Wright State University CORE Scholar
Browse all Theses and Dissertations Theses and Dissertations
2008
Poly(arylene ether)s with Truly Pendant Benzene Sulfonic Acid Groups
Mohamed Moustafa Abdellatif Wright State University
Follow this and additional works at: https://corescholar.libraries.wright.edu/etd_all
Part of the Chemistry Commons
Repository Citation Abdellatif, Mohamed Moustafa, "Poly(arylene ether)s with Truly Pendant Benzene Sulfonic Acid Groups" (2008). Browse all Theses and Dissertations. 877. https://corescholar.libraries.wright.edu/etd_all/877
This Thesis is brought to you for free and open access by the Theses and Dissertations at CORE Scholar. It has been accepted for inclusion in Browse all Theses and Dissertations by an authorized administrator of CORE Scholar. For more information, please contact [email protected].
POLY(ARYLENE ETHER)S WITH TRULY PENDANT BENZENE SULFONIC ACID GROUPS
A thesis submitted in partial fulfillment Of the requirements for the degree of Master of Science
By
MOHAMED ABDELLATIF B.S. Wright State University, 2006
2008 Wright State University
WRIGHT STATE UNIVERSITY
SCHOOL OF GRADUATE STUDIES
September 4, 2008
I HEREBY RECOMMEND THAT THE THESIS PREPARED UNDER MY SUPERVISION BY Mohamed Abdellatif ENTITLED Poly(arlyene ether)s with Truly Pendant Benzene Sulfonic Acid Groups BE ACCEPTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF Master of Science .
______Eric Fossum, Ph.D. Thesis Director
______Kenneth Turnbull, Ph.D. Department Chair
Committee on Final Examination
______Eric Fossum, Ph.D.
______Kenneth Turnbull, Ph.D.
______Daniel M. Ketcha, Ph.D.
______Joseph F. Thomas, Jr., Ph.D. Dean, School of Graduate Studies
ABSTRACT
Abdellatif, M. Mohamed M.S., Department of Chemistry, Wright State University, 2008. Poly(arlyene ether)s with Truly Pendant Benzene Sulfonic Acid Groups.
A series of new sulfonated poly(arylene ether)s containing one sulfonic acid group,
located on a pendant phenyl sulfonyl moiety, at every repeat unit was synthesized using
3-sulfonated-3’, 5’-difluorophenyl Sulfone, 4, and a variety of bisphenols. A
conventional nucleophilic aromatic substitution (NAS) was utilized for the
homopolymerization and copolymerization reactions. Polymerization reactions of 4 in
NMP at 180 oC provided the corresponding linear sulfonated poly(arylene ether)s, s-
PAEs, with number average molecular weight, Mn, ranging from 10,000 to 38,000
Daltons. All of the polymers were characterized by 1H and 13C NMR spectroscopy,
thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The Tg
values ranged from 264 oC to 296 oC for the salt form and 183 oC to 240 oC for the acid
o o o form. The Td5% values ranged from 311 C to 472 C for the salt form and 307 C to 367
oC for the acid form. All of the samples possessed excellent thermal stability.
Furthermore, higher accessibility of the sulfonic acid groups was evident by observing increased solubility in hot water, which is presumed to construct a more efficient conductive pass way, thus increasing the proton conductivity of the newly developed systems. The polymer samples also formed tough peelable films when DMSO solutions were slowly evaporated on a glass slide. These new sulfonated polymers possess good properties and are excellent candidates for proton exchange membrane in fuel cell applications.
iii Table of Contents
I. Introduction 1
Background Information 3
Alternative to Nafion 6
Sulfonated poly(arylene ether sulfone)s, s-PAES 7
Pendant Versus Backbone Substitution 12
Current Project 16
II. Results and Discussion 19
Synthesis of 3-Sulfonated-3’, 5’-difluorophenyl Sulfone, 4 19
Synthesis of 3-Sulfonated- 4, 4’-difluorophenyl Sulfone, 8 22
Homopolymer Synthesis and Characterization of 5a-d 24
Copolymer Synthesis and Characterization of 6 26
Thermal Analysis 33
Conclusions 38
III. Experimental 39
Instrumentation and Chemicals 39
Synthesis of 3,5-difluorophenyl Sulfone, 1 40
Synthesis of 3-sulfonated-3’,5’-difluorophenyl Sulfone, 4 41
Synthesis of 3-sulfonated-4,4’-difluorophenyl Sulfone, 8 42
iv General procedure for polymerization reactions of 5a-d 43
Procedure for copolymerization reaction with 6 44
IV. References 45
VI. Appendix 53
1H and 13C NMR spectra of 5b to 5d 53
v
List of Figures
Figure 1: Nafion structure…………………………………………………………...5
Figure 2: Generic scheme of sulfonated PAE ...……………………………………7
Figure 3: Examples of PAEs prepared from bis-phenol monomers designed to afford pendant sulfonic acid groups. Sulfonic acid groups are introduced on the pendant phenyl rings after the bisphenol is incorporated into the PAE..…………………………………………………………………………….12
Figure 4: Structure of sulfophenoxybenzoyl PSU (PSU-sphb), and sulfonaphthoxy- benzoyl PSU (PSU-snb) 55..……………………………………………...13
Figure 5: Representations of a rigid polyaromatic backbone with pendant side chains containing two sulfonic acid groups, and with a branched pendant side chain containing four sulfonic acid groups..………………………..15
Figure 6: Comparison of the effects of incorporating the proton conducting group on the backbone or in a truly pendant position…………………………………………17
1 Figure 7a: 300 MHz H NMR (D2O) spectrum of 4………………………………...20
13 Figure 7b: 75.5 MHz C NMR (D2O) spectrum of 4……………………………….21
1 Figure 8: 300 MHz H NMR (D2O) spectrum of 8………………………………...23
Figure 9: SEC traces of polymers at 38 weight % and 64 weight % concentrations for reactions 2 and 5……………………………………………………...29
Figure 10: Free standing films of the salt form of 5b unperturbed (left) and folded (right)………………………………………………………………….....30
vi
Figure 11a: 300 MHz 1H NMR of 4,4’-bisphenol homopolymer…………………….31
Figure 11b: 75.5 MHz 13C NMR of 4,4’-bisphenol polymer…………………………31
Figure 12a: 300 MHz 1H NMR of 4,4’-bisphenol copolymer………………………..32
Figure 12b: 75.5 MHz 13C NMR of 4,4’-bisphenol copolymes...…………………….32
Figure 13: Stacked DSC traces of polymers 5a-5d salt form………………………..33
Figure 14: Stacked DSC traces of polymers 5a-5d acid form……………………….34
Figure 15: Influence of polymer composition (salt form) on the TGA thermograms, under nitrogen at 10oC/min………………………………………………35
Figure 16: Influence of polymer composition (acid form) on the TGA thermograms, under nitrogen at 10oC/min………………………………………………36
vii List of Schemes
Scheme 1: Fuel Cell Schematic……………………………………………………..2
Scheme 2: General synthesis of PAEs via NAS……………………………………..8
Scheme 3: Post-polymerization sulfonation technique……………………………...9
Scheme 4: Pre-polymerization sulfonation technique………………………………10
Scheme 5: Synthesis of 3-Sulfonated-3’,5’-difluorophenyl sulfone, 4……………...20
Scheme 6: Synthesis of 3-sulfonated-4,4’-difluorophenyl sulfone, 8……………….23
Scheme 7: Homopolymer Synthesis of 5a-d.………………………………………..26
Scheme 8: Copolymer Synthesis of 6………………………………………………..27
viii List of Tables
Table 1: Mw, Mn, PDI and percent yield of s-PAES polymers under various concentration conditions………………………………………………….....27
Table 2: Ion Exchange Capacities (IEC) and Equivalent Weights (EW) of Sulfonated PAEs prepared from 4 and a variety of bisphenols………………………....28
Table 3: TGA and DSC values for 5a-d polymers and 6a copolymer………………..36
ix ACKNOWLEDGMENTS
I would like to thank my family and friends for their great support and encouragements, throughout my life and studies. Without their love and support I would have not been successful. I would also like to thank my advisor Dr. Eric Fossum for his support, guidance and shared knowledge. He gave me the opportunity to grow and gain everlasting experiences. I would also like to thank Dr. Kenneth Turnbull and Dr. Daniel
M. Ketcha for helping me defend this thesis. Last, but not least I would like to thank the chemistry department faculty and staff for their extended help and cooperation.
x I. Introduction
Over the past decade humans have become extremely dependent on fossil fuels
for the production of energy. Fossil fuels are used for the majority of our current energy
production. No other material can come close to the enormous impact in which fossil
fuels have on our lives today. Fossil fuels are now used in every aspect of our lives such as industrial, agriculture, and transportation systems. With the increasing world population and the expansion of many world markets, such as China and India, and our ever-growing need for energy, our dependency on fossil fuel is now greater than it has ever been.
