Multilayered Pei-Based Films for Co2 Adsorption And

Home , Polyetherimide

MULTILAYERED PEI-BASED FILMS FOR CO2 ADSORPTION AND

DIFFUSION

A Thesis

Presented to

The Graduate Faculty of The University of Akron

In Partial Fulfillment

of the Requirements for the Degree

Master of Science

Jing Liu

May, 2013

MULTILAYERED PEI-BASED FILMS FOR CO2 ADSORPTION AND

DIFFUSION

Jing Liu

Thesis

Approved: Accepted:

Advisor Dean of the College Dr. Steven S.C. Chuang Dr. Stephen Z.D. Cheng

Co-Advisor or Faculty Reader Dean of the Graduate School Dr. Li Jia Dr. George R. Newkome

Department Chair Date Dr. Coleen Pugh

ABSTRACT

Polyethyleneimine (PEI), composed of the amine group on the repeating unit,

o o can bind CO2 at 50–75 C and dissociate it at 100–130 C. This unique characteristic

allows PEI to serve as an active molecule in solid sorbents for the thermal swing and

membrane processes for CO2 separation. The nature and concentration of CO2

adsorption on amine sites of PEI can be observed and investigated by Fourier

Transform Infrared Spectroscopy (FTIR) 15. In-situ FTIR was used to evaluate and

determine i) the CO2 adsorption on PEI layers containing 1, 8, 21 and 35wt% of water

and ii) the rate of diffusion of CO2 through multiple layers of PEI partially crosslinked

with glutaraldehyde (GA). Each sequential PEI layer was prepared by increasing the

degree of cross-linking with GA, generating a gradient amine concentration

throughout the multiple layers thickness. Initial results show an increase of CO2

adsorption at a lower concentration of water. The multiple layered amine gradients

enhanced the rate of CO2 diffusion compared to a single layer of crosslinked PEI of

similar thickness. The fundamental information obtained from this study will be

used to fabricate multilayer PEI-based films having rapid diffusion of CO2 and

enhanced CO2 capture capacity at moderate temperatures.

iii

ACKNOWLEDGEMENTS

I would like to express my deepest appreciation to my research advisor,

Professor Steven S.C. Chuang, who continually and convincingly gives me guidance,

supervision and help throughout my master studies.

I would like to give my high gratitude to my friends and colleagues, Mr. Mathew

Isenberg, Miss. Shirin Norooz Oliaee, Miss. Uma Tumuluri, Mr. Ernesto Silva Mojica,

Mr. Chris Wilfong and Mr. Nader Hedayat. I acquired many important skills

including experimental skill, data analysis capacity and writing ability from them.

And thank all of them for encouraging me when I felt confused or un-confident during

research process.

TABLE OF CONTENTS

Page

LIST OF FIGURES ...... viii

LIST OF TABLES ...... x

CHAPTER

I. INTRODUCTION ...... 1

II. LITERATURE REVIEW ...... 3

2.1 CO2 Removal ...... 3

2.2 CO2 membranes ...... 4

2.2.1 Membrane Materials for CO2 Removal ...... 4

2.2.2 Membrane Permeation ...... 6

2.2.3 Membrane Structure...... 9

2.2.4 PEI-based membranes...... 11

2.3 Layer-by-layer assembly technology ...... 14

III. EXPERIMENTAL SECTIONS ...... 18

3.1 Materials ...... 18

3.2 FTIR study ...... 19

3.3 Water effects on high MW PEI films ...... 20

3.3.1 H2O adsorption to PEI films ...... 20

3.3.2 H2O desorption from PEI films...... 21

v 3.3.3 CO2 adsorption on PEI/H2O films ...... 22

3.4 Water effects on Low MW PEI ...... 22

3.5 Crosslinked PEI/GA films...... 23

3.5.1 Single layer of crosslinked PEI/GA film ...... 23

3.5.2 CO2 adsorption on single layer of crosslinked PEI/GA film ...... 23

3.5.3 Multilayer of crosslinked PEI/GA film ...... 24

3.5.4 CO2 adsorption on multilayer of crosslinked PEI/GA film ...... 25

IV. RESULTS AND DISCUSSION...... 26

4.1 Water effects on high MW PEI ...... 26

4.1.1 Water adsorption to PEI film ...... 26

4.1.2 Water desorption from PEI film ...... 27

4.1.3 CO2 adsorption on PEI/H2O film ...... 30

4.2 Water effects on low MW PEI ...... 32

4.3 Crosslinked PEI ...... 35

4.3.1 Single layer of PEI/GA film...... 35

4.3.2 CO2 adsorption on Single layer of PEI/GA film ...... 39

4.3.3 Multilayer of crosslinked PEI/GA film ...... 41

4.3.4 CO2 adsorption on multilayer of crosslinked PEI/GA film ...... 44

4.3.5 CO2 diffusion through PEI-based films ...... 47

V. CONCLUSIONS...... 50

REFERENCES ...... 51

APPENDICES……………………………………………………………………….55

vi APPENDIX A. ESTIMATION OF WATER CONTENT DURING WATER DESORPTION...... 56

APPENDIX B. WATER RESISTIVITY OF PEI/GA FILMS ...... 59

APPENDIX C. LOW Mw PEI THIN FILM ...... 63

vii

LIST OF FIGURES

Figure Page

1 The multilayered PEI/GA film with amine concentration gradient ...... 2

2 SEM photographs of cross-section of the asymmetric polyimide membranes .. 10

3 The MFI zeolite structure ...... 13

4 The fabrication process of the PVPy/PVPh LBL membrane ...... 16

5 The process of taking IR scan for PEI/GA films ...... 20

6 (a) IR Single beam spectra and (b) absorbance spectra of PEI film during the first minute water bubbling in an enclosed system...... 27

7 (a) IR Single beam spectra and (b) absorbance spectra of PEI film during water evaportation in an ambient environment...... 28

8 Absorbance of the PEI film during water evaporation from 30oC to 200oC in the ambient condition...... 29

9 Absorbance of PEI/H2O spectra with different water content...... 30

10 Absorbance of PEI/H2O spectra at initial state and after 5min purge...... 31

11 Absorbance of PEI/H2O spectra after 20min CO2 flowing...... 32

12 Absorbance of PEI/H2O spectra at initial state ...... 33

13 Absorbance of PEI/H2O spectra after 20min CO2 flowing ...... 34

14 PEI/GA solutions after being prepared (a) 5min, (b) 24h and (c) 48h...... 36

15 Absorbance of PEI/H2O spectra in an enclosed system...... 37

16 The intensity ratio of NH2/CH2 and C=N/CH2 versus molar ratio of C=O/CH2 . 39

viii 17 Absorbance of cross-linked PEI spectra with CO2 flowing at 75C ...... 40

18 Absorbance of crosslinked PEI spectra with CO2 at different temperatures...... 40

19 Photographs of PEI/GA films before and after washing...... 43

20 Absorbance of PEI/GA spectra (a) before and (b) after being washed (Ph=6).... 43

21 Absorbance of PEI/GA spectra before flowing CO2...... 44

22 Absorbance of PEI/GA spectra after flowing 15min CO2...... 45

23 The intensity ratio of C=N/CH2 for PEI/GA films ...... 47

24 The intensity ratio of 1573/CH2 for PEI and PEI/GA films...... 47

25 Peak intensity of absorbance spectra at 1573cm-1 ...... 48

26 Temperature change versus time in the FTIR and TGA measurements...... 55

27 Photographs of (a) the metal foil for IR and (b) the pan for TGA...... 56

28 TGA of PEI film from 30oC to 400oC with 10oC/min ...... 56

29 Absorbance of PEI/GA-4 spectra after washing with different washing PH...... 58

30 Absorbance of PEI/GA-6 spectra after washing with different washing PH...... 59

31 Absorbance of PEI/GA-8 spectra after washing with different washing PH...... 60

32 The crosslinking degree of PEI/GA films before and after washing...... 60

33 Absorbance spectra of low Mw PEI thin film ...... 61

LIST OF TABLES

Table Page

1 Examples of layer-by-layer PEI films...... 17

2 The information of materials ...... 19

3 The composition of crosslined PEI solution ...... 19

4 The composition of H2O/PEI solution ...... 22

5 Technical parameters of different films ...... 24

6 IR band assignments ...... 26

7 Peak intensities and intensity ratio of single layer film in Figure. 15 (a) ...... 38

8 Peak intensities and intensity ratio of single layer film in Figure. 15 (b) ...... 38

9 Averaged ratio of peak intensities ...... 39

10 The composition of multilayered PEI/GA films and related thickness...... 41

11 Peak intensities and ratios after CO2 flowing...... 46

12 Diffusion rates and thickness of films ...... 48

13 Water content of PEI film at different temperatures ...... 57

14 The physical characteristics of PEI/GA films ...... 60

CHAPTER I

INTRODUCTION

Polymer-based membranes recently has been used for CO2 capture and

sequestration in natural gas and hydrogen processing. To promote the commercial

applications of these CO2 separation membranes, approaches of reducing the cost of

membrane separation technology and increasing the CO2 flux through membrane has

been intensively investigated.

The CO2 flux through the polymer-based membranes can be explained by the

P × ∆ρ Fick’s Law ( J = ) which is related to membrane-dependent portion (P/l ) and l a process-dependent portion ( ∆ρ ). The high differential pressure ( ∆ρ ) can be achieved by applying compressors or vacuum pumps, however increasing the

operational costs of membranes and limiting the commercial applications. To obtain

the high CO2 flux even at low differential pressure, the hypothesis of introducing

chemical potential into membranes was applied in this report.

A multilayer of film was prepared by sequential casting PEI/GA cross-linked

layers with different degree of cross-linking. By adjusting the cross-linking degree,

an amine concentration gradient was created through the thickness of films. Figure.

1 illustrates the multilayer of PEI/GA film and the gradient of amine concentration across the film. The amine concentration is increased from the feed

1 side to the permeate side. The adsorbed CO2 at the feed side will be driven by higher amine concentration near the permeate side. Herein, the gradient of amine concentration is the chemical potential which drives the CO2 flux through the film.

