barton, 1983] 2 Parameter Oxygen Nitrogen logx -2.7536 -2.7959 2 V , cm3 -mol” 1 46 53 2 6 , MPa^ 11.7 10.6 The calculated results applying the Flory-Huggins correction (equations (3.11) and (3.12)) are also given in Table 3.6. It can be seen that better results are achieved, although the estimated separation factor is slightly less than the experimental value. The terms which represent the molar volume difference between solute and solvent, ln(V / V ) and (1 - V /V ) in this equation, are greater than 1 2 1 other additive terms to the ideal solubility. In conclusion, for a system with considerably different molecular sizes a better result can be obtained from equation (3.11). The permeability calculated from the equations (3.11) is still smaller (by about 25%) than the experimental result. The reason for this is possibly the assumed values of porosity and tortuosity for the GVHP membrane. After the membrane is imbibed with the solvent, it may act less tortuously as a continuous phase. So the porosity of Durapore GVHP membrane can be taken as 0.75 (as indicated in the catalogue) because the solid part cannot contribute to the gas transport but the tortuosity could be taken as 1 instead of the assumed V2 = 1.4142. Calculations on this basis, i.e. t; = 0.75 and t - 1, were done for the same system and the calculated results are given in Table 3.7. The calculated permeabilities are now 6% to 12% higher. -95- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. Table 3.6 The calculated results from equations (at 20°C) (at = 0.75 and r = \f2 ). Calculated results Exper From eq.(3.10) From eq.(3.11) Solubility Oxygen 0.807x10” 4 1.583x10’ 4 (mol/m3-Pa) Nitrogen 0.661x10“4 1.178x10" 4 Diffusivity Oxygen 0.855x10"9 (m2 /sec) Nitrogen 0.754x10" 9 Permeability Oxygen 0.690x10"13 1.354x10" 1 3 (mol/m-s-Pa) Nitrogen 0.499x10"13 0.888x10"13 Permeability Oxygen 206 404 540 (Barrer) Nitrogen 149 266 337 Separation factor 1.385 1.524 1.61 Solubility contribution to separation factor 1.221 1.344 Diffusion contribution to separation factor 1.134 -96- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. Table 3.7 The calculated results from equations (at 20°C) (at f = 0.75 and r = 1 ). Calculated results Exper Parameter From eq.(3.10) From eq.(3.11) Solubility Oxygen 0.807x10”4 1.583x10"4 (mol/m3-Pa) Nitrogen 0.661x10”4 1.178x10”4 Diffusivity Oxygen 1.210x10”9 (m2 /sec) Nitrogen 1.067x10”9 Permeability Oxygen 0.977x10”13 1.715x10”13 (mol/m-s-Pa) Nitrogen 0.705x10”13 1.256x10”13 Permeability Oxygen 292 572 540 (Barrer) Nitrogen 211 376 337 Separation factor 1.385 1.524 1.61 Solubility contribution to separation factor 1.221 1.344 Diffusion contribution to separation factor 1.134 From the permeabilities in Tables 3.6 and 3.7 it is suggested that the tortuosity is a value between 1 and /2. The tortuosity of the GVHP membrane which can concile the experimental and calculated results is 1.06, as shown in Table 3.8. This table also includes the tortuosities required to reconcile the results for hydrophilic membranes. -97- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. Table 3.8 Comparison between experimental results and calculated results and reconciled tortuosities. P P P a a a Membrane Type Exp Calc Exp Exp Calc Exp P a Calc Calc GVHP Hydrophobic 540 540 1.00 1.62 1.524 1.063 1.06 GSTF Hydrophilic 950 950 1.00 1.60 1.524 1.050 0.56 HVLP Hydrophilic 540 540 1.00 1.62 1.524 1.063 0.99 VSWP Hydrophilic 350 350 1.00 1.60 1.524 1.050 1.53 From this table, it can be seen that the GSTF and HVLP membranes have effective tortuosities less than one. This contradicts the previous comments that even if all the pores are straight the tortuosity should be not less than one. A question rises here concerning the assumed concentration profile (Figure 3.3) which may only be valid for the hydrophobic membranes but not for the hydrophilic ones. Possible concentration profiles for the solvent imbibed membranes are schematically depicted in Figure 3.10. In the hydrophobic membrane (a), the liquid will completely occupy the whole pore length as indicated in Figure 3.3. In the hydrophilic membranes, on the other hand, it is possible that the liquid only occupies thin layers close to the surface but not the middle of the pores (b). As shown in the graphs the average thickness of the liquid layers is 6 and S ' on the upstream and downstream sides respectively. The thicknesses of these two layers depend upon the nature of the surface material, the pore size of the membrane and the nature of the liquid. These layers are not necessarily the same on both sides of the -98- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. membrane due to the different pore distributions. However, average values for these two layers exist. The driving force, measured by AC/Sl (1 is the thickness of the liquid layers), is higher in the hydrophilic membrane than that in hydrophobic membrane due to the reduced thickness of the liquid layer. Considering (b) of Figure 3.10, concentration (at x = <5 ) and (at x=<$+L') will be essentially the same in value because the diffusivity in the gas phase is about a thousand time greater than that in the liquid phase. Therefore, the driving force for the hydrophobic membrane (a) is C /L and for the hydrophilic o membrane is C /(6+6') and, o C /(S+S') > C /L (3.13) o o The measured permeabilities are based on the whole membrane thickness, L and for case (b) this thickness is greater than actually experienced. This means that the tortuosity required to reconcile the data will be small, and possibly less than one. -99- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES concentration concentration (a) (b) Figure 3.10. Schematic Concentration profiles of hydrophobic (a) and hydrophilic (b) membranes Both the experimental and calculated results for the GVHP membrane are depicted in Figure 3.11 (for the conditions f = 0.75 and r = 1.06). From this graph it can be seen that both experimental and calculated permeabilities increase with increasing temperature but the calculated ones show a stronger temperature dependence than the experimental ones. The reason for this is the viscosities of the solvent calculated from a graph (Appendix VI) are smaller than the -100- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. real value, so the diffusivities (from equation 3.5) and the permeabilities are larger. On the other hand, separation factor decreases with temperature, and this is found experimentally and predicted by calculation. The major influence on this is the variation of solubility with temperature, which favours oxygen at lower temperature. -101- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES roqopg: uoTqejpdes O lo co CVJ — a a a oo o CD Temperature CD o OJ OJ cvj [5HUIO-D0S- ^UID/UID-(dis) eUI3] 0 T _0IX A^TITqP0UIJ0d Figure 3.11 Calculated(C) and experimental(E) results of permeability and separation factors. -102- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. 3.8. MEMBRANE STABILITY. The membranes will be functional until the solvent is evaporated or the transmembrane pressure exceeds the bubble point. The evaporation rate of the solvent depends upon the vapour pressure of the solvent. The vapour pressure of FC- 43 and silicone oil is very low (vapour pressure of FC-43 is 2.5 mmHg at 25 °C). This is one of the reasons these chemicals are chosen as separation media. Usually, the liquid remains in membranes for a few hours which is longer than other solvents. The pre-saturating device provides a longer life for the liquid membranes. However, it is difficult to stop solvent losses. In practice, the pressure on both sides of the membrane will be different in a separation process, with the upstream slightly compressed and the downstream always kept at a reduced pressure which, for instance, may be one-twentieth or one-tenth of an atmosphere. It is possible to pre-saturate the upstream of a membrane only and the downstream flow will still carry some solvent away. Sometimes this becomes so serious that the solvent loss and recovery become a main obstacle to the membrane process. There are several other factors influencing the stability of immobilized liquid membranes: the interaction between solvent and membrane; compatibility of solvent and membrane; contact angle of solvent and membrane and carrier resistance towards irreversible oxygenation in the case of carrier-facilitated transport. In this study it was observed that the liquid did not chemically interact with the membranes. All the membrane materials tested in the experiments was stable in both FC- 43 and silicone oil. The membranes apparently returned to their original shape when the solvents evaporated from the membranes after being tested. -103- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. The compatibility of membrane and liquid could be seen when the liquid immobilized membranes were made. Originally, all the membranes used in the experimental studies were white in colour. When a hydrophilic membrane was imbibed with FC- 43 or silicone oil, its colour darkened and it became translucent. On the other hand, the hydrophobic Durapore GVHP membrane became nearly completely transparent after imbibing. Accordingly, this liquid membrane could bear a higher pressure difference than the others. This indicates the contact angle on this membrane is smaller than that on the other membranes if the same hydrophobic solvent is used. Considering the balance between the pressure difference and the surface tension, the maximum pressure, referred to as the bubble point, is given by Cantor's equation [Cheryan, 1986 ] , 27COS0 AP = ------(3.14) r where 7 - Surface tension of liquid, Newton/metre, (N/m); 6 - Contact angle between liquid and membrane; r - Pore radius, m. FC-43 (7 = 16.8 mN/m) had a low bubble point as expected because it was a lower surface tension solvent than water and many hydrocarbons. On the other hand, the silicone oil (7 = 21.2 mN/m) membranes withstood a relatively higher pressure drop than FC-43. In fact, both FC-43 and silicone oil are low surface tension solvents. Usually, the maximum pressure attainable can be calculated from equation (3.14) for the known pore size assuming a contact angle of zero. However, this kind of calculation -104- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. was only valid for the hydrophobic Durapore GVHP membrane, because only in this case can the assumed zero contact angle be fulfilled. For the hydrophilic membranes, on the other hand, a larger contact angle was expected. As mentioned before, the Durapore GVHP membrane is made from polyvinylidene difluoride. The critical surface energy of this material is 25 mN/m (25 dynes/cm) [Wu, 1982] which is larger than that of the solvents used in the experiments. Furthermore, both polymer and solvents have the non-polar hydrophobic nature. This implies that the liquids are ready to spread with a zero contact angle. The other membranes from cellulose acetate and nitrate (GSTF and VSWP membranes) contain the polar groups -OH and NO in their constituent unit renderring the hydrophilic nature to the polymers. This implies that with this kind of material, a contact angle will be observed when non-polar solvent (FC-43 or silicone oil) is used. Calculations were made using the data of bubble point from the Millipore catalogue (with liquid methanol, 7 = 22.65 mN/m), compared with that from the experiments with FC-43 and the silicone oil. The calculated results are given in Table 3.9. It should be noted that the calculated pore sizes are referred to as "big pore" sizes which are larger than the mean pore sizes. -105- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. Table 3.9. Calculated pore size from the data of bubble point with methanol, FT-43 and silicone oil as the liquids. Calculated Mean Durapore GVHP membrane Mean pore pore size bubble size (/im) (Aim) point (KPa) Catalogue 0.22 118* Calculated form catalogue* 0.77 118* Calculated from FC-43 0.87 77.3** Calculated from silicone oil 0.91 93.3*** * Methanol-wet, 7 = 22.65 mN/m ** Obtained from Figure 3.5. *** Obtained from Figure 3.7. The calculated results based on the bubble point and maximum pressure give similar pore sizes with a maximum variation 16.7%. Frequently, oxygen enrichment is carried out at normal pressure upstream and at reduced pressure downstream. This means that the pressure drop can be an atmosphere (or lxlO5 Pa) or less. It would be stable for the immobilized liquid membrane if the bubble point were higher than an atmosphere. From the experiments, one can conclude that only the hydrophobic membrane tested in the experiments was stable and the others were not. In fact, most of the membranes only withstood about 30 cmHg (40 KPa) pressure difference. The reason is the hydrophilic nature of these membranes and the hydrophobic nature of the organic liquids. It can be concluded that the compatibility between membrane and liquid is an important factor to be considered. -106- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. 3.9. CONCLUSIONS ON IMMOBILIZED MEMBRANES. The high permeability of the immobilized liquid membrane makes this kind of membrane attractive. At the present stage, it could be used in a small scale oxygen enricher, but a highly selective solvent still needs to be found in order to compete with the cryogenic process for the large- scale production of oxygen enriched air. Predictions of solubility and diffusivity can give a reasonable assessment of liquid-membrane performance before experimentation. The phase equilibrium data are recommended to carry out this kind of calculation if they are available. For a more general prediction, equation (3.11) provides a more accurately predictive base than (3.10) if a solvent of a high molecular weight is used. Furthermore, it is also possible to roughly estimate the permeation properties of a compound even before it has been synthesized. The lifetime of immobilized liquid membranes based on perfluorocarbon or silicone oil as the separation media is good because the absorption process is completely reversible and the vapour pressure is low for these kinds of chemicals. In addition, these materials are non-toxic. The low surface tension and the bubble points are a difficulty, because this limits the applicable pressure drop (unless a very "small pore" hydrophobic membrane is used). A single stage oxygen enricher using immobilized liquid membranes based on perfluorocarbons or silicone oil would not be able to reach high concentrations because the selectivity is low. It would lose its advantages if a multistage oxygen enricher has to be employed. However, assuming an a ~ 2.0, a product of about 35% oxygen is -107- CHAPTER III. IMMOBILIZED LIQUID MEMBRANES. achievable which could find limited application in enhanced combustion or biological treatment processes. As pointed out by Baker et al (1987), Johnson et al (1987) and Matson et al (1987), the future of the immobilized liquid membrane for oxygen enrichment may rely on facilitated transport in which the oxygen transfer is enhanced by reversibly reacting species. Organic solvent carriers, on the other hand, rely upon solubility and diffusivity properties which appears to limit their selectivity. -108- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. 4.1. INTRODUCTION. A "gas plasma" is an electrically conducting gas in which gas molecules are partially ionized. It is in fact a gas mixture containing ions, electrons, atoms and molecules in both ground state and excited. Usually, the gas plasma refers to the high temperature gaseous state. However, a low-temperature gas plasma can be generated by combustion, flames, electrical discharge, controlled nuclear reactions, shocks, etc. in a laboratory. A low pressure electrical discharge is the most convenient method to carry out the low-temperature plasma treatment process in a laboratory [Yasuda, 1981] . Plasma treatment is widely used in many processes to produce thin adhesive films for the purpose of modifing the surface properties of the substrate. The properties of the plasma-treated surface depend upon the nature of the gaseous plasma mixture, the type of plasma equipment and the conditions used in the process [Yasuda, 1981]. Plasma treatment has been known for a long time in laboratory practice. In scanning electron microscopy, for instance, this treatment is employed to produce an electrically conducting layer. The potential of plasma polymerization and plasma treatment for membrane preparation has only been appreciated in recent times. The effort given to the application of this process nowadays is large and many novel membranes have been made by depositing different monomers [Kramer et al, 1989]. The attraction of plasma polymerization is that the selective "skin" layer can be produced from a wide range of -109- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. materials and deposited on the substrate of choice. Very thin layers are feasible and this offers the potential for moderately high flux. In this study experiments have been carried out to examine the permeability and selectivity of plasma-treated membranes with materials containing organic silane and fluorine. Dense silicone rubber and microporous Millipore and Sartorius membranes were used as substrates in these experiments. The operating conditions, such as monomer flow rate, the composition of feed monomers and power supplied to the polymerization chamber, were also investigated. An infrared spectroscopic study was carried out to characterize the nature of the film produced. In addition characterization, based on surface energy, was attempted using surface tension measurement to determine dispersive and polar components of the surface energy of the plasma-treated membranes. 4.2. CHARACTERISTIC OF PLASMA TREATMENT. A gas plasma can affect a solid surface in contact with it in different ways. Firstly, etching is a process that removes some materials from a solid substrate. The process can affect a few monolayers of the solid surface. Secondly, plasma polymerization is a method for coating a monomer onto a solid surface. The method employs electrical discharge instead of using initiators to form polymer. One advantage of this method is that the coated layer can be as thin as 0.5 jum, which is at least 1/100 the thickness of typical dense membranes used in gas separations. Another advantage of the method is that many saturated organic chemicals, which are normally impossible to polymerize because of a lack of functional groups, can be deposited onto a solid substrate. This includes alkanes, perfluorocarbons and siloxanes. Indeed almost all volatile organic molecules can be used in this process [Inagaki and Ohkubo, 1986; Kramer et al, 1989]. -110- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. Thirdly, surface treatment with a plasma can modify a solid surface. This method can rearrange, remove and redeposit a few monolayers on the solid surface. In contrast to the former two processes, which are characterized by loss or increase in substrate weight, this process does not cause considerable change in the substrate weight. However, the property of the solid surface is changed. In some cases, the gaseous plasma mixture could be some inert gas, even a noble gas instead of a monomer. It needs to be noted that the three processes mentioned above are not distinguished from one another in some circumstances. The surface modifying, for instance, could be considered as the combination of etching and plasma polymerization. Even in the plasma polymerization, the etching can still occur to a significant extent. It is important to note that although the term plasma polymerization is used in many references, it is quite different from the conventional process of polymerization. One can certainly obtain quite different polymers with one species of monomer if only the polymerization conditions are changed. Even the geometry of the reaction vessel can affect the composition and structure of the plasma- polymerized polymers. A polymer formed by plasma polymerization usually has no discernible constituent repeating units, as expected in the conventional sense of polymerization. The plasma- polymerized polymer of ethylene, for example, does not contain the regular constituent repeating group, -(CH —CH )-, but ethylene units, unsaturated groups, aromatic rings and branched or crosslinked groups instead. -Ill- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. 4.3. EQUIPMENT AND MATERIALS. 4.3.1. EQUIPMENT. The equipment with which the plasma treatment was carried out in the CSIRO Division of Chemicals and Polymers, Melbourne, is shown schematically in Figure 4.1 and photographically in Figure 4.2. The whole system was placed in a fume cupboard without temperature control. The main part of the equipment was a glass bell jar in which the plasma was generated and the samples were fixed. The effective volume of the bell jar was 7.6 litres and measured 15 centimetres in diameter and 40 centimetres in height. The power source was a HPG-2 model radio frequency generator which could supply adjustable energy up to 150 watts and adjustable frequency. A BARATRON MRS 390 HA model gauge was connected to measure the pressure. The flow rate of monomer was controlled by needle valves and measured by the pressure gauge according to the pressure increases. A mechanical pump was used to create vacuum down to 0.02 torr (i.e. 0.02 mmHg) absolute pressure. Substrates ranging in size from 59 to 100 millimetres in diameter were fixed on one electrode and an aluminium foil was fixed on another electrode. For the horizontal electrodes the sample was fixed on the bottom electrode and the aluminium foil on the top electrode and for the vertical electrodes there were no differences between the two electrodes (see Section 4.4.2). Usually, the electrodes were set up to 30 mm apart. The whole system was evacuated to less than 0.05 torr and purged twice with argon in order to get rid of water vapour and oxygen before any glow discharge experiment was carried out. Then the flow rates of monomer and methane were set -by the pressure measuring method. Following this, the power -112- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. was supplied to the system. Usually, the pressure increased after plasma was generated. A glow discharge could be seen at this stage that appeared either between the electrodes or beyond the electrodes depending upon the total gas pressure of the chamber, i.e. at a pressure of less than 0.1 torr the glow discharge mostly occurred beyond the electrodes (Figure 4.4)r otherwise between the electrodes (Figure 4.3). -113- Figure BELL JAR CHAPTER 4.1. Plasma IV. PLASMA-POLYMERIZED treatment - 114 - equipment. MEMBRANES. W RF GENERATOR * CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. Figure 4.2. Front view of plasma equipment (top) and power supply unit (bottom) -115- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. Figure 4.3. Glow discharge occurring between electrodes. (at 0.5 torr, a mixed gas of methane and argon) Figure 4.4. Glow discharge beyond the electrodes. (at 0.05 torr, a mixed gas of methane and argon) -116- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. 4.3.2. MATERIALS USED IN THE EXPERIMENT. In the preliminary experiments, some fluorinated hydrocarbons and silicon organic compounds were found to form selective plasma-polymerized layers on silicone rubber dense membranes (see Section 4.5.2). Therefore, they were chosen as monomers in the following experimental studies. These plasma polymerizable species, substrates and other materials were: PFS Pentafluorostyrene. From Imperial Smelling, England. (This compound was stocked in 1969. Some precipitate, possible dimer or polymer, was found in the liquid but it is unlikely that this would affect the process of plasma polymerization) TEVS Triethoxyvinylsilane. From Aldrich Chemical Company, Inc., Boiling point 160-161°C. Argon From Commonwealth Industrial Gases Ltd. Methane From Commonwealth Industrial Gases Ltd. (Spectrum purity). RTV 3110 Dow Corning silicone rubber. Millipore VSWP - cellulose acetate and cellulose nitrate membrane with typical pore size 0.025 ^m. Sartorius SM 113 - cellulose acetate and cellulose nitrate membrane with typical pore size 0.05 /im. -117- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. 