While there are many opinions and skeptics that doubt that there is anything that
can wean us away from our dependency on fossil fuels, it is becoming clear that
alternative forms of energy are needed. As time passes the need to find alternative
sources for energy production is not only becoming greater, it is becoming a necessity to the continued existence of life. Great efforts are being undertaken by both industrial and academic research to find alternative energy sources such as solar, wind, nuclear and fuel cells.
Fuel cells are quite attractive because of their clean way of producing electricity.
They have zero emissions, high fuel efficiency, low noise, and diverse applications2.
Some of the disadvantages accompanied with fuel cells are their high cost, low power density, and durability issues. The high cost of a fuel cell is a result of using specialized components such as precious metals at the anode and cathode in order to carry out the
1 oxidation and reduction reactions. Thus, when constructing a fuel cell certain measures must be taken to reduce the cost and increase the reliability of the cell.
Fuel cells operate in an analogous manner to batteries by converting chemical energy to electrical energy without the use of combustion. The main difference between fuel cells and batteries is that batteries run out and become depleted, although some can be recharged1. Fuel cells provide a continuous supply of electricity as long as a continuous fuel source, such as hydrogen or methanol, is provided and a source of oxygen is readily available2,3. The single step conversion of chemical energy to electrical energy allows for greater efficiencies, up to 80 %, compared to a maximum efficiency of
30% from a common combustion engine.
As illustrated in scheme 1 a typical fuel cell consists of an anode, cathode and an electrolyte. Oxidation occurs at the anode where hydrogen is oxidized into protons and electrons are generated. The electrons are passed through an external circuit while the protons are conducted through a proton exchange membrane4 onto the cathode where oxygen is reduced to form water and heat as the only byproducts5.
Scheme 1
- e e- - e- e - e- e - O2 e e-
+ from H + + + + + + H + H H H H H H air 2 + H H O2 + Fuel H polymer Anode electrolyte Cathode Catalyst membrane Catalyst
PEM H2O
2 Six different types of fuel cells are currently under investigation and they are
categorized by the electrolyte used: 1) direct methanol (MFC) or 2) hydrogen fueled
proton exchange membrane Fuel Cells (PEMFC), 3) solid oxide fuel cells (SOFC), 4) phosphoric acid fuel cells (PAFC), 5) alkaline fuel dells (AFC) and 6) molton carbonate fuel cells (MCFC).
Proton exchange membrane fuel cells are mostly investigated by academic and industrial research because of their relatively small size and low weight, which make them best suited for portable and automotive applications. Also they operate at moderate temperatures (60-120oC), which allows for a fast start up and shut down time3,6,7.
Background Information
PEMFC were first developed in the early 1960’s, through the work of Thomas
Grubb and Leonard Niedrach. They were incorporated into NASA’s project Gemini in the early days of the U.S. piloted space programs1,8. In the 1970’s, the U.S. Navy
developed an oxygen generating plant by the use of proton exchange membrane (PEM)
water electrolysis technology for undersea life support. In the 1990’s, PEMFCs were
seriously considered for terrestrial transportation applications9. In 1995, Ballard systems
tested PEM cells in buses in Vancouver and Chicago followed by experimental vehicles
made by Ford, GM and DaimlerChrysler9.
The proton exchange membrane, PEM, is considered one of the most important
components of a PEMFC. The electrolyte of a PEMFC consists of an ionic polymer
membrane which serves to conduct protons as well as preventing oxidant and fuel from
mixing, a process called fuel cross-over4. In order for a PEM to be considered for use in
3 fuel cell applications it must fulfill certain requirements including10-17: 1) high proton conductivity (S/cm), 2) low fuel and water permeability, 3) mechanically robust at higher temperatures, 4) high chemical, electrochemical and oxidative stability, 5) competitive price. Therefore any new PEM material that exhibits these characteristics must also be easily synthesized and cost efficient to be feasible as a replacement for existing PEM materials
Proton conductivity is measured in units of Siemens per centimeter (S/cm). Proton conductivity is directly proportional to Ion Exchange capacity (meq/g), (IEC), and
Equivalent Weight (g/mol), (EW). IEC is defined as the milliequivalents of acid groups per gram of polymer, while the EW is defined as the mass of polymer affiliated with the acid group (g/mol). IEC and EW are inversely related, thus a high IEC corresponds to a low EW, both of which should lead to higher conductivity values.
Proton conductivity in the sulfonated membranes is highly dependent on the amount of water present in the membrane. They become conductive when hydrated.
Researchers have investigated the minimum amount of water molecules needed to conduct protons. Some have shown that at least 6-7 water molecules are needed for every sulfonic acid moiety in order to generate reasonable conductivities for PEM applications20-22. Thus, hydration of the ionic polymers is necessary. Unfortunately, the ionic polymer membranes such as sulfonated poly(arylene ether sulfone)s (s-PAES) are very hydrophilic which causes some of these materials to lose some of their mechanical properties at high levels of hydration. Furthermore, at higher hydration states the ionic membranes swell considerably resulting in significant stress on the system. Thus, membrane swelling must be controlled for efficient PEM applications.
4 The percent of membrane water uptake can be measured by the following equation:
Water uptake = (Wwet – Wdry)/ Wdry x 100%
Where Wwet is defined as the wet weight, which will be measured by equilibrating
the membrane in de-ionized water at roon temperature and weighing the sample immediately after removing any residual water on the film surfaces. Wdry is defined as the
dry weight of the membrane. This is typically measured by drying the sample at 120 oC for 1hr and then weighing.
The most widely studied systems for PEM applications are perfluorinated copolymers, such as Nafion, that carry sulfonic acid groups. Because of their high chemical and thermal stability, they are attractive for PEM applications. E.I. DuPont de
Nemours first developed Nafion for space exploration programs in the late 1960s18. The structure of Nafion is illustrated in Figure 1.
F2 F2 C C C CF n F2 O
CF2
CF F2 F2 F3C O C C SO3H m
Figure 1. Nafion structure
It received large attention as a proton conductor for PEMFC due to its selectivity,
low water permeability, as well as excellent thermal and mechanical stability of the
membrane. Nafion is considered to be the “gold standard” membrane for PEMFC
5 applications. It is a highly fluorinated system which allows for a high ratio of sulfonic
acid groups on its conducting membrane. The IEC value for Nafion 117 is
0.91(meq./gram)19, the conductivity is relatively high because of its pendant sulfonic acid
groups. These materials displayed excellent proton conductivities ranging from 90-120
mS/cm at temperatures below 80 oC and relative humidity ranging from 34-100%15.
Even though Nafion is the most commonly used material for PEMs, some
limitations exist including: 1) its high cost because of the highly fluorinated structure23-25
and 2) a maximum operating temperature of 80oC which limits the efficiency of the
catalysts. Platinum catalysts can easily be poisoned by the presence of any traces of
carbon monoxide, CO, which is often present as an impurity in the fuel. Increasing the
operating temperature to 120 oC greatly reduces the opportunity for CO poisoning3. In
turn, this will increase the lifetime of the cell as well as giving more flexibility to the type of fuel used. Thus, new membranes that exhibit high thermal and chemical stability at
temperatures above 120 oC are currently under investigation.