Figure. 1 The multilayered PEI/GA film with amine concentration gradient

According to the CO2 adsorption reactions (1) (2), CO2 adsorption products are

different at dry and moist conditions. Besides, H2O can interact with amine groups

on PEI by forming hydrogen bonding. To lower the CO2 adsorption strength and

release CO2 much easier at the permeate side, the effect of water on CO2 adsorption

properties was studied in this report.

CHAPTER II

LITERATURE REVIEW

2.1 CO2 Removal

The increasing public concern over global warming is focused on the emission of greenhouse gases, especially carbon dioxide (CO2) emitted from the power plants, gas

processing industries, refineries, chemical and petrochemical industries, iron and steel

industries, and cement industries.1 Carbon dioxide, in combination with water, is

highly corrosive and destructive for pipelines and equipments if it cannot be partially

2 removed or exotic. Therefore, CO2 capture and sequestration are receiving significant attention and being recognized as a third option. Currently, a number of

CO2 gas removal technologies are under investigations, including chemical and/or

3 physical absorption, adsorption, membrane separation, and cryogenic distillation.

Compared to other CO2 capture technologies, the membrane separation has the

advantages of operational simplicity and low-energy consumption (there is no need of

chemicals or heating to regenerate an absorbent/adsorbent) , high reliability, and an

absence of moving parts (it can be retrofitted easily from end of flue gas streams

without complicated integration)9. Therefore, the membrane separation are

especially favored for applications that have large flows, high CO2 contents, or are in

remote locations 2. Although membranes have not yet been commercially used in

3 CO2 removal from coal-fired power emissions, they have already been applied in some applications including natural gas purification and hydrogen processing, and show promise for other applications such as metabolic CO2 removal from enclosed

breathing atmospheres, microalgal cultivation by capturing CO2 and CO2 transport

biological membranes4-8.

2.2 CO2 membranes

2.2.1 Membrane Materials for CO2 Removal

Currently, the commercially viable membranes used for CO2 removal are using

polymeric materials, such as cellulose acetate, polyimides, polyamides, polysulfone, polycarbonates, and polyetherimide. The Cellulose acetate membranes were initially

developed for reverse osmosis but are now the most rugged CO2 removal membrane

available. The polyimide membranes, initially used for hydrogen recovery, now are widely used for CO2 removal with modifications. However, both the cellulose

acetate and the polyimide based membranes suffer from low CO2 permeability and selectivity, because the CO2 separation on these membrane s are based on a

solution-diffusion mechanism and rely primarily on the subtle size differences of the

penetrants to achieve separation 10. -12. (The solution-diffusion mechanism will be

described later in the review). As an alternative to these polymeric membranes, facilitated transport membranes have been investigated to promote the CO2

permeability and selectivity 14. . Polyethyleneimine (PEI), a widely used polymer

4 material for the facilitated transport membranes, will be discussed in the later section.

Scheme. 1 shows the CO2 separation process in the facilitated transport membranes.

The principle of facilitated transport membrane of improving the gas selectivity and permeability is incorporating carrier agents (e.g amine or hydroxyl groups), which can react reversibly with the target gas component, into the membranes.

Scheme. 1 facilitated transport membranes

Here, I only discuss reaction of CO2 gas with the amine carrier which is abundant on the Polyethyleneimine (PEI) chains. According to the zwitterions mechanism, the reaction between CO2 and the amine groups can be described as following 13. :

If water is included, the products is bicarbonate and ammonium ion.

+ R1R2NH+ H2O + CO2 (aq) ↔ R1R2NH2 + HCO3- (1)

If water is not included, amine itself is considered as the base and the products is

5 carbamate and ammonium ion.

+ - R1R2NH + R1R2NH+ CO2 (aq) ↔ R1R2NH2 + R1R2NCO2 (2)

Based on the equations above, under the humid condition, the CO2 hydration

reaction in a water-swollen membrane would be enhanced in the presence of amino

groups, which work as weak base catalysts. While under the dry condition, amine

carriers required for CO2 capture is increased and as a result the CO2 capture

efficiency is lower than that in humid condition. Therefore, CO2 transport was

14. facilitated in the forms of carbamate and bicarbonate with the presence of water.

2.2.2 Membrane Permeation

Different from the filter which separates components by the molecular size, the

membranes used for CO2 removal operate on the basis of how well different

compounds dissolve into the membrane and how fast they diffuse through it. This

principle is called the solution-diffusion mechanism. For examples, carbon dioxide,

hydrogen, helium, hydrogen sulfide, and water vapor, permeate quickly and as a result

are called “fast” gases. Carbon monoxide, nitrogen, methane, ethane and other

2 hydrocarbons permeate less quickly and as a result are called “slow” gases.

Therefore, the membranes can selectively remove fast gases (e.g CO2) from slow gases (e.g CH4) using the permeation difference.

k × D × ∆ρ J = (3) l

Fick’s Law 2, 19. , described in the equation (4), is widely used to approximate the solution-diffusion process (CO2 are used as an example of the target gas):

Where J is the flux of CO2 through the membrane, k is the solubility of CO2 in

the membrane, D is the diffusion coefficient of CO2 through the membrane, ∆ρ is

the pressure difference of CO2 between the feed side (high pressure) and permeate

side (low pressure) of the membrane，and l is the membrane thickness.

To simplify this equation, the solubility k and diffusion coefficients D are

combined into a new variable called permeability (P) and the equation is changed

P × ∆ρ to J = . Therefore, the CO2 flux only depends on two factors: a l membrane-dependent portion (P/l ) and a process-dependent portion ( ∆ρ ). To achieve a high flux, either the ideal membrane materials or the enough pressure

difference will be needed.

The another important variable, selectivity (α), can also be deduced from the equation (3) :

P(CO ) α = 2 (4) P(other gases )

Where P(CO2) and P(other gases) are the permeabilities of CO2 and other

components in the flux gas stream, respectively. The selectivity is always used to

measure how much better the membrane permeates CO2 compared to the other

compounds.

Bot h permeability and selectivity are important considerations when selecting a

membrane. The higher the permeability, the less membrane area is required for a

given separation and therefore the lower the system cost. The higher the selectivity,

the lower the losses of hydrocarbons as CO2 is removed and therefore the higher the

7 volume of salable product. For a polymeric membrane to be economically attractive,

it should provide both high gas permeability and selectivity. Most study has focused

on setting high pressure difference to achieve a large driving force for CO2

permeation 9,10. . An example of the membrane separation system driven by the

pressure difference are shown in Scheme. 2: the flue gas is compressed and fed to a

membrane module 10. . During the process, the retentate gas remains more or less

on pressure. Hence, an expander is used to recover the energy contained in the

retentate stream. Finally, the permeated CO2 is recompressed to a pressure of 60

bar to meet uansport requirements.

10. Scheme. 2. Lay out of a single membrane stage

However, Minh T. Ho and co-workers found that the cost of membrane

separation will be increased because of the high costs of compressors which account

for over 50% of the capital and operating costs.17. ,18. In addition, they also showed that relative improvements in membrane permeability and selectivity resulted in only a small reduction of the capture cost. This is because the membranes material costs

8 are less than 10% of the total capital cost. They later reported 15. that the cost of

capturing CO2 using gas-separation membranes under vacuum conditions can be reduced by 35% and the cost can be even lower if the CO2 permeability reaches 300 barrer and CO2/N2 selectivity is above 250.

2.2.3 Membrane Structure

The structure and properties of separation membranes have received much

attention because of their importance in practical applications. The structure widely

used is the asymmetric structure which is expected to improve both of the

permeability and selectivity of membranes. This asymmetric structure consists of an

extremely thin nonporous layer mounted on a much thicker and highly porous layer.

Figure. 2 shows an example of the asymmetric structure 22. . The thin nonporous layer

is highly selective and the porous layer provides mechanical support and allows the

free flow of compounds that can permeate through the nonporous layer.

M. Niwa and co-workers 22. synthesized an asymmetric polyimide membrane

with an oriented surface skin layer by a dry-wet phase inversion process at different shear stresses. Interestingly, both the gas permeability and selectivity of the asymmetric membranes increases with an increase in the shear rate. The effect of surface thin layer on the membrane permeation was also discussed by Kawakami. H et al.50. They synthesized asymmetric membrane with an ultrathin skin layer and

spongelike porous layer and suggested the gas selectivity increases with a decrease in

9 the thickness of the surface skin layer, perhaps because the membrane with a thinner

surface skin layer forms a more packed structure.

Non-porous layer

porous layer

22. Figure. 2 SEM photographs of cross-section of the asymmetric polyimide membranes

To lower the fabrication costs of membranes, the nonporous layer and porous

layer are always prepared by different polymeric materials. The resulting membrane is called the composite membrane. This composites membrane incorporates a thin selective layer made of one polymer mounted on an asymmetric membrane composed of another polymer. The composite structure allows membrane manufacturers to use available materials for the asymmetric portion and to use specially developed polymers, highly optimized for the required separation, for the selective layer.

S ince the properties of the selective layer can be adjusted readily without

increasing membrane cost too significantly, composite structures are being used in

most of the newer advanced CO2 removal membranes. Gaurav Shil and

co-workers23. studied a series of composite membranes using polyethylenimine as

the interfacial selective layer and commercial polyetherimide and Matrimid as the

10 microporous supports. In this report, the membrane showed high CO2 /N2 selectivity at 100-140 °C under a suitable polyethylenimine concentration and solution immersion times.

The porous layer can also be fabricated by the mesoporous inorganic and the selective polymers are confined to the inorganic environment. This is also a very attractive system for gas separations. Koros and co-workers24. studied a variety of glassy polymers with inherently good diffusivity-based separation characteristics and found that the permselectivity in O2/N2 separation improved by 20-100% when the polymers were confined in the mesoporous silica environment. They suggests that specific and nonspecific energetic interactions between the polymer and silica modify the packing of the polymer to create a better material for sieving. For the inorganic materials, ordered meso- and microporous silica are intensively studied because of their unique advantages that their pore size can be fine-tuned in the 1.6-3.0 nm range

25. -27. , which can assist in the grafting of large polymeric molecules in the pores.

2.2.4 PEI-based membranes

Polyethyleneimine (PEI), which is composed of a large number of primary and secondary amine groups, exhibits the great ability for carbon dioxide adsorption.