4.4. CONSIDERATION OF EXPERIMENTAL CONDITIONS. 4.4.1. CHOICE OF SUBSTRATES. Silicone rubber was selected as a substrate because it is popular in the oxygen-enrichment process based on membranes on a commercial scale and in field tests. Plasma-coated silicone rubber could have an advantage since both skin and substrate may be oxygen selective. In fact, silicone rubber membranes have already been modified by plasma treatment for oxygen enrichment [Zhu et al, 1986] and hydrogen/ methane separation [Stancell and Spencer, 1972]. The silicone rubber substrates were prepared by casting them on a polyethylene covered glass plate and cured at room temperature for two hours. Then they were placed in a vacuum oven at 70 degrees centigrade for 24 hours. The vacuum oven treatment was necessary for two reasons. It stabilized the weight of silicone rubber substrate by taking away any volatile material from the surface, so that the thickness determination by measuring the weight could be accurate. Secondly, it took the moisture away from the surface to make the plasma treatment easier. For the silicone rubber substrates, the thickness of the deposited layers was determined by the weight increase before and after the plasma treatment. This method is good for measuring the thickness of silicone membranes because of the stability of the substrates and the formed membranes after the plasma treatment. However, it cannot be applied to the microporous membranes tested in the experiment. A decrease in weight was always found in these membranes, presumably due to etching. Usually, the silicone rubber substrates had a thickness of 100 to 150 micrometers. They did not shrink nor expand -118- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. during the plasma treatment procedures. Any microporous membrane could be used as a substrate. However, those which have a larger pore size have a disadvantage because they need longer periods of plasma treatment in order to cover the pores. The Millipore membrane VSWP and Sartorius SM 113 used in the experimental studies are the smallest pore size membranes that can be found on the present market with typical pore size 0.025 and 0.05 micrometers respectively. 4.4.2. POSITIONS OF THE ELECTRODES AND SUBSTRATES. There are essentially two types of glow discharge equipment - capacitive and inductive glow discharge. They have both been employed in membrane treatment [Yasuda, 1981]. It seems that the capacitive method which is equipped with two parallel electrodes is used more than the inductive method, because the plasma coating between electrodes is easy to grow and control. The capacitive method was used in this study. Substrates were fixed on the electrodes as a target when plasma coating was carried out. There were several ways to arrange the electrodes. Two electrodes were placed vertically in the beginning. This arrangement was good for a small target (10 mm or less in diameter) or for a mobile strip-shaped target. There were difficulties in coating a relatively large target (50 mm or more). The deposited layer was not uniformly distributed because it was difficult to keep the substrate on the electrode. Some sticky materials were used which looked good when a substrate was fixed on the electrode but peeled off in many places when a vacuum was applied. Sticky tapes were also tried but the tapes could only be applied on the edge of the target. This peeling, uneven surface of the substrate -119- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. caused a distortion of the electrical field. After starting plasma treatment, those places which were still in contact with the electrode got a thicker deposited layer than those places which had peeled off the electrode. Sometimes the peeled places had no deposited layer at all. The vertical arrangement of the electrodes caused another source of uneven distribution of the plasma-polymerized layer, that was the flow pattern of the monomer. The monomer was introduced from the bottom of the bell jar and flowed upwards to the vent. When a large target was used, the lower half of the target touched the monomer first and got more deposited polymer than the higher half. (It might be good if the target could be rotated). For the reasons mentioned above, the electrodes were modified to be horizontal and the target was fixed on the lower electrode. There were some problems with this arrangement. Some places peeled off again when the vacuum was applied. The reason was that a small amount of air was unavoidably trapped between the target and electrode. This air would expand nearly a hundred thousand times when the vacuum reached 0.01 torr and therefore caused a bubble beneath the target. An electrode with tiny grooves (less than 1 millimetre between grooves) proved to be a good electrode which provided an exit for the trapped air. The target substrate was placed on the electrode naturally, i.e. any extra force was not necessary, and indeed could be fatal when fixing a substrate due to blocking of the exit for the trapped air. The geometry of the reaction vessel where the plasma polymerization is carried out plays a significant role. The deposition of polymer occurs anywhere inside the bell jar. However, the thickness and properties are usually different for the polymer formed in the different regions. When a -120- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. capacitive coupling is used, the intensity of the electrical field between the electrodes is stronger than that in the other region. So the surface of a substrate in this region will be more affected than that in other regions, although the surface of a substrate elsewhere inside the bell jar could be affected if a sample was fixed. Also, the edge of the electrodes may be even more affected because of the high intensity of the electrical field in this area. For these reasons, the sample (target) was fixed on the middle of the electrode and the distance between the electrodes was adjusted until a relatively uniform deposition was observed. The target was usually larger than the sample prepared for the permeation test which was taken from the region which had an apparently uniform deposited layer. The method used for determining the flow rate of the monomers had limited accuracy. A flow rate decline was often observed during deposition and it was necessary to check the flow rate before and after the experiment. Some experiments were discarded because the flow rate was too unstable. 4.4.3. CHOICE OF OPERATING CONDITIONS. It is difficult to give satisfactory criteria to describe or predict in advance what kind of polymer will be produced and what the properties of a plasma-polymerized polymer will be, because the properties of the formed deposited layer change depending upon the conditions. Up to now, investigators working on plasma treatment mostly rely on their experimental studies rather than theoretical predictions. Yasuda and Hirotsu [1978] and Yasuda [1981] have given detailed descriptions of the conditions and parameters used -121- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. in plasma polymerization. According to Yasuda, the flow rate is better described by the combined parameters, F M/Ap v or F /A, where F and F are the volume and weight flow w V w rate respectively, M is the molecular weight of the monomer, A is the area of the cross section of a vessel where the plasma is generated and p is the pressure in the plasma region. In effect, the former, F M/Ap, represents v the time for the monomer to remain in the treatment vessel. The letter, F /A, represents the weight of monomer passing w the region where the polymer deposition occurs. The system pressure, along with the flow rate, is an important factor affecting the property of the deposited layer. In turn, the system pressure is affected by the nature of the monomer introduced. The power used to initiate and maintain the plasma is also important factor which affects the property of the an important factor which affects the property of the plasma layer. When considering the power (W watts), Yasuda uses the combined parameter, W/F or W/F M to express the power w v consumed by a unit monomer. On the other hand, the surface area of electrodes, U, needs to be considered too. So, it would probably be better to use the parameter, W/F U, which w represents the power per unit area of electrodes and unit monomer fed into the vessel. However, for a given bell jar, the parameters related to the geometry are constant. The adjustable variables are flow rate of monomer, power supplied to initiate and maintain the plasma, pressure of the glow discharge vessel and perhaps the temperature. Therefore, the analysis of plasma-polymerized membranes has been done by varying these parameters, which are used by most investigators working on plasma treatments. -122- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. 4.5. GENERAL OBSERVATIONS AND DISCUSSIONS. 4.5.1. SILICONE RUBBER MEMBRANES. The rate of deposited layer formation using pentafluorostyrene and methane plasma is plotted in Figure 4.5. It was found to be easier to form the deposited layer with the plasma generated from a gas mixture with a higher percentage of methane. The deposition rate decreased when the methane content was decreased, and it was impossible to have any deposition in the experimental study when the methane content was below fifty percent. In other words, the pentafluorostyrene (PFS) alone could not be polymerized in the processes carried out, although there is a functional group in its molecule ready to form the repeating unit in a normally initiated polymerization process. It may be possible to polymerize it under certain conditions, but in this work the experimental result did not show any polymerization in the glow discharge process. Other investigators observed a similar phenomenon. As Yasuda [1984] pointed out, fluorine and fluorine-containing compounds reduce the rate of polymer formation in the glow discharge process. The silicone rubber substrate was softened and even became sticky when pentafluorostyrene was used as a monomer alone. This shows that some high molecular structure of the polymer substrate was degraded and some cross links were destroyed by the gas plasma. This softening effect did not occur when methane was used, possibly due to the beneficial effect of nascent hydrogen radicals and hydrogen ions which preferentially combined with the fluorine radicals and ions. -123- Figure Pentafluorostyrene feed rate: 2.0 seem Substrate: silicone rubber 3110 4.5. The CHAPTER deposition (uTui/er^euiojoTui) IV. PLASMA-POLYMERIZED rate - 124 vs e^ej - ratio uoxgxsodea of MEMBRANES. methane I to OJO I PFS . Percentage of PFS in feed (seem) rate flow Monomer co OJ CD CD TEVS of CD MEMBRANES. rate feed CD uox^Tsodaa - vs 125 - a^pj rate ) CD PLASMA-POLYMERIZED IV. CD deposition The UTur/aj^auiojDTui CHAPTER ( CD 4.6. 90'0 Figure CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. The deposition of triethoxyvinylsilane and methane follows a different rule, with the deposition rate being simply dependent upon the feed of triethoxyvinylsilane monomer, as depicted in Figure 4.6. The flow rate of methane does not show any significant influence on the polymer formation. However, the feed of methane gives a slight additional deposition to the polymer. It was observed that the colours of the plasma-polymerized layer were more intense when the methane content in the gas phase was increased. The silicone rubber substrates were originally white. They turned brown after being plasma polymerized when a mixture of pentafluorostyrene and methane was used as a feed gas and became dark black when only methane was used as a feed gas. It is quite obvious that it is much easier to polymerize methane than pentafluorostyrene. The phenomenom supports the remark made above: it is easier to make a free radical of methane and the association process such as CH -> CH + H can go 4 3 further till all the hydrogen atoms in the molecule of methane are gone, leaving a naked carbon atom deposited on the silicone rubber substrate. It was also observed that the coated films were not uniform, especially when the diameter of the film was the same diameter as the electrodes. In this case, the edge of a film usually had more polymer deposition than the centre. This presumably occurs because the strength of an electrical field is always higher at any sharp point than on a flat surface, so the deposition would be more at that part. In preliminary studies with vertical electrodes the lower part of the substrate had a thicker deposited layer than the higher * part of the substrate, even when the substrate was smaller than the electrodes. This may be because the -126- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. low part contacted monomers earlier than the other parts and therefore had more chance to catch monomers and to form deposited layers. After the horizontal electrodes were constructed, an even deposited layer was obtained. However under some conditions this causes another problem because the deposited polymer fell off the top electrode covered with aluminium onto the bottom electrode covered with a sample, making irregular spots on the prepared film. In order to overcome this problem, a rough aluminium foil with grids was found to be better than a plain one. Care was needed as the plasma treatment proceeded, i.e. the coating process had to be stopped before the falling off was visibly observed. It has been reported previously that the higher the power used in plasma polymerization, the more cross-linked the polymer layer formed, and the higher the selectivity achieved. The power supplied to plasma systems has been reported to range from several watts to hundreds of watts. However, the influence of power supplied to the system on the separation factor was not obvious in this study. Many reports have pointed out there will be a limit for the power applied. In this study, four different power supplies (15, 25, 30 and 45 watts) have been selected. The results show that the best power applied to the system was 30 watts, forming a fast deposition rate of the polymer. In fact, a low energy supply causes a power deficiency so that a certain portion of the monomer flows to the exhaust and this will decrease the deposition rate. On the other hand, too much energy caused a monomer deficiency with some materials being removed from the deposition layer which decreased the net deposition process. The influence of power supply on the deposition rate for PFS /Mcthd/16 treated membrane is depicted in Figure 4.7. -127- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. (watts) plasma maintain to supplied Power O O O (uTui/ajq.euioj3Tui) uoT^xsodaa Figure 4.7. The influence of power on deposition rate. -128- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. Pressure played an important role in the polymerization process. In practice, plasma formation needs very low pressure, usually less than 1 torr. At a relatively high pressure, larger than 0.1 torr, glow discharges occur between the electrodes, and the deposited layers grow faster than those at a very low pressure (less than 0.01 torr), in which the glow discharge occurs outside of the electrode area. In this study, very low pressures could not be reached because of the limits of the equipment. If a more powerful diffusion vacuum pump were used, the influence of pressure on the permeation properties could be examined. The frequency supplied to the system is less important in the process than the other parameters. In fact, the breakdown energy necessary to initiate the glow discharge of an organic compound mainly depends upon the nature of the compound and the voltage applied. In practice, frequencies ranging from audio to radio frequency have been used in various laboratories. In the present system, the frequency of 700 kHz was selected because it was observed that this frequency did not show any influence on both initiating and maintaining the plasma state. The results of the plasma treated are given in Table 4.1. The separation factor of the deposited layer of plasma polymerized silicone rubber membranes ranges from 2.23 to 2.96. The best result was 3.01, obtained in the preliminary experiment, when a mixture of pentafluorostyrene, trimethoxybenzylsilane and methane was used as the feed gas (Table 4.2). -129- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. Table 4.1. Experimental results with silicone rubber membranes treated with gas plasma. Membrane description a a l 2 Power Percent Deposition 6 e < f P d Watts of PFSC rate (/n/min) i 2 i 3110a 15 0.38 0.016 154 1.18 3.156 2.087 2.25 3110a 15 0.30 0.022 147 0.88 3.447 2.086 2.28 3110a 20 0.18 0.052 133 0.78 4.83 2.097 2.96 3110a 20 0.23 0.0303 157 1.03 3.67 2.063 2.23 3110b 150 800 2.0 Silicone 3110 substrate coated with PFS and methane. b Silicone 3110 without plasma treatment. The volume portion of PFS in the feed gas. d Units of Barrer. - Thickness of substrates in micrometers. f - Thickness of coating layer in micrometers. a - Separation factor of the composite membrane, a - Separation factor of the coating layer. Table 4.2. Preliminary experimental results with silicone rubber 3110 treated with gas plasma. Membrane description Separation factor a 2 Power, watts 3110 PFSa TEBSb Methane 15 3.01 3110 DMDCSc Styrene Methane 15 2.67 3110 PFIOd methane 15 2.34 3110 PFDMCH6 Methane 15 1.61 Pentafluorostyrene. b TEBS - Triethoxybenzylsilane. DMDCS - Dimethyldichlorosilane. d PFIO - Perfluoroisooctane. PFDMCH - Perfluorodimethylcyclohexane. -130- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. It seems that the aromatic fluorocarbons could be used to form high performance active layers in the early experimental study, but a later experiment with pentafluorostyrene gave a less encouraging result. In fact, pentafluorostyrene has been used by Nomura et al [1984] with the same methods. The separation factor obtained by them was not very encouraging either. His conclusion was that the higher the molecular weight used, the higher the separation factor achieved. The separation factors of the composite membranes were probably limited because of the thick non-porous substrate. The following demonstration explains why a composite membrane with a thick substrate of only modest permeability cannot reach a high separation factor. Example I. Two layer composite: Suppose there is a composite membrane of two layers: the substrate of silicone rubber with a thickness of 150 micrometers having a permeability of 800 (Barrer) and a separation factor of 2, and the deposited layer with a thickness of 1 micrometer having a permeability of 10 (Barrer) and separation factor of 4. The permeability of the composite membrane of the above two layers can be calculated using equation (2.49), 0.0151 P = ------= 525 (Barrer) (4.1) ° 0.0150/800 + 0.0001/10 0.0151 P = ---- —------= 195 (Barrer) (4.2) 0.0150/400 + 0.0001/2.5 and the separation factor can be calculated using equation (2.50), -131- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. a = 523/195 = 2.69 (4.3) composite ' 7 This calculation shows that the composite does not approach the separation factor of the selective skin layer. In order to achieve its potential the substrate needs to have a much larger permeability. For example, if the substrate had a very high permeability, preferably as high as that in microporous membrane, the composite could achieve a much higher selectivity. A substrate permeability of (say) 80000 Barrers (this is the permeability similar to microporous glass, see Table 2.2) will be good as a substrate for this purpose. One problem with this approach is the difficulty in coating microporous substrate (see Section 4.5.3). An ingenious solution to this has been proposed by Kramer et al [1989]. Firstly, a silicone rubber layer of 2 to 3 micrometers was attached to the polysulfone. Then an argon plasma was applied onto the silicone surface for a short period of time; this produces a three layer composite. The key to this work involves two procedures, firstly a thin and highly permeable substrate was made and secondly a highly selective plasma polymerized layer was made on it. The overall performance can be estimated from the procedure used in example I, i.e., Example II. Three layer composite: The membrane comprises, (1) 100 micrometers microporous membrane as a substrate (Pi= 80000 and a =1) , (2) 2 micrometers silicone rubber as the first coating layer (P^ =800 and a =2), (3) 1 micrometer high selective material as the second coating layer (P =10 and a = 4); 3 3 -132- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. The overall permeability for this composite membrane will be, 0.0103 P =------=9 97 (Barrer) (4.4) ° 0.0100/80000+0.0002/800+0.0001/10 0.0103 P =------=254 (Barrer) (4.5) N 0.0103/80000+0.0002/400+0.0001/2.5 and the overall separation factor will be, a = 997/254 = 3.93 (4.6) overrall It can be seen from this demonstration that only in the last case can the overall separation factor reach the highest value of the separation factors of the deposited layers. Many papers have reported that the plasma-polymerized treatment makes a membrane more selective than the substrate. This may only occur if the membrane substrate is also affected by the plasma treatment. In other words, not only is the deposited layer formed but also the substrate itself is modified. Therefore, the permeation properties of the substrate are also affected. This could happen because the particles in the plasma, including ions, electrons, radicals and excited molecules are so active that they penetrate the surface they come into contact with to a certain extent and change the structure of the substrate. Under these conditions the membrane permeability may be decreased and the selectivity increased because the substrate becomes more compact than in its original condition. Many investigators use silicon containing monomers since the deposited layer formed by these monomers has a high permeability. However, the plasma-polymerized skin layer -133- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. does not form a high separative membrane. In fact, the high separation barriers are usually produced by a compact hydrocarbon, especially linear hydrocarbons. The problem of hydrocarbon polymer membranes is that a compact hydrocarbon film always has too low a permeability and it may be difficult to make a thin membrane with it in a conventional coating method. For example, Yamamoto et al [1984] used silanes and hexenes as monomers to coat microporous polypropylene membranes and proved that a high separation barrier was formed by hexenes but not by silanes. 