Alternatives to Nafion
Due to their interesting properties sulfonated poly(arylene ether)s, s-PAEs, have
been widely studied as an alternative to Nafion. PAEs are engineering thermoplastic
materials, which exhibit excellent thermal and mechanical properties, relatively good
resistance to hydrolysis and oxidation, and, depending on the structure of the backbone of
the polymer, moderate glass transition temperatures ranging from 180- 250oC are
obtainable40, This combination of properties has enabled them to become perfect
candidates for PEM materials10. A variety of sulfonated PAEs have been investigated as
6 alternatives to Nafion. Examples include Sulfonated poly(arylene ether sulfone)s, s-
PAES10,26-39, and sulfonated poly(ether ether ketone)s, s-PEEK39-41. A generic structure of
a s-PAE is given in Figure 2.
SO3H
* O X O Y * n
HO3S
X = nil, isopropylene, etc.
Y = SO2, CO, etc.
Figure 2: Generic structure of a sulfonated PAE
Sulfonated Poly(arylene ether sulfone)s, s-PAE
The synthesis of PAEs is generally carried out by nucleophilic aromatic
substitution, NAS (Scheme 2). This process is only feasible because of the presence of a
strongly electron withdrawing group and either choride or fluoride as a leaving group.
Common electron withdrawing groups used in PAEs synthesis include the phosphoryl, sulfone and ketone groups that withdraw electron density away from the activated carbon
(electrophile). The presence of the halogen as the leaving group further induces the
reactivity of the electrophile by inductively withdrawing electron density away from the
activated carbon, thus making it more susceptible to attack by a nucleophile.
7 Scheme 2
Dipolar Aprotic Solvent Y X Y + HO Ar OH Base
Y X O Ar O H n
X = CO, SO2 and RPO Y = F or Cl
As demonstrated above, PAE step growth polymerization is generally carried out by NAS reactions where the dihalides (fluorides or chlorides) are reacted with a variety of different bis-phenols.
One of the most widely investigated class of polymers for PEM applications are the sulfonated poly(arylene ether sulfone)s, s-PAES. Hundreds of research articles have described the investigations of these polymers. Furthermore, some researchers were able to prove that a number of these systems showed proton conductivities comparable to that of Nafion. For the preparation of the sulfonated analogues the sulfonic acid group can be introduced at the polymer stage (post-polymerization), by using fuming sulfuric acid26.35.36, followed by hydrolysis or at the monomer stage (pre-polymerization)46-48.
In the post polymerization process up to two sulfuric acid groups are substituted on the electron rich bisphenol moiety resulting in less acidic membranes (scheme 3); higher pKa values for the acid groups. Therefore, the stability of the conjugate base is
greatly reduced due to the presence of a strong electron donating group resulting in
8 lowering the proton conductivity of the membrane. Furthermore, the presence of the
sulfonic acid group ortho to the more electron rich aromatic ether linkage causes the polymers to become more vulnerable to degradation. At high ratios of acid to polymer cross-linking might occur during the sulfonation process.
Scheme 3
DCDPS Bis-A
O CH3
Cl S Cl + HO C OH
O CH3
K2CO3
NMP/Toluene Δ
O CH3
Cl S O C O H n O CH3
"SO3"/H2SO4
HO3S
O CH3
Cl S O C O H n O CH3
Alternatively, the pre-polymerization sulfonation technique enhances the acidity
of the membrane by adding the sulfonic acid groups on both electron poor rings ortho to
the chlorine moiety. Thus allowing every repeat unit to contain two acid groups, which
increases the acidity, and consequently increasing the IEC and proton conductivity of the
9 membrane after polymerization. However, due to the fully sulfonated polymer being
soluble in water, a 50/50 copolymer is required.
Introduction of the sulfonic acid group at the monomer stage is achieved by
sulfonation of the 4,4’-dichlorophenyl Sulfone monomer DCDPS resulting in the substitution of two acid groups on the sulfone ring (electron poor aromatic ring) to produce 3,3’ disulfonated-4,4’-dichlorodiphenyl Sulfone, SDCDPS26,35-37. Once the
sulfonated monomer is obtained, the monomer is reacted with a variety of different bis-
phenols. This process is illustrated in scheme 4 with bis-A.
Scheme 4
SDCDPS DCDPS SO3H
O O "SO3"/H2SO4 Cl S Cl Cl S Cl
O O
HO3S
SDCDPS SO3H Bis-A
O CH3
Cl S Cl + HO C OH
O CH3
HO3S
K2CO3
NMP/Toluene Δ
SO3H
O CH3
Cl S O C O H n O CH3
HO3S
Sulfonated PAES showed very promising results as an alternative to Nafion reported by McGrath et al. s-PAES membranes were prepared using a variety of different bis-phenols. They showed IEC ranging from 0.88 to 2.2 mequiv/g36. The proton
conductivities for two of their samples range from 10 to 170 mS/cm, which exceeds that
10 of Nafion. However, it was observed that higher IEC membranes possessed higher water solubility.
Further investigations on s-PAES have led Dang and Bai49 to increase the proton conductivities of these membranes. The polymerization of 100 mol % of the 3,3’ disulfonated-4,4’-difluorodiphenyl Sulfone, SDFDPS, with a bis-thiophenol derivative showed a dramatic increase in conductivity values compared to that of Nafion. Their polymer showed a proton conductivity value of 360 mS/cm at 85% humidity and 65oC,
which is significantly higher then that of Nafion, (80 mS/cm ), under the same
conditions49.
Although, s-PAES membranes have revealed great thermal and mechanical
properties as well as improved proton conductivities compared to that of Nafion,
investigations on these systems have shown that the backbone of the polymer is
vulnerable to hydrolysis and oxidation due to the presence of two strong electron
withdrawing groups (sulfonic acid and sulfone group) in the ortho and para positions
relative to the aryl ether bonds, respectively50-54. Furthermore, these polymers showed
higher proton conductivities at elevated IECs, which increases water uptake of these
membranes thus leading to loss of mechanical properties that are not suitable for PEMFC
applications. These discoveries have led researchers into looking at ways to improve the
overall stability of these membranes. Some believe that placing the sulfonic acid group
pendant to the backbone of the polymer will enhance the stability of the membrane50-54.
Pendant versus Backbone Substitution
Placing the acid group in a truly pendant position to the main chain is believed to
enhance the nanophase separation between the hydrophilic and hydrophobic domains
11 resulting in higher accessibility of the acid moiety. Thus a more efficient conducting
pathway will be produced55. The resulting s-PAEs possessed high thermal and oxidative
stabilities. Good proton conductivities at elevated temperatures were observed50-54, 56.
Examples of monomers that allow an acid group to be incorporated as a pendant moiety are tri- and tetraphenyl methane53, 54, and 9,9-bis-(4-hydroxyphenyl) fluorine52. Please
note that the SO3H groups are added during a post-polymerization sulfonation step.
SO3H
R R R R
H
O O O O
R R R R
SO3H SO3H
Sulfonated-triphenyl methane Sulfonated-tetraphenyl methane R = H or CH R = H or CH3 3
O O
HO3S SO3H
Sulfonated-9,9-bis-(4-hydroxyphenyl)fluorene
Figure 3. Examples of PAEs prepared from bis-phenol monomers designed to afford pendant sulfonic acid groups. Sulfonic acid groups are introduced on the pendant phenyl rings after the bisphenol is incorporated into the PAE.
All of the polymers shown in Figure 3 are bis-phenols resulting from introducing the sulfonic acid group to the electron rich phenyl rings (post sulfonation). This is
presumed to reduce the acidity of the membrane compared to that with the same group
12 present on the electron deficient phenyl rings. However, placing the SO3H group on the pendant rings should allow it to be more accessible. By comparison, introduction of the acid group on an electron poor pendant phenyl ring should enhance the acidity of the system. Therefore higher proton conductivity could be achieved.