The commercially available PEI are in linear and hyperbranched forms.

PEI first used in CO2 capture in the space life support applications was reported by Sunita Satyapal and co-works 31. . To enhance CO2 adsorption and desorption rates, PEI is always bonded to a high-surface-area, solid polymethyl methacrylate

11 polymeric support and this material also incorporated poly(ethylene glycol) (PEG).

After that, Various adsorbents, such as carbons molecular basket 34. , glass fibers 35. ,

metal oxides, and zeolites 20. ,21. , have been investigated to replace the polymeric

support. Xiaochun Xu 32. developed a nanoporous solid adsorbent MCM-41-PEI, serving as a "molecular basket" for CO2 in the condensed form. In addition, he

found that the loading of PEI into the MCM-41 pore channels significantly increased

the CO2 adsorption capacity and CO2 desorption rate compared with the pure

33. MCM-41 or pure PEI. R. Sanz prepared CO2 adsorbents by impregnating the

pore surface of SBA-15 mesoporous silica with different amounts of branched PEI.

These branched PEI-impregnated materials are very efficient for CO2 capture even at low pressure and after several adsorption-desorption cycles.

Parveen Kumar and co-workers 28. investigated the use of polyethyleneimine

(PEI) confined to the ordered mesoporous MCM-48 silica membranes for CO2

separation from N2. This PEI-modified MCM-48 membranes were highly N2/CO2

selective at room temperature in the presence of 2.6% water vapor, and the selectivities increased with CO2 concentration in the feed. Here, PEI were selected

because of its high affinity for acidic CO2 and high CO2 adsorption capacity but

29. ,30. negligN ible 2 adsorption capacity .

Among these inorganic support membranes, zeolitic membranes have gained

considerable attention during the last decade because of their high selectivity 21. .

Their small pore sized and narrow pore size distribution make the zeolites a unique

material for designing thin films, coatings and membranes. Besides, the high

12 thermal and chemical stability of these inorganic crystals make them ideal materials for use in high temperature applications. Different from other membranes, zeolites membranes separate gases based on molecular size and shape rather than the

solution-diffusion mechanism. Figure. 3 is an example of MFI zeolite structure 49. .

Figure. 3 The MFI zeolite structure 49. .

Small-pore zeolites, such as zeolite T, SAPO-34, and DDR, have pores size

similar than that of the CH4 molecule, but larger than the CO2 molecule. For that

20. reason, they are suitable for nature gas separation. Shiguang Li and co-workers

synthesized the zeolite membranes called SAPO-34 in flue gas treatment and found

that at a trans-membrane pressure drop of 138 kPa and at an atmospheric pressure in

the permeate side, a membrane had a CO2 permeation of 1.2×10-6 mol/m2·s·Pa with

CO2/N2 separation selectivity of 32 for a 50/50 feed at 22°C. Shuji Himeno and

co-workers21. synthesized the DDR-type zeolite membranes with 2-3 μm layers on

porous α-alumina tube supports. The best CO2/CH4 separation selectivity was

-7 2 achieved at 298 K with a CO2 permeance of 3×10 mol/m ·s·Pa.

13 Besides, PEI has been widely used for nonviral transfection in vitro and in vivo

and has a strong DNA compaction capacity with an intrinsic endosomolytic activity.36.

Due to the rich primary and secondary amine groups, PEI shows outstanding adsorption ability for heavy metals. A novel hybrid adsorbent D001-PEI for selective Cu(II) removal, reported by Yiliang Chen and co-workers37. , was fabricated

by immobilizing PEI nanoclusters within a macroporous cation exchange resin D001.

2.3 Layer-by-layer assembly technology

Fabrication of Layer-by-Layer (LBL) assembly polymeric membrane is based on

the alternate deposition of oppositely-charged polyelectrolytes on a substrate surface

including metal, silicone, glass, and inorganic/organic colloid. The attractive feature

of this technology is its ability to construct multilayers at the nanometer scale and to

integrate polymers of interest within a hierarchical porous architecture. Recently,

LBL assembly has been used to create novel nano- and microcapsules with tunable

and reversible permeability.38. The materials used can be small organic molecules

or inorganic compounds, macromolecules, bio-macromolecules such as proteins,

45. DNA, or even colloids.

The permeability of the polyelectrolyte microcapsules can be readily tuned by

factors such as layer number 39. , pH value 40. , ionic strength 5341. , and polarity of the

solution 42. . Bingbing Jiang and co-workers 43. fabricated the innovative amine-multilayered sorbents using LbL nanoassembly technology via alternate

14 deposition of the polyethylenimine and the oppositely-charged polymer (e.g.

polystyrene sulfonate or PSS). They found that these amine-multilayered sorbents

had much faster CO2 desorption rates compared to single-layered sorbents prepared

using the current impregnation approach. This report also mentioned that the CO2

capture capacity was enhanced by the strong polyelectrolyte, increased layer numbers

and improved distribution of polymers inside and outside the substrate. Jodie L and

co-workers40. discovered that the structure of porous polyelectrolyte multilayers

consisting of linear poly(ethylenimine) (LPEI) and poly(acryli c acid) (PAA)

exhibited dramatic differences with small changes in the pH value, yielding a series of

pore sizes from nanometers scale to micrometers scale and pore volume fractions from 0 to 77%. Besides, they thought that the high mobility of LPEI can produce the asymmetric structure of membranes.

The LBL nanoassembly structure mentioned above are organized by the static

electrostatic interaction between polyelectrolyte materials. Apart from the classical

electrostatic interaction, covalent bonding and hydrogen bonding are also utilized in the fabrication of LBL structures.

Weijun Tong and co-workers 44. reported the fabrication of poly(ethyleneimine)

microcapsules via the glutaraldehyde(GA)-mediated covalent LbL assembly, which utilized GA to cross-link the adsorbed PEI layer. These PEI/GA multilayers grew nearly linearly along with the layer numbers and their thickness was controlled at the nanometer scale. The resulting microcapsules can be employed for gene delivery, and utilized to anchor cell-binding ligands onto PEI molecules. Hongyu Zhang and

15 co-workers45. investigated the buildup of hydrogen-bonding-directed poly(4-vinylpyridine)/poly(4-vinylphenol) (PVPy/PVPh) multilayer film via LbL assembly of PVPy and PVPh from an ethanol solution. They found that the increased polarity of the adsorption solutions.resulted in a marked decrease of the amount of polymers adsorbed. Figure. 4 shows the fabrication processes of the

hydrogen-bonded PVPy/PVPh LBL membrane.

45. Figure. 4 The fabrication process of the PVPy/PVPh LBL membrane

Compared with multivalent weak hydrogen bonding interactions, covalently

bounding yields the higher stability due to the cross-linked networks and therefore are

not susceptible to disassembly under varying solution conditions (e.g., strongly acidic,

strongly basic, or high ionic strength solutions).46. However, covalent bonded

membranes are less sensitive to acid or base etching. In contrast, the hydrogen

bonded membranes may be destroyed easily since the hydrogen bonds are basically

pH or temperature sensitive.47. ,48. But the weak strength of hydrogen bonding can

be used for the biological and medical application in some cases, such as the d rug

16 degradation at certain PH value. Table. 1 summarizes technical methods, physical characteristics and applications of some reported LBL PEI films.

Table. 1 Examples of layer-by-layer PEI films

PEI Structures Substrate LBL Tech. Thickness Appli. ref. Mw

PEI/PDMS/ PDMS~0.74µm PEBA1657/ dip-coating CO2 separation 52. PEBA~0.5µm PDMS

MWNT/PEI [(PEI/MWNT) high strength 5 glass 70K dip-coating ~13nm 53. (PEI/PAA)]nGA CNTs n=20,t=1.5μm

(PEI /GA) 750 10 (PEI/GA) 750K ~30nm medicine, 10 MnCO3 dip-coating 54. mircrocapsue 25K (PEI25/GA)10 catalysis ~17nm

(PEI/PAA)nGA medicine, MnCO3 25K dip-coating ~1 nm /layer 55. microcapsule catalysis

silicon, medicine, (PEI/PAA) 25K dip-coating 1~7µm (total) 56. 30 ITO-glass catalysis

(PEI/PAA) polymer 60 ITO 25K dip-coating ~33 nm / layer 57. (PEI/PAA)90 electrolyte

(PEI/PAA)60 photo-voltaic ITO 25K dip-coating ~2µm (total ) 58. OEGDA device

PMMA PMMA 10K ~8-10nm/bilayer CO2 sorbents 42. (PEI/PSS)n

(HBPO-NO /PE quartz 2 25K dip-coating ~4.5nm/bilayer drug delivery 45. I)n /HBPO-NO2 slides

quartz biology, (PVPy/PVPh) 50K dip-coating ~3.4 nm/bilayer 44. n slide, CaF2 medicine

PEI/(PSS-b-PE 25K not mentioned protein ads. 59. G/PAH)21

CHAPTER III

EXPERIMENTAL SECTIONS

3.1 Materials

The chemical used in this study are listed in Table. 2. For the simplicity of

discussion, the polyethyleneimine (PEI) mentioned without special notes refers to the

high MW PEI (Mw=750,000). PEI-based films were casted on the substrate (metal

foil or CaF2 disk) by using PEI or PEI/glutaraldehyde (GA) solutions. 30, 10 and

0.5(w/v)% PEI solutions were prepared by diluting 50(w/v)% PEI solution with

deionized water. 92, 79 and 65(wt)% low MW PEI (Mw=600) solutions were

prepared by diluting 99(wt)% PEI solution with deionized water. 1.25(wt)% GA

solution was prepared by diluting 50(wt)% GA solution with deionized water.