4.5.2. PROPOSED MECHANISM OF REACTION UNDER PLASMA. According to Yasuda's competitive ablation and polymerization (CAP) mechanism, polymer formation and ablation will take place at the same time. Whether the substrate gains or loses materials depends upon which of these two processes dominates the whole process. In this study the weight of substrates was decreased when PFS gas plasma was used alone under the 15, 20 30 and 45 watts power supply. Being attacked by the PFS plasma, the surface of the substrate roughened and some material was removed. There are at least two mechanisms occurring simultaneously: plasma-induced and plasma state polymerization. The letter refers to a process in which the ions or excited state of molecules are generated by the imposed electrical field. Then the excited state of molecules joins and deposits onto the substrate. The plasma state polymerization can be schematically represented as [Yasuda, 1984]: -134- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. Mechanism Is Plasma state polymerization, (i) Initiation or reinitiation, M. -»• M x , (4.7) M - M x . (4.8) (ii) Termination, M x + M x -*• M -M , (4.9) i j i j where M x and M x represent the excited molecules and subscripts i and j represent the number of repeating units. Mechanism 2: Plasma-induced polymerization is a process whereby the monomers are initiated, then the excited molecules join unexcited molecules and the polymer long chains start to grow. This mechanism can be represented as, (i) Initiation or reinitiation, M - M x , (4.10) M M x . (4.11) (ii) Propagation, M x + M M / M + M ^ M (4.12) i + 1 i + 1 i + 2 M x + M -+ M i M + M ^ M (4.13) j + 1 j + 1 j + 2 (iii) Termination, M + M x -*• M -M (4.14) i+n j+m i+n j+m It should be noted that (1) the process may be repeated many times and (2) as mentioned before, the whole process may be a combination of two mechanisms, but only one -135- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. dominates the whole process. For example, if mechanism 1 is a dominant process many atoms of the monomer may be lost in the plasma treatment process as the initiation or reinitiation is repeated a few times. On the other hand, there will not be many atoms lost if mechanism 2 is the main process. Therefore, the experimental results suggested that mechanism 1 be the main course of the plasma treatment when pentafluorostyrene was used as one component of the feed gas (see below). Based on the results of infrared spectrum analysis (see Section 4.6), one can draw the conclusion that (i) the aromatic ring structure does not appear in the formed polymer and (ii) very few fluorine atoms are attached to the polymerized polymer. From the deposition process, it can be concluded that the methane plays an important role in the polymer formation. Actually, methane provides a source of hydrogen in the process of pentafluorostyrene plasma polymerization. Considering the bond strength for these molecules, Yasuda [1984] concluded that the hydrogen and fluorine atoms joined to become H-F and then could be removed from the discharge system because the bond strength of H-F (565 J/mol) is larger than either the bond strength of F-F (155 J/mol) or of C-F (427 J/mol). In the discharge system, the bond C-F may have been broken since the energy imposed by the electrical field is strong enough to break such bonds. The most stable product in this system would be favourably formed as a final product in several competitively possible reactions. Presumably this mechanism caused the deficiency of fluorine atoms in the final polymer. Perhaps this is one of the reasons that the plasma treated membrane produced in this experiment did not form a highly selective membrane. Actually, it is normal practice to put fluorine atoms into polymers to form oxygen selective membrane materials. -136- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. 4.5.3. MICROPOROUS MEMBRANES. It was proved that all the microporous membranes treated by plasma polymerization had a very high permeability and a separation factor of one. This indicated that after these membranes were treated with a plasma either many pores were still open or the coating layer was cracked. Whatever the reason, these kinds of membranes are not suitable for oxygen enrichment. The microporous membranes coated in the experimental study, Millipore VSWP and Sartorius SM 113, are made from cellulose acetate and cellulose nitrate, which is quite sensitive to heat. In some cases, membranes were deformed and became very fragile after being coated. All the membranes lost weight although there was a coating layer seen on the top surface. There seemed to be an upper temperature limit of 50°C to 60°C for coating these kinds of membranes. It has been reported [Inagaki and Kawai, 1986] that coating layers need to be as thick as two times larger than the biggest pore size of the corresponding membrane. This is very necessary because of the pore size distribution. In electron microscopic studies, big pores had a pore size of about 0.2 ^m which was ten times larger than the typical pore size (Figure 4.8). Inagaki and Kawai [1986] and Inagaki et al [1988] used coating layers of about twenty times larger than the typical pore size. Therefore, the deposited layer for the Millipore VSWP membranes needs to be more than half a micrometer, and for the Sartorius 113 one micrometer. The deposited layers calculated from the silicone membrane deposition experiment suggested that they could be thick enough (about 1 ^m) to cover all the pores. However, the results of a separation factor of about 1.0 -137- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. have shown that the pores are still open. Inagaki and Ohkubo [1986] used a hexafluoropropene and methane mixture to deposit onto Millipore VSWP membranes and concluded that a thickness of 0.5-0.7 ^m was enough to cover the pores. One reason for unsuccessful coating in this study could be the effect of uneven coating of the substrate. Thus, although the "average" deposition may be large enough, the local variation may leave thin regions. Another explanation could be that the substrates were deformed by the treatment, and that the process of testing in which the sample was fixed between permeation cell and a rubber o- ring may have caused cracks to appear in the deposited layer. Figure 4.8. The surface view of VSWP by SEM. A longer period of deposition time will be necessary to produce a pore-free membrane, especially for membranes with a bigger pore size, but this may produce more heat which can destroy the pore structure of the cellulose based membranes. This indicates the importance of temperature -138- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. controlling of electrodes. High stress levels were found in all the coated membranes in this study, similar to the phenomena observed by Yasuda et al [1977]. It was evident that the physical properties of the membrane were changed to different extents after the plasma treatment. Stress only affected the silicone membranes a little because of the flexibility of silicone rubber. However, it was significant for Millipore VSWP and Sartorius 113 membranes because of their fragile nature. In fact, some membranes were seriously deformed and became so weak that further testing for permeation became very difficult, even if special care was taken. In summary, the instrument for plasma treatment was not suitable for a microporous substrate if there was no temperature controlling device to moderate the temperature of the electrode and then a target fixed on the electrode. 4.6. INFRARED SPECTRUM INTERPRETATION. Compositions and structures of the plasma-polymerized film have been studied by Fourier transform infrared spectroscopy (FT-IR). The instrument used was an FTS 7 FT-IR Spectrometer from BIO-RAD equipped with a total attennuated reflectance accessary (ATR). Two kinds of samples were prepared: potassium bromide disks for infrared transmission and ATR samples. It has been proved that the contact between an ATR sample and the crystal is good so that the ATR spectrum provides interesting information on the surface structure. The spectra of two plasma-treated films polymerized by pentafluorostyrene (PFS) and triethoxyvinylsilane (TEVS) are shown in Figures 4.9-4.10. As a comparison, the spectra of silicone rubber 3110 [the main component of the silicone rubber is poly(dimethyl- siloxane)] are given in Figure 4.11. -139- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. O r~ o _ o _ o _ o CVJ P 2*9063--- _ o _ o aoueqaosqv Figure 4.9. ATR spectra of PFS and methane treated membrane -140- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. O tlMl 0*688 _ O 8*91^ _O 0 * 119 _o _ o _ o _ o eoupqjosqv Figure 4.10. ATR spectra of TEVS and methane treated membrane -141- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES O j— o 1000 1500 2000 Wavenumbers 2500 0*296* 3000 SSl£B zizse- < 3500 oo UD oo o 4000 o o d o' o soupqjosqv Figure 4.11. ATR spectra of poly(dimethylsiloxane) . -142- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. The ATR spectra can reflect the structure of thickness of about two micrometers. The depth of penetration of infrared light can be calculated by the equation, A/n1 d = ------«------. . (micrometer) (4.15) p 2?r[sin^ 9 -(n2/n1) where A - wavelength of light (micrometer); 9 - angle of incidence; n^ - refractive index of ATR crystal; n^ - refractive index of the sample. The crystal used to obtain the ATR is made of zinc selenide (ZnSe) with a refractive index 2.4 at wavenumber 1000 cm"1 . Applying equation (4.15) the calculated range of the penetration is from 0.3 ^m to 1.8 ^m. Therefore, the spectra of coated membranes still contain the substrate spectra if the peaks are strong. On the other hand, a peak of the substrate may not appear if it is weak. There are several references available to aid interpretation of the silicon compound infrared spectra. The interpretation of the infrared spectra in this study mostly relies on the work of Zelei et al [1988], Wright and Hunter [1947] and Segui and Ai [1976]. Silicone rubber and plasma-polymerized membranes have similar spectra to poly(dimethylsiloxane) (PDMS) (in the computer-installed library). An interpretation of these spectra is given in table 4.3 and summarized here: The sharp peak at 2963 cm" 1 is the contribution of the C-H stretching vibration of the -CH group. The band at 2905 cm" 1 is assigned to the stretching mode of the vibration of -CH . -143- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. The strong sharp peak at 1258 cm" 1 for silicone rubber and the PFS polymerized membrane and at 1259 for the TEVS polymerized membrane is less affected by the polymer conditions. These peaks are assigned to methyl rocking vibrations which means the methyl group stretches as a single entity. This peak shows the abundance of the methyl group in both the silicone rubber substrate and the plasma- treated surface. In fact, it can be expected that all monomers including methane would produce methyl groups. It is observed that infrared spectra of the membrane prepared by TEVS as monomer overlap heavily in the region between 1700 cm"1 and 1400 cm"1 and it is difficult to recognize any peaks. A strong and broad peak lying between 1000 cm" 1 and 1100 cm" 1 may shelter some peaks which are characteristic of aliphatic and aromatic fluorine containing groups. Therefore, this analysis cannot be used to identify the presence of C-F bonds in the case of triethoxyvinylsilane as the polymerized species. The spectra of two coated membranes contain peaks which do not appear in the spectra of silicone rubber. There are very weak peaks at 1501 and 1482 cm" 1 which are due to fluorine substituted benzene. This indicates that most benzene rings were converted to the aliphatic from the aromatic. Otherwise, there would be strong peaks in this region because of the strong influence of fluorine on the vibration. -144- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. Table 4.3. Spectra of silicone rubber and plasma-treated membranes. Wavenumbers vibration mode Silicon PFS TEVS description rubber polymerized polymerized 3442 s* (polymerized OH) stretching 2963 vs sh 2962 vs sh Si(CH ) Rocking 2961 s sh stretching 2907 w br 2906 s Si(CH ) asymmetric 2905 w 3 3 stretching 2857 Si(CH ) symmetric 2851 w stretching 2357 w 2347 vw CO in air 2 2346 w 1734 1726 w br C=0 stretching 1641 w 1640 s C=C stretching 1501 vw C=F stretching 1482 vw C=F stretching 1447 w Si(CH ) rocking 1446 w 1413 s Si(CH ) , Si(CH ) 1412 w 1412 w br C-H bfen&ing 1258 vs 0-Si(CH)2-0 rocking 1257 vs 1257 vs sh 1070 vs brl070 vs 1069 vs CCC stretching 1040 s Si-O-C, Si-O-Si 1040 s stretching (continued next page) -145- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. Table 4.3. (continued) Wavenumbers vibration mode Silicon PFS TEVS description rubber polymerized polymerized 999 vs 998 vs Si-0 stretching 991 vs sh 864 s 864 vs Si(CH ) stretching 863 SiC stretching 812 s Si(CH ) rocking SiC Stretching 773 vs 772 vs Si(CH ) rocking 771 vs SiC Stretching 2 703 vs Si(CH ) 702 vs 702 vs stretching 665 vs sh 665 vs sh 665 vs sh Si(CH3)3 stretching * vs - very strong; s - strong; sh - sharp; w - weak; vw - very weak; br - broad. Double bond stretching bands were found at 1640 for PFS and 1641 cm-1 for TEVS as monomers which is evidence that the vinyl group is joined onto the polymer surface. This may be easy to explain for the pentafluorostyrenen and triethoxyvinylsilane coating membrane because both the monomers contain a vinyl group. In the case of pentafluorostyrene, in fact, a double bond could also be formed when the aromatic ring is broken down. The absence of C-F bending at 1020 and 995 cm” 1 may be caused by the strong band of Si-0 stretching at near 998 or 999 cm”1 which overlaps the former. The spectra of PFS polymerized film show that no new intensive peaks appear which were expected before the -146- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. experiment was conducted: the halogen substituted hydrocarbons would show some strong bands in the corresponding spectra. The spectra indicate that the deposited layer still contains CH3~, -CH^-, Si-0 and Si-C groups. It is interesting that the fluorine containing group does not strongly appear. This supports the mechanism mentioned above: most of the fluorine atoms in the PFS were knocked out and a long chain of hydrocarbon and siloxane formed the main composition of the deposited layer. Terada et al [1983, 1984, 1985 and 1986] found that the aromatic structure was replaced by aliphatic structure when perfluorobenzene was polymerized. A similar replacement was found in the experimental studies using PFS as monomer. It seems there were neither spectra of aromatic hydrocarbons, which would appear at 1500 cm" 1 , nor spectra of substituted benzene rings, which would also appear at 1500 cm-1 . 4.7. SURFACE ENERGY ANALYSIS. The surface characteric has been studied in terms of contact angle. The theoretical background of surface energy and contact angles is summarized in Appendix III. This is a somewhat qualitative characterization because the practical measurement of contact angles and then the interfacial energy analysis can not be expected to have stable values, because many factors such as surface contamination and roughness can affect the result. The contact angles of silicone rubber membranes and plasma- polymerized membranes were measured by a drop-on-plate method using the Kernco model G-ll contact angle metre with a goniometer. The liquids used were water, methylene iodide LR from AJAX Chemicals Ltd, tricresylphosphate LR from AJAX Chemicals Ltd, formamide from L. Light & Co. Ltd and Glycerol AR from BDH Chemicals, Australia Pty. All the -147- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. chemicals were used as supplied without purification, except for water which was distilled three times. The properties of these liquids are given in Table 4.4 in which are included the dispersive and polar components of the surface energy. Table 4.4. The surface tension of the liquids used to measure the surface energy, at 20°C [Wu, 1982]. Liquid Surface tension, dyne/cm Total Dispersive Polar component component d 1 V 1 V 71i V Water 72.8 22.1 50.7 Glycerol 63.4 40.5 22.9 Formamide 58.2 36.0 22.2 Methylene iodide 50.8 44.1 6.7 Tricresylphosphate 40.9 39.7 1.2 The contact angle data were analysed in terms of dispersive and polar component contributions to the surface energy. Typical values of surface analysis are given in Table 4.5. It can be seen from this table that the surface energy of silicone rubber and plasma-treated membranes is usually small in value in comparison with an organic solvent, such as the solvent used to measure the surface tension. Particularly, it is much smaller than water. -148- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. Table 4.5. The surface tension of the silicone rubber and some plasma-polymerized membranes Membrane Surface tension, dyne/cm Total Dispersive Polar component component d p 7, 7. 1 V 1 V 1 V Silicone rubber 27.6 24.7 2.9 3110 PFS treated membrane3 23.5 19.3 4.1 3110 TEVS treated membrane6 29.0 23.0 6.0 Power, 20 watts, 18% PFS in feed gas. b Power, 20 watts, TEVS in feed gas. The concentration of feed gas mixture does not have significant influence on either the dispersive or polar component of surface energy as shown in Table 4.5. However, the silicone rubber and plasma-treated membranes are such low surface tension materials that neither dispersive nor polar component of the surface energy could change a great deal with the plasma treatment. The polar component is very small, as expected, which indicates that either the substrate or deposited layer is a nonpolar polymer. 4.8. MORPHOLOGY OF THE PLASMA-POLYMER I ZED MEMBRANES. Morphological studies have been carried out using electron microscopy. A CAMBRIDGE model S 240 scanning electron microscope was used to observe the surface. It can be seen from the photographs that the plasma-polymerized membrane formed by PFS and methane strongly depends on the composition of the monomer. The surface of the membrane from a high percentage of methane as monomer forms a smoother deposited layer (Figure 4.12) than that from lower methane content feed gas (Figure 4.13). ; -149- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. The power used to maintain the plasma state also plays an important role in the plasma-polymerization process. The high power used in the system tends to speed up the polymerization process and as a result a rough surface was formed (compare Figures 4.15" and 4.14). A cross section of the plasma-polymerized membrane was observed under an electron microscope. A sample was prepared by immersing the membrane in liquid nitrogen and bending it. Cracking occurred and a cross section of the crack was observed. The photograph shows that the thickness estimated by the weight method provides quite an accurate measurement. The thickness of the deposition layer of PFS and methane treated membrane is about one micrometer (Figure 4.15, compared with the thickness of 1.03 measured by weight increase method). It was found that there were many cracks in the membrane treated with triethoxyvinylsilane and methane with a high energy input (Figure 4.16). There are two factors contributing to the cracks: one is the thickness of this deposited layer which reaches 2.09 ^m; the other is the high stress formed by the plasma treatment in the deposited layer. It is clear that the energy supplied to maintain the plasma has a limitation. For example, the best power supplied in this system is 20-25 watts for the PFS and methane treatment, although the optimum power supplied could be slightly different if a monomer other than PFS or TEVS was used. -150- CHAPTER IV. PLASMA-POLYMER I ZED MEMBRANES. Figure 4.12. PFS and methane treated, 20 watts, PFS/Methane=l/3. Figure 4.13. PFS and methane treated, 20 watts, PFS/methane=l/l. -151- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. Figure 4.14. PFS and methane treated, 30 watts, PFS/Methane=1/3. Figure 4.15. Cross section of the membrane in Figure 4.12. -152- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. Figure 4.16. TEVS and methane treated membrane. TEVS/methane=2/l, 30 watts, TEVS feed 2.5 SCCM. 4.9. CONCLUSIONS ON THE PLASMA-TREATED MEMBRANES. A thin uniformly deposited layer of polymer can be made by plasma polymerization under carefully controlled conditions with horizontal electrodes. The deposited layer possesses a high resistance towards the permeation of both oxygen and nitrogen. The permeation resistance of nitrogen is higher than that of oxygen. As a result, the separation factors of the deposited layers are high, but not of the whole membrane in which the substrate is included. A thin substrate would be the most important factor for production of a highly permeable and permselective membrane, especially for a dense membrane. The selectivity -153- CHAPTER IV. PLASMA-POLYMERIZED MEMBRANES. of a composite membrane will not be very high if the dense membrane is thick. The thickness of the dense membrane needs to be comparable to the coating layer in order to produce a good separation barrier. An improved concept is to make three layers for the plasma- treated membranes. The base would be microporous and able to withstand a high pressure difference but have a very low permeation resistance. The middle layer would make the membrane pore free and also have low permeation resistance. The third layer would possess high permselectivity. Both the second and third layers need to be very thin. The overall separation factor in this case will be near the highest separation factor of three. Silicon containing monomers, such as triethoxyvinylsilane, could be good for coating microporous membranes to produce pore free membranes but not for coating dense membranes. The former need more flexible material, such as silicone rubber, to cover their pores. The latter, which are pore free already, on the other hand, need a highly selective barrier. In this case, a monomer, such as methane, which can form a denser compact deposited layer has an advantage. In terms of equipment design for plasma polymerization it is important to have a temperature controlling system, because some membrane materials are heat sensitive, especially in the case of the cellulose acetate and cellulose nitrate membranes. It is also necessary to have a means for precise determination and control of the monomer flow rates, especially if more than one monomer is used. -154- CHAPTER V. PHOTOSENSITIVE MEMBRANES. CHAPTER V. PHOTOSENSITIVE MEMBRANES. 5.1. INTRODUCTION. The early experimental work of Scholander [1960] and the theoretical considerations of Schultz [1986] suggest that oxygen/nitrogen separation may be feasible using photosensitive membranes (Section 2.6). This chapter describes the development of oxygen-coupling compounds and attempts to incorporate these into a membrane device. Initially, some mathematical analyses are presented which show the potential application of a photochemical reaction for oxygen enrichment (Section 5.2) under ideal conditions. After that, chemical considerations are described, examining the possible performance of compounds when used as oxygen carriers. These compounds are basically new oxygen carriers derived from anthracene (Section 5.3). Chemical syntheses are presented in Section 5.4. Some anthracene derivatives including 1,4-dimethoxyanthracene, 1,4-dimethoxy-9,10-diphenylanthracene and 1,4-dimethoxy- 9,10-bis(4'-bromophenyl)anthracene have been synthesized and investigated. Since the chemicals which are used as oxygen carriers play an essential role in the gas separation process, synthesis of a nitrated anthracene carrier was attempted in order to improve their separation performances. Spectrophotometric studies are described in Section 5.5 which examine the conditions under which the reversible reactions may be carried out inside the membranes. Different carriers and solvents have been tested, and then 1,4-dimethoxy-9,10-diphenylanthracene has been chosen as the carrier and a less volatile solvent has been chosen as -155- CHAPTER V. PHOTOSENSITIVE MEMBRANES. the solvent. Permeability and selectivity of immobilized liquid membranes with and without carriers have been determined and discussed in Sections 5.6 and 5.7. In addition, polymer membranes incorporating fixed carriers have also been studied. Since no positive results were obtained in the permeation tests, an alternative process to carry out oxygen enrichment using a photochemical reaction was assessed, i.e. oxygen absorption and desorption experiments (Section 5.8). The results from the absorption and desorption experiments give an interpretation of the phenomena encountered in the permeation experimental studies. The kinetics of the corresponding chemical reaction has also been studied and is reported in the last section (Section 5.9) and the conclusions are given in Section 5.10. 