Very few studies on s-PAES have been conducted in which the sulfonic acid groups are located on the pendant phenyl groups, however, those studies have shown relatively high proton conductivity values. Jannasch reported proton conductivity values for sulfophenoxybenzoyl polysulfone, PSU-sphb, and sulfonaphthaoxybenzoyl polysulfone, PSU-snb, bearing an acid group pendent to the main chain in the range of
11-32 mS/cm at 120 oC, with IEC values of 0.57- 1.02 meq/g respectively as illustrated in figure 455.
O O
S S
O O
O O
and
O O
SO3H
SO3H
PSU-sphb PSU-snb
Figure 4: structure of sulfophenoxybenzoyl PSU (PSU-sphb), and sulfonaphthoxy- benzoyl PSU (PSU-snb)55.
13 A more recent study by Guiver reported the synthesis of newly developed comb- shaped copolymers with two or four acid groups pendent to the main chain showing high proton conductivities ranging from 34-147 mS/cm and 63-125 mS/cm, with IEC values of 1.23- 2.03 meq/g, respectively, as illustrated in figure 557. These polymers showed low methanol permeability and low water uptake. They also showed high thermal and oxidative stability at elevated temperatures.
14 O O
HO3S HO3S
O O
SO3H SO3H
O O
O S O O S O
CF3 CF3
O O O O
HO3S HO3S HO3S HO3S
O O O O
SO3H SO3H SO3H SO3H
Figure 5: Representations of rigid polyaromatic backbone with pendant side chains containing two sulfonic acid groups, and with branched pendant side chains containing four sulfonic acid groups.
15 Current Project
Functionalized Poly(arylene ether)s with Truly Pendant Conducting Groups
This research project has focused on providing answers to two fundamental
questions associated with sulfonated PAE systems. First, which is the best position for the conducting group - on the backbone or truly pendant? Second, what is the optimum microstructure for the sulfonic acid groups - random or evenly distributed along the polymer chain? These two questions were probed by synthesizing two geometric isomers of the well studied 50/50 copolymer of SDFDPS and DFDPS as outline in Figure 6.
A newly developed PAE bearing a truly pendant phenyl sulfonyl group58 provided
the opportunity to introduce the sulfonic acid group on an electron poor pendant moiety.
A direct comparison of pendant versus backbone sulfonation was achieved by the
synthesis of 3-sulfonated-3’,5’-difluorodiphenyl Sulfone and 3-sulfonated-4,4’- difluorodiphenyl Sulfone. Polymerization of either of these monosulfonated monomers with a variety of bis-phenols give rise to the geometric isomers of the 50/50 copolymer of
SDFDPS and DFDPS and all of the polymers will possess identical IEC and EW values.
A direct comparison of random versus evenly spaced sulfonation sites will be afforded by comparison 3-sulfonated-4,4’-difluorodiphenyl sulfone with the same 50/50 copolymer.
16 Hydrolytically stable
F F O O
Polymerization O S O O S O Truely pendant
SO H Enhanced acidity SO3H 3
3-sulfonate-3', 5'- difluorodiphenyl sulfone
Hydrolytically less stable O Polymerization O F S F O S O O O HO S 3 SO3H HO3S SO3H
3,3'-disulfonate- 4, 4'- difluorodiphenyl sulfone Sterically hindered Decreased Acidity
Hydrolytically less stable O Polymerization O F S F O S O O O SO3H SO3H
3-sulfonate- 4, 4'- difluorodiphenyl sulfone Sterically hindered Decreased Acidity
Figure 6: Comparison of the effects of incorporating the proton conducting group on the backbone or in a truly pendant position.
The sulfonated PAEs with truly pendant sulfonic acid groups are expected to
provide some key advantages over the currently available materials including: 1) higher
proton conductivity because of the presence of the acid group on the pendent phenyl
sulfonyl moiety allowing the system to be more accessible and have higher acidity due to
the increased stability of the conjugate base, 2) higher stability to oxidation and
hydrolysis due to the lack of two highly electron withdrawing groups in the backbone of
17 the polymer, 3) high loadings of conducting groups with one acid group at every repeat unit.
18 II. RESULTS AND DISCUSSION
Monomer Synthesis
3-Sulfonated-3’,5’-difluorophenyl Sulfone, 4
The synthesis of 4 was achieved via electrophilic aromatic substitution35 on 3,5- difluorophenyl sulfone 1, in fuming sulfuric acid as illustrated in scheme 5. The reaction was carried out at 40 oC for 6 hours, which afforded a homogeneous reaction solution.
The reaction mixture was then diluted with water and the organic material was precipitated by the addition of NaCl. Because of intermolecular forces between water and
NaCl, the more soluble NaCl goes into solution forcing out the less soluble organic material (product). The precipitate was then isolated via filtration, re-dissolved in water and then neutralized with sodium hydroxide to a pH of 6-7 while keeping the solution in an ice-cold water bath. The ice bath was utilized to prevent any hydroxide ions attacking the activated halide to produce unwanted side products such as phenolic impurities. The strong acid-base reaction of the sodium hydroxide with the acidic proton of the sulfonic group allowed for quantitative exchange from the acid to salt form. A monomer grade product was obtained in good yield (67%) after recrystallization from 250 mL (9:1) isopropanol: water. The pure, recrystallized product was dried under vacuum and its identity was confirmed by 1H, 13C, and 19F NMR spectroscopy. The 1H and 13C NMR spectra for 4 are shown in Figures 7a and 7b, respectively.
19 Scheme 5
F F F F
fuming "SO " 1) H2O O S O 3 O S O o 40 C, 6 Hours 2) NaCl (excess)
SO3H 1 3
F F
H O NaOH NaCl Recrys. 2 O S O pH = 6-7 isopropanol/ H2O (9:1 ratio)
- + SO3 Na 4
1 Figure 7a: 300 MHz H NMR (D2O) spectrum of 4
20 From the proton spectrum of 4 the integration in the aromatic region shows the correct ratio of 1: 2: 1: 2: 1 and five distinct peaks are observed in the aromatic region as expected. Proton a appears as a triplet of triplets around 7.3 ppm due to the fluorine and proton b couplings. Protons b also show up as a multiplet around 8.15 ppm because of the fluorine and proton a coupling, the signal appeared further down-field because of the presence of the sulfone group ortho to proton b. Proton c appeared as a triplet around
8.37 ppm because of the weak coupling from protons d and f. Proton c was furthest down field because of the presence of the strong electron withdrawing groups (sulfone group and sulfonic acid group) ortho to it as expected. Finally protons d and f appeared as overlapping signals at 7.65 ppm; this is expected as both protons are located in chemically similar environments. They thus appear to be an overlap of two doublets, which will be expected because of the strong coupling to proton e.
13 Figure 7b: 75.5 MHz C NMR (D2O) spectrum of 4
21
Ten 13C NMR signals for 4 were assigned using the relative chemical shifts and
splitting patterns. Carbon a was easily recognized as a triplet at 110 ppm because of the
strong coupling to the two fluorine atoms in the ortho positions. Carbon b was found as a
wide doublet of doublets at 161-164.5 ppm due to the coupling with the fluorine attached
to the carbon and the fluorine in the meta position. Carbon c was found at 112 ppm as an
unusual doublet of doublets due to the splitting of both fluorine atoms in the ortho and
para positions. Carbon d was found at 142 ppm as a triplet due to the coupling with the
two meta fluorine atoms. Carbons e, f, g, h, j, and i were all identified as singlets and were found at 144,131.3, 131, 124.5, 130.5, 139.5 ppm, respectively.