The PEI/GA solutions were prepared by mixing 10(w/v)% PEI and 1.25(wt)%

GA solutions. The mixed solutions with varying weight ratio of PEI/GA are listed in

Table. 3. For example, the PEI/GA-8 solution with weight ratio of PEI/GA of 8 was

prepared by mixing 500ul 10(w/v)%PEI and 0.5g 1.25(wt)%GA solutions. The

molar ratio NH2/C=O in PEI/GA-8 was calculated as

NH W (PEI ) / 43 8 / 43 2 = = = 9.3 , where 43 is the molecular weight of unit C = O W (GA) * 2 /100 1* 2 /100

(—CH2—CH2—NH—) of PEI and 100 is the molecular weight of GA. According the values of NH2/C=O, the crosslinking degree of solutions can be ranged as

PEI/GA-8< PEI/GA-6

Table. 2 The information of materials Company Item Description Item # CAS #

Fluka Polyethyleneimine, 50 wt% sln, Mw=750,000 P3143 9002-98-9

Alfa Aesar Polyethyleneimine, 99% sln, branched, Mw=600 40527 9002-98-6

Alfa Aesar Glutaraldehyde, 50 wt% AAA10500-36 500

Table. 3 The composition of crosslined PEI solution

Solutions V (10(w/v)%PEI) W (1.25%GA) WPEI/WGA NH2/C=O

PEI/GA-8 500ul 0.5g 8 9.3

PEI/GA-6 375ul 0.5g 6 7.0

PEI/GA-4 250ul 0.5g 4 4.7

3.2 FTIR study

The Fourier Transform Infrared Spectroscopy (FTIR) (Nicolet-6700 FTIR spectrometer) was used to study the nature of PEI-based films and their CO2 adsorption properties. The film/substrate unit was placed in a DRIFT (Diffusive

Reflectance Infrared Fourier Transform) cell enclosed by a dome containing two ZnSe windows. Figure. 5 illustrates details of FTIR study for PEI/GA films. The IR spectra of PEI-based films were collected by 3 or 10 co-added scans with a resolution of 4 cm-1.

Figure. 5 The process of taking IR scan for PEI/GA films

3.3 Water effects on high MW PEI films

3.3.1 H2O adsorption to PEI films

The PEI film was casted on the metal cup (D=10.04mm) by using 5ul of 0.5%

PEI solution. H2O adsorption to PEI films was carried out by two steps: (i) pretreatment by heating PEI films at 90oC for 5min with Ar purge (150cc/min), (ii)

o H2O adsorption by bubbling H2O vapor over PEI films at 30 C. Scheme. 1 shows the saturated bottle used for water bubbling. The water vapor was transported by continuous Ar (150cc/min) flowing from the outlet of the saturated bottle to inlet of the DIRFT cell. The in-situ FTIR was operated during H2O adsorption. The IR spectra were collected by 10 co-adding scans, resolution of 4 cm-1, at a rate of 6 scans/min.

Inlet outlet

Scheme. 3 The saturated bottle for water bubbling.

3.3.2 H2O desorption from PEI films

The PEI film was casted on the metal cup (D=10.04mm) by using 5ul of 10%

PEI solution. H2O desorption from PEI films was carried out by two steps: (i) surface water evaporation by purging PEI films with Ar (40cc/min) at 30oC for 42min,

(ii) internal water evaporation by heating PEI films from 29oC to 200oC with the heating rate of 10 oC/min accompanied by Ar purge. The in-situ FTIR was operated during H2O desorption. The IR spectra were collected by 10 co-adding scans, resolution of 4 cm-1, at a rate of 6 scans/min.

3.3.3 CO2 adsorption on PEI/H2O films

Three kinds of PEI/H2O films were casted on the metal cup (D=10.04mm) by using 5ul of 10, 30 and 50% PEI/H2O solutions, respectively. CO2 adsorption on

PEI/H2O films was carried out by two steps: (i) Ar purge (22.5cc/min) over these

o films at 25 C for 5min, (ii) CO2 adsorption by flowing CO2 (22.5cc/min) over these

o films at 25 C for 20min. The in-situ FTIR was operated during purge and CO2 flowing. The IR spectra were collected by 10 co-adding scans, resolution of 4 cm-1, at a rate of 6 scans/min.

3.4 Water effects on Low MW PEI

Table. 4 The composition of H2O/PEI solution

H2O% 1% 8% 21% 35%

Weight of sample (g) 0.0023 0.0025 0.0029 0.0035

PEI wright (g) 2.3*10-3 2.3*10-3 2.3*10-3 2.3*10-3

Four kinds of PEI (Mw=600) films were casted on the metal cup (D=10.04mm) by using PEI/H2O solutions listed in Table. 4. CO2 adsorption was carried out by

o o flowing CO2 (22.5cc/min) over these films for 20min at 25 C or 75 C. The in-situ

FTIR was operated to during CO2 flowing. The IR spectra were collected by 10 co-adding scans, resolution of 4 cm-1, at a rate of 6 scans/min.

3.5 Crosslinked PEI/GA films

3.5.1 Single layer of crosslinked PEI/GA film

Three kinds of crosslinked PEI/GA films were casted on the Al foil (D=2.5cm) by using 100ul of PEI/GA-4, PEI/GA-6 and PEI/GA-8 solutions (Table. 3), respectively. Treatments of PEI/GA films include three steps: (i) drying films on the hot plate (70oC) for 5min, (ii) washing films in water (pH=2, 6 and 9) for 10min at room temperature, (iii) drying washed films on the hot plate (70oC) for 5min. The IR spectra of dried PEI/GA films before and after washing were collected by 3 co-adding scans, resolution of 4 cm-1.

3.5.2 CO2 adsorption on single layer of crosslinked PEI/GA film

Three kinds of crosslinked PEI/GA films were casted on the metal cup

(D=10.04mm) by using 10ul of PEI/GA-4, PEI/GA-6 and PEI/GA-8 solutions

(Table. 3), respectively. CO2 adsorption was carried out by three steps: (i) drying films by Ar purge (150cc/min) at room temperature for 10min, (ii) flowing 15% CO2

(22.5cc/min) at 25, 50 and 75oC for 3min in sequence, (iii) operating Temperature

Programmed Desorption (TPD) with batch during heating to 130oC and with purge during cooling down to the room temperature. The in-situ FTIR was operated to during CO2 flowing and TPD operation. The IR spectra were collected by 3 co-adding scans, resolution of 4 cm-1.

3.5.3 Multilayer of crosslinked PEI/GA film

The multilayered PEI/GA films were casted layer-by-layer on the Al foil

(D=2.5cm) by using PEI/GA-4, PEI/GA-6 and PEI/GA-8 solutions. The PEI film and the single layered PEI/GA film were used as the reference. The composition and drying time of these films are included in Table. 5.

Table. 5 Technical parameters of different films

Drying time Drying time a Films Composition NH2/C=O (Al foil) (CaF2)

PEI-1L 60ul PEI 6min 40min -

PEI/GA-1L 60ul PEI/GA-8 6min 40min 9.3

30ul PEI/GA-8 3min 20min PEI/GA-2L 7 30ul PEI/GA-4 3min 20min

20ul PEI/GA-8 2min 13min

PEI/GA-3L 20ul PEI/GA-6 2min 13min 7

20ul PEI/GA-4 2min 14min a. the averaged NH2/C=O of overall PEI/GA layers.

For the multilayered films, the next layer was casted over the previous layer which had already been dried on the hot plate (70oC). All of these films were washed in water (pH=6) for 10min and then dried on the hot plate (70oC) for 5min.

The IR spectra of dried films before and after washing were collected by 3 co-adding scans, resolution of 4 cm-1.

3.5.4 CO2 adsorption on multilayer of crosslinked PEI/GA film

The multilayer of crosslinked PEI/GA films were casted layer-by-layer on the

CaF2 (D=2.5cm) disk by PEI/GA-4, PEI/GA-6 and PEI/GA-8 solutions (Table.3).

The next layer was casted over the previous layer which had already been dried on the hot plate (70oC). The composition and drying time of PEI and PEI/GA films are summarized in Table. 5. Scheme.4 illustrates the structures of PEI and PEI/GA films casted on CaF2 disk. The CaF2 disk was placed on and bounded to the micro-cell by rubber tape to avoid gas leaking.

Scheme. 4 PEI and PEI/GA films casted on the CaF2 disk

CO2 adsorption was carried out by two steps: (i) purging film with Ar

o (150cc/min) at room temperature for 10min, (ii) flowing CO2 over the film at 50 C for

15min. The in-situ FTIR was operated to during purge and CO2 flowing. The IR spectra were collected by 10 co-adding scans, resolution of 4 cm-1, at a rate of 6 scans/min.

CHAPTER IV

RESULTS AND DISCUSSION

4.1 Water effects on high MW PEI

4.1.1 Water adsorption to PEI film

Table. 6 IR band assignments

wavenumbers(cm-1) assignment species 3700-3000 Hydrogen bonding Water 3355, 3280 NH stretching PEI 2933, 2810 CH stretching PEI

2360, 2349 CO2 gas phase CO2 1675 C=O/C=N Amide, nitride

1655 H2O bend Water 1650 C=N Imine + 1630 NH3 deformation Ammonium ion 1600 NH bend PEI 1575, 1488 (O=C=O)- stretching Carbamate 1460 CH bend PEI 1410 C-N stretching Carbamate, PEI 1310 NCOO- skeletal vibration Carbamate

Water adsorption was done by bubbling water vapor to dried PEI layers at room temperature. The changes of PEI single beam and absorbance spectra during the first minute of water bubbling were showed in Figure. 6. The band assignments for

PEI and adsorbed H2O have been summarized in Table. 6. The single beam spectrum of PEI film at 0min of water bubbling exhibits NH2 stretching bands at 3355 and 3280cm-1, sharp CH2 stretching bands at 2933 and 2808cm-1, and tiny water band

at 1655cm-1. Bubbling water to the PEI film results in a significant broadening of

-1 the NH2 and OH stretching bands in 3000-3800 cm region, suggesting the interaction of H2O molecules and amine species. An increase of water peak at

1652cm-1 indicates the increased water content in the PEI film.

3355 3280 2933 2808 1655 (a) 0.5

1min 0.8min 0.6min H2O 0.4min bubbling Single Beam (a.u.) 0.2min 0.1min 0min

(b) 0.5

1min 0.8min 0.6min H2O 0.4min bubbling

0.2min 0.1min Absorbance (a.u.) 0min

4000 3500 3000 2500 2000 1500 1000 -1 Wavenumber (cm ) Figure. 6 (a) IR Single beam spectra and (b) absorbance spectra of PEI film during the first minute

water bubbling in an enclosed system. Abs=log(I0/I) where I0 is the single beam spectrum of dried

PEI layer and I is the single beam spectra during bubbling H2O.