5.2. MATHEMATICAL ANALYSIS OF FACILITATED TRANSPORT. It is interesting to assess the potential achievement for the ideal case. The following analysis expresses the necessary factors for achieving a practical separation membrane. The facilitated process is based on the reaction scheme, A + 02 (5.1) For this reaction scheme to apply to a membrane it is necessary for the feed side to be illuminated and the downstream side to be in darkness. -156- CHAPTER V. PHOTOSENSITIVE MEMBRANES. Considering equation (2.67), one can simplify it under the assumed conditions that the reaction rate for the forward reaction, which occurs at the region near the illuminated face, is much larger than the reverse reaction rate in the same region. On the other hand, the reaction rate of the reverse reaction, which takes place in the region near the downstream face (without light), will be much larger than that of the forward reaction in this region. In fact, it is a logical speculation from the preliminary experiments (Section 5.5.1) that the forward reaction can be completed under illumination and the reverse reaction can be completed in the dark at above 45°C. The equilibrium constants under these conditions can be expressed mathematically as, K^>> 1 (at upstream face |-d[A]/dt| >> |-d[AO]/dt|) (5.2) and K-^<< 1 (at downstream face |-d[AO]/dt| >> |-d[A]/dt|) (5.3) Then equation(2.72) will be reduced to, F (5.4) This equation consists of two terms. The first one is the Henry's law transmission. The facilitated contribution is represented by the second term and depends only on the total concentration, diffusivity of the carrier and the thickness of the membrane. This equation gives the maximum transmission and facilitated transport across the membrane. It can be seen from equation (5.4) that the concentration difference of oxygen does not affect the facilitated transport. This implies that transport against the concentration gradient is possible in this case [Schultz, 1986; Kim and Stroeve, 1989; Cussler, 1971]. -157- CHAPTER V. PHOTOSENSITIVE MEMBRANES. On the other hand, if one considers the dual mode sorption equation (2.20), the complete forward reaction means the hole affinity constant b is very large and then the adsorption is in a saturated state. The hole saturation constant will be equal to the total concentration of the carrier in this case, i.e., CH = [A]t (5.5) Therefore, at the surface close to the upstream where the equilibrium constant is large, which means nearly all the carrier is oxygenized, there will be, Ch=Sph+[A]T (5.6) and at the surface close to the downstream where the equilibrium constant is very small, i.e. all the carrier is in the unoxygenized form, there will be, ^1 (5.7) The total mass transmission rate in this case will be, daCa)t F Ph~Pl (5.8) L L This is similar to equation (5.4) and shows that a similar behaviour is expected for facilitated mass transfer from the dual mode sorption model. These equations will remain valid if the following conditions are obeyed: -158- CHAPTER V. PHOTOSENSITIVE MEMBRANES. (i) The forward reaction is completed so all the carrier becomes an oxygenized form in the area close to the upstream surface; (ii) The reverse reaction is completed so all the carrier becomes a non-oxygenized form in the area close to the downstream surface; (iii) The carrier is totally mobile (because it is in liquid phase); (iv) The oxygenation/deoxygenation reaction is reversible. These conditions are hereafter called ideal conditions. The concentration profile under the ideal conditions is schematically depicted in Figure 5.1. Concentration downstream upstream Figure 5.1. Concentration profile of Henry's law diffusion and facilitated transport by photochemical reaction under ideal conditions. -159- CHAPTER V. PHOTOSENSITIVE MEMBRANES. It can be seen that the driving force for facilitated transport, ([AO] - [AO] ) will reach a upstream downstream maximum when in the upstream [AO] reaches a maximum, i.e. [AO]=[A] , while at downstream [AO] reaches zero. In Total the photochemical reaction, these conditions will be fulfilled by the lightness and darkness on the opposite surfaces of the membrane. On the other hand, for a non photosensitive facilitated transport the concentration of the oxygenized carrier at the upstream surface is somewhat lower than [A] depending upon the equilibrium constant Total and at the downstream side is higher than zero because of equilibrium conditions. This implies that the driving force in this case is smaller than that in a photochemical reaction. Furthermore, the equilibrium constant is usually temperature dependent, i.e. the equilibrium constants on both sides of a membrane tend to decrease with increasing temperature. Therefore, the driving force will be reduced accordingly. The driving force reaches a maximum at any temperature only in the case of a photochemical reaction and so does the augmentation factor, F, which is defined as, Carrier enhanced permeability G = ------Henry's law permeability Calculations have been carried out under these ideal conditions and are described below. The results show the potential advantage of using photochemical reaction in oxygen enrichment. On the basis of these results an experimental assessment of facilitated transport has been made using synthesized carriers such as 1,4-dimethoxy-9,10- diphenylanthracene. The solvent proposed is diethylene glycol dibutyl ether (dibutyldigol or DBDG). The physical properties of this solvent are given in Table 5.1. -160- CHAPTER V. PHOTOSENSITIVE MEMBRANES. Table 5.1. Physical properties of dibutyldigol [Barton, 1983 and Curme and Johnston, 1952] Molecular weight 218.3 Boiling point (bp),°C (at 760 torr) 255 Vapour pressure, mbar (at 20°C) 0.01 Density, gm/ml (at 20°C) 0.885 Viscosity, cp (at 20°C) 2.39 Solubility parameter, MPa^ 17.1 Molar volume, cm3/mol (at 20°C 248 The membrane substrate used is Millipore HVHP (specification shown in Table 5.4). The diffusion coefficient and solubility of oxygen in this solvent have been calculated using equations (3.2) and (3.6) respectively. The transmission rate of oxygen ( m3/m2-sec) has been calculated using equation (5.8). The photochemical reaction is assumed to be totally reversible, i.e. no carrier deterioration occurs. As mentioned above, both the forward and reverse reactions could be completed because, for example, a metal filter as used by Schultz [1986] can stop the light penetration effectively. The solution inside the membrane is saturated with DMPA, the concentration of which is about 0.04 molar. Results of the calculation are given in Table 5.2 and Figure 5.2. The details of the calculation are given in Appendix VI. -161- CHAPTER V. PHOTOSENSITIVE MEMBRANES. Table 5.2. The calculated permeability and the separation factor. Temperature(° C) 23 45 75 95 Solubility of oxygen 9.75 9.93 10.09 10.26 (mol/m3-Pa)xl0"5 Solubility of nitrogen 5.54 5.79 6.06 6.20 (mol/m3-Pa)xl0“ 5 Diffusivity of oxygen 10.39 13.86 20.59 25.19 (m2 /sec ) xl0~ 10 a Diffusivity of nitrogen 9.23 13.32 18.29 22.41 (m2/sec)xlO- 10 a Concentration of carrier 40 39.23 38.14 37.37 (mol/m3) Diffusion coefficient of 1.883 2.514 3.754 4.598 of carrier (m2/sec)xlO* 10a Oxygen Permeability Henryb 101.3 137.6 207.8 258.4 Nitrogen (mol-m/m2-sec-Pa) Henryb 51.13 70.12 110.8 138.9 xlO" 15 Carrier0 353.9 463.5 672.9 807.9 Oxygen Permeability Henryb 303 441 621 773 Nitrogen (Barrer) Henryb 153 210 331 415 Carrier0 1058 1385 2012 2416 Augmentation factor, G 3.49 3.37 3.23 3.13 Separation factor Henryb 1.98 1.95 1.88 1.85 Overalld 8.90 8.55 7.95 7.68 The tortuosity is assumed to be /2 = 1.41 and porosity is 0.75. Represents the Henry's law contribution. Represents the facilitated contribution. d Overall separation factor. -162- Total permeability Separation factor contributed by Henry's law by Henry's Figure 0 photochemical t OT CD oooooooooo x [6 5.2. law huio CHAPTER 00 -O0 sorption Permeability s - reaction. z uio/uio-(dJ,S) joqopj V. CD PHOTOSENSITIVE and uoi^eiedes calculated and LG separation E uio] - 163 ' AqT-[Tqeeui;rea - MEMBRANES. facilitated factor — o due transport o to oo Temperature, CHAPTER V. PHOTOSENSITIVE MEMBRANES. It can also be seen from Table 5.2 and Figure 5.2 that the permeation of oxygen can be enhanced as much as 3.1 to 3.5 times that of the Henry's law permeation. The separation factors at the different temperatures are essentially the same. This is because the diffusion coefficient of carriers in both free and oxygenized forms, and thus the permeability of oxygen, increases significantly with increasing temperature, while the nitrogen transfer is still governed by Henry's law permeation which is not affected by the chemical reaction at all. The equilibrium constant does not play any part in this process because of the assumed completion of photochemical reaction. For this reason, a high temperature is always preferable. High temperature has two functions favouring this process. Firstly, the diffusion coefficient will be increased which is the result of the decrease of viscosity of the solution. Secondly, a high temperature would help to increase the reverse reaction constant. This would help to balance the forward and reverse reaction rates and thereby increase the separation factor. On the other hand, the high temperature may bring about a side reaction which may cause deterioration of the oxygen carriers. 5.3. CHEMICAL ASPECTS OF THE CARRIERS. The compounds which absorb light energy and transfer it to oxygen molecules are called sensitizers. The compounds which are often used to combine with singlet oxygen in a photochemical reaction are termed acceptors. Sometimes the acceptor can absorb light energy, and in this case we say this compound is a self-sensitive acceptor. It should be noted that there is a special requirement for the acceptor used in the facilitated transport: the reversibility to react with oxygen. A general review of this subject can be found in review papers such as those of Monroe [1985]; -164- CHAPTER V. PHOTOSENSITIVE MEMBRANES. Wilkinson and Brummer [1981]; Gollnick and Kuhn, [1979]; Satio and Matsuura [1979]. Usually, acceptors of singlet oxygen in photochemical studies are allyl hydrocarbons, conjugated dienes and aromatic hydrocarbons. The compounds of the first type form hydroperoxides [Gollnick, 1978] whereas the compounds of the second and third types form endoperoxides (when the oxygen atoms form bonds in a ring structure) when attacked by singlet oxygen. It has been shown that many polycyclic aromatic hydrocarbons can be used to carry out the forward and reverse reactions [Saito and Matsuura, 1979]. However, in this study attention has been focussed on anthracene and its derivatives. This is because some of these compounds have already shown stability against degradation reactions [Turro et al, 1979; Turro et al, 1981; Wilson et al, 1986]. A brief summary of the chemical aspects of the carriers synthesized is given below; details of preparative methods are given in Section 5.4. The aim has been to seek the optimum carriers to provide rapid and reversible oxygen uptake without being liable to degradation. The simplest member of this class is anthracene. It can undergo an oxygenation reaction to form an endoperoxide at its 9 and 10 positions. As a general rule, the electrophilic singlet oxygen attacks anthracene to form endoperoxide at the meso-positions 9 and 10 [Gollnick, 1978] as shown in the following reaction equation (5.9), -165- CHAPTER V. PHOTOSENSITIVE MEMBRANES. Heating of the endoperoxide leads to cleavage of the 0-0 bond to form free radicals which can undergo a side reaction that destroys the anthracene ring, as-well as the regeneration of the original compound by giving off oxygen [Saito and Matsuura, 1979]. Therefore, this compound cannot be used as an acceptor for long term use. 1,4-dimethoxyanthracene (DMA) has been reported to form an endoperoxide in the 1,4 positions as the methoxy groups increase the electron density at these sites. In fact, powerful electron-donating substituents raise the electron density at sites of bonding (1 and 4 positions in this example). Therefore, singlet oxygen, an electrophilic reagent, attacks these points to form an endoperoxide in the 1,4 positions exclusively through the following reaction [Saito and Matsuura, 1979]. (5.10) It has been found in this study (Section 5.5.1) that this compound also suffered an irreversible oxygenation in chloroform and in dibutyldigol. The second compound examined in this study is 1,4- dimethoxy-9,10-diphenylanthracene (DMPA), some of the chemistry of which has been examined in several solvents by many investigators [Wilson, 1969? Wilson et al, 1986] since Dufraisse and Velluz [1942] discovered the reversible reaction of this compound with oxygen. It may be more stable in dibutyldigol than the DMA mentioned above, due to the electron-donating effect of the additional phenyl groups which stabilizes the endoperoxide from side -166- CHAPTER V. PHOTOSENSITIVE MEMBRANES. reactions, thus making the release of the oxygen molecule more efficient. The endoperoxide formed by DMPA is in 1 and 4 positions as in 1,4-dimethoxyanthracene, although the phenyl groups bring about a weaker electron-donating effect. The experimental studies of 1,4-dimethoxy-9,10-phenyl- anthracene showed that this compound bound an oxygen molecule quickly after the illumination but it took a long time to be regenerated (see Section 5.5). This phenomenon can be interpreted as due to the electron density being too high in the sites of binding singlet oxygen. In fact, it is necessary to keep relatively high electron density in the 1 and 4 positions which are used to direct the endoperoxide formation at these sites, otherwise the 9,10 endoperoxide will be formed in which the 0-0 cleavage side reaction will destroy this oxygen carrier. However, the high electron density tends to bind oxygen tightly and make the reverse reaction slow. This postulation led to examination of the next compound. The third compound studied is 1,4-dimethoxy-9,10-bis(4'- bromophenyl)anthracene (DMBPA). This compound was prepared to allow determination of whether an electronegative group, bromine for instance, introduced to the 4'-positions of the phenyls would reduce the electron density in the anthracene ring sufficiently to facilitate the release of oxygen from endoperoxide. -167- CHAPTER V. PHOTOSENSITIVE MEMBRANES. R = H, Br The experiment with the bromophenyl anthracene did not show any significant difference from the unbrominated one (see Section 5.5). It was postulated that a more electron withdrawing group introduced onto the phenyls, could probably further decrease the electron density in the anthracene ring and increase the oxygen release rate. Therefore, synthesis of 1,4-dimethoxy-9,10-bis(4'- nitrophenyl)anthracene was attempted via the Grignard reaction. Unfortunately, after introducing a nitro group into the benzene ring the compound, p-bromonitrobenzene, was so deactivated that further Grignard reaction became impossible, although various synthetic methods were tried. 5.4. SYNTHESIS OF ANTHRACENE DERIVATIVES. The anthracene derivatives used as oxygen carriers in the experiment were synthesized in the CSIRO Division of Food Processing. These were 1,4-dimethoxyanthracene, 1,4- dimethoxy-9,10-diphenylanthracene, and 1,4-dimethoxy-9,10- bis (4'-bromophenyl)anthracene. Syntheses of the nitro- anthracene were attempted but were not successful. -168- CHAPTER V. PHOTOSENSITIVE MEMBRANES. 5.4.1. MATERIALS USED IN THE SYNTHESIS. The chemicals used are listed below: Quinizarin from Alderich Chemical Company, Inc. Dimethylsulphate from AJAX Chemical Pty. Ltd. Sodium carbonate from BDH Chemical (Australia) Pty. Ltd. Acetic acid from BDH Chemical (Australia) Pty. Ltd. Bromobenzene from AJAX Chemical Pty. Ltd. 1,4-Dibromobenzene from Merck-Schuchardt. Potassium iodide from AJAX Chemical Pty. Ltd. Other chemicals including solvents are: Toluene, Tetrahydrofuran, Benzene and Sodium hypophosphite. Most chemicals were used as supplied, except for dimethylsulphate which was distilled before use. Some solvents were purified; this is mentioned in the description of the synthesis process. 5.4.2. SYNTHESIS PROCEDURES. Methylation of quinizarin was carried out according to the Dufraisse and Velluz [1942] method. Sodium carbonate was dried at 200°C for an hour. Quinizarin, 10 grams, and sodium carbonate, 50 grams, were ground in a mortar and then mixed with 50 grams of dimethylsulphate in a 200 ml round bottom flask. The mixture was heated for 4 hours with constant stirring. Crushed ice was added to the mixture. The precipitate was filtered and washed with 10% potassium hydroxide and ethanol alternately. The solid was recrystallized from ethanol. Yellow needle crystals were collected. The melting point of the product, 1,4-dimethoxy- 9,10-anthraquinone, was measured as 170-171°C (literature -169- CHAPTER V. PHOTOSENSITIVE MEMBRANES. 170-171°C) [Dufraisse and Velluz, 1942]. The yield was about 20%, while Dufraisse was reported to reach 70%. 1.4- dimethoxv-9,10-diphenylanthracene was also synthesized according to the Dufraisse method. The Grignard reagent of bromobenzene was prepared first. Tetrahydrofuran (THF) was dried with metallic sodium wire. Dried magnesium, 5.6 grams, and 100 ml of THF were mixed in a 200 ml flask for the Grignard reaction. Bromobenzene, 39.2 grams, was added dropwise. A few drops of methyliodide were added to start the reaction. The mixture was heated slightly when it was necessary to maintain the reaction and then refluxed for two hours. Toluene, 60 ml, was added to the solution to replace the THF which was evaporated using a water jet pump. The solution was heated to boiling point. 1.4- dimethoxy-9,10-anthraquinone, 5.6 grams, was mixed with 50 ml of toluene and heated to boiling point and then mixed with the Grignard reagent in small portions. The mixture was refluxed for two hours with constant stirring. Crushed ice was added to destroy the excess Grignard reagent. The precipitate was taken and the product in the oil phase was separated by a rotary evaporator. The product, 1.4- dimethoxy-9,10-diphenyl-9,10-dihydroxyanthracene, was collected. The reduction of the above product was carried out by adding 100 ml acetic acid, eight grams of sodium hypophosphite and four grams of potassium iodide. The mixture was heated for a quarter of an hour. It was easy at this stage to identify the final product by its fluorescent colour. The final product was collected by filtration and air-dried at atmosphere conditions. There was about four grams of product, yield 43%. The boiling point was 189- 194°C (literature 203-204°C [Dufraisse and Velluz, 1942]). -170- CHAPTER V. PHOTOSENSITIVE MEMBRANES. 1.4- dimethoxy-9,10-bis(4'-bromophenvl)anthracene was synthesized using a similar method to that of Arshady [1981]. Three grams of magnesium were mixed with 31.6 grams of p-dibromobenzene and then put into a 300 ml flask. 100 ml anhydrous THF was added. The reaction occurred after addition of a few drops of methyliodide. A water bath was used to control the reaction because the reaction needed cooling initially and needed heating when nearly finished. Toluene was used to replace THF after the reaction was completed. Then the mixture was heated again to boiling point. Three grams of 1,4-dimethoxy-9,10-anthraquinone were dissolved in 100 ml of toluene and heated to boiling point and mixed in small portions with the solution prepared above. Then the solution was refluxed for 4 hours with continuous stirring. The product was 1,4-dimethoxy-9,10-bis(4'-bromophenyl)- 9,10-dihyhroxyanthracene which was collected as a precipitate and from an oil phase by removing the toluene. A production procedure similar to that described above was used to convert this compound to 1,4-dimethoxy-9,10-bis(4'- bromophenyl)anthracene. The final product was 1.49 grams, yield 21%. The boiling point was 266-268°C. 1.4- dimethoxy-9.10-bis(4 '-nitrophenyl)anthracene synthesis was also tried using the following procedure. Firstly 4- bromonitrobenzene was prepared according to Vogel [1956] and then further Grignard reaction failed due to the deactivating effect of the nitro group. The lithium metal conversion method was tried with 4-bromonitrobenzene, but failed also. 5.5. SPECTROPHOTOMETRIC STUDIES. -171- CHAPTER V. PHOTOSENSITIVE MEMBRANES. All solutions absorb light when the light passes through the solution and different solutions absorb different wavelengths. The acceptor in both free and oxygenized forms absorbs light but at different wavelengths. It is easy to examine the condition of an acceptor in a solution, i.e. whether the acceptor is in its free form, oxygenized form or already degraded according to its absorption spectra. It is well known that monochromatic light will be absorbed when it passes through a medium. Therefore, the light intensity will be reduced accordingly. The quantitative description of the phenomenon is given by the Beer-Lambert law in which the light intensity I after passing through a solution is governed by an equation [Owen, 1980], I A = log(——) = £ CL (5.12) I where A - Absorbance of solution; £ - molar decadic extinction coefficient, litre/mol-cm; I - intensity of incident light; o L - the optical path length, cm; C - the concentration of absorbing molecules, mol/litre. The quantity of £ is a constant characteristic of a particular substance and at a particular wavelength. It is a measure of light absorption efficiency and therefore of the efficiency of a light-initiated chemical reaction. A Lambda 2 UV/VIS spectrophotometer from Perkin-Elmer was used to carry out this study. As an example, the spectra of the solution of tetraphenylporphine (TPP), which ; has been chosen as a sensitizer, in dibutyldigol are given in Figures 5.3 and 5.4. -172- most wavelength. converted data range Figure The Absorbarv in (5.12). 0.5000 0.3900 0.2800 0.1700 0.0600 0.0500 Figure 300.0 dibutyldigol absorbance of to -j - - - is 5.5. Later the the 5.3. to 350.0 proportional Therefore, time It corresponding the CHAPTER in 1 dibutyldigol The can of concentration. the at in 400.0 different spectra be i wavelength this V. kinetics i Wavelength ! 1 seen ! ! ! ! I! i A it ■ PHOTOSENSITIVE 458.0 part to is of concentration that at concentrations - easy the 173 ( TPP, only studies, 24° nanometers) 500.0 418 However, the - C. to absorbance 0.888x10 nanometers for concentration convert 550.0 convenience. MEMBRANES. all absorbance data of 6 absorbance , the 600.0 for TPP from is dissolved absorbance a plotted dissolved equation selected in is 650.0 used this in is in Figure Absorbance Absorbance Figure i J.8050 ) i.0150 i.8288 0.00 i.0858 0.50 2.00 .0180 .0000 .00 388.0 0.00 Concentration 5.5. 5.4. extinction dibutyldigol 3S0.0 The The CHAPTER absorption y amplified .00 480.0 of coefficient. Wavelength V. - for TPP 450.0 • PHOTOSENSITIVE ~T~ the spectra versus in 2.00 - 508.0 ( 174 dibutyldigol calculation nanometers) - concentration of \ at 550.8 \ I \i\ 'j TPP cm A 23° A MEMBRANES. 3.00 c dissolved uvelte //O'' of 688.0 (1x10 A molar at 650.0 7 molar) 418 decadic in 4.00 nm 708.8 CHAPTER V. PHOTOSENSITIVE MEMBRANES. The molar decadic extinction coefficients of this solution are shown in Table 5.3 and were calculated using equation (5.12) and data obtained from Figure 5.4. Table 5.3. Molar decadic extinction coefficient of TPP in DBDG Wavelength (nm) 418 513 547 591 649 Extinction coefficient 541666 20495 8446 5405 4391 (litre/mol-cm) Solvents play an important role in photochemical reactions. As shown in early work by Dufraisse and Velluz [1942] and later by Lundeen and Adelman [1970], 1,4 dialkoxy peroxides behaved differently in different solvents. 1,4-dimethoxy- 9,10-diphenylanthracene endoperoxide, for example, underwent an irreversible reaction in benzene. In tetrahydrofuran, on the other hand, the dissociation reaction was found to be quantitatively reversible. Therefore, the forward and reverse reactions can be examined using a spectrophotometer before a carrier is subjected to a permeation test. Less volatile solvents are usually preferred for the membrane process because they remain inside the membrane longer than benzene, tetrahydrofuran, dioxane or chloroform which are often used in photochemical reactions. However, other solvents were tested as a preliminary consideration. The solvents used in the experimental studies are: chloroform from Merck-Schuchardt; Xylene from BDH; diethyldigol from Alderich; dibutyldigol (DBDG) from Alderich and Merck-Schuchardt. -175- CHAPTER V. PHOTOSENSITIVE MEMBRANES. Sensitizers are tetraphenylporphine (TPP) from Sigma; erythrosine and methylene blue of BDH. The spectra of the solvent dibutyldigol used for the photo chemical reaction are given in Figure 5.6. It can be seen that this solvent does not have any strong absorption band between the wavelengths from 350 to 700 nanometers. Several optical filters have been examined in order to provide some protection for the carriers. The reason for this can be described as follows. The short wavelengths which have high energy may cause a side reaction of the carriers and the long wavelengths may introduce extra heat to the system. The filters tested include polypropylene filters, coloured glass and copper sulphate aqueous solution. The spectra of copper sulphate solution are given in Figure 5.7. It can be seen that it is transparent only between wavelengths 310 to 680 nanometers. Therefore, a light of less than 310 ^m and higher than 680 ^m will be absorbed by the copper sulphate solution. -176- Figure Absorban - 5.8800 7.0000 4.6000 2.2800 3.4000 1.0000 0.1000 2.3400 3.5600 4.7800 1.1200 200.0 280.0 Figure 5.7. -4 - f The 300.0 * 5.6. CHAPTER spectra The 300.0 400.0 V. spectra of PHOTOSENSITIVE Wavelength 500.0 Wavelength copper - 177 of 400.0 - ( solvent sulphate nanometers) 600.0 ( nanometers) / / MEMBRANES. dibutyldigol 700.0 aqueous S r 580.0 800.0 solution "T £ orbance , Absorban 2.0000 3.1000 9.7200 Figure 0.7400 1.5400 1.5800 2.3600 3.1800 4.0000 0.3200 1.1600 0.1000 200 200.0 -\ . 