3-sulfonated-4,4’-difluorophenyl sulfone, 8
The synthesis of 8 was achieved via electrophilic aromatic substitution35 on 4,4’-
difluorophenyl sulfone, DFDPS, in fuming sulfuric acid as illustrated in scheme 6. To
afford only mono-substituted product a ratio of 1: 0.9 of DFDPS to fuming sulfuric acid
was necessary to only substitute one acid group. The reaction was carried out at 70 oC for
20 hours, which afforded a homogeneous reaction solution. The reaction mixture was then diluted with water and the organic material was precipitated by ”salting out”. The precipitate was then isolated via filtration, re-dissolved in water and then neutralized with sodium hydroxide to a pH of 6-7 while keeping the solution in ice-cold water bath. Many efforts have been taken to recrystallize the monomer; unfortunately we were unable to purify it. Its identity was confirmed by 1H NMR spectroscopy. The 1H NMR spectrum for
8 is shown in Figure 8.
22 Scheme 6
O O
"SO3" F S F F S F 70 oC O 20 hours O
SO3H DFDPS 7
O
H2O NaCl H2O NaCl F S F PH = 6-7 O
+ SO3Na 8
1 Figure 8: 300 MHz H NMR (D2O) spectrum of 8
From the proton spectrum of 8 the integration in the aromatic region shows the expected ratio of 1: 1: 2: 1: 2 and five distinct peaks were observed. Proton a appears as a doublet of doublets (more of a triplet) around 7.35 ppm due to the fluorine and proton b
23 couplings. Protons b showed up as doublet of doublets around 8.05 ppm because of the
fluorine and proton a coupling; the signal appeared further down-field because of the presence of the sulfone group ortho to proton b. Proton c appeared around 8.17 ppm as a
complex multiplet because of the coupling from protons e, d and the fluorine meta to it.
Proton c was further down-field because of the presence of the strong electron
withdrawing groups (sulfone group) ortho to it. Proton d appears as a doublet of doublets
at 8.35 ppm. Proton d appeared furthest down-field due to the presence of two electron
withdrawing groups (sulfone and sulfonic acid goup) ortho to it as expected. Finally
proton e is observed at 7.51 ppm as a triplet due to the coupling from the ortho fluorine
and proton c as anticipated.
As illustrated in Scheme 6, 8 is the geometric isomer of 4. Synthesis of 8 was
designed to allow a direct comparison of two isomeric systems where 4 is bearing the
acid group in the pendant position while 8 is bearing the acid group on the backbone of the polymer at every repeat unit. Thus, the influence of the acid group position on the proton conductivity, water uptake and stability for the corresponding PEM can be studied. Therefore, polymerization of 8, with a variety of bisphenols will provide a series of sulfonated PAEs that contain identical IEC and EW to those prepared from 4, thus providing the ability to compare, directly, the effects of acid group location on membrane performance.
Homopolymer, 5a-d, Synthesis and Characterization
Nucleophilic aromatic substitution polycondensation (Scheme 7) was utilized to
synthesize high molecular weight sulfonated poly(arylene ether)s bearing pendant phenyl
sulfonyl groups with sulfonic acid moieties located in the meta positions of the pendant
24 moiety. The polymerization reactions were carried out using a “one pot reaction” under
static flow of nitrogen. The presence of 2.5 equivalents of K2CO3 allowed the conversion
of the phenols to the corresponding phenoxides required for the NAS reaction. The
reaction mixture was heated to 150 oC for the first 4 hours using toluene to dehydrate the
reaction system. The toluene was then drained and the temperature of the system was slowly increased to 180 oC and held there for 16 hours. An additional 5 mole percent of 4
was added and the mixture was heated for 2 more hours. The reason for the addition of 5
mol % of 4 at the end of the reaction was to balance out the one-to-one stoichiometry due
to the high hygrophilicity of 4 as mentioned below.
In order to produce high molecular weight polymers certain requirements need to
be satisfied such as: one-to-one stoichiometry, high monomer and reagent purity and very high (> 99 %) conversions. Because of the hygroscopic nature of 4, addition of 5-weight
% extra of 4, at the end of the reaction allows the system to compensate for any error
affiliated with the stoichiometry of the reaction mixture. This is important when
considering the degree of polymerization, DP, equation shown below:
DP = 1/ (1- p)
Where p is the degree of conversion. Thus the higher the purity of 4, the higher the DP will be. Therefore, 5% addition takes care of the one-to-one stoichiometry and the high conversion associated with the reaction.
These reaction conditions were successful to produce high molecular weight polymers using a variety of bisphenols such as: 4,4’-biphenol, 4,4’-diphenyl ether, bisphenol A and bisphenol AF. The high molecular weight sulfonated polymers were isolated by precipitating from vigorously stirred methanol for (4,4’-biphenol and
25 diphenyl ether) and deionized water for (Bis A and Bis AF) after diluting the viscous
reaction mixture with 5 ml NMP.
Scheme 7
F F F O Ar O H n
2.5 eq K2CO3 O S O + HO Ar OH O S O 1.5 ml NMP/ 1.5 ml Toluene 2a-d reflux 150 oC/ 4hr Heat 180 oC/ 16hr 5% addition/ 2hr
+ SO Na+ SO3Na 3 4 5a-d
Ar =
CF3
CF3 2a 2b
O
2d 2c
Copolymer, 6, Synthesis and Characterization
The synthesis of a 50/50 copolymer was carried out in an effort to reduce the solubility issues associated with the homopolymers, which will be discussed in the following section. The same procedure described above was utilized for the synthesis of a copolymer consisting of a 50: 50 mixture of 4 and 1. Only one polymerization reaction was carried out using 4,4’-bisphenol. The reaction sequence is illustrated in scheme 8.
The relatively high molecular weight (Mn= 13,597 Dalton) copolymer was precipitated as
chunks from deionized water after diluting the reaction mixture with 5 ml NMP. 1H and
13C NMR spectroscopy were used to characterize this polymer.
26 Scheme 8
F F F F
O S O ++O S O OH OH
- + SO3 Na 4 1 3a
2.5 eq K2CO3
6 ml NMP/ 6 ml Toluene reflux 150 oC/ 4hr Heat 180 oC/ 16hr 5 % B2 addition/ 2hr
F O O O O H
n m y
O S O O S O
- + SO3 Na
6
After drying the polymers under vacuum for 48 hours in a drying pistol at 111°C using toluene as the drying solvent, Size Exclusion Chromatography (SEC) analysis was utilized to determine the number average molecular weight, Mn, average molecular weight, Mw and PDI for all of the polymers and the results are compiled in table 1.
27 Table 1: Mw, Mn, PDI and percent yield of s-PAES polymers under various concentration conditions
Reaction A2 Concentration % Mn Mw PDI (wt/v) % yield 1 5a 38 79 17,578160.897 9.153 2 5b 38 79 14,73779,384 5.386 3 5d 38 81 15,70646,733 2.975 4 5a 64 75 10,27533,438 3.254 5 5b 64 64 38,422196,073 5.103 6 5c 64 62 11,97937,996 3.172 7 5d 64 71 14,02257,945 4.132 8 6a 38 80 13,59727,983 2.058
• Reaction temp. 180 oC • Solvent NMP • Time 22 hours • 2.5 equivalent K2CO3 • 5 weight % was added at the end of each reaction
From reactions 2 and 5 it is fair to assume that increasing the concentration
increases the intermolecular polymerization and decreases the intramolecular
cyclization resulting in high molecular weight polymers. As can be seen the Mn
value for 5b (14,737 Daltons) is doubled to (38,422 Daltons), while the amount of
cyclic species is dramatically reduced when the concentration is increased from 38-
weight % to 64-weight %. The SEC traces illustrated in figure 9, show a major
reduction in cyclic species and a dramatic increase in the average number molecular
weight, Mn, and average weight molecular weight values, Mw.
28
Figure 9: SEC traces of polymers at 38 weight % and 64 weight % concentrations for reactions 2 and 5
The increased accessibility of the acid group became apparent when the salt form of the polymers showed solubility in hot water. The systems were dissolved in water and precipitated out of H2SO4 to obtain the acid form. While dissolving the samples it was
observed that the solubility of the polymers increased with decreasing equivalent weights,
as expected. The Ion Exchange Capacities and Equivalent Weights for s-PAEs prepared
from a variety of bisphenols are listed in Table 2. All of the IEC and EW values are
within the range of the best materials that are currently available26.