4.1.2 Water desorption from PEI film

Water desorption experiment was done by continuously purging liquid PEI films with Ar. The changes of PEI single beam and absorbance spectra during water evaporation at room temperature were showed in Figure. 7. The results show the

opposite situations compared with those in the water adsorption experiment. NH2 stretching bands was narrowed during water evaporation, suggesting the break of hydrogen bonding between NH species and water molecules. The decreased intensity of water peak at 1652cm-1 indicates lose of water from the PEI film.

According to these results, water adsorption and desorption on PEI films are reversible. 3280 1652 0.5

10min 5min 3min H O 2min 2 1.5min evap. 1min

Single Beam (a.u.) 0min 0.5 10min 5min 3min 2min H O 2 /I) (a.u.) 0 1.5min 1min evap. log(I 0min

4000 3000 2000 1000 -1 Wavenumber (cm )

Figure. 7 (a) IR Single beam spectra and (b) absorbance spectra of PEI film during water

evaportation in an ambient environment. Abs=log(I0/I) where I0 is the single beam spectrum of liquid

PEI layer and I is the single beam spectra during evaporating H2O.

The PEI spectrum of 1min water bubbling in Figure. 6 and PEI spectrum of 0min water evaporation in Figure. 7 are different in the 3000-3800 cm-1 region. It results from the different water concentrations in PEI films. Actually, the water

concentration of 0min water evaporation (~90%) was much higher than that after

1min water bubbling.

NH2 asym. NH2 sym. shoulder C-H str. CH2 C-H+N-H N-H bend CH2 bend CH. C-N str

1.6 3352 3278 3178 2935 2881 2816 1675 1601 1459 1322 1089

1.4

1.2 200oC 170oC 1.0 160oC 0.8 o 150 C 0.6 30oC

log(1/sb)(a.u) 0.4

0.2

0.0

-0.2 4000 3000 2000 1000 Wavenumber (cm-1)

Figure. 8 Absorbance of the PEI film during water evaporation from 30oC to 200oC in the ambient condition. Abs=log(1/I) where I is the single beam of PEI spectra at different temperatures.

The changes of PEI spectra during water evaporation at high temperatures were showed in Figure. 8. From 30oC to 150oC, the shape of PEI spectra show little change because large amount of water had already been evaporated at room temperature before. However, the intensities of amine species at 3352, 3278 and

1601cm-1 is lowered during heating, illustrating the losing of water. Above 160oC, the decreased intensities of CH2 bending band, CH2 stretching band and NH2 stretching band were accompanied with the increment of sharp peak at 1675cm-1.

According to Srikanth's report 15, the peak around 1670cm-1 belongs to the amide and imide species which result from the oxygen degradation of secondary amines. The

water concentration of the PEI film at each temperatures estimated by TGA data are included in the Appendix Section (A).

4.1.3 CO2 adsorption on PEI/H2O film

The spectra of PEI film with water content of 50, 70 and 90% are showed in

Figure. 9. The intensity of water peak at 1648cm-1 of 90% water content was larger than that of 50 and 70% water content. OH stretching band shifted from 3606cm-1 to

-1 3450cm when water content was reduced from 90% to 50%. CH2 stretching band at 2953 and 2852cm-1, and CH bending band at 1463cm-1 was overlapped by water when water content was 90%.

2.8 3606 3450 2953 2852 1648 1463 2.6 2.4 water% 2.2 2.0 50% 1.8 1.6 1.4 1.2 70% 1.0 log(1/sb) (a.u.) 0.8 0.6 90% 0.4 0.2 0.0 -0.2 4000 3000 2000 1000 -1 Wavenumber (cm ) Figure. 9 Absorbance of PEI/H2O spectra with different water content in an enclosed system.

Abs=log(1/I) where I is the single beam of PEI spectra.

Since the water content of raw PEI solution are 50%, the casted PEI films were purged with Ar to achieve low water content. Figure. 10 illustrates the spectra of PEI

films before and after 5min Ar purge. Both the spectra of 50 and 90% H2O/PEI show the red shift in the 3000-3800 cm-1 region, indicating loss of water from PEI films. The blue shift in this region for 70% H2O/PEI might be caused by the gas leaking and as a result water vapor in the air could be adsorbed to the film. Besides,

-1 the overlapping of CH2 band at 2845cm indicates the water adsorption on 70%

H2O/PEI film.

initial state and after 5min purge

3466-3402 1652 2845 5min Ar 3 initial,50% H O 2

3558-3582 5min Ar 2 initial,70% H O 2

3590-3463 5min Ar 1 log(1/sb)(a.u) initial, 90% H O 2

4000 3000 2000 1000 Wavenumber (cm-1)

Figure. 10 Absorbance of PEI/H2O spectra at initial state and after 5min purge in an enclosed system.

The purged PEI film was then flowed with pure CO2 for 5min at room temperature in an enclosed system. The absorbance spectra of CO2 adsorption in

Figure. 11 exhibits the increased intensity of carbamate species at 1578, 1411 and

-1 1286cm . The IR bands of gas CO2 are observed at 2342cm-1 with overtones at

-1 3727 and 3614cm . Much higher CO2 adsorption was observed in 90% H2O/PEI

film. To simplify the later discussion of CO2 adsorption bands, the spectra range is divided into three regions: A-branch from 4000 to 2800 cm-1, B-branch from 2800 to 2100 cm-1 and C-branch from 1680 to 1200 cm-1.

CO2 ads. after 5min Purge

2.5 3727 3614 2342 1578 1411 1291

2.0 initial water% 50% 1.5

70%

1.0 Absorbance(a.u) 90% 0.5

0.0 4000 3000 2000 1000 Wavenumber (cm-1)

Figure. 11 Absorbance of PEI/H2O spectra after 20min CO2 flowing in an enclosed system.

Abs=log(I0/I) where I0 is the single beam spectra of purged PEI layer and I is the single beam spectra

after 20min CO2 flowing.

4.2 Water effects on low MW PEI

To study the CO2 adsorption properties on the PEI with low water concentrations, low molecular weight (MW 600) PEI (99%) was utilized. To control the same amount of amine sites, each film had the same weight of pure PEI. Figure. 12 illustrates the absorbance spectra of PEI films with 1, 8, 21 and 35% water

-1 concentrations. NH2 stretching bands at 3365 and 3300cm were observed for 1, 8 and 21% H2O/PEI films but overlapped by water for 35% H2O/PEI film. Water band at 1653cm-1 were not obvious for all the films

abs. of PEI before CO2 at 75C

1.8 3365 3300 1653 1603 1528 1461 water%

1.6 1% 1.4

1.2 8%

1.0 21% 0.8

log(1/sb)(a.u) 0.6 35% 0.4

0.2

0.0

-0.2 4000 3000 1000 Wavenumber (cm-1)

Figure. 12 Absorbance of PEI/H2O spectra at initial state

Comparing with the spectra of high MW PEI (Figure. 9), CH2 stretching bands around 3000-2500cm-1 in Figure. 12 did not show the doublet features. The large thickness (23μm) of these films might be responsible for the disappearance of CH2 stretch features. Figure. 33 (Appendix.C) shows the low Mw PEI film with small thickness (6μm), which exhibits apparent doublet feature at CH2 stretch bands.

Figure. 13 shows absorbance spectra of PEI films after 20min CO2 flowing.

The increased intensity of bands at C branch indicates the formation of CO2 adsorbed species (Figure. 13a). An increase of CO2 adsorption was observed at lower water concentration (Figure. 13b), which might be explained that amine sites occupied by water is no longer available for CO2.

20min CO2 ads. at 75C 1407 (a) 3363 3300 1655 1528 1461 water% 0.2 1% 8% 21% 35% log(1/sb)(a.u)

0.2 (b) water% 1% 8% 21%

log(I0/I) (a.u) log(I0/I) 35%

4000 3000 2000 1000 Wavenumber (cm-1)

Figure. 13 Absorbance of PEI/H2O spectra after 20min CO2 flowing in an enclosed system.

Abs=log(1/I) and Abs=log(I0/I) where I0 is the single beam spectra of initial PEI layer and I is the

single beam spectra after 20min CO2 flowing.

This conclusions are contrast to that using high MW PEI which obtain higher

CO2 adsorption with higher water content. One suggested reason is that the amine

sites were still abundant for CO2 adsorption in PEI film with high water content.

Because of the high molecular weight, PEI chains might be entangled badly and amine sites were blocked in the polymer chains. When in the water environment,

PEI chains can be relaxed and stretched, exposing more amine sites to CO2. Scheme.

5 illustrates the condition of PEI chains in the water conditions and dry conditions.

However for the low MW PEI chains, the amine sites are not as much as those on high MW PEI. Therefore, the water adsorption occupy most of amine sites and inhibit the CO2 adsorption.

Scheme. 5 Water induced PEI chains (hydrate) and randomly tangled PEI (melt) 51

4.3 Crosslinked PEI

4.3.1 Single layer of PEI/GA film

The single layer of PEI/GA film was prepared by casting PEI/GA solutions on the Al foil. Figure. 14 shows the PEI/GA solutions after being prepared 5min, 24h

and 48h. The solutions almost transparent once they were prepared (Figure. 14 (a)) and became deeper after 24h (Figure. 14 (b)) and 48h (Figure. 14 (c)).

For the three crosslinked solutions, the deepest color was observed in PEI/GA-4 solution which has the highest degree of crosslinking. This color was resulted from the imine species formed in the crosslinking reaction.

(a)

(b)

(c)

Figure. 14 PEI/GA solutions after being prepared (a) 5min, (b) 24h and (c) 48h.

For each of single layered PEI/GA film, the IR spectra was collected twice individually. Figure. 15 shows the absorbance spectra of PEI/GA films with the molar

ratio NH2/C=O of 4.7, 7.0 and 9.3. Figure. 15(b) includes the spectra of PEI film with the similar thickness. Different from spectrum of PEI film, C=N band at 1660cm-1 was observed in spectra of PEI/GA films. The appearance of imide species results form the crosslinking reaction between amine and aldehyde groups. Simultaneously,

PEI/GA films had lower intensity of NH2 stretch bands than PEI film because of the consumption of NH2 group during crosslinking.