5.8. 0 (DMPA Figure The (2.287x10 CHAPTER 2.287x10" spectra 5.9. 300.0 / / A V. The 4 \ \ molar). 4 of PHOTOSENSITIVE A Wavelength molar spectra DMPA Wavelength - 178 400.0 (\J and dissolved - ( of nanometers) ( TPP mixed nanometers) MEMBRANES. 1.1x10" in solution 500.0 500.8 dibutyldigol 4 A molar). v./ \ CHAPTER V. PHOTOSENSITIVE MEMBRANES. The spectra of 1,4-dimethoxy-9,10-diphenylanthracene dissolved in dibutyldigol are given in Figure 5.8. It can be seen that a strong absorption occurs between the wavelengths of 370 to 450 nanometers. The light energy in this region is strong enough to initiate singlet oxygen formation and then start corresponding photochemical reactions. TPP, which has a strong absorption band at 418 nanometers, is added to the solution in order to increase the efficiency of singlet oxygen formation and the spectra of the mixed solution are given in Figure 5.9. 5.5.1. OXYGENATION AND DISSOCIATION EXPERIMENTS BY SPECTROPHOTOMETRIC STUDIES. Spectrophotometric studies of endoperoxide formation and dissociation have been carried out using a Pye Unicam PU 8800 UV/VIS spectrophotometer. The optical cell was placed in a temperature controlled system. The experiments showed that the light from a 500W photographic projector is sufficient for the forward reaction. The temperature does not have a significant effect at this stage but the rate of the reverse reaction is substantially affected by the temperature (shown later in this section). Ideally a high temperature is necessary only on the downstream side of the membrane to accelerate the reverse reaction, but in practice it is nearly impossible to make a substantial temperature difference between the two sides of a membrane. Therefore, both sides of the membrane have been kept at the same temperature in all the experimental studies. It should be noted that all the absorbances in these figures could be converted to the corresponding concentrations as mentioned before. However, the use of absorbance is equally explicit as the use of concentration for interpretation of the oxygenation and regeneration processes of the carriers in the photochemical reaction. -179- CHAPTER V. PHOTOSENSITIVE MEMBRANES. Most spectrophotometric studies were carried out using tetraphenylporphine (TPP) as a sensitizer. It was found that the solution of DMPA in xylene and chloroform can pick up and release oxygen, but suffers irreversible oxidation. In contrast, when dibutyldigol was used as a solvent to form a dilute solution, DMPA was quantitatively regenerated and there was no degradation detected except for the first run. Typical oxygenation spectra are given in Figure 5.10. The first line from the top of the graph is the initial solution (4x10"5 molar), then every 30 seconds down to the bottom. The last line on the bottom was illuminated for 21 minutes. The spectrophotometric experiment also showed the temperature effect on regeneration of the oxygen carrier. The dissociation process showed a strong temperature dependence. There was no observable regeneration of the oxidized carrier at room temperature. The minimum temperature for carrying out a reversible photochemical reaction at low concentration of carrier is 45°C. The dissociation rate increases with temperatures above 45°C. The endoperoxide formation and dissociation curves are depicted in Figure 5.11. The solution of 1,4-dimethoxy-9,10-bis(4'-bromophenyl)- anthracene dissolved in dibutyldigol has also been examined under similar conditions but irreversible oxygenation has been found which is depicted in figure 5.12. A comparison of these two compounds is given in Figure 5.13. The irreversible oxygenation of DMBPA is given in Figure 5.14. -180- CHAPTER V. PHOTOSENSITIVE MEMBRANES O WWW "0W TW3 W T)G T)G T)G W O O O O O O 3 u o o 30000000 H rl -rl rl rl rl tI tI '•HWcdaJaJrtctiaj—I 4-» 4-» 4-> 4-> 4-» 4-» 4-J Drlrlrlrlrlrlrl 033 PQMMMMMMM (nanometer) -trvinsriOvONCO Wavelength LO O LO O — — o o aouHqaosqy Figure 5.10. The oxygenation of DMPA dissolved in dibutyldigol under atmospheric conditions (4x10" 5 molar, every 30 seconds from the top curve) -181- Figure Figure Absorbance 5.12. 5.11. (Broken ( 1.648x10" (Broken (2.05x10" Oxygenation Oxygenation CHAPTER line 4 line 4 molar, V. indicates molar, indicates PHOTOSENSITIVE and and self-sensitized - dissociation dissociation self-sensitized 182 the - the 100% Time, 100% 00 Time, MEMBRANES. regeneration) of of regeneration) oxygenation). second DMBPA oxygenation). DMPA second 1000 solution solution 4-1 £ ■P u § S' S o 2 C CD e 0 o e Percentage of regeneration Figure Figure 0.80 0.90 O.bO 0.60 0./0 0.00 0.10 0.30 o.-ic 0.80 n.oo 1 .009 at 5.14. 5.13. 60° C The The (DMPA CHAPTER degradation comparison 1.05x10 ■ o. V. PHOTOSENSITIVE Time o of - of molar, 183 of ° "o- O dissociation 5 DMBPA - Regeneration Regeneration Times DMBPA regeneration, DMBPA 6 (1x10" of in MEMBRANES. illumination dibutyldigol 1.03x10 7 4 of molar). Of of endoperoxides minute 8 DMBPA DMPA molar). 9 10 CHAPTER V. PHOTOSENSITIVE MEMBRANES. The experiments on the dilute DMPA or DMBPA solutions have also shown that they undergo self-sensitized reaction without dyes. This could be very interesting because the dyes could cause some side reactions in the dye-initiated chemical reactions, in which the dyes absorb photons first then pass the energy to the molecules of oxygen to form the singlet state. In this case, the sensitizer and acceptor are the same material in the self-sensitized reaction. A solution of dimethylnaphthalene (DMN) in dibutyldigol with TPP has also been investigated under similar conditions, but the reaction rate is much slower than the others mentioned above. In fact, other investigators have already noticed this slow oxidation process. In the experiment conducted by Wilson et al [1986], it took four days to convert 90% 1,4-dimethylnaphthalene to its oxidized form in the presence of methylene blue as a sensitizer. As shown in Figure 5.10, the endoperoxide formation only took two minutes at 23°C, for 90% of the DMPA to become oxidized. But, in the reverse process at the same temperature, it took a few days to regenerate (see Figure 5.23) and at 45° took 100 minutes. In other words, with this compound, it is easy to take up oxygen but hard to release it. Therefore, it is very necessary to keep the reversible reaction at a high temperature. It should be borne in mind that a high temperature may cause some side reactions. 1,4-dimethoxyanthracene was tested in the same way and it degraded more seriously than 1,4-dimethoxy-9,10-bis(4 ' - bromophenyl)anthracene in the solvents, such as chloroform, diethyldigol and dibutyldigol. -184- CHAPTER V. PHOTOSENSITIVE MEMBRANES. 5.5.2. SUMMARY OF THE SPECTROPHOTOMETRIC STUDIES. A summary of the spectrophotometric studies is made below. All these factors were considered when conducting permeation tests. (1) 1,4-dimethoxy-9,10-diphenylanthracene was better than the other anthracene derivatives because it was more stable than the others in the dibutyldigol solutions. (2) Dibutyldigol was the best solvent among those examined. (3) A high temperature was preferred in order to balance the forward and reverse oxygenation of the carrier. In fact, most experimental observations were carried out at 95°C which was the limit of a water bath although sometimes a low temperature was used. An even higher temperature could be used if an organic liquid was used instead of water. A silicone oil, for instance, was purchased for this purpose but was not put to use. (as explained in Section 5.7.4. ) (4) A copper sulphate aqueous solution was necessary in order to reduce side reactions. (5) TPP was a good sensitizer in this combination of carrier and solvent. 5.6. PERMEATION TEST FACILITY. The equipment developed for testing photochemical reaction membranes is shown in Figure 5.15. It is similar to the immobilized liquid membrane testing equipment, but it contains more components. The permeation cell has the same -185- CHAPTER V. PHOTOSENSITIVE MEMBRANES. dimensions as used before, but a glass window is fitted to one side to permit light to enter (Figure 5.16). The effective permeation area of the cell is 11.95 cm2 which was calculated according to the effective diameter (3.9 cm). A projector with a 500 watt bulb is used as a light source to illuminate the membrane. The whole permeation cell is placed in a water bath in order to operate at above ambient temperature. The temperature is controlled by a thermostat with an accuracy of ±0.5 °C. There is a glass window on the water bath to permit light to pass through. An optical filter with an aqueous solution of sodium sulphate is placed between the light source and the window to eliminate the heat (i.e. infrared light) and light of unwanted wavelengths. Both feed and sweep gases are heated with a copper tube coil, and then pre-saturated in tubes with the same solvent as used for dissolving the carrier. The feed air passes through the upstream side of the permeation cell and the sweep gas through the downstream side respectively. After that, the feed gas goes to the vent through a rotameter and the sweep gas goes to an analysis loop, a rotameter and the vent. The volume of the loop is 1.16 cm3 which was calibrated using a syringe injecting air. The membrane sample was usually cleaned and prepared before use and then loaded into the permeation cell. Initially, the water bath was pre-heated to a certain temperature and then the flows of feed air and sweep gas (helium) were introduced and adjusted by valves according to the pressures and flow rates. The upstream pressure was set at 2 - 3 cm Hg higher than downstream because the membrane could not withstand reverse pressure and the downstream pressure was set at 1-2 cm Hg higher than ambient pressure in order to avoid backward diffusion from -186- CHAPTER V. PHOTOSENSITIVE MEMBRANES. the vent to the loop. The permeability and separation factors under the immobilized liquid membrane conditions were examined without illumination. After that the cell was illuminated and the permeation performance under facilitated transport was measured. Experiments were continued for up to six hours after which the membrane was usually still wet. Gas samples were analysed using a Shimadzu gas chromatograph model GC-4B RTF (from Shimadsu Seisakusho) through a sampling loop. Delta Junior software (from Digital Solutions) was installed in an Osborne computer to give digital analytical results. A typical result obtained from the Delta Junior analysis is given in Figure 5.17. Mixed gases with lower concentrations of oxygen than that of air were sometimes used as the feed in the permeation tests instead of air to check if concentration affected the permeation rate of oxygen. Helium was used as the sweep. All the gases were purchased from CIG. -187- CHAPTER V. PHOTOSENSITIVE MEMBRANES. Projector. 2 permeation cell. 3 Membrane. Thermostat. 5 Heating coils. 6 Saturating devices. 7 Manometer. Rotameters. 9 Valves. 10 Analysis loop. Helium Air Figure 5.15. Permeation test equipment for photosensitive membrane -188- CHAPTER V. PHOTOSENSITIVE MEMBRANES. Glass window 7 ? Sintered metal ZZ / / Figure 5.16. Permeation cell for photosensitive membrane. -189- CHAPTER V. PHOTOSENSITIVE MEMBRANES. Retention time, minutes Figure 5.17. A typical gas chromatogram of a sample of air. (First peak - oxygen. Second peak - nitrogen) 5.7. GENERAL OBSERVATIONS AND DISCUSSION. This section describes the experimental permeation test using 1,4-dimethoxy-9,10-diphenylanthracene (DMPA) as a carrier. The membrane was illuminated on one side where the forward reaction proceeded and the other side was in darkness where the reverse reaction occurred. The various configurations of the membrane assembly were: (i). Immobilized liquid membranes incorporating the carrier in a solution of dibutyldigol held in the pores of microporous membranes (Section 5.7.1). -190- CHAPTER V. PHOTOSENSITIVE MEMBRANES. (ii) . Dense silicone rubber 3110 RTV membranes coupled with fixed carrier DMPA and a light absorber, charcoal (Section 5.7.2) . (iii) . Dense silicone rubber 3120 RTV membranes swollen with a carrier solution (Section 5.7.3). The specifications of the substrate membranes used in the photosensitive membrane test are given in Table 5.4. Table 5.4. Substrate membranes used in the experimental studies Membrane Specifications Material Pore size Porosity Thicknes /xm % fjLm Durapore Polyvinylidene 0.45 75 100 (HVHP) * difluoride Millipore- Mixed cellulose 0.02 79 100 MF (HABG)* acetate and nitrate PALL Stainless steel 2 70 300 MOTT Stainless steel 0.2 24 1000 Memtec Polypropylene 2 120 Van Leer Polypropylene 120 3120** Silicone rubber (dense membrane) 3110** Silicone rubber (dense membrane) * From Millipore Ltd. ** Made from Dow Corning silicone rubber RTV 3110 or 3120. 5.7.1. IMMOBILIZED LIQUID MEMBRANES SUPPORTED IN POROUS MEMBRANES. The challenge has been to determine under what conditions the facilitated transport of oxygen can be achieved, the extent of oxygen enrichment that can be reached and under -191- CHAPTER V. PHOTOSENSITIVE MEMBRANES. what conditions the best result can be obtained. Most effort in this section is given to creating different illumination environments on both sides of the membrane because this is essential for the photochemical reaction. The configurations in this test include: (a) One layer of Millipore HVHP membrane; (b) One layer of dyed Millipore membrane; (c) Multi-layer sandwich membrane configuration; (d) Three layers metal filter configuration. Although the results for these configurations were not encouraging it is useful to describe the concepts attempted (see following section). The possible reasons for inadequate separation are discussed in Sections 5.7.4, 5.7.5 and 5.8. 5.7.1.1. ONE LAYER MILLIPORE MEMBRANE. The Durapore HVHP membrane was washed with ethanol and dried at atmospheric conditions before use. The solvent, dibutyldigol, was distilled to get rid of acid which might affect forward or reverse reactions. DMPA was dissolved in dibutyldigol at room temperature with different concentrations. A membrane sample was prepared by immersion of the membrane in the appropriate solution. The problem with the use of Durapore HVHP membranes was that they could not block the light effectively. In fact, the membrane became transparent after imbibing the solution. In this case, the light could penetrate the membrane and initiate the singlet oxygen formation at the -192- CHAPTER V. PHOTOSENSITIVE MEMBRANES. downstream surface. Therefore, the forward photochemical reaction could occur in both surfaces of the membrane sandwich. As a result, the oxygenized form of the carrier would be very high throughout the membrane. Considering the relatively slow dissociation process, one can conclude that if the dissociation of endoperoxides happens, the reformation of the endoperoxide would soon take place due to the light passing through the membrane, so light- enhanced mass transfer could not be achieved. The results of these kinds of membranes are presented in Table 5.5. In Figure 5.18, the permeabilities and separation factors at different temperatures are plotted. The results show that the membrane tested in this case behaves like an immobilized liquid membrane with the dibutyldigol solution rather than a photosensitive membrane even though carrier is dissolved in this solution. The same permeabilities and separation factors were obtained with or without illumination. The permeation measurements were carried out using carrier concentrations from low concentration (0.01 molar) to the saturated solution (0.04 molar) with the thought that a high concentration might block the light more effectively. However, the concentration of the saturated solution was not high enough to block the light. The permeabilities and separation factors are given in Table 5.5. A Millipore-MF membrane which was black in colour was prepared in a similar way. The ease with which the Millipore-MF is attacked by the solvents used rendered it an unsuitable membrane material for use in this facilitated transport experiment, although it has a favourable black colour to block the light. This membrane is made of mixed cellulose acetate and nitrate which is attacked by dibutyldigol. The membrane was in the solvent to test its -193- CHAPTER V. PHOTOSENSITIVE MEMBRANES. stability and the results showed that this microporous medium became a poreless, paste-like material. Tetraphenylporphine (TPP) was used as a sensitizer in these experimental studies because TPP was a more effective sensitizer than the carriers themselves, but it is also a quencher of singlet oxygen. All the tests involving TPP proved that there was no facilitated transport of oxygen, perhaps because of quenching effects. In order to avoid this quenching effect, a test under the self-sensitized conditions was carried out. According to the spectrophotometric studies, there are two strong absorption bands for DMPA at wavelengths 377nm and 407nm (Figure 5.10). Absorption of light of these wavelengths can cause self-sensitized photochemical reactions to form the corresponding endoperoxides. However, no facilitated transport was observed in this case. The possible reason for this could be the slow reverse reaction, light penetration and precipitation of the carrier as explained in Sections 5.7.5 and 5.8. -194- CHAPTER V. PHOTOSENSITIVE MEMBRANES uoi^piedes o oo cd cvj o (\J -— ’ '— '— [°C] Temperature, [ Bh^o-obs- wd/uid- (diS ) cuio] ' AgTTTqpeuLiaa Figure 5.18. Permeability and separation factor of dibutyldigol solution with DMPA saturated solution, (about 0.04 molar) -195- CHAPTER V. PHOTOSENSITIVE MEMBRANES. 5.7.1.2. ONE LAYER DYED MILLIPORE HVHP. Dyes were included in the membrane either dissolved in the reagent solution or deposited in the pores in order to block the light passing through membranes. Methylene blue, rose bengale, erythrosine and oil red-0, for instance, were been dissolved in ethanol and the Millipore HVHP was dyed with the solution. The membrane was prepared according to the procedures described above after it was dried under ambient conditions. But again, no facilitated transport was observed (see Table 5.5). This may be due to the quenching effects of these dyes on the singlet oxygen, resulting in prevention of the forward reaction to form the carrier endoperoxide. Alternatively the poor transport could be due to the slow reverse reaction and precipitation of the carrier as explained in Section 5.8. 5.7.1.3. MULTI-LAYER SANDWICH MEMBRANE CONFIGURATION. In this experiment two layers of membranes were put together for the immobilized liquid membrane test. The first layer was HVHP, which provided the forward chemical reaction zone. The second layer was a material with high light resistance. A polypropylene or dyed polypropylene microporous membrane was used as the second layer. The result of this kind of membrane is given in Table 5.5. It can be seen from the table that there is no facilitated transport in this case. The possible reason could be the slow reverse reaction, light penetration and precipitation as explained in Sections 5.7.5 and 5.8. 5.7.1.4. THREE LAYERS METAL FILTER CONFIGURATION. In this approach, a MOTT filter (thickness 0.5 mm) or PALL metal filter (thickness 0.3 mm) was placed between two -196- CHAPTER V. PHOTOSENSITIVE MEMBRANES. layers of HVHP membranes in order to create the effective light blockage. The third layer of Millipore membrane HVHP in this case was designed to help in holding the liquid because the metal membranes had a large pore size and tended to drain. The experimental result is also given in Table 5.5. It can be seen that no facilitated transport has been achieved. The possible reason for this could be the slow reaction and precipitation as explained in Section 5.8. Table 5.5. Separation factors for different approaches, (all the test used saturated solution of DMPA, 0.04 molar, and 0.5x10”5 molar TPP) Separation Membrane device Permeability3 factor Temp. Offb Onc Offb Onc ° C 0.01 Molar DMPA 350 370 1.30 1.28 95 One layer HVHP 0.02 Molar DMPA 360 370 1.30 1.27 95 0.04 molar DMPA 360 380 1.28 1.25 95 One layer dyed HVHP 340 350 1.31 1.29 95 Multi-layer sandwich 380 400 1.28 1.25 95 Three layers metal 350 370 1.30 1.27 95 Units - Barrer. b Off - Permeation test without light. On - Permeation test with light. -197- CHAPTER V. PHOTOSENSITIVE MEMBRANES. 5.7.2. DENSE MEMBRANES WITH FIXED CARRIERS. Charcoal was added to silicone rubber Dow Corning 3110 in order to block light more effectively. The carrier was dissolved in chloroform and the solution was mixed with silicone rubber 3110 RTV and then the charcoal was added. The mixture was stirred thoroughly and the catalyst was added. After a quick stirring the paste-like material was cast on a polyethylene film. It took overnight for the silicone rubber to be cured. The resulting silicone rubber membrane contained 1.75 w% carrier, 4.27 w% charcoal. It seemed that the properties of silicone rubber did not change at all. The experimental results for both without and with carriers are given in Figures 5.19 and 5.20 respectively. There was no facilitated transport observed. It is postulated that the molecules of the carriers inside the silicone rubber crystallized and became large solid particles so they could not reversibly react with oxygen and work as an oxygen carrier in this case. In fact, crystals were observed in membrane samples under an optical microscope. What percentage of carriers is crystallized has not been examined. 5.7.3. DENSE MEMBRANES SWOLLEN WITH CARRIER SOLUTION. Dow Corning 3120, swollen with carrier solutions was tested but no improvement was observed. Firstly the carrier was dissolved in dibutyldigol and then a silicone membrane was immersed into the solution overnight. The membrane was significantly swollen, i.e. the volume was increased a great deal. The significance of this experiment was that (1) the carrier in this case was still in liquid solution and (2) this silicone rubber has a dark brown colour which blocks the light efficiently. -198- CHAPTER V. PHOTOSENSITIVE MEMBRANES. The result is depicted in Figure 5.21. Again, it was found that there was no facilitated transport in this case. The reason was that the solvent diffused into the membrane but not the carrier which tended to crystallize at the membrane surface. More solid crystals were found when a saturated solution was used to swell the membrane instead of dilute solution. This indicated that the solvent was absorbed by the silicone rubber membrane but not the solute (carrier). -199- CHAPTER V. PHOTOSENSITIVE MEMBRANES. uoTq.pjpd0S , Temperature o < □ o cvj O 00 QD ^ CVJ Oix[bHuio-oas- mo/mo- (aiS) cuia] 1 Aq.T-[Tcp29Ui;i0a Figure 5.19. Permeability and separation factor of silicone rubber membrane (no additives) -200- CHAPTER V. PHOTOSENSITIVE MEMBRANES. joqoeg: uoTq.pj^des in• co••••• — cd r- lo , Temperature 0Tx[6huio-33S-3uid/uid-(diS) euio] /Aq.TXTqeeuiJ0d Figure 5.20. Permeability and separation factor of charcoal added silicone rubber membrane( 1.75 w% carrier, 4.27 w% charcoal) -201- CHAPTER V. PHOTOSENSITIVE MEMBRANES. uoTq.pjedas , Temperature Oix [ 6huio-O0S- wd/wd- (dI,S ) €uio] ‘AiTXTqeeuuea Figure 5.21. Permeability and separation factor of silicone rubber membrane swollen by 0.04 mol DMPA in DBDG solution) -202- CHAPTER V. PHOTOSENSITIVE MEMBRANES. 