Table 2. Ion Exchange Capacities (IEC) and Equivalent Weights (EW) of Sulfonated PAEs prepared from 4 and a variety of bisphenols.
Bisphenol IEC (mequiv./g) EW (g/mol) 4,4’biphenol 2.09 480.5 hydroquinone 2.46 406.4 Bisphenol A 1.91 523.6 Bisphenol AF 1.59 628.9 4,4’-dihydroxyphenyl ether 2.10 497.5
29 Tough peelable films were solution cast from dilute DMSO solutions
(approximately 5 weight percent) on a glass slide and vacuum dried at 150°C. Figure 10 demonstrates freestanding films of the salt form of 5b unperturbed (left) and folded
(right), prepared in our lab.
Figure 10: Free standing films of the salt form of 5b unperturbed (left) and folded (right).
1H and 13C NMR spectroscopy were used to characterize the four sulfonated polymers and the 4,4’-bisphenol copolymer. The NMR results allowed for structural determination and confirmation for each polymer. Integration and Chemdraw estimations were utilized to assign the 1H NMR signals of all the polymers. Representation of the homopolymer/ copolymer 4,4’-bisphenol polymer for 1H and 13C NMR are illustrated in figures 11 – 12 respectively.
30
Figure 11a: 300 MHz 1H NMR 4,4’-bisphenol homopolymer
m n n m
a F O O H k l l k b b n m n n m c c
d O S O e g f
h m j SO3H n i
b k l d j i h g f e c
a Cyclic Species
160 155 150 145 140 135 130 125 120 115 ppm
Figure 11b: 75.5 MHz 13C NMR 4,4’-bisphenol homopolymer
31 F O O O O H n m y H2O
O S O O S O
- + SO3 Na
6
DMSO
8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 ppm
Figure 12a: 300 MHz 1H NMR 4,4’-bisphenol homopolymer
k w F O O O O H b n p m y d r O S O O S O e s
- + SO3 Na
6
160 155 150 145 140 135 130 125 120 115 ppm
Figure 12b: 75.5 MHz 13C NMR 4,4’-bisphenol homopolymer
32 Thermal Analysis
The thermal properties of the sulfonated polymers were analyzed using differential
scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA).
DSC was utilized to determine the glass transition temperature, Tg. The Tg is highly affected by intermolecular interactions and the rigidity of the A2 and B2 monomer units.
Both factors are present in this, thus the increased sulfonation afforded more
intermolecular interaction by pendant ions and the addition of a rigid A2 monomer such
as 4,4’-biphenol, hinders the free rotational ability of the membrane thus increasing the
59 glass transition temperature . In other words, the more rigid the A2 monomer, the higher
the Tg will be. Stacked traces of the glass transition temperature for the salt and acid
forms of 5a-d and 6 are illustrated in figures 13 and 14, respectively.
Figure 13: stacked DSC traces of polymers 5a-5d salt form.
33
Figure 14: stacked DSC traces of polymers 5a-5d acid form.
o As expected the rigid 4,4’-bisphenol displayed the highest Tg (295 C), while the
o least rigid diphenyl ether possessed the lowest Tg (263 C). The salt forms of the
polymers possessed higher Tg values than the acid forms due to the fact that the
intermolecular forces between the ionic bonds in the salt forms are much stronger than
the hydrogen bonds in the acid forms. Thus, it will require more heat to rotate portions of
the backbone in the salt form resulting in a higher Tg value. The new polymers also seem
36 to possess lower Tg values compared to the 50/50 copolymers (SDFDPS) . This is due to the presence of the acid group on the pendant moiety, which can rotate more easily around the backbone of the polymer compared to the copolymer. This is presumed since there is considerably more steric hindrance in the SDFDPS system than in 4.
S-PEAs are of particular interest for PEMFC applications due to their high thermal stability. Therefore, the Thermo-oxidative behavior of all the sulfonated polymers/copolymers was investigated by TGA programmed from 100 to 600 oC at a
34 heating rate of 10 oC/min under nitrogen. A two-step degradation profile was observed for
all polymers in their salt-form and acid form as illustrated in Figure 15 and 16 respectively. The first weight loss is likely due to a desulfonation process, while the second weight loss peak is most likely due to main chain polymer degradation33. The 5% weight loss temperatures for 5a-5d, 6 and literature data for SDFDPS are given in table
3. Polymers 5a-5d showed excellent thermal stability. Even though these polymers were dried for 48 hours, about 1% weight loss was observed around 150 oC. This is possibly
due to the presence of tightly bound water molecules associated with the sodium salt
form33 .
Figure 15: Influence of polymer composition (salt form) on the TGA thermograms, under nitrogen at 10 oC/min.
35
Figure 16: Influence of polymers composition (acid form) on the TGA thermograms, under nitrogen at 10 oC/min.
From the TGA scans above one can observe that the copolymer, 6, displayed in
figure 16, possesses three degradation steps; we believe that the first degradation
occurring around 200 oC is due to the presences of NMP in the sample. The second
degradation temperature does not start until about 415 oC, which is indicative that the
copolymer is very thermally stable at elevated temperatures.
The T5% and Tg values of 5a, 5b, 5c, 5d, 6a for both the acid and salt forms are
listed in table 3 in comparison to the 50/50 copolymer PBPS-5036. From the table, one
can conclude that the new polymers do possess thermal stability comparable to the currently available membranes. As expected the new systems showed slightly higher
thermal stability than that of the PBPS-50 membranes.
36 Table 3: TGA and DSC values for 5a-d polymers and 6a copolymer
o a o b Reaction T5% ( C) Tg ( C)
Sodium Acid Sodium Acid
PBPS-50 c 498 356 277, 303 -----
5d 472 367 296 183
5b 394 358 279 199
5a 403 307 265 231
5c 311 333 264 240
6a 263 -- 275 --
a The 5% weight loss temperatures by TGA at 10 oC/min under nitrogen for acid and sodium form membranes b o Glass transition temperature, Tg, by DSC at 10 C/min under nitrogen for acid and sodium form membranes c Literature value for 50/50 copolymer PBPS-50 using 4,4’-bisphenol36
37 Conclusions
The one step syntheses of the 3-sulfonate-3’, 5’-difluorophenyl sulfone, 4, and the
3-sulfonated-4, 4’-difluorophenyl sulfone, 8, were achieved by utilizing fuming sulfuric
o acid (< 27 % SO3) at 40 C. The sulfonic acid group was substituted in the meta position on the pendant phenyl sulfonyl moiety. The sulfonic acid group was then titrated to the
sodium sulfonate with sodium hydroxide. The monomers were characterized by GC/MS,
1H and 13C NMR spectroscopy.
Polymerization of 4 under typical nucleophilic aromatic substitution conditions
with a variety of different bisphenols, 3a-d, afforded high molecular weight polymers
with Mn values ranging from 13,000 to 38,000 Daltons and PDIs ranging from 2.1 to 9.2.
The polymers were also characterized by 1H NMR and 13C NMR spectroscopy.
Tough, ductile membranes were successfully cast from DMSO.
The new polymers displayed Tg values of 264, 265, 275, 279, 296, for the salt form
and from 183 oC to 240 oC for the acid form, for samples prepared from diphenol ether,
bisphenol A, copolymer 4,4’-bisphenol, bisphenol AF and 4,4’-bisphenol, respectively.
They also possessed high thermal stability with 5% weight loss temperatures from 310 oC to 472 oC for the salt version and from 307 oC to 367 oC for the acid form.
In summary a series of thermally stable s-PAEs bearing the acid group on a pendant
phenyl sulfonyl moiety has been developed and such compounds are promising for use in
PEMFC.
38 III. Experimental
Instrumentation and chemicals
All reactions were carried out under nitrogen and all chemicals were transferred
using syringes.