100ul soln. 3281 2948 1660

2.5 (a) NH /C=O= 4.7 2.0 2

1.5 NH /C=O= 7.0 2 1.0 log(1/sb) (a.u.) NH /C=O= 9.3 2 0.5

0.0 2.5 (b) PEI

2.0 NH /C=O= 4.7 2

1.5 NH /C=O= 7.0 2

NH /C=O= 9.3 1.0 2 log(1/sb) (a.u.)

0.5

0.0

4000 3000 2000 1000 Wavenumber (cm-1)

Figure. 15 Absorbance of PEI/H2O spectra in an enclosed system. Abs=log(1/I) where I is the single beam spectra of PEI/GA films.

The peak intensities of NH2 band, CH2 band and C=N band were measured from

Figure. 15(a) and Figure. 15(b), and were summarized in Table. 7 and Table. 8,

respectively. The ratio of peak intensities were calculated as well. For the same kind of film sample, the intensities for a certain band are different in these two measurements. For example, the intensities of NH2 band for 100ul PEI/GA were

0.5070 in Table. 7 and 0.3046 in Table. 8, respectively. This difference was resulted from the nonuniform of film thickness. To evaluate the crosslinking degree by IR spectra, values of intensity ratios were averaged in Table.9

Table. 7 Peak intensities and intensity ratio of single layer film in Figure. 15 (a)

Single NH2/ NH2 CH2 C=N C=N/ NH2/ layer C=O (3281) (2942) (1660) CH2 CH2

100ul 9.3 0.5070 0.9149 0.4600 0.503 0.554 PEI/GA 100ul 7 0.3455 0.9002 0.4123 0.458 0.384 PEI/GA 100ul 4.7 0.2171 1.0052 0.5391 0.536 0.216 PEI/GA 50ul - 0.5293 0.958 - 0.553 PEI

Table. 8 Peak intensities and intensity ratio of single layer film in Figure. 15 (b)

Single NH2/ NH2 CH2 C=N C=N/C NH2/ layer C=O (3281) (2942) (1660) H2 CH2 100ul 9.3 0.3046 1.1823 0.3047 0.258 0.258 PEI/GA 100ul 7 0.1266 1.2595 0.4497 0.357 0.101 PEI/GA 100ul 4.7 0.3342 1.2135 0.8976 0.740 0.275 PEI/GA

The relationships between molar ratio (C=O/NH2) and peak ratios (C=N/CH2 and NH2/CH2) were plotted in Figure. 16. The reduced peak ratio of C=N/CH2 and

the increased peak ratio of NH2/CH2 can be observed going with the higher C=O/NH2 values. These correlations can be explained by the crosslinking reaction between

C=O and NH2 groups. That is, the higher degree of crosslinking, the less amount of unreacted NH2 species left and larger amount of imine group (C=N) formed.

Table. 9 Averaged ratio of peak intensities Averaged ratio of peak intensities

Single layer C=O/NH2 C=N/CH2 NH2/CH2 100ul PEI/GA 0.108 0.378 0.406 100ul PEI/GA 0.143 0.408 0.242 100ul PEI/GA 0.213 0.638 0.245 50ul PEI 0 0 0.553

0.7 0.8 0.6 0.6 0.5 0.4 0.4 0.3

NH2/CH2 0.2 0.2 C=N/CH2 0.1 0.0

0.0 -0.2 -0.05 0.00 0.05 0.10 0.15 0.20 0.25 -0.10 -0.05 0.00 0.05 0.10 0.15 0.20 0.25

C=O/NH2 C=O/NH2

Figure. 16 The intensity ratio of NH2/CH2 and C=N/CH2 versus the molar ratio of C=O/CH2

4.3.2 CO2 adsorption on Single layer of PEI/GA film

To study the effect of crosslinking degree on the CO2 adsorption, each of single

o layered PEI/GA films were flowed CO2 at 25, 50 and 75 C in an enclosed system.

o Figure. 17 illustrates the spectra of PEI/GA films after being flowed 3min CO2 at 75 C.

-1 The consumption of NH2 band at 3261cm and formation of bands at C branch

indicate the CO2 adsorption to the crosslinked films. The largest CO2 adsorption was obtained by the PEI/GA film of NH2/C=O =7.0. C branch

0.30 3261 1573 1476 1413 1310

0.25

0.20 NH /C=O= 9.3 2

0.15 /I) (a.u.) 0 NH /C=O= 7.0 0.10 2 log(I

0.05

0.00 NH /C=O= 4.7 2

-0.05 4000 3000 2000 1000 -1 Wavenumber (cm )

Figure. 17 The absorbance of cross-linked PEI spectra with CO2 flowing at 75C in an enclosed

system. Abs=log(I0/I) where I0 is the single beam spectra of initial PEI layer and I is the single beam

spectra after 3min CO2 flowing.

CO2 adsorption at different temp.

0.60 1563 1476 0.55 M :M =9.3 0.50 PEI GA M :M =7.0 o 0.45 PEI GA 75 C M :M =4.7 0.40 PEI GA 0.35 M :M =9.3 0.30 PEI GA

/I) (a.u.) o 0 M :M =7.0 50 C 0.25 PEI GA M :M =4.7 0.20 PEI GA log(I 0.15 M :M =9.3 0.10 PEI GA o M :M =7.0 25 C 0.05 PEI GA M :M =4.7 0.00 PEI GA -0.05 4000 3000 2000 1000 -1 Wavenumber (cm )

Figure. 18 The absorbance of crosslinked PEI spectra with CO2 flowing at different temperatures.

Abs=log(I0/I) where I0 is the single beam spectra of initial PEI layer and I is the single beam spectra

after 3min CO2 flowing.

Figure. 18 shows the CO2 adsorption on PEI/GA films at different temperatures.

At each temperature, the CO2 gas was flowed for 3min. For all the crosslinked films,

o no CO2 adsorption was observed at 25 C. The CO2 adsorption on each PEI/GA film at 50oC is still much lower than that at 75oC.

4.3.3 Multilayer of crosslinked PEI/GA film

To create the gradient of amine concentrations across films, the crosslinked

PEI/GA solutions listed in Table. 3 were applied for preparing the multilayered films.

By adjusting the crosslinking degree of single layered PEI/GA film, the amine concentration can be varied through the thickness of multilayered film. To evaluate the CO2 adsorption properties of this kind of multilayered film, the single layer PEI and PEI/GA film were also prepared for comparison.

Table. 10 The composition of multilayered PEI/GA films and related thickness

Estimated Films Composition NH /C=O 2 Thickness PEI/GA-1L 60ul PEI/GA-8 9.3 6.11um 30ul PEI/GA-8 PEI/GA-2L 7 5.09um 30ul PEI/GA-4 20ul PEI/GA-8

PEI/GA-3L 20ul PEI/GA-6 5.14um 7 20ul PEI/GA-4

The composition of PEI and multilayered PEI/GA films are listed in Table. 10.

PEI/GA-1L, PEI/GA-2L and PEI/GA-3L presents the one layer, two layers and three layers of crosslinked films. To eliminate the effects of film thickness on CO2

adsorption properties, all the PEI/GA films have similar thickness (5.1~6.1um).

Since the largest CO2 adsorption was obtained by the PEI/GA film of NH2/C=O =7.0

(Figure. 17), the averaged values of NH2/C=O in PEI/GA-2L and PEI/GA-3L are controlled to 7.0.

Scheme. 6 demonstrates two mechanisms of cross-linking reactions taken place between PEI and GA: a) the reaction of the primary amine groups (-NH2) with aldehyde groups of GA and b) the reaction of secondary amine (-NH-) with aldehyde groups. The N-C bond, C-O band and -OH can be observed in the FTIR around

1415cm-1, 1203cm-1 and 3000cm-1 respectively. Since the crosslinking reactions between PEI and GA are reversible, it’s necessary to study the water resistance properties of PEI/GA films. Thus, the PEI/GA films were washed in water for

10min to see if these films can be dissolved by water or not.

(a)

(b)

Scheme. 6 The mechanisms of crosslinking reactions

Photographs in Figure. 19 (a) and (b) show that the PEI/GA film can be easily washed away from the Al foil but can not be dissolved completely in water.

Photographs in Figure. 19 (c) were for the fresh and washed PEI/GA films of 1 layer,

2 layers and 3 layers. The films washed off were then spread on the Al foil and dried

on the hot plate (70oC) for 5min. The IR spectra of washed films on the AL foil were collected individually (Figure. 20b).

(b)

(a)

(c)

PEI/GA-1L PEI/GA-2L PEI/GA-3L Figure. 19 Photographs of PEI/GA films before and after washing

3283 2936 1660 (a) 2 3Layers 2Layers

1 1Layer log(1/sb) (a.u.)

0 2 (b) 3Layers

2Layers 1 1Layer log(1/sb) (a.u.)

4000 3000 2000 1000 -1 Wavenumber (cm )

Figure. 20 Absorbance of PEI/GA spectra (a) before and (b) after being washed in the water (Ph=6).

Abs=log(1/I) where I is the single beam spectra of PEI/GA films.

The spectra changes of PEI/GA films before and after being washed are showed in Figure. 20 (a) and (b). For all the crosslinked films, the decreased intensity of C=N band at 1656cm-1 can be observed after washing, indicating the reversibility of crosslinking reaction. Compared with spectra of 2 layers and 1 layer PEI/GA film, spectra of the 3Layers film changed least after being washed. Therefore, the 3 layers film has stronger resistance to water solubility. The study of water resistance properties for PEI/GA films with different PH of washing environment are explained in Appendix Section (B)

4.3.4 CO2 adsorption on multilayer of crosslinked PEI/GA film

2.5 3352 3280 2937 2820 1655 3L PEI/GA

2.0 2L PEI/GA

1.5 1L PEI/GA

1.0

1L PEI 0.5 Absorbance (a.u.) 0.0

-0.5

-1.0 4000 3000 2000 1000 Wavenumber (cm-1)

Figure. 21 Absorbance of PEI/GA spectra before flowing CO2. Abs=log(1/I) where I is the

single beam spectra before flowing CO2.