5.7.4. CARRIER STABILITY. The facilitated transport effect was not observed even though much effort was put into this process. Accordingly, absorption and desorption experiments were conducted in order to find an alternative way to achieve oxygen enrichment. Some conclusions can be drawn from these experiments to explain why the membrane process could not bring about the expected enrichment. The absorption and desorption experiments which will be described in section 5.8 can give some answers. Several tests were carried out in addition to the permeation tests and these included testing of the stability of the carriers and the light-blocking efficiency. The stability of the carriers under the expected use conditions has been studied in order to ensure that the permeation result could be correctly interpreted. The results show that at low concentration, the change in concentration of DMBPA solution in 6 layers (to increase the sensitivity of the spectrophotometer measurement) of HVHP Durapore membrane took place quickly (Figure 5.22) under atmospheric conditions with ambient lighting, even before strong illumination. Therefore, this compound has a strong tendency in solution to undergo the forward reaction to form the endoperoxide and perhaps even further irreversible reactions. The DMPA was relatively stable under similar conditions, as can be seen by comparison of the curves in Figure 5.23. In this case, a solution had been illuminated and placed in a dark temperature room. The carrier concentration in the sample regained a level of more than 90% of this value after 60 hours at 35°C under atmospheric conditions. This -203- membrane then temperature dibutyldigol Figure reaction. membrane DMPA this The high than stability regeneration an scarcely result Wavelength (nanometer) such optical every process. or results in — as a i — 5.22. also dibutyldigol similar was expected. observable system slow of 20 (23°C). shows under The impregnated the process cell CHAPTER --- minutes. showed In rate to and ,i l ______decrease endoperoxide fact, that atmospheric The silicone for From when needs V. is under takes that one probable the too a PHOTOSENSITIVE with left this of a may the quite few stable rubbers. high these reverse - concentration 204 to 1.05x10' plays reaction conditions: say separation reason hours. - right: temperature a to that conditions long an These reaction 3 carry was MEMBRANES. for molar the the important time In factors of 6 data this other carried original out endoperoxide . layer at at DMBPA occurred that the were is 35° room words, Absorbance were HVHP part that C in reverse out and not in it less the was the the of as in at in CHAPTER V. PHOTOSENSITIVE MEMBRANES 5-1 0 _ihj: IIC Absorbance (a) 5-1 0 Absorbance Figure 5.23. The regeneration of DMPA solution at 35°C: 6 layers of HVHP membranes, (a) Before illumination (b) After illumination (nearly 100% of carrier was oxidized) and then the sample was placed in a dark temperature room for 60 hours. 5.7.5. CONTROL OF LIGHT PENETRATION. Since the forward reaction proceeds only in the presence of light, it is necessary for the downstream side of the -205- CHAPTER V. PHOTOSENSITIVE MEMBRANES. membrane to be in darkness in order to ensure that no reformation of the endoperoxide in downstream occurs. In practice the difference in light intensity between two sides of the membrane needs to be as large as possible. The following paragraphs will discuss whether the light was effectively blocked using the different methods. There would not be enough difference if only one layer of Durapore HVHP membrane was used, because the membrane becomes translucent after absorbing the carrier solution. There was a difference in light intensity of only two-fold to three-fold measured by a spectrophotometer even when four layers of membrane were used and impregnated with a solution of DMPA, 1.05x10”3 molar in dibutyldigol (Figure 5.24). As the photochemical reaction proceeds, the concentration of the unoxidized form decreases, so the efficiency of light absorption decreases, because it is the unoxidized form of the carrier which absorbs light. The Durapore HVHP membrane impregnated with the solvent could not block the light, as mentioned in the earlier spectrophotometric studies, and so the forward photochemical reaction occurred at both surfaces. The result of this is that the desired difference in value of the equilibrium constant on going from one side of the membrane to the other could not be achieved. In other words, nearly all carrier molecules are oxidized and can not lead to improved separation. -206- CHAPTER V. PHOTOSENSITIVE MEMBRANES. L.__. r . t Absorbance Figure 5.24. Absorbance of four layers of HVHP membrane impregnated with 1.05x10”3 molar of DMPA in dibutyldigol. What would be a good membrane material for absorbing the light? In this part of the experimental study, emphasis was placed on studying the light-absorption efficiency of various membranes. Several membranes were examined including silicone rubbers (Dow Corning 3120 and 3110), Durapore (HVHP), black-pigmented Millipore-MF (HABG), polypropylene porous membranes (from MEMTEC and Van Leer) and stainless steel membranes from PALL (PMF-FH, 200) and MOTT (Porous Metal Medium). Some dyes were also put into carrier solutions for the purpose of light absorption. The system of a dye dissolved in dibutyldigol solution was chosen to test the available methods of measuring light penetration of the membrane; i.e. (a) measurements with a UV spectrophotometer; (b) a radiometer used to measure the intensity of light from a projector as a light source; (c) a radiometer to measure the intensity of light from a high pressure mercury lamp passing through a monochromator. -207- CHAPTER V. PHOTOSENSITIVE MEMBRANES. The extent of light blocking using these methods gave dissimilar results. A polypropylene microporous membrane was selected because the spectrophotometric study showed that a high light blocking efficiency had been reached. The absorbance of this membrane yielded more than 3.323 (the maximum value of the instrument), i. e. the light intensity behind the filter remained less than 1/103'323 ( ~ 1/2100) of the original light intensity. This membrane was prepared using the following procedures: the membrane was dyed with oil red 0 first, dried under ambient conditions and then impregnated with the carrier solution. No facilitated transport was observed for the dyed and impregnated polypropylene membrane. The light blocking efficiency of the membrane was tested again using method (b) and yielded only 25% of the light intensity. It was suspected that the disagreement between these two methods might be caused by the much wider range of spectrum of light emitted by the projector from infrared to near ultraviolet radiation. In fact, all the radiation in this range could initiate the photochemical reaction because the energy absorbed by the carrier in this region was sufficient to sensitize this kind of reaction. If this was the fact, there would certainly be no significant facilitated transport in this case. However, there was a big disagreement between the two methods. Therefore, a third method (c) was used to test the light intensity with the same sample and the results are given in Table 5.6. The results in the table show the ratio of intensity of incident light from a high pressure mercury lamp with a monochromator to that of transmitted light was about eight-fold measured by a radiometer. The disagreement -208- CHAPTER V. PHOTOSENSITIVE MEMBRANES. between this method and the spectrophotometric method was probably caused by measurement of infrared radiation by the radiometer but not by the spectrophotometer. The former used a relatively wide range of wavelengths of light (about ± 5 nanometer) but the latter used a short range of light (about ± 0.5 nanometer). Table 5.6. The ratio of light intensity on both sides of wetted polypropylene (Two layers of polypropylene from Van Leer). Wavelength* light intensity, milliwattsxlO3/cm2 Intensity nanometers ratio With membrane,I^ Without membrane,I 0 l09 [ I„ Z1 ! 800 3.23 0.47 0.88 555 22.3 3.80 0.77 430 25.0 3.60 0.84 407** 13.4 1.90 0.85 377** 6.75 0.87 0.89 300 0.0144 0.0029 0.70 210 0.0153 0.0031 0.69 * A mercury lamp with monochromator as light source. **these two wavelength are maxima of the carrier's spectrum. In summary, the Durapore HVHP membranes became tranparent when the membrane pores were impregnated with an organic solvent. The light intensity ratio was only two or three even when four layers of membranes were put together. Polypropylene membranes, on the other hand, can block light more efficiently than Durapore and retained their white colour after being wetted with organic solvents. It is believed that some of the finer pores were still dry when they were tested and therefore could not contribute to gas separation. The dyed polypropylene membrane had a better light blocking efficiency than the undyed one, but still could not block the light sufficiently. As mentioned above, in testing the efficiency of light blocking by the -209- CHAPTER V. PHOTOSENSITIVE MEMBRANES. spectrophotometric and radiometric measurement methods, it was found that a considerable fraction of the light passed through the wetted or dyed polypropylene membrane, providing conditions for the unwanted forward reaction to occur on the downstream side of the membrane. The PALL metal membrane was the ideal medium to carry out the permeation process incorporating a photochemical reaction. It is thicker and has lower porosity than other membranes. No penetrant light was observable on the downstream side of the membrane. However, no facilitated transport was observed in this membrane. The probable reason for this is due to precipitation which is given in Section (5.8) . 5.8. ABSORPTION AND DESORPTION EXPERIMENTS. It has been shown spectrophotometrically that DMPA and DMBPA can undergo reversible oxidation under a variety of conditions in a dilute solution. Usually, the forward reaction is affected by the light intensity and the reverse reaction is substantially affected by the temperature. However, the reaction pattern may be different when a high concentration of reagent is used. At low concentration (1x10”4 to 1x10"5 molar), the reaction is reversible to a large extent. The early experiments showed that more than 99% of the original carrier could be regenerated when the endoperoxide of DMPA was heated to 75°C. However, when a 0.04 molar solution in a 1 cm pathlength cuvette was illuminated at room temperature and evaluated visually, the self-sensitized forward reaction was scarcely observed. It may need a long time to convert a significant portion to the oxygenized form of the carrier. On the other hand, the forward reaction rate can be increased by a sensitizer such as tetraphenylporphine -210- CHAPTER V. PHOTOSENSITIVE MEMBRANES. (TPP). Therefore, all the absorption and desorption experiments were conducted in the presence of TPP. A high temperature may be used in order to increase the reverse reaction rate, but the degradation reaction (irreversible oxidation) also took place when the temperature was higher than 75°C. A 0.04 molar solution was tested at 75°C without a sensitizer. The results of this experiment showed that the carrier's fluorescent colour disappeared and turned to a yellow solution after 30 minutes of illumination. No precipitated endoperoxide was observed. (It is shown later in this section that the endoperoxide has a very low solubility in dibutyldigol.) An absorption and desorption experiment was set up in order to examine why no facilitated transport was observed in the membrane permeation test. The equipment for this purpose is shown in Figure 5.25. A reaction tube, 10 ml, was connected to a pressure transducer, BHL-4400-13-01MO from Transamerica Instruments, and then connected to a Datataker, DT 100. The tube was immersed in a water bath in which the temperature was controlled by a thermostat with an accuracy of ± 0.5 °C. -211- CHAPTER V. PHOTOSENSITIVE MEMBRANES X) o O G P U W d) £ -P -t~i Du w O 0) H CN U P p C p CnX3 O C P -H X5 O «—H CO <—Ip 0) h cn n tj- m Figure 5.25. Absorption(a) and desorption(b) equipment. -212- CHAPTER V. PHOTOSENSITIVE MEMBRANES. Eight millilitres of a 0.04 molar solution of 1.4- dimethoxy-9,10-diphenylanthracene and 1x10” 5 molar TPP with respect to dibutyldigol were placed in the reaction tube. Air was introduced bubblewise through a nozzle when the tube was illuminated by a projector at room temperature. A great deal of precipitate could be seen at this stage. It took about 10 to 15 minutes to finish the reaction. The water bath was pre-heated to four different temperatures, i.e. 60°C, 65°C, 70°C and 75°C. The reaction tube was evacuated to a minimum of 1.913 KPa and then placed in the water bath and the pressure of the gas phase was recorded with the Datataker. A sample of the gas phase was taken from the port with a syringe after the pressure had stopped rising and the composition was analyzed by gas chromatograph GC-4B RTF. When low concentrations of acceptor were used previously (section 5.5), the reverse reaction could be observed with an UV/Visible spectrophotometer when the temperature was higher than 45°C. However, the reverse reaction rate was still too low to be visible until the temperature reached 60°C if 0.04 molar DMPA solution was used. The reverse reaction rates between 60°C to 75°C are shown in Figures 5.26 and 5.27. The figures show that the reaction rate does not change much at temperatures between 60°C to 75°C, but then the reaction rate increases considerably when the temperature reaches 75°C. It was observed that many gas bubbles rose from the precipitate. The precipitate was 1,4-dimethoxy-9,10-diphenylanthracene- 1.4- endoperoxide in the form of white crystals. The solubility of this oxidized form was very low. Precipitation was observed soon after illumination started and it was evaluated that the precipitation was -213- CHAPTER V. PHOTOSENSITIVE MEMBRANES. quantitatively formed, i.e. nearly all the carrier precipitated as the solid form. The precipitate was not substantially dissolved even when the temperature was increased to 60°C until the bound oxygen was released. This may explain why the oxygenation could not occur at 75°C. At this temperature, the degradation process would dominate the oxygenation/dissociation process. The carrier was effectively converted to a chemically new species which could not undergo the reverse reaction. It is concluded from this study that only the unoxygenized acceptor can move freely in the solution but not the oxygenized form. In a photosensitive membrane process, the carrier will be aggregated after the illumination has been started. This solid form of acceptor will reside somewhere in the membrane and lose its mobility. This phenomenom can give an explanation as to why the membrane process could not reach the expected enrichment of oxygen. This is because of the precipitation of the oxygenized form of the carrier, which could not contribute to the facilitated transport of oxygen in the solid state. -214- CHAPTER V. PHOTOSENSITIVE MEMBRANES. 800 ib \<\ o Second 600 , Time 400 200 Figure 5.26. Pressure increase due to dissociation of peroxides, (between 60°C to 75°C) -215- Figure o 0 1 b b 5.27. Desorption rate b peroxides, Pressure CHAPTER ryx V. increase PHOTOSENSITIVE (at ‘ ejnssejj 75°C) - 216 due - to dissociation MEMBRANES. _J O ______ L_ LO of - - O 1 CD OJ CD O 400 600 800 Time, Second CHAPTER V. PHOTOSENSITIVE MEMBRANES. It is known that a carrier in a membrane matrix can be either mobile or fixed (see Section 2.6). The carrier in this case lost its mobility when it was precipitated. The solid state of the acceptor in this instance cannot be considered as a fixed carrier in the commonly-used sense. In fact, the oxygenized form of a carrier has been shown to work as a fixed carrier only if it is evenly distributed in the membrane material as molecules. In this situation, oxygen molecules can jump from one carrier site to another, and therefore, the facilitated transport is performed among the carrier molecules. The experimental results in this study do not support this mechanism since the crystals formed in the precipitation are big enough to be seen and cannot be effective in carrying out facilitated transport. After the first absorption and desorption experiment, about 70% of active carrier remained. This indicated that 30% of the carrier was permanently oxidized in one absorption/ desorption circle. As the experiment proved, it was the reverse reaction which caused the degradation of the carrier, since the forward reaction was shown to produce a quantitative precipitate of the oxygenized form of the carrier. The absorption experiment demonstrated that the method could be used to enrich oxygen utilizing the forward and reverse reactions. A gas concentration of 60.5% oxygen was obtained from the dissociation process when the reverse reaction was finished. It would be possible to use this absorption/desorption device to store oxygen. As shown in the absorption and desorption experiments, the device should include a sensitizer (TPP, for example) and carrier (DMPA as an example) in a solution. Oxygen will be absorbed by the carriers under illumination at room temperature. As an -217- CHAPTER V. PHOTOSENSITIVE MEMBRANES. example, one millilitre of a 0.04 molar DMPA solution can absorb 0.9 ml of oxygen at room temperature. Sunlight can be used as a light source for the illumination. The oxygenized form of the carrier can be stored at room temperature. The absorbed oxygen can be released at an elevated temperature whenever it is needed. This process will be more efficient if a high concentration of carrier is used. 5.9. KINETICS OF THE PHOTOCHEMICAL REACTION. The evidence from the membrane tests and from the absorption/desorption experiments suggested that the limiting factor may be the difference in the kinetics of the forward and reverse reactions. This section analyses these kinetics in terms of the lifetimes of singlet oxygen. The reaction constant for DMPA and DMBPA in dibutyldigol has been examined using erythrosine tetrabutylammonium salt as a sensitizer. The reaction can be described by the following kinetic processes [Davidson and Trethewey, 1977]. In the following expressions, Sq and T^ refer to the ground, singlet excited and triplet excited state of the sensitizer respectively. The sensitizer (sens) in its ground state firstly absorbs light energy (5.13) and becomes an excited species. The excited species undergoes three processes: fluorescence to release its energy (5.14), internal conversion to become the ground state (5.15) and inter-system conversion (5.16) to become a triplet excited state. These photochemical processes can be represented by, Senss + hu Sens, (5.13) Sens, SenSg+hi/ (5.14) 0 -218- CHAPTER V. PHOTOSENSITIVE MEMBRANES. Senss Senss (5.15) i 0 SenSg SensT (5.16) i i where I - Rate constant of light absorption, sec 1; a k - Rate constant of fluorescent emission, sec' ; F k - Rate constant of internal conversion, sec' 1 . I c k - Rate constant of intersystem crossing, sec'1; ISC The triplet excited species has two possible reactions, either decay reaction to ground state (5.17) or initiating a singlet oxygen and losing its energy (5.18). These two reaction can be represented by, k, i SensT ---- -*■ Senss (5.17) 1 0 k n 30 + Sens„ ---- -+ Sens„ + 30 (5.18) 2 0 0 2 T 0 where k - Decay constant of excited sensitizer, sec' 1 ; d 1 k - Singlet oxygen formation rate constant, m3/mol-sec. o The formed singlet undergoes two further reactions, either the decay reaction (5.19) or reaction with an acceptor. These two reactions can be given as, i 0 (5.19) 2 1 0 + A AO (5.20) 2 2 where A - Acceptor (i.e. carrier), mol/m3; k - Decay constant of singlet oxygen, sec' 1 ; d k - Reaction constant of oxygenation of acceptor, m /mol-sec; r k - Reverse reaction rate constant of oxygen release, sec' -219- CHAPTER V. PHOTOSENSITIVE MEMBRANES. By a steady state kinetic treatment the reaction constant can be related to the concentration of the acceptor in an equation as [Davidson and Trethewey, 1977], d[A]’ -1 i k 1 1 + d (5.21) dt k [A] ^ r -* where t/> = k /(k + k + k ), the quantum yield of the T ISC P I C ISC7 excited form of the sensitizer. This equation leads to a linear relation between (-d[A]/dt)_1 and [A]-1 and can be used to determine k /k . This equation is valid for an irreversible oxygenation of the acceptor. However, the reverse reaction will soon occur after illumination is introduced, especially at a high temperature. Therefore, the reverse reaction needs to be taken into account when developing a general kinetic equation. In this case, the reaction rate equation will be determined by the simultaneous equations, d[ A] ----- =k ([A]-[A])-k [A][10 ] (5.22) dt 0 r 2 d [1 0 ] ------= k ([A] -[A))-k [A] CO 1-k [:0 1 (5.23) where k is the rate constant of the reverse reaction; - r [A]+[A02 ]=[A]o is total concentration of the acceptor; [x0 ]+[30 ]=[0 ] is the saturated concentration of 2 2 2 0 oxygen in the solution. Therefore, in the reversible reaction, these equations will replace equation (5.21). ; However, equation (5.21) is still valid at low -220- CHAPTER V. PHOTOSENSITIVE MEMBRANES. concentration of the oxidized form of the acceptor. In other words, it will be valid at the beginning of the reaction when the oxygenation reaction starts to take place. In this case, the extent of the oxygenation of the acceptor is so small that it could not bring about a high level of the reverse reaction. It would not make any difference in calculating the reaction constant whether the reverse reaction is taken into account if the forward and reverse reactions are at a low temperature, because the reverse reaction is so slow that it does not affect the kinetics significantly. However, a small portion of the oxidized form of the carrier is preferable for the kinetic studies. In order to measure the kinetics, the concentration of the acceptor should be very low, otherwise the activity coefficient has to be taken into account. Furthermore, a high concentration may form dimers or trimers which would reduce the effective concentration of the acceptors. Therefore, a study of the reaction kinetics has been carried out with different initial concentrations of the acceptor. The reaction rate -d[A]/dt can be determined at time t=0, at the point when the concentration of the oxidized form is actually equal to zero. The reaction constant of DMPA in chloroform and dibutyldigol has been calculated according to the conditions mentioned above, by employing the singlet oxygen decay constant from the literature [Bellus, 1978]. Usually, the reaction constants are solvent dependent, but they are of the same order of magnitude. Therefore, the decay constant of the singlet oxygen in dibutyldigol has been calculated, assuming that the reaction constant is the same in chloroform and in dibutyldigol. -221- CHAPTER V. PHOTOSENSITIVE MEMBRANES. For a particular wavelength, normally 407 nm, the absorbance of light by a solution was measured and is given in Figures 5.28 to 5.29 for 1,4-dimethoxy-9,10-diphenyl- anthracene and 1,4-dimethoxy-9,10-bis(4'-bromophenyl)- anthracene dissolved in chloroform. The rate of the corresponding reaction has been calculated from the curves by determining -dA/dt=-£L(dC/dt) according to equation 5.12. Then the ratio of k /k can be calculated by linear d r regression of -(dC/dt)-1 on (1/C) as given in Figures 5.30 and 5.31. -222- Absorbance 0 * . 0.0 Figure 50 Figure - 5.29. 5.28. CHAPTER The The V. 1.0 oxygenation oxygenation PHOTOSENSITIVE - 223 •Q-- ' Q ' - □- - Q Time, of A D * of O C A □ * DMBPA DMBPA DMBPA DMBPA DMPA DMPA DMPA DMPA 2.0 DMBPA DMPA Time MEMBRANES minutes 0.9x10 2.1x10 3x10 6x10 0.9x10' 2.1x10' 3x10' 6x10 , in in minutes chloroform. 4 chloroform. molar molar molar 5 5 molar molar molar molar molar 3.0 Figure -(dc/dt)'1 xlO'6 3 “(dc/dt)'1 xlO 5.31. 5.30. c a DMDPBP DMDPfl in -(dC/dt) -(dC/dt) CHAPTER in chloroform. chloroform. IN IN CHLOROFORM CHLOROFORM V. 1 1 versus versus PHOTOSENSITIVE - 224 (1/C) (1/C) - for for MEMBRANES. solutions solutions of of DMBPA DMPA Absorbance Absorbance Figure 0 0.60 , o.oo 1 OQ go .20 Figure ~0 1 ------j- r 0 ------ — ------5.33. 