Solvent Purification
N-methyl pyrrolidinone, NMP, was dried over CaH2 and distilled under nitrogen
before use. Tetrahydrofuran, THF, was dried over sodium/ benzophenone and distilled
before use. Toluene was dried over and distilled from sodium/ benzophenone before use.
o Powdered K2CO3 was dried at 130 C in an oven before use. Fuming sulfuric acid containing oleum (30% SO3) was purchased from Aldrich chemical Co and was used as
received.
The 1-bromo-3,5-difuorobenzene and benzene-sulfonyl chloride used in the
synthesis of 1 (B2 monomer) were purchased from Aldrich. 4,4’-Bisphenol and diphenyl
ether were recrystallized from toluene and dried under vacuum before use. Bis-phenol-A
and bis-phenol AF were recrystallized from ethanol/water and dried under vacuum prior
to use.
Proton, 1H, and Carbon, 13C, Nuclear Magnetic Resonance, NMR spectra were
acquired using a Bruker AVANCE 300 MHz instrument operating at 300 and 75.5 MHz,
respectively. Samples were dissolved in appropriate deuterated solvents (DMSO-d6,
CDCl3, or D2O), at a concentration of (30 mg / 0.7 mL). SEC analyses were performed
using a Viscotek Model 300 TDA system equipped with refractive index, viscosity, and
39 light scattering detectors operating at 70 oC. Two polymer laboratories 5 μ m PL gel
Mixed C columns were used with NMP (with 0.5% LiBr) as the eluent and a Thermo
separation Model P1000 pump operating at 0.8 mL/ minute. DSC and TGA analysis were
carried out under nitrogen on TA Instruments DSC Q200 and TGA Q500, respectively, at
a heating rate of 10 oC/ min.
Synthesis of 3,5-difluorophenyl Sulfone, 1
The synthesis of the 3,5-monomer was carried out according to a previously
published procedure61. In a 500 mL round-bottomed flask equipped with a stir bar, condenser, and an addition funnel; under continuous flow of nitrogen, 2.86 g of Mg turnings and approximately15mL of THF enough to cover the metal were added. A solution of 22.78g of 1-bromo-3, 5-difluorobenzene and 62 mL of THF was added slowly through an addition funnel to the stirred Mg at room temperature. The solution was then allowed to react for 3 hours upon the complete addition of the mixture to the Mg. The resulting solution of 3,5-difluorophenylmagnesium bromide was transferred to an addition funnel and added drop-wise to 31.28g (22.60 mL) of benzenesulfonyl chloride and 85 ml of THF at 0oC. The reaction mixture was then allowed to react over-night
followed by refluxing for 2h. 1 M HCl solution was used to acidify the reaction mixture;
two layers were separated (organic and water). The water layer was drained out and was
washed three times with 100 mL increments of ethyl ether. The water layer was then
separated and the ethyl ether layer was added to the organic layer, followed by 3 washes
with distilled water, 5% NaHCO3, and again with 3 washes of distilled water. The ethyl ether layer was then dried over MgSO4 (2 hrs), filtered, and then driven to dryness by the
40 use of a rotary evaporator. The resulting solid was then recrystallized from EtOH/H2O followed by EtAcO/Hexane. The precipitate was then dried under vacuum for two days to
o 1 afford 19 grams (63 %) of 1, mp = 101-103 C. H NMR (CDCl3, δ): 7.00 (tt, 1H); 7.45
13 (m, 2H); 7.58 (d, 2H); 7.62 (m, 1H); 7.95 (d, 2H); C NMR (CDCl3, δ): 108.8 (t); 111.2
(d); 128.0 (s); 129.6 (s).
Synthesis of 3-sulfonated-3’,5’-difluorophenyl Sulfone, 4
Sulfonation of 3,5-difluorophenyl Sulfone was carried out by electrophilic
aromatic substitution. In a round bottom flask equipped with a stir bar and condenser, 8.0
g (31.45 mmol) of 1 was added followed by the addition of ~12 mL (32.40 mmol) of
fuming sulfuric acid. The reaction was heated under nitrogen for 6 hours at 70oC.
The resulting dark brown solution was diluted with 300 mL of DI H2O. The
solution was completely soluble in water, which indicated that all of the starting material
was sulfonated. The solution was then “salted out” as follows:
Approximately 60 g of NaCl was added slowly to the solution. The solution changed
color to a light cream color and a considerable amount of organic material precipitated.
The precipitate was then isolated by vacuum filtration and left to dry under vacuum over
night. The powder was then dissolved in 300 ml of water. Some insoluble materials were
present in solution; vacuum filtration was then utilized to remove any insoluble materials.
The solution was then neutralized with 5 M NaOH to a pH of 7 [~ 5ml NaOH (5M)] was
needed to change the solutions pH from pH 2 to pH 7). The solution was kept in an ice
bath to avoid the fluorine from reacting. The solution also changed color from light
brown to very dark brown. The solution was then immediately “salted out”. It took about
41 30 grams of NaCl until it reached the end point. Vacuum filtration was then utilized to
isolate the crystals, which were left to dry under vacuum overnight. The product was then
recrystallized from 9:1 isopropanol/water and dried in a vacuum at 140 oC for 48 hours to
1 provide 7.5 grams (67 %) of white powder. H NMR (D2O, δ): 7.3 (tt, 1H); 8.15 (m, 2H);
13 8.37 (t, 1H); 7.65 (m, 2H); 7.65 (m, 1H). C NMR (D2O, δ): 110 (t); 161-164.5 (dd); 112
(dd); 142 (t); 144 (s); 131.3 (s); 131 (s); 124.5 (s); 130.5 (s); 139.5 (s).
Synthesis of 3-sulfonated-4,4’-difluorophenyl sulfone, 8
The monosulfonation of 4,4’-difluorophenyl Sulfone was carried out by electrophilic aromatic substitution, similar to the procedure described previously. In a round bottom flask equipped with a stir bar and condenser, 1.001g (3.9 mmol) of 4,4’- difluorosulfone was added followed by the addition of 0.5 mL (3.5 mmol) of fuming sulfuric acid. The reaction was heated under nitrogen for 20 hours at 70oC.
The resulting dark brown solution was diluted with 100 mL of DI H2O. The
solution was completely soluble in water. The solution was then “salted out” as follows:
Approximately 30 g of NaCl was added slowly to the solution. The solution changed
color to a light cream color and all of the organic materials were precipitated out of
solution. The precipitate was then isolated by vacuum filtration and was left to dry under
vacuum over night. The filtered precipitate was then dissolved in 100 mL of water. Some
insoluble materials were present in solution and vacuum filtration was then carried out to
get raid of any insoluble materials. Approximately 5 mL of 5M NaOH was utilized to
neutralize the cooled solution to pH of 7. The solution was then immediately “salted out”
with 30 grams of NaCl until it reached the end point. Vacuum filtration was then utilized
42 to isolate the crystals, which were left to dry under vacuum overnight. Efforts were taken to recrystallize the product but unfortunately we were unable to purify the product. 1H
NMR (D2O, δ): 7.35 (dd, 2H); 8.05 (dd, 2H), 8.17 (m, 1H); 8.35 (dd, 1H); 7.51 (t, 1H).