Before flowing CO2, PEI film shows the highest intensities of NH2 band at

3284cm-1 because of its large amount of un-reacted amine groups. A small but sharp peak at 1653cm-1 observed in all the PEI/GA spectra were for C=N band (Figure. 21).

o 15% CO2 gas was continuously flowed to PEI/GA films for 15min at 50 C. From the absorbance spectra of CO2 adsorption (Figure. 22), two key findings can be concluded. First, the single layer of PEI film had the highest intensities of CO2 adsorption bands compared with any of PEI/GA films. Second, the PEI/GA film with 3 layers had higher intensities of CO2 adsorption species compared with 1 layer or 2 layers PEI/GA films.

CO2 adsorption (15min) 1310 1409 3286 2945 2814 1573 1482 (a) 1655 3L PEI/GA 1 2L PEI/GA 1L PEI/GA 1L PEI log (1/sb) (a.u.)

1 (b)

3L PEI/GA

2L PEI/GA /I) (a.u.) 0 1L PEI/GA

log(I 1L PEI

4000 3000 2000 1000 -1 Wavenumber (cm )

Figure. 22 Absorbance of PEI/GA spectra after flowing 15min CO2. (a) Abs=log(1/I) and (b)

Abs=log(I0/I) where I0 is the single beam spectra of layers before flowing CO2 and I is the single beam

spectra after CO2 flowing.

To analyze the CO2 adsorption properties quantitatively, the peak intensities of

PEI/GA spectra after flowing 15min CO2 were compared in Figure. 23. The peak

-1 intensity was normalized by the CH2 stretching band at 2937cm and presented as the peak ratio NH2/CH2, C=N/CH2 and 1573/CH2.

Figure. 23 illustrates the crosslinking conditions for each of PEI/GA films.

PEI/GA-2L and PEI/GA-3L have the similar values of C=N/CH2, which is consistent with same values of their molar ratio NH2/C=O (Table. 10). PEI/GA-1L that has the lowest crosslinking degree shows the lowest C=N/CH2 value.

Table. 11 Peak intensities and ratios after CO2 flowing

3278 2937 1653 1573

films NH2/CH2 C=N/CH2 1573/CH2

(NH2) (CH2) (C=N) (carbamate)

PEI 0.776 2.072 - 1.105 0.374 - 0.533

PEI/GA-1L 0.411 1.283 0.459 0.520 0.320 0.358 0.405

PEI/GA-2L 0.092 0.561 0.376 0.244 0.165 0.671 0.435

PEI/GA-3L 0.126 0.606 0.366 0.264 0.208 0.604 0.436

Figure. 24 shows the highest value of 1573/CH2 for PEI film, which is consistent with the conclusion that PEI film obtains the largest CO2 adsorption. Therefore, the

CO2 adsorption on crosslinked PEI film was lower than that on PEI film.

For 2layers and 3 layers PEI/GA films, the values of 1573/CH2 are almost same and a little bit higher than that of 1 layer PEI/GA film.

Figure. 23 The intensity ratio of C=N/CH2 for PEI/GA films

Figure. 24 The intensity ratio of 1573/CH2 for PEI and PEI/GA films

4.3.5 CO2 diffusion through PEI-based films Figure. 25 illustrates the relationship between intensity of band at 1575cm-1 and time. The diffusion rates of CO2 through films were summarized in Table. 12 For example, the rate of CO2 diffusion through the PEI film was calculated as

 -13 vd  .0 04 min3/ 13 3.  10 min . To determine how fast CO2 gas diffuse through

films, the peak intensity of adsorbed CO2 species versus time during flowing CO2 were plotted.

Composition Thickness

0.09 3L PEI/GA 5.14um 15min 0.08 1L PEI 12.23um

0.07 7min

0.06 5min 2L PEI/GA 5.09um 0.05 3min

0.04 1L PEI/GA 6.11um

0.03 abs. intensitypeak (a.u) 0.02

0.01

0.00 22 24 26 28 30 32 34 36 Time

Figure. 25 Peak intensity of absorbance spectra at 1573cm-1

Table. 12 Diffusion rates and thickness of films

Estimated diffution rate Layers thickness (um) (min-1)

PEI 12.33 13.3*10-3

PEI/GA-1L 6.11 2.7*10-3

PEI/GA-2L 5.09 5.7*10-3

PEI/GA-3L 5.14 8.0*10-3

Although the PEI film has the largest thickness (12.23um), it obtains largest diffusion rate (13.3*10-3min-1). The reason might be that the crosslinking network formed in PEI/GA films inhibit the CO2 diffusion. Comparing these three PEI/GA films, the PEI/GA film with 3 layers shows the largest diffusion rate (8.0*10-3min-1).

According Table. 10, PEI/GA films have the similar thicnkess, which weakens the effects of thickness on CO2 diffusion. Besides, the averaged degree of crosslinking for 2 layers and 3 layers PEI/GA film are same (NH2/C=O =7). Hence, the difference of diffusion rate between 2 and 3 layers was mainly affected by the amine concentration gradient. These results suggest that the gradient in multilayered film may improve the CO2 diffusion rate through films.

CHAPTER V

CONCLUSIONS

Polyethyleneimine (PEI) films with different water concentrations were prepared

and tested for CO2 adsorption. The results showed that water can improve CO2

adsorption in the high MW PEI films while inhibit CO2 adsorption in the low MW

PEI films. To fabricate the gradient multilayered PEI films, single layer of

crosslinked PEI/GA films were casted using the molar ratio NH2/C=O of 4.7, 7.0 and

o 9.3. By flowing CO2 at 75 C, the molar ratio NH2/C=O of 7 showed the highest CO2

adsorption. Comparing with the PEI/GA films, the PEI film obtains higher CO2

adsorption due to its large amount of amine sites. However, the water resistance of

3layers PEI/GA film was largely improved. Besides, the 3layers PEI/GA film shows fast CO2 diffusion rate compared with 1layer and 2layers films.

REFERENCES

1. Samanta, A.; Zhao, A.; Shimizu, G. K. H.; Sarkar,P.; Gupta. R. Ind. Eng. Chem. Res. 2012, 51, 1438-146, url

2. Dortmundt, D.; Doshi, K. Chemical engineering world, 2003, 38, 55-66, url

3. Ravanchi, M. T.; Sahebdelfar, S.; Zangeneh, F. T. Front. Chem. Sci. Eng. 2011, 5, 173-178, url

4. Yampolskii, Yuri. Macromolecules, 2012, 45, 3298-3311,url

5. Koros, W. J.; Fleming, G. K. J. Membr. Sci. 1993, 83,1-80. url

6. Xomeritakis, G.; Tsai, C.-Y.; Brinker, C. J. Sep. Purif. Technol. 2005, 42, 249-257, url

7. Abd, R.; Muhammad, S.; Cheng, L.H.; Xu, X.H.; Zhang, L.; Chen, H.L. Renewable and Sustainable Energy Reviews 15 (2011) 4002–4012, url

8. Blank, M. E.; Ehmke, H. J Physiol, 2003, 550. 419–429, url

9. Ho, M.T.; Allinson, G.W.; Wiley, D. E. Ind. Eng. Chem. Res. 2008, 47,1562-1568, url

10. Scholes, C. A.; Stevens, G. W.; Kentish, S. E. Fuel, 2012, 96,15-28, url

11. Meisen, A.; Shuai, X. Energy ConVers. Manage.1997, 38, S37-S42. url

12. Koros, W. J.; Mahajan, R. J. Membr. Sci. 2000, 175, 181-196, url

13. Gray, M. L. et al. Fuel Chemistry Division Preprints, 2002, 47, 63, url

14. Huang, J.; Zou, J.; Winston, Ho. W. S., Ind. Eng. Chem. Res.2008, 47,1261-1267, url

15. Srikanth, C. S.Chuang, S. S. C.; ChemSusChem. 2012, 5,1435-1442, url

51 16. Van Der Sluis, J. P.; Hendriks, C. A.; Blok, K. Energy Conversion and Management, 1992, 33, 429. url

17. Ho, M. T.; Allinson, G.; Wiley, D. E. Desalination, 2006, 192, 288. url

18. Ho, M. T.; Leamon, G.; Allinson, G.; Wiley, D. E. Ind. Eng. Chem. Res. 2006, 45, 2546. url

19. Ho, M.T.; Allinson, G.W.; Wiley, D. E. Ind. Eng. Chem. Res. 2008, 47,1562-1568, url

20. Li, S.G.; Fan, C.Q. Ind. Eng. Chem. Res. 2010, 49, 4399–4404, url

21. Himeno, S. J. Ind. Eng. Chem. Res. 2007,46, 6989-6997, url

22. Niwa,M.; Kawakami,H.; Kanamori, T.; Shinbo,T.; Kaito,A.; Nagaoka. S. Macromolecules, 2001, 34,9039-9044. url

23. Shil, G.; Winston Ho. W.S. Journal of Environmental Engineering and Management, 2006, 16, 233-241，url

24. Moaddeb, M.; Koros, W. J. J. Membr. Sci. 1997, 125, 143, url

25. Ogawa, M.; Masukawa, N. Microporous Mesoporous Mater. 2000, 38, 35. url

26. Yao, N.; Ku, A.Y.; Nakagawa, N.; Lee, T.; Saville, D. A.; Aksay, I. A. Chem. Mater. 2000, 12, 1536-1548, url

27. Klotz, M.; Ayral, A.; Guizard, C.; Cot, L. Sep. Purif. Technol.2001, 25, 71. url

28. Kumar, P.; Kim, S.; Ida, J.;Guliants, V. V. Ind. Eng. Chem. Res. 2008, 47, 201-208, url

29. Xu, X.; Song, C.; Anderson, J. M.; Miller, B. G.; Scaroni, A. W. Energy Fuels, 2002, 16, 1463.url

30. Xu, X.; Song, C.; Miller, B. G.; Scaroni, A. W. Fuel Process. Tech. 2005, 86, 1457.url

31. Satyapal, S.; Filburn, T.; Trela,J.; Strange, J. Energy & Fuels, 2001, 15, 250-255. url

32. Xu, X. C.; Song, C.S; Wincek, R.; Andrésen, J. M.; Miller, B. G.;Scaroni, A. W. Fuel Chem. Div. Prepr. 2003, 48, 162-163, url