5.32. 10 CHAPTER 1 ------ 10 1 ------ The The 20 oxygenation 1 ------ V. 20 oxygenation 1 ------" PHOTOSENSITIVE ^ ^ 30 ■ ------ - 225 Time, 30 1 ------ of — - 40 1 ------ of Time, minutes DMBPA DMPA 40 1 ------□ O A * minutes MEMBRANES 50 1 ------0.824x10 0.577x10 0.32x10 1.648x10 in O D A * in 2.05x10 4.1x10 7.2x10 1.03x10 dibutyldigol. dibutyldigol 50 60 1 ------ 1 4 ------ 4 4 4 5 5 molar molar molar molar 4 4 molar molar molar molar 70 60 J J . Figure Figure - (d C /)" d t 1000000 1500000 500000 2000000 1500000 1000000 500000 5.35. 5.34. o o DMPfl in -(dC/dt)" -(dC/dt) in CHAPTER DMBPR dibutyldigol. dibutyldigol in 10000 IN DBDG V. 10000 1 DBDG 1 versus versus PHOTOSENSITIVE at RT 23° - . 226 20000 23 C (1/C) (1/C) - C 1/C for 20000 for MEMBRANES. 30000 solutions solutions of of 40000 30000 DMBPA DMPA ; CHAPTER V. PHOTOSENSITIVE MEMBRANES. Figures 5.32 and 5.33 show the absorbance of 1,4-dimethoxy- 9,10-diphenylanthracene and 1,4-dimethoxy-9,10-bis(4'- bromophenyl)anthracene dissolved in dibutyldigol respectively.-They have been used to calculate the lifetime of singlet oxygen in the solvent dibutyldigol according to similar regression graphs as given in Figures 5.34 and 5.35. Table 5.7. The reaction constants in chloroform. DMPA DMBPA Reaction constant(s" 1 )(at 2 3°C) 3.15xl08 * 3.62x10s Lifetime of singlet oxygen in dibutyldigol (sec)(at 23°C) 1.16x10"5 1.48x10"5 * The literature value 3.20xl08 L/mol-sec [Monroe, 1978] (The lifetime of singlet oxygen in chloroform is 6x10" 5 seconds [Bellus, 1979]). The lifetime of singlet oxygen in dibutyldigol calculated from the experiments with both DMPA and DMBPA in dibutyldigol is also given in this table. It can be seen that the lifetime of singlet oxygen is not affected by the solute but depends on the solvent only. In fact, the solutions contain very little solute (about 10"4 mol), so the solute would not bring about a significant influence on the lifetime of singlet oxygen in such a dilute solution. 5.10. CONCLUSIONS ON PHOTOSENSITIVE MEMBRANES. As an immobilized liquid membrane incorporating chemical reactions, the photosensitive membrane potentially has an unique feature in comparison with other facilitated immobilized liquid membranes - namely completion of the chemical reaction. -227- CHAPTER V. PHOTOSENSITIVE MEMBRANES. Carriers play a critical role in this process. Generally speaking, a carrier with a fast oxygenation rate can be easily used to carry out the facilitated transport. However, as shown in the experimental studies, the facilitated transport can be utilized only if the forward and reverse reaction rates are similar to each other, i.e. the reactions not only to be fast but also to be comparable with each other. Otherwise, there would be either a high concentration of the oxidized form of the carrier for the fast forward and slow reverse chemical reaction on both sides of the membrane, or a low concentration of the oxidized form for the contrary situation with slow forward and fast reverse chemical reaction. Thus facilitated transport could not be achieved in either situation. It is concluded that there will be no significant photosensitive transport of oxygen if the carrier concentration is very low. In this case, the permeation of oxygen and nitrogen through an organic solvent will dominate the whole process. The separation factor is merely the contribution of the organic solvent, which, as mentioned in Chapter III of the thesis, would not be high. On the other hand, a high carrier concentration causes precipitation of the oxidized form of the carrier that cannot bring about facilitated transfer either. This indicates the importance of the solvent chosen to dissolve the carrier and to impregnate the microporous membranes in photosensitive membranes. To evaluate a solvent the following terms should be included: the stability of the carrier in the solvent and the solubility of carrier in both free and oxygenized form. An alternative approach would be to incorperate the acceptor as a repeating unit in a polymer molecule In this case where no liquid solution is involved one may put a -228- CHAPTER V. PHOTOSENSITIVE MEMBRANES. high concentration of the carrier into a membrane matrix and avoid precipitation. This approach has been discussed by Meyer [1969], Beinert et al [1970], Saito et al [1985], Twarowski and Dao [1988] and Twarowski et al [1988]. In order to achieve facilitated transport in the photosensitive liquid membranes studied in this work, there are three main things to be considered: (1) carrier degradation of the endoperoxide, (2) light penetration and (3) crystallization of the endoperoxide. The carrier, 1,4-dimethoxy-9,10-diphenylanthracene, can work for a period of time before it degrades to a significant level. The light penetration and blocking is crucial but can be solved if a metal medium is used. The third one is a real obstacle for this carrier, i.e. the very low solubility of the oxygenized form of the carrier stops any possible function of the facilitated transport by the carrier. The experimental results have proved that the proposed equilibrium conditions everywhere inside the membrane are scarcely established. In fact, the experimental observations show fast forward and slow reverse reactions. Furthermore, the equilibrium conditions could not be reached at the surface close to the upstream where the liquid is illuminated, even if the forward reaction is much faster than the dark side. The experimental observations have proved that the forward reaction for a 0.04 molar solution takes about ten minutes to complete, i.e. all the free form of the carrier have been converted to the oxidized form. In other words, the forward reaction can be completed under illumination for such a long time that no equilibrium can be reached anywhere in the membrane. The reverse reaction, on the other hand, can be completed without illumination, but it needs an even longer time than the forward reaction. In fact, the slow reaction rare has been reported in literature. For example, the oxygenation -229- CHAPTER V. PHOTOSENSITIVE MEMBRANES. of 1,4-dimethoxy-9,10-diphenylanthracene in pyridine (0.01 mol) at 10°C took 20 minutes to complete [Wilson et al, 1986] (the light source was a 500 W projector), but it took about one and half hours to regenerate 80% and 3 hours to regenerate it completely. A device working on the same chemical principle as the photosensitive membrane process can be used to absorb and store oxygen. Once again, a high concentration of acceptor is necessary to store a large amount of oxygen. It is less important for high solubility of the oxygenized form of the acceptor because precipitation can be allowed. -230- CHAPTER VI. COMPARISON OF DIFFERENT MEMBRANES. CHAPTER VI. COMPARISON OF DIFFERENT MEMBRANES. Most work on gas separations using membrane technology is focussed on searching for a better separation barrier, as mentioned in the introduction of this thesis. Individually, different membranes have their special features in _ comparison with others. The polymeric membranes, for example, are still the most popular ones, and perhaps, the first choice in many membrane processes. Other membranes have been studied more intensively in recent years. Comparisons of the different types of membranes studied in this work are made in this chapter in terms of their effectiveness and feasibility. 6.1. EFFECTIVENESS OF DIFFERENT MEMBRANES FOR GAS SEPARATION. Polymer membranes have been intensively studied by many investigators for several decades. They are widely used in membrane processes such as reverse osmosis, dialysis and ultrafiltration. There are several successful examples in applying polymer membranes to gas separation. However, the low selectivity of polymer membranes limits their use in oxygen enrichment. Nevertheless, polymer membranes are still the most active research objective in all areas of membrane science including gas separation. Theoretically, the immobilized liquid membrane incorporating a chemical reaction for gas separation shows the greatest potential prospect among the membrane processes. By introducing carriers, the transport of some species can be enhanced while the transport of others is unchanged. As a result, the separation factors should be increased. As an example, Johnson et al [1987] conducted intensive studies on different immobilized liquid membranes -231- CHAPTER VI. COMPARISON OF DIFFERENT MEMBRANES. and found a promising potential for such membranes in oxygen enrichment. The life span of their liquid membranes was as long as six months. As immobilized liquid membranes, the photosensitive membranes studied in these experiments could be very interesting because they could potentially achieve a high selectivity. The reason for this is that in principle both the forward and the reverse reactions can go to completion. This can be achieved by maintaining illumination at the upstream surface and darkness at the downstream surface respectively. Under ideal conditions, calculations show that the enhancement of the separation factor could reach 7 to 8 times Henry's law permeation and the separation factor could reach about 12. Of the four different types of membrane which have been examined (including the SILICON RUBBER/COPPER COMPLEX MICELLE MEMBRANES described in Appendix VI) in this study, the plasma polymerized membrane shows the best separation factor. It should be noted that this is based on limited results and it is not conclusive that the plasma treated membranes would be the most effective type. For example, the photosensitive liquid membrane could be further assessed with different carriers and solvents. The selection of better carriers and solvents involves a compromise between oxygen selectivity and operational limitations. As an example, the Schiff base complex could reach high separation factors [Johnson, 1987] but such membranes may suffer solvent problems which could become a main obstacle in a real process. 6.2. FEASIBILITY OF MANUFACTURE OF DIFFERENT MEMBRANES. Polymer membranes are still the most feasible for production on an industrial scale. Therefore, many -232- CHAPTER VI. COMPARISON OF DIFFERENT MEMBRANES. investigators have focussed their attention on this kind of membrane. As a result, considerable progress has been made in the last few decades. Composite membranes made from polysiloxanes, polysulfones and more recently disubstituted acetylene [see Section 2.8] represent the major advances in this area. However, these membranes still cannot compete with the cryogenic process in large-scale production of oxygen due to their poor selectivity. In other words, they are still economically inefficient. However, there are some viable applications for small quantities of the production of oxygen such as in clinical applications and for conditions which require inert or low oxygen gas. Plasma polymerized membranes, which are a kind of polymer membrane, are usually much easier to handle than immobilized liquid membranes. It is relatively easy to make this kind of membrane in a laboratory although more complicated than conventional polymer membranes. It may be difficult to make this type of membrane on a large scale due to the problems of maintenance of vacuum and achieving even distribution of the deposited layer. This may be the reason that the plasma polymerized membrane has not appeared in commercial applications. From this study there are two major problems which need to be considered whenever an immobilized liquid membrane is put into use. Firstly, the carrier stability may be an obstacle encountered in the process design. Irreversible oxygenation of complexes in an oxygen enricher, for instance, occurs in all carriers, although some carriers work longer than others. Therefore, a new carrier with a long life is usually the first target for a membrane investigator. There has been great progress in searching for better carriers in recent years [Johnson, 1987], However, more work is still needed to make membrane processes competitive with the cryogenic process. Secondly, -233- CHAPTER VI. COMPARISON OF DIFFERENT MEMBRANES. solvents used to dissolve the carriers are equally important in a liquid membrane process. The solvent will inevitably be lost and need to be recovered from the product, no matter how low the vapour pressure. This may be a serious problem because many solvents are volatile and toxic. In some cases a low solvent content may not be essential. For example, oxygen enriched air used in combustion processes could tolerate some solvent which would be burned in the furnace . However, it is often unacceptable to lose a large quantity of solvent or to recover a solvent from a large amount of product using methods which may be more complicated than oxygen enrichment itself. -234- CHAPTER VII. GENERAL CONCLUSIONS AND RECOMMENDATIONS. CHAPTER VII. GENERAL CONCLUSIONS AND RECOMMENDATIONS. 7.1. CONCLUSIONS. Using FC-43 or silicone oil, the immobilized liquid membranes are less selective than polymer membranes such as silicone rubber due to the high solubility of oxygen and nitrogen in the organic solvent. The silicone oil membranes have better selectivity, which reaches a separation factor of 2, than the FT-43 ones, which has a separation factor of 1.6. FC-43 and silicone oil may only be used as liquid membrane materials for the production of low concentrations of oxygen. They can remain in a membrane for a significant time because of their low vapour pressure. The permeability of such membranes could be as high as that of silicone rubber. The advantages of plasma treated membranes are apparent. The results from the membranes treated with plasma are more encouraging than those from other membranes which have been examined in these experimental studies. This kind of membrane gives improved selectivity which reaches about 3.0 while the permeation is only slightly reduced. This technique is now being widely used in membrane manufacturing in laboratories because it is relatively easy to handle and could achieve a better separation effect. The substrates of plasma polymerized membranes can be either microporous or dense membranes. With microporous membranes, the selectivity could be higher but the deposition conditions are more difficult than dense membranes. With dense membranes the thickness needs to be very low. A more attractive compromise would be to use at least three layers, a microporous substrate, a thin dense intermediate -235- CHAPTER VII. GENERAL CONCLUSIONS AND RECOMMENDATIONS. layer and a thin plasma-polymerized top layer. The metal complex micelle membranes were not stable and suffered phase separation in a few hours although they had an improved separation factor of 2.35 compared with the separation factor of 2.0 for silicone rubber. The facilitated transfer of photosensitive membranes is potentially higher than non-photosensitive membranes although in this experimental study facilitated transport has not been observed. The analytical result under ideal conditions shows the prospects of this kind of membrane if a suitable carrier and solvent are used. As the analysis shows, it may be feasible to achieve high flux and a good separation factor. Although the carriers developed in the course of this study did not have the ability to facilitate membrane transport, they were found to have absorptive/ desorptive properties. They offer the possibility of trapping (solar) energy and storing oxygen in a solid form. 7.2. RECOMMENDATIONS. Based on these studies of oxygen enrichment, the following recommendations are made. Polymers are fundamental membrane materials which need further investigation. Since the separation factors of the existing polymers are not high for C^/^, it is necessary to develop new membrane materials, and the most likely success could be with organic silicone and fluorine containing polymers. Plasma polymerization may be a very effective method for membrane preparation. It can be used to modify the surface properties of different substrate membranes to suit various applications. The enhancement of the separation factor -236- CHAPTER VII. GENERAL CONCLUSIONS AND RECOMMENDATIONS. for oxygen enrichment is an example. Coatings containing fluorocarbons are potentially the most interesting. The design of equipment for plasma polymerization of membranes requires attention to the temperature control and flow rate measuring devices. The former is needed to avoid overheating of the substrate by the electrical current when plasma treatment is carried out, especially for microporous membranes which are normally sensitive to heat. The temperature of the electrodes may become so critical that plasma treatment becomes impossible. For example, cellulose acetate membranes which are very fragile under the influence of heat can only be treated at room temperature. Accurate flow control is important because it provides a means of controlling deposit thickness. The photosensitive membrane is potentially very interesting. In principle this process could give a very high separation factor because of reversible oxygenation. It is interesting to consider that this process could also lead to trapping solar energy, if a truly reversible oxygen carrier can be found. That is to say the carriers can be oxygenated under the illumination of the sun at room temperature, and then release oxygen in darkness and at an elevated temperature. It is worthwhile trying to find a polymerized carrier which inherits the simplicity of polymer membranes while bringing about facilitated transport by the fixed carriers. As an example, some work is already reported on the polymerization of a vinyl-substituted naphthalene, which can react photochemically with oxygen. Twarowski et al [1988] and Twarowski and Dao [1988] have shown that singlet oxygen could be released from the 1,4-endoperoxide of a thin solid film of 1,4-dimethyl-2-poly(vinyl- naphthalene) into a gas phase. The dissociation reaction of -237- CHAPTER VII. GENERAL CONCLUSIONS AND RECOMMENDATIONS. the endoperoxide is reported quantitatively with a significant reaction rate, i.e. more than half of the oxygen bound to the carriers was released in less than 1 second at above 98°C. There would be some advantages to this kind of carrier. Firstly, it is a polymer, and it may be possible to use conventional methods of membrane manufacture to formulate a dense membrane. Secondly, there is no need for a solvent to form the liquid membrane, and this avoids some problems of solvent loss. Thirdly, because there will not be any solvent incorporated in the membrane device, the concentration of carrier, which is a component of polymer, can be very high and this should significantly enhance the oxygen permeation. The only limitation could be the reduced diffusivity in the membrane matrix, compared with that in an immobilized liquid membrane. This can be partially offset by using a very thin skin layer. -238- APPENDIX I. PROPERTIES OF OXYGEN AND NITROGEN APPENDIX I. PROPERTIES OF OXYGEN AND NITROGEN. THE PROPERTIES OF OXYGEN AND NITROGEN [Isalski, 1989]. Oxygen Nitrogen Molecular weight 32.00 28.01 Boiling point at 101.3kPa -182.97° C -195.8°C Specific volume at 20°C 0.755 m3 kg" 1 0.855 m3 kg" 1 101.3 kPa Critical temperature -118.6°C -146.95° C Critical pressure 5043 kPa 3400 kPa Heat capacity at 25°C, 918 Jkg" 1 K" 1 1040 Jkg" 3K' 1 101.3 kPa The composition of air. Normal With maximum impurities Nitrogen 78.084 mol.% 78.028 mol% Oxygen 20.946 mol.% 20.931 mol% Argon 0.934 mol.% 0.933 mol.% Carbon dioxide 350 v.p.m. 1000 v.p.m. water variable* saturated* Neon 18 v.p.m. 18 v.p.m. Krypton 1.1 v.p.m. 1.1 v.p.m. Xenon 0.08 v.p.m. 0.08 v.p.m. Helium 5.3 v.p.m. 5.3 v.p.m. Hydrogen 0.5 v.p.m. 0.5 v.p.m. Acetylene 0.1 v.p.m. 1.0 v.p.m. Ethylene 0.01 v.p.m. 2.0 v.p.m. Propylene nil 0.2 v.p.m. Methane 2.5 v.p.m. 10 v.p.m. Ethane 0.02 v.p.m. 0.1 v.p.m. Propane nil 0.1 v.p.m. Butane nil 0.1 v.p.m. Carbon monoxide nil 35 v.p.m. Nitrogen oxides nil 0.5 v.p.m. Sulphur compounds nil 0.1 v.p.m. * Air composition data are on dry basis. -239- APPENDIX II. PERMEATION PROGRAM IN COMPUTER LANGUAGE C. APPENDIX II. PERMEATION PROGRAM IN COMPUTER LANGUAGE C. /* PERMEATION.C This program is designed to calculate permeabilities and separation factors from the experimental data. Program below is written on IBM PC compatible computer in Language C, which is a middle level language. */ #include puts(CLEAR); if (argc !=3) { printf("usege: filename datafile outputfile"); exit(1); > if ((fpl = fopen(argv[1], "r")) == NULL) { printf("cannot open %s\n", argv[l]); exit(1); > if ((fp2 = fopen(argv[2], "w")) == NULL) { printf("cannot open %s\n", argv[2]); exit(1); } printf("Enter the number of data point\n"); fscanf(stdin, "%d", &n_point); ; fprintf(fp2, "Permeation Permeation Separation\n"); fprintf(fp2, "of oxygen of nitrogen factor \n"); -240- APPENDIX II. PERMEATION PROGRAM IN COMPUTER LANGUAGE C. calcul(fpl, fp2); fclose(fpl); fclose(fp2); > void calcul(data, output) FILE *data, *output; { float temp; /* temperature of the sampling loop in K */ float feedo; /* feed area of oxygen by loop (pure oxygen) */ float feedn; /* feed area of nitrogen by loop(pure nitrogen)* float thick; /* thickness of filter in cm*/ float sweep; /* flow rate of helium of sweep, cc per second * float analo[10]; /* permeate area of oxygen */ float analn[10]; /* permeate area of nitrogen */ float permo[10]; /* permeability of oxygen */ float permn[10]; /* permeability of nitrogen */ float factor[10]; /* separation factor */ float dif; /*pressure difference between two sides*/ float sum; /*sum of the separation factors */ float x; /* oxygen concentration of feed */ int i; i = 0; sum = 0; fscanf(data, "%f%f%f%f", &sweep, &thick, &feedo, &feedn); fscanf(data, "%f%f%f", &x, &dif, &temp); for ( i = 0; i < n_point; i++) { fscanf(data, "%f%f", &analo[i], &analn[i]); permo[i] = analo[i]*sweep*thick/(feedo*11.95*(76.0+dif)*x)* (273/temp); permn[i] = analn[i]*sweep*thick/(feedn*11.95*(76.0+dif)*(1.0 -x))*(273/temp); factor[i] = permo[i]/permn[i]; fprintf(output, "%10g%16g%14g\n", permo[i], permn[i], factor[i]); sum = sum + factorfi]; fprintf (output, "______\n\n") ; fprintf (output, "Average %g\n\n", sum/n__point) ; > fprintf(output, "%d\n", n_point); } -241- APPENDIX III. SURFACE ENERGY AND CONTACT ANGLE. APPENDIX III. SURFACE ENERGY AND CONTACT ANGLE. The surface characteristics have been studied in terms of contact angles. The following paragraphs will summarize the theoretical and practical aspects of this interfacial phenomenum [Kaelble, 1971 and Wu, 1982]. It is well known that the molecules in the surface possess an extra force caused by the internal attraction of molecules that is not balanced by the molecules around them. Therefore, a solid or liquid surface tends to decrease its surface area. The quantitative measure of this is called surface tension, expressed as dynes/cm, which is numerically equivalent to the SI unit mN/m. Zisman et al [Bascom, 1988] conducted a series of experiments and concluded that there was a critical surface tension for a solid surface. In spite of argument about this concept, the critical surface tension of a solid provides a criterion that the surface can be totally wetted by a liquid if the liquid it comes into contact with has a surface tension less than the critical surface tension, otherwise a certain contact angle will be formed [Bascom, 1988]. The following table shows the critical surface tension of some materials [Barton, 1983]. -242- APPENDIX III. SURFACE ENERGY AND CONTACT ANGLE. Material Critical surface tension (dyne/cm) Cellulose, regenerated 44 polycarbonate 42, 29 PVDF 40 Poly(dimethylsiloxane) monolayer 24 Poly(tetra fluoroethylene) 18.5 It is assumed that the surface tension equal the surface energy expressed as ergs/cm2 . Considering a piece of flat surface with a drop of liquid, one can recognise three phases: air saturated with vapour, liquid and solid. There would be three surface tensions involved: the surface tension of the liquid in equilibrium with saturated air, 7 , the surface tensions of the solid/vapour (7 ) and 1 s liquid/solid (7 ). In the equilibrium state, the three 1 B forces will be in balance, as given by Young's equation [Wu, 1982], 7 COS (9=7 - 7 (AIII.l) 1 sis where 9 is called a contact angle. The contact angle represents the interaction between liquid and solid. There are several forces contributing to the surface tension, namely the dispersion forces, dipole, induced dipole and hydrogen bonding. It is assumed that all the forces causing the surface tension are additive. In other words, these forces can be considered individualy and then added together. Therefore, the surface tension of solid, 7^ can be represented by dispersive and polar components as, 7=7d+7P (AllI.2) -243- APPENDIX III. SURFACE ENERGY AND CONTACT ANGLE. and the polar component can be further separated as, P dipole induced dipole H - b o n d 7 7 + 7 + 7 (AllI.3) 8 8 8 8 However, one can consider the surface energy as having the two components 'dispersive' and 'polar'. The work done in separating a liquid from a solid surface is defined as the work of adhesion and this equals the two component surface energies minus the corresponding liquid/ solid surface energy as, w = 7 + 7 - 7, (AllI.4) 1 s Is The work here can be separated into two components due to the dispersion and polar forces. Therefore, the above equation can be modified to, 7 =7 +7 - W*1 - Wp (All 1.5) Is 1 8 where and Wp are the dispersive and polar components of the adhesive work respectively. The work for cohesion can be obtained if one supposes that the surface of the solid is the same as that of the liquid, so, 7 = 7, , 7, =0 (AIII.6) B 1 Is and then W = 2y . The solid cohesion work can be derived i i by using a similar assumption, W =27 s s . There are two approaches to applying the individual surface energy to describe the interaction of the liquid and solid. In the harmonic mean approximation [Wu, 1982] it is assumed -244- APPENDIX III. SURFACE ENERGY AND CONTACT ANGLE. that the work of adhesion equals the harmonic mean of cohesion work of the liquid and the solid. Applying this assumption to the dispersion component, one has, Wd (AllI.7) The work can be replaced by the corresponding surface tensions this equation becomes, d d 47 1 W (AllI. where 7^andd 7dg are the dispersive components of the liquid and the solid respectively. The polar component can be written in the same way as, P P 4 7 7 Wp 1 S (AllI.9) where 7^and 7P are the polar components of the liquid and the solid respectively. Joining equations (AllI.5) with equations (AllI.8), (AIII.9) and Young's equation (AIII.l), one can obtain, ! 