52 33. Sanz, R.; Calleja, G.; Arencibia, A.; Sanz-Perez, E. S. Appl. Surf. Sci. 2010, 256, 5323-5328, url

34. Wang, D. X. et al., Energy Fuels, 2011, 25, 456–458, url

35. Li, P. Y; Ge, B. Q; Zhang, S. J; Chen, S. X; Zhang, Q. K; Zhao, Y. N. Langmuir, 2008, 24, 6567–6574, url

36. Peng, Q; Zhong, Z. L; Zhuo, R. X. Bioconjugate Chem. 2008,19,499–506, url

37. Chen, Y. L, Pan, B. C.; Li, H. Y.; Zhang, W. M.; Lv, L.; Wu, J. Environ. Sci. Technol. 2010, 44,3508–3513, url

38. Decher, G. Science,1997, 277, 1232. url

39. Antipov, A. A.; Sukhorukov, G. B.; Donath, E.; Moehwald, H. J. Phys. Chem. (B), 2001, 105, 2281.url

40. Lutkenhaus, J. L.; McEnnis, K; Hammond, P. T. Macromolecules, 2008, 41, 6047-6054, url

41. Ibarz, G.; Dahne, L.; Donath, E.; Moehwald, H. Advanced Materials, 2001, 13, 1324. url

42. Lvov, Y.; Antipov, A. A.; Mamedov, A.; Mohwald, H.; Sukhorukov, G. B. Nano Lett. 2001, 1, 125. url

43. Jiang, B.B; Kish, V; Fauth, D. J.; Gray, M. L.; Pennline, H. W.; Li, B. Y . International Journal of Greenhouse Gas Control, 2011, 5, 1170–1175, url

44. Tong, W. J.; Gao, C. Y; Mohwald, H. Polym. Adv. Technol. 2008, 19, 817–823,url

45. Zhang, H.Y.; Wang, Z.Q; Zhang, Y.Q; Zhang, X. Langmuir, 2004, 20 , pp 9366–9370, url

46. Hu, X.F; Ji, J. Biomacromolecules, 2011, 12, 4264–4271, url

47. Sukhishvili, S.A.; Granick, S. J. Am. Chem. Soc. 2000, 122, 9550. url

48. Sukhishvili, S.A.; Granick, S. Macromolecules, 2002, 35, 301. url

49. Welk, M. E. Microporous Zeolite Membranes and Their Potential for H2 Production, url

53 50. Kawakami,H.; Mikawa, M.; Nagaoka. S. Macromolecules, 1998, 31, 6636-6638, url

51. Hashida, T. et al., Polymer, 2003, 44, 1721–1724, url

52. Ren, X. L.; Ren, Ji. Z.; Li, H.; Feng, S. C.; Deng, M. C, International Journal of Greenhouse Gas Control, 2012, 8, 111-120, url

53. Olek, M.; Ostrander, J.; Jurga, S.; Möhwald, H.; Kotov, N.; Kempa, K.; Giersig, M, Nano Letters, 2004, 4, 1889-1895, url

54. Tong, W.; Gao, C.Y.; Möhwald, H, Polymers for Advanced Technologies, 2008, 19, 817-823, url

55. Tong, W.J.; Gao, C.Y.; Möhwald, H, Macromolecules, 2005, 39, 335-340, url

56. Lutkenhaus, J. L.; McEnnis, K.; Hammond, P. T, Macromolecules, 2008, 41, 6047-6054, url

57. Lowman, G. M.; Tokuhisa, H.; Lutkenhaus, J. L.; Hammond, P. T, Langmuir, 2004, 20, 9791-9795, url

58. Tokuhisa, H.; Hammond, P. T, Advanced Functional Materials, 2003, 13, 831-839, url

59. Cortez, C.; Quinn, J. F.; Hao, X. J.; Gudipati, C. S.; Stenzel, M. H. Davis, T. P.; Caruso, F, Langmuir, 2010, 26, 9720-9727

APPENDICES

APPENDIX A

ESTIMATION OF WATER CONTENT DURING WATER DESORPTION

A PEI film was casted on the surface of a TGA pan (D=10.04mm) by using 5ul of a 10% PEI solution. The TGA process involved two steps i) purging the film with

N2 (40ml/min) to partially evaporate the surface water of the PEI film at room temperature for 42min and ii) heating the film from 29oC to 400oC with a heating rate

of 10oC/min under Ar flowing. The weight change of PEI film was measured by the

TGA balance and recorded by the computer automatically.

400

o Heat to 400C o with 10 C/min C) o

(

. 200 emp t

100

Evaporate water at RT

0 0 10 20 30 40 50 60 70 80

time (min)

Figure. 26 Temperature change versus time in the FTIR and TGA measurements.

Figure. 26 shows the temperature change of the PEI film versus time during the

in-situ IR scanning and TGA measurements. Since the surface area of metal cup

56 (Figure. 27 a) and TGA pan (Figure. 27 b) are same, the rates of water evaporation at a

certain temperature in TGA and IR process are same. Therefore, the water content calculated from the TGA curve (Figure. 28) can be used to estimate water content

during IR process.

5mm 5mm

Figure. 27 Photographs of (a) the metal foil for IR and (b) the pan for TGA.

0.7

0.6

o o o 30 C 150 C 160 C o 0.5 200 C (starting

heating) )

g 0.4

t 0.3

weigh 0.2

0.1

0.0 0 50 100 150 200 250 300 350 400 temp. (oC)

Figure. 28 TGA of PEI film from 30oC to 400oC with 10oC/min

According to Figure. 28, the initial weight of 5ul of 10% PEI solution was 5.29mg.

The water content of PEI film at different temperatures are listed in Table.1 and the

57 corresponding calculation process are showed as following:

The initial weight of pure PEI solution: WPEI = 5.29 *10% = 0.529mg

For example, at 150 ° C , the weight of the PEI film was 0.592mg

W = 0 . 5 9m2 g− 0 . 5m2=g9 0 .m0 g6 3 H 2 0

W % = (0.063 / 0.592) *100% = 1 0 . 6 4 % H 2 O

Table. 13 Water content of PEI film at different temperatures

Temp.(oC) 30 90 150 160 170 180 190 200 Water content% 12.8 11.6 10.6 10.4 10.2 10.1 10.1 10.0

APPENDIX B.

WATER RESISTIVITY OF PEI/GA FILMS

3286 2943 2833 1655 1573 1467 3.0 NH /C=O= 4.7 2 ph=2 washed 2.5 fresh

.) u . 2.0 (a

) ph=6 washed b fresh

/s 1.5 1 ( og l 1.0 washed ph=9 fresh 0.5

0.0

1.0

ph=2 NH2/C=O= 4.7

washed .) 0.5 u . ph=6

) washed I

/ 0

I ( ph=9 washed og l 0.0

-0.5 4000 3000 2000 1000 -1 Wavenumber (cm )

Figure. 29 Absorbance of PEI/GA-4 spectra after washing with different washing PH. Abs=log(1/I)

and Abs=log(I0/I) , where I0 is the single beam spectra of PEI/GA-4 layer before washing and I is the single beam spectra of PEI/GA-4 layer after washing.

The single layer of crosslinked films PEI/GA-4, PEI/GA-6 and PEI/GA-8 were

casted on the Al foil. Each kind of PEI/GA films was washed in water (25ml) with

pH of 2,6 and 8 for 10min. The washed films were then dried on the hot plate (70oC) for 5min. The IR spectra were collected for the PEI/GA films before and after being

59 washed. Figure. 29 - Figure. 31 show the absorbance of fresh PEI/GA films and washed PEI/GA films with different pH of washing environments.

3286 2943 2812 1655 1573 1467 3.0 NH /C=O= 7 ph=2 2

2.5 washed ) . u . fresh (a

2.0

)

b ph=6 washed /s

1 1.5 fresh ( og

l 1.0 washed ph=9 fresh 0.5

0.0

1.0

ph=2 NH2/C=O= 7

)

. u . washed (a

ph=6

) 0.5 I /

I ( washed og l ph=9 0.0 washed

4000 3000 -1 2000 1000 Wavenumber (cm )

Figure. 30 Absorbance of PEI/GA-6 spectra after washing with different washing PH. Abs=log(1/I)

and Abs=log(I0/I) where I0 is the single beam spectra of PEI/GA-6 layer before washing and I is the single beam spectra of PEI/GA-6 layer after washing.

3286 2943 2812 1655 1573 1467 3.0 NH /C=O= 9.3 2

2.5 ph=2 washed fresh

.) 2.0 u

. washed (a ph=6

) fresh

b 1.5 /s 1

(

l 1.0 washed ph=9 fresh 0.5

0.0

1.0

ph=2 .) NH /C=O= 9.3

u 2 . (a

) washed I

/ 0.5 ph=6 0

( washed og

l ph=9 0.0 washed

4000 3000 2000 1000 Wavenumber (cm-1)

Figure. 31 Absorbance of PEI/GA-8 spectra after washing with different washing PH. Abs=log(1/I)

and Abs=log(I0/I) where I0 is the single beam spectra of PEI/GA-8 layer before washing and I is the single beam spectra of PEI/GA-8 layer after washing.

Table. 14 The physical characteristics of PEI/GA films

Crosslinking degree (C=N/CH2) Molar ratio Films Thickness (NH2/C=O) wash wash wash fresh ph=2 ph=6 ph=9 PEI/GA-4 4.7 3.6um 0.340 0.207 0.546 0.538 PEI/GA-6 7.0 3.7um 0.226 0.254 0.515 0.500 PEI/GA-8 9.3 3.6um 0.324 0.206 0.259 0.357

Figure. 32 The crosslinking degree of PEI/GA films before and after washing.

APPENDIX C.

LOW Mw PEI THIN FILM

3357 3290 2933 2816

0.8 1652 1603 1528 1459

0.6

) .

u . a

( 0.4

e c

n a

b r o

s 0.2

0.0

-0.2 4000 3000 2000 1000 Wavenumber (cm-1)

Figure. 33 Absorbance spectra of low Mw PEI thin film