7d / 7P 7P 7l(l + COSO— = d 1 d- + p p--- (AIII.10) ' 1 1 s ' 1 s The are two variables 7P and 7^ in this equation when a s s known surface tension liquid is used. In order to determine the surface tension the surface tension of a solid, two known surface tension liquids are needed to form simultaneous equations, -245- APPENDIX III. SURFACE ENERGY AND CONTACT ANGLE. ( 71) i ( 1 + 1 (71)2(l + cose9)— = (AllI.12) z 4 P s A program for calculating the surface tension or energy has been written in computer language C on IBM PC as shown below. /* SURFACE ENERGY CALCULATION PROGRAM IN LANGUAGE C This program is designed to calculate the surface energy according to the contact angle measurement. The calculation is based on the simultaneous equations of (AIII.ll) and (AIII.12). V #include "stdio.h" #include "math.h" #define CLEAR "\033[2J" main(argc, argv) int argc; char *argv[]; { FILE *fpl, *fopen(); /* fpl = datafile */ float anglel; /* the contact angle of liquid 1, degree */ float angle2; /* the contact angle of liquid 2, degree */ float rl; /* surface tension of liquid 1, dyne/cm */ float r2; /* surface tension of liquid 2, dyne/cm */ float rid; /* dispersion component of liquid 1, dyne/cm*/ float r2d; /* dispersion component of liquid 2, dyne/cm*/ float rip; /* polar component of liquid 1, dyne/cm */ float r2p; /* polar component of liquid 2, dyne/cm */ float xl,x2; /* dispersion component of solid surface, dyne/cm */ -246- APPENDIX III. SURFACE ENERGY AND CONTACT ANGLE. float yl,y2; /* polar component of solid surface, dyne/cm */ float sd; float m, p, pi, p2; float n, q, ql, q2; float a, bl, cl; double b, c, w; puts(CLEAR);- if (argc !=2) { printf("usege: filename datafile"); exit(1); > if ((fpl = fopen(argv[1], "r")) == NULL) { printf("cannot open %s\n", argv[l]); exit(1) ; } fscanf(fpl, "%f%f%f%f", &anglel, &angle2, &rl, &r2); fscanf(fpl, "%f%f%f%f", &rld, &rlp, &r2d, &r2p); m = ( 1 + cos(anglel/57.296))*rl/4; n = ( 1 + cos(angle2/57.296))*r2/4; printf("%10g%10g\n", m, n); p = rid + rip - m; q = r2d + r2p - n; pi = rlp*rld - m*rld; p2 = rlp*rlp*rld; ql = r2p*r2d - n*r2d; q2 = r2p*r2p*r2d; a = rlp*rlp*q - r2p*r2p*p - (rip - r2p)*p*q; bl = p2*q + rlp*rlp*ql - r2p*r2p*pl -p*q2 - (rip - r2p)*(pl*q + p*ql); cl = p2*ql - pl*q2 - (rip - r2p)*pl*ql; b = bl/a; c = cl/a; w = sqrt(b*b/4 - c); xl = -b/2 + w; x2 = -b/2 - w; yl = (rlp*rlp*(xl + rid))/(p*xl + pi) - rip; y2 = (rlp*rlp*(x2 + rid))/(p*x2 + pi) - rip; printf( "%10g\n", w); printf( "%10g%16g%14g\n", xl, yl, xl + yl); printf( "%10g%16g%14g\n", x2, y2, x2 + y2); fclose(fpl); > -247- APPENDIX IV. SILICON RUBBER/COPPER COMPLEX MICELLE MEMBRANES APPENDIX IV. SILICON RUBBER/COPPER COMPLEX MICELLE MEMBRANES AIV.1. INTRODUCTION. This Appendix briefly describes attempts to improve the selectivity of the silicone rubber membrane by incorporating metal complexes capable of reacting reversibly with oxygen. It is known that it is possible to increase permeability and selectivity using a reversible chemical reaction. Iron, cobalt and copper complexes, for instance, have been shown to reversibly react with oxygen but not nitrogen. The idea of the complex micelle membrane is that a complex can be dispersed into a membrane matrix in the form of micro-micelles in a similar way to liquid membranes but the dispersed phase in this case is solid polymer instead of liquid. The transport of some gas species may be promoted by the metal complex, whilst transport of others may not be influenced. As a result, facilitated transport could be obtained and the separation factor increased. A copper complex has been chosen as carrier in this application. There could be several advantages in this process. Firstly, there is a wide variety of available complexes. In fact, many complexes have proved to be capable of carrying oxygen by virtue of a reversible chemical reaction. They are stable towards the irreversible oxidation in either solid or organic solvent but not in aqueous solutions. Secondly, surfactants are needed in this process which would help to disperse the metal complex into other solvents. Several surfactants can be used in this process. They have two functions: to disperse the metal complex and -248- APPENDIX IV. SILICON RUBBER/COPPER COMPLEX MICELLE MEMBRANES. to maintain the membrane material property as described later. AIV.2. MEMBRANE MATERIALS AND COMPLEX PREPARATION. The membrane material used in the experiments was silicone rubber: DOW CORNING 3110 RTV and 3120 RTV, which are paste like materials. The former is white in colour and the latter is brown. When making a membrane, the procedures were as follows: (a) adding catalyst (a curing agent) to silicone rubber, (b) mixing them were thoroughly, (c) casting the mixture on a piece of glass, (d) standing for 2 hours (the 'curing time'), and the paste-like substance became an elastic membrane. Copper thiocyanate was prepared by putting 0.4 grams (A.R.) copper sulphate pentahydrate into a 250 ml beaker, and dissolving it in 50 ml water. A few drops of hydrochloric acid and 25 ml of 1:10 ammonia bisulphite solution were added. Then the solution was diluted to 200 ml and heated to nearly boiling point. After that, freshly prepared 10 percent ammonia thiocyanate solution was added and stirred slowly and constantly. The white coloured precipitate of copper thiocyanate was left to stand overnight. After filtering the precipitate it was washed with dilute ammonium thiocyanate solution(l: 1000). The product of copper thiocyanate was dried in a vacuum oven for two hours. The complex prepared in these experiments is copper thiocyanate reacted with tetraethylenepentamine (tetren). No solvent was added. It was reported [Kawakami, 1982] that Cu(I)/tetren complex should be colourless or slightly purple coloured if the preparation process was carried out under vacuum conditions. It was also possible to make Cu/ tetren complex under atmospheric conditions to form a blue -249- APPENDIX IV. SILICON RUBBER/COPPER COMPLEX MICELLE MEMBRANES. coloured solution at first. After being evacuated at 40°C, the solution became colourless and the final solution was no different from that prepared under vacuum conditions. This provided a possible way to regenerate an oxidized complex after it lost its reversible oxygenation property. However, the complex prepared in this study was light blue in colour. Thus it might have contained the oxidized form before it was put into the membranes. The oxidized form could be converted to the oxygen free form by heating the complex in a vacuum oven, although this conversion was not necessary since the complex would be partially in its oxidized form when used in the membrane process. Surfactants were involved in preparing the micelle membranes. They formed a boundary layer between the silicone rubber and the copper complex solution. The surfactants which were examined were Teric N2, N3, N4, N10, 16M15, PE61, PE62, etc. (from ICI Australia Ltd.). They are nonionic surfactants which are essentially chemically inert. This means they usually do not ionise in an aqueous environment nor interact with any components in the solutions. Physically, all the nonionic surfactants used contain the ethylene oxide hydrophile, but their hydrophobes are different. The N series surfactants, for instance, contain a nonyl phenol hydrophobe, and the PE series contains a polypropylene glycol hydrophobe. The surfactant specification is given in Table AIV.l. According to the ICI catalogue, all the surfactants here can be used as a dispersing agent. -250- APPENDIX IV. SILICON RUBBER/COPPER COMPLEX MICELLE MEMBRANES. Table AIV.l. Surfactant specification (from ICI catalogue). Appearance Melting Specific Vise. Type E0d at 20°C PH6 HLB point gravity,20° 20°,CP PE6 la 5.7 Liquid 6 3 <0 1.017 388 PE62 17 Liquid 6 7 <0 1.032 440 PE64 25.5 Liquid 6 15 8 1.051 1332 N2b 2 Liquid 6-8 5.7 <0 1.001 620 N3b 3.5 Liquid 6-8 8.2 <0 1.017 395 N4b 4 Liquid 6-8 8.9 <0 1.023 370 N10b 10 Liquid 6-8 10.5 5 1.063 360 16M15C 15 Liquid 8-10 13.8 -18 1.021 266 PE series contain polypropylene oxide hydrophobe N series contain nonyl phenol hydrophobe 16M series contain a refined, primary soya-amine base hydrophobe. EO is the average number of moles of ethylene oxide per mole of hydrophobe. 1% aqueous solution The required property of the surfactant was that it allowed formation of a stable emulsion (dispersion) of the copper complex in the silicone rubber. Of the surfactants listed in Table AIV.l it was found that most were miscible in Cu- tetren except PE61 and PE62 which form emulsions, although they were not very stable. When making the complex/surfactant/rubber membrane, a surfactant and complex were mixed together. After being shaken for a period of time, an emulsion was formed. The complex/surfactant/rubber membranes were made by mixing the emulsion with silicone rubber before the curing agent was added. The compositions of the prepared membranes are shown in Table AIV.2. AIV.3. DISCUSSION OF THE RUBBER/COMPLEX MICELLE MEMBRANES. The permeabilities and separation factors for pure silicone rubber membranes and rubber/complex membranes are shown in Figures AIV.l and AIV.2 respectively. -251- APPENDIX IV. SILICON RUBBER/COPPER COMPLEX MICELLE MEMBRANES. It can be seen from Figure AIV.l that the permeabilities of silicone rubber membranes declined slightly when the partial pressure increased. This result shows that the assumption of constant solubility and diffusivity is only approximately correct over the range of pressures examined. The separation factors of the silicone rubber membranes are about two (Figure AIV.2). Composite membranes were prepared with different compositions. If no surfactant was added to a rubber/ complex composite membrane, the property of the silicone rubber was considerably influenced by the Cu-tetren ingredient. The tensile strength, for instance, decreased. Furthermore, the silicone rubber membranes could not be formed at all when the complex constituent was increased to about 10 wt% in the membrane. Membranes made with PE61 and PE62 were found to have the elastic properties of pure silicone rubber, but the curing times were two or three times longer than the pure one. Because of the added surfactant, the silicone rubber property was less affected than that of the rubber/complex membranes. It is shown in Figures IV.1 and IV.2 that the permeability is independent of the partial pressure over the pressure range examined. However, at higher pressure, permeability may decrease if the pure rubber behaviour is followed. -252- Figure APPENDIX o l 0T COMPLEX AIV.l. x o CO LU cr QQ m cr Z) * IV. membrane SILICON Oxygen z uio / permeability RUBBER/COPPER uid (No3). - ( dJ,S - 253 ) - of AqTTTqeeuiuad COMPLEX o o rubber/complex MICELLE MEMBRANES. micelle _ _ _ - o o O 1 OJ O O O 00 O CD O O ^r cvj o o Time (minutes) APPENDIX IV. SILICON RUBBER/COPPER COMPLEX MICELLE MEMBRANES. O hi cr _J LU 0- OQ o * (minutes) Time joq-opg: uoTgejedes Figure AIV.2. Separation factor of rubber/complex micelle membrane (No3) -254- APPENDIX IV. SILICON RUBBER/COPPER COMPLEX MICELLE MEMBRANES. A summary of the experimental results is given in Table AIV.2. It can be seen that the separation factor for oxygen over nitrogen has slightly increased from 2.00 for the pure silicone rubber membrane to about 2.35 for the complex/ rubber membrane. Table AIV.2. Experimental results of copper complex/surfactant/silicone rubber membranes Membrane composition, %wt Thick Permea No Colour ness , bility, Separat. Cu-tetren PE61 Rubber cm at 2 0° C factor 1 0 0 3110 White 0.020 800 2.00 2 0 0 3120 brown 0.017 500 2.00 3 9.65 1.45 88.90* Blue 0.015 380 2.24 4 18.52 2.78 78.70* Blue 0.015 330 2.35 5 16.08 2.40 81.52* Blue 0.017 330 2.33 *silicone rubber and catalyst were included. It was clear that the permeability of the silicone rubber/ - complex micelle membrane decreased after surfactant was added to the rubber membrane. The reason for this was the inert character of this nonionic surfactant. In the process, the surfactant was only a dispersing agent and it did not bring about any additional mass transfer function, in fact it could provide a mass transfer barrier around the complex. It should be possible to find the minimum surfactant content, which could form stable micelles inside the silicone rubber. The surfactants used in the experiments did not adequately disperse the copper complex into the silicone rubber. As a result, the liquid droplets of the emulsion were neither small nor stable. However, there was a noticeable difference between the various surfactants. For example, the PE61 was better than the PE62 and both were better than N series surfactants, which seemed to dissolve the copper -255- APPENDIX IV. SILICON RUBBER/COPPER COMPLEX MICELLE MEMBRANES complex and tetraethylenepentamine and did not form an emulsion. A worthwhile direction for this kind of work is to assess surfactants with co-polymer structure. A-B or A-B-A type co-polymers, for example, are of interest because they have a similar structure to the PE series. In particular, A-B-A type surfactants are attractive since there are two functional groups in one surfactant molecule. The metal complex used in this experimental study was a copper complex. However, other complexes such as Cobalt(salicylidene)ethylenediamine and Cobalt(methoxy- salycilidnene)ethylenediamine, may be good candidates to facilitate transfer of oxygen [Johnson, 1987]. The solvent used for making the membrane also played a significant role because the evaporation rate of the solvent could influence the membrane structure which then could influence permeation properties. AIV.4. CONCLUSION ON RUBBER/COMPLEX MEMBRANES A potentially promising alternative to silicone rubber was the complex solution/surfactant/rubber membrane which could reach a higher selectivity than the rubber. Preliminary experiments with the nonoptimised surfactants showed that the selectivity of the membrane was enhanced but the permeability was decreased considerably. Further improvement might be possible if a more suitable surfactant could be found. In addition, other complexes could be used in this kind of process to give better selectivity. -256- APPENDIX V. CALCULATION ON FACILITATED TRANSPORT. APPENDIX V. CALCULATIONS OF HENRY' S LAW PERMEATION AND FACILITATED TRANSPORT UNDER IDEAL CONDITIONS. This calculation is based on ideal conditions. The permeabilities of Henry's law contribution and facilitated transport are calculated and then the augmentation factors are given under these conditions. The solubility of gases in dibutyldigol is calculated according to equation (3.11) instead of equation (3.10) because the volume difference between solvent and gas is large. The solubility parameters are obtained from Table 3.5. For example, the solubility is first calculated for oxygen, 46*(11.7 - 17.I)2 46 46 -lnX^= 6.3404+------+ln --- + (1--- ) (AV.l) 8.314*296 248 248 and for nitrogen, 53*(10.6 - 17.I)2 53 53 -lnx!:= 6.4378+------+ln --- +(1----) (AV.2) 8.314*296 248 248 The calculated results are in molar fraction and need to be converted to (mol/m3-Pa) using the equation below, S(mol/m3-Pa) = M1*P1*106/101325, (AV.3) where p^ - Density of dibutyldigol, g/ml; - Molecular weight of dibutyldigol The density of dibutyldigol at 20°C is 0.885 g/ml. The densities of dibutyldigol at elevated temperatures are calculated according to Benson's equation [Perry and Chilton, 1973], -257- APPENDIX V. CALCULATION ON FACILITATED TRANSPORT. (pl>l T1 -,0.29 (AV.4) 2 J where subscript 1 and 2 refer to two temperatures and T is c the critical temperature of dibutyldigol which is calculated according to Eduljee's additive contribution equation [Perry and and Chilton, 1973], T, T = ------(AV.5) C SAt/100 where are additive components, whose value for carbon is -55.32, for hydrogen 28.32 and for oxygen 1.59 [Perry and Chilton, 1973]. The calculated critical temperature is T = c 640.4 ° K and the calculated densities of dibutyldigol at different temperatures are listed in Table AV.l. Table AV.l. Calculated densities of dibutyldigol. Temperature, ° K 296 318 348 368 Density, g/ml 0.883 0.866 0.842 0.825 Then the calculated solubilities are listed in Table AV.2. Table AV.2. Calculated solubilities of oxygen and nitrogen. Temperature, ° K 296 318 348 368 Solubility of oxygen, mol/m3-PaxlO'5 9.75 9.93 10.09 10.26 Solubility of nitrogen, mol/m3-PaxlO"5 5.54 5.79 6.06 6.20 -258- APPENDIX V. CALCULATION ON FACILITATED TRANSPORT. The diffusivity is calculated according to equation (3.5). For example, the diffusivity of 1,4-dimethoxy-9,10- diphenylanthracene at 23°C is computed by, (218.3)0*5 x296 D 7.4x10 lT7G (AV.6) MX(V ) 1 where /x is viscosity of dibutyldigol in cp and V is the molar volume of the solute at normal boiling point in cm3/ mol. The viscosity of dibutyldigol is 2.39 cp [Curme and Johnston, 1952]. The viscosities at elevated temperatures are calculated using the generalized chart [Perry and Chilton, 1973] and the results are given in Table AV.3. Table AV.3. Calculated viscosities of dibutyldigol. Temperature, ° K 296 318 348 368 Viscosity, cp 2.36 1.90 1.40 1.20 The molar volume of 1,4-dimethoxy-9,10-diphenylanthracene at normal boiling point is computed using the structural contribution method [Perry and Chilton, 1973]. The calculated result of the molar volume is 440.3 cm3/mol. The diffusivities of the carrier at different temperatures are listed in Table AV.4. -259- APPENDIX V. CALCULATION ON FACILITATED TRANSPORT. Table AV.4. Calculated diffusivities of the carrier. Temperature, ° K 296 318 348 368 Diffusivity of the carrier, in /secxlO” 1 0 3.55 4.74 7.04 8.67 Similarly, the diffusivities of oxygen and nitrogen are computed and the computation results are listed in Table AV.5. Table AV.5. Calculated diffusivity of oxygen and nitrogen. Temperature, ° K 296 318 348 368 Diffusivity of oxygen, m2/secxlO" 1 0 19.59 26.14 38.83 47.50 Diffusivity of nitrogen, m2/secxlO" 10 17.40 23.23 34.49 42.55 Considering the porosity (f=0.75) and tortuosity (r=V2), all the diffusivities need to be multiplied by a factor ($■/?■). The results of the diffusivity are given in Table 5.2. Equation (5.8) can be modified to, °a[A]t Ph'pj F (AV.7) Ph" Pi L The term inside the square brackets can be considered as the overall permeability including the facilitated transport and the term SD as the Henry's law contribution. D The augmentation factor will be, -260- APPENDIX V. CALCULATION ON FACILITATED TRANSPORT. DA [a1t G (AV.8) It can be seen from the above two equations that the Henry's law diffusion depends on the pressure drop across the membrane but the facilitated transport is independent of the pressure. The permeability contributed by Henry's law, on the other hand, is independent of the pressure but the permeability contributed by the facilitated transport depends on the pressure. The calculated results listed in Table 5.2 are based on the fact that the upstream is atmosphere absolute and the downstream is in vacuum, i.e. p is 101325*21% (Pa) = 21278.3 (Pa) and p is 0. h 1 -261- NOMENCLATURE. NOMENCLATURE. Unit name A - Area b - Hole affinity constant Pa" 1 C - Concentration mol/m3 C - Concentration of bound carrier mol/m3 A 0 C - Concentration of absorbed component mol/m3 D C - Concentration of adsorbed component mol/m3 H C ' - Hole saturation constant mol/m3-Pa H C - Concentration of oxygen in free form mol/m3 o C^ - Concentration of oxygen at t=0 mol/m3 C - Total concentration of carrier mol/m3 T C - Concentration in a chamber mol/m3 V D - Diffusivity m2 /sec D - Diffusivity of bound carrier AO m2/sec AO 2 D - Diffusivity of free carrier m2/sec D - Diffusivity of absorbed component m2 /sec D D - Diffusivity of adsorbed component m2/sec d - Depth of penetration of light micrometer p - Activation energy of diffusion J/mol E - Activation energy of permeation J/mol p F - Molar diffusion rate mol/m2 -s. f G - Fugacity of gas spicies mol/m3 f^L - Fugacity of gas spicies at hypotheticacl liguid condition mol/m3 G - Augmentation coefficient Dimensionless g - Free energy J/mol -262- NOMENCLATURE. AH - Heat of solution J/mol s AH^- Latent heat of evaporation J/mol AH^- Heat of mixing J/mol I - Light intensity Lumin/m2-sec J - Gas flux m3 /m2 -s K - A constant defined by eq.(2.23) Dimensionless - Henry's law dissolution constant mol/m3-Pa. K - Equilibrium constant of reaction Dimensionless e q k - Forward reaction constant m3/mol-sec" 1 k - Reverse reaction constant sec" 1 - i L - Thickness of membrane m M - Molecular weight gram/mol n - Refractive index Dimensionless P - Permeability cm3(STP)-cm/cm2-s-cmHg P - Permeation coefficient in SI units mol-m/m2-sec-Pa in P - Permeation coefficient in SI units S I m3 (STP)-m/m2-sec-Pa P - Transmmision rate cm3 ( STP)/cm2-s-cmHg 2 p - Partial pressure Pa. p^ - Partial pressure in upstream Pa. p^ - Partial pressure in down stream Pa. - Total amount of gas diffussed mol/m2 R - Mobility constant defined by eq.(2.24) Dimensionless r - radius of pores m S - Solubility mol/m3-Pa. T - Absolute temperature Kelvin -263- NOMENCLATURE. - Critical temperature Kelvin t - Time second V - Volume m 3 V - Molar volume m 3 X,Y - Molar fraction Dimensionless x, y, z - Coordinate in Cartesion m a - Separation factor Dimensionless p - Contact angle Radius 7 - Activity coefficient Dimensionless 7 - Surface tension N/m 6 - Solubility parameter of solvent MPa*s 8^ - Solubility parameter of solute MPai$ f - Porosity Dimensionless rj - Viscosity N-sec/m3 6 ~ Fractional thickness of dense layer Dimensionless B - Contact angle Radain A - Wavelength Nanometer n - Viscosity N-sec/m3 p - Chemical potential J/mol v - Frequency of vebration cm" 1 £ - Molar decadic extinction coefficient Litre/mol-cm p - Density Kg/m3 o - Time lag second r - Tortuosity Dimensionless -264- NOMENCLATURE. 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