General procedure for polymerization reactions of 5a-d
A typical polymerization reaction for all sulfonated polymers will be described
using the 4,4’-bisphenol system: 0.2645g (1.4 mmol) 4,4’-bisphenol and 0.4993g (1.4
mmol) 4 were added to a round bottom flask equipped with stir bar, nitrogen inlet and a
Dean Stark trap. Next, 2.5 mole equivalent of potassium carbonate was added, followed
by the addition of 1.5 mL NMP to afford a 64 weight % (w/v) concentration. 1.5 mL of toluene was also added, which was used as an azeotropic drying agent. The reaction mixture was heated under reflux at 150ºC for 4 hours to dehydrate the system. The temperature was raised slowly to 180ºC by controlled removal of the toluene. The reaction mixture was reacted for 16 hours during which time the solution became very viscous. After 16 hours, an additional 5 mole % of 4 were added and the mixture was heated for two additional hours to afford a highly viscous solution. The solution was cooled to room temperature and diluted with 5 mL of NMP. The solution was isolated as swollen strings by addition to stirred (500mL) methanol. Finally, the filtered polymer was vacuum dried at 140ºC for 48 hours. The homopolymers with other bis-phenols were also prepared by this procedure. The polymers were converted to the acid form by dissolving the polymers in deionized water and precipitating them out by acidifying the solution with concentrated sulfuric acid. Swollen membranes were observed. The resulting precipitates were washed several times with deionized water and filtered to
43 1 afford a 75 % yield. H NMR (DMSO-d6, δ): 7.02 (t, 1H); 7.8 (d, 1H); 8.05 (s, 1H); 7,95
13 (dd, 1H); 7.67 (t, 1H); 7.95 (dd, 1H); 7.25 (m, 1H); 7.25 (m, 1H). C NMR (DMSO-d6,
δ) also detected 14 signals as expected.
Procedure for copolymerization reaction to 6
The copolymer synthesis was carried out using the same procedure used for the
homopolymerization reactions: 0.7461g (4.0 mmol) 4,4’-bisphenol, 0.5211g (2.0 mmol)
of 1, and 0.7048g (2.0 mmol) 4 were added to a round bottom flask equipped with stir
bar, nitrogen inlet and a Dean Stark trap. Next, 1.364g (2.5 mole equivalent) of potassium
carbonate were added. Followed by the addition of 6 mL of NMP to afford 38-weight %
concentration. 6 mL of toluene was also added, which was used as an azeotropic drying
agent. The reaction mixture was heated under reflux at 150ºC for 4 hours to dehydrate
the system. The temperature was raised slowly to 180ºC by controlled removal of the
toluene. The reaction mixture was reacted for 16 hours during which time the solution
became very viscous. After 16 hours, an additional 5 mole % of 4 were added and the
mixture was heated for two additional hours to afford a highly viscous solution. The
solution was cooled to room temperature and diluted with 5 ml of NMP. The solution
was isolated as swollen strings by addition to stirred (500 mL) deionized water. Finally,
the filtered polymer was vacuum dried at 140ºC for 48 hours to afford 70 % yield.
44 IV. References
1. Liebhafsky, H.A.; Cairns, E.J., Fuel Cells and Fuel Batteries, John Wiley and
Sons, Inc., New York1968; p.7
2. Appleby, A.J., Ed. Fuel Cells: Trends in Research and Applications; Hemisphere
Publishing Corp.: New York, 1987; p.281
3. Zalbowitz, M.; Thomas, S. "Fuel Cells: Green Power," Department of Energy,
1999 LA-UR-99-3231.
4. Appleby, A.J.; Foulkes, R.L., Fuel Cell Handbook, Van Nostrand Reinhold, New
York, 1989.
5. Barbir, F.; Gomez, T., Int. J. Hydrogen Energy 1997, 22, 1027.
6. Dhathathreyan, K.S.; Sridhr, P.; Sasikumar, G.; Ghosh, K.K.; Velayuthan, G.;
Rajalakshmi, N.; Subramaniam, C.K.; Raja, M.; Ramya, K., Int. J. Hydrogen
Energy 1999, 24, 1107.
7. Korgesch, K.; Simader, G. Fuel Cells and Their Applications, Wiley-VCH,
Weinheim, 1996.
8. Okada, O.; Yokoyama, K. “Development of Polymer Electrolyte Fuel Cell
Cogeneration Systems for Residential Applications”, Fuel Cells 2001, 1 (1) 72.
9. Dunwoody, D; Leddy, J. Proton Exchange Membranes: The View Forward and
Back. J. Electrochem. Soc. interface. 2005, 37-39.
10. Hickner, M.A.; Ghassemi, H.; Kim, Y. S.; Einsla, B.R.; McGrath, J.E.,
Alternative Polymer Systems for Proton Exchange Membranes (PEMs). Chem.
Rev. 2004, 104, 4587-4612.
45 11. Kreuer, K. D., On the Development of Proton Conducting Polymer Membranes
for Hydrogen and Methanol Fuel Cells J. Membr. Sci. 2001, 185, 29-39.
12. Kerres, J. A., Development of Ionomer Membranes for Fuel Cells J. Membr. Sci.
2001, 185, 3-27.
13. Heinzel, A.; Nolte, R.; Lefjeff-Hey, K.; Zedda, M., Membrane Fuel Cells -
Concepts and System Design. Electrochimica Acta 1998, 43., 3817-3820.
14. Gamburzev, S.; Appleby, A. J., Recent Progress in Performance Improvement of
the Proton Exchange Membrane Fuel Cell (PEMFC). J. Power Sources 2002,
107, 5-12.
15. Neburchilov, V.; Martin, J.; Wang, H.; Zhang, J., A Review of Polymer
Electrolyte Membranes for Direct Methanol Fuel Cells. J. Power Sources 2007,
169, 221-238.
16. Silva, V. S.; Mendes, A.; Madeira, L. M.; Nunes, S. P., Proton Exchange
Membranes of Direct Methanol Fuel Cells: Properties Critical Study Concerning
Methanol Crossover and Proton Conductivity. J. Membr. Sci. 2006, 276, 126-134.
17. Bock, T.; Mohwald, H.; Mulhaupt, R., Arylphosphonic Acid-Functionalized
Polyelectrolytes as Fuel Cell Membrane Material. Macromol. Chem. Phys. 2007,
208, 1324-1340.
18. Church, Steven. "Del. firm installs fuel cell", The News Journal, Jan 2006 p. B7.
19. Allcock, H. R.; Hofmann, M. A.; Ambler, C. M.; Lvov, S. N.; X.Y., Z.;
Chalkova, E.; Weston, J., Phenyl Phosphonic Acid Funtionalized
Poly[aryloxyphosphazenes] as Proton-conducting Membranes for Direct
Methanol Fuel Cells. J. Membr. Sci. 2002, 201, 47-54.
46 20. Yeo, R. S. J. Electrochem. Soc. 1983, 130, 533.
21. Pourcelly, G.; Oikonomou, A.; Hurwitz, H. D.; Gavach, C. J. Electroanal. Soc.
1990, 287, 43.
22. Randin, J. J. Electrochem. Soc. 1982, 129, 1215.
23. Steck, A., Proc. of the 1st Internat. Symp. on New Materials for Fuel Cell
Systems, O. Savadago, P.R. Roberge, T.N. Vezioglu, Eds., Montreal, July 1995,
p. 74.
24. Hirschenhofer, J.H.; Stauffer, D.B.; Engleman, R.R., Fuel Cells: A Handbook
for the Department of Energy; B/T Books: Orinda, CA, 1996; p.1-1.
25. Carrette, L.; Friedrich, K.A.; Stimming, U., J. ChemPhysChem 2000, 1, 162-193.
26. Harrison, W. L.; Hickner, M. A.; Kim, Y. S.; McGrath, J. E., Poly(Arylene Ether
Sulfone) Copolymers and Related Systems from Disulfonated Monomer Building
Blocks: Synthesis, Characterization, and Performance - A Topical Review. Fuel
Cells 2005, 5, 201-212.
27. Kim, Y. S.; Wang, F.; Hickner, M.; Zawodzinski, T. A.; McGrath, J. E.,
Fabrication and Characterization of Heteropolyacid (H3PW12O40)/Directly
Polymerized Sulfonated Poly(arylene ether sulfone) Copolymer Composite
Membranes for Higher Temperature Fuel Cell Applications J. Membr. Sci. 2003,
212, 263-282.
28. Kim, Y. S.; Hickner, M.; Dong, L.; Pivovar, B. S.; McGrath, J. E., Sulfonated
Poly(arylene ether sulfone) Copolymer Proton Exchange Membranes:
Composition and Morphology Effects on the Methanol Permeability. J. Membr.
Sci. 2004, 243, 317-326.
47 29. Yang, Y.; Shi, Z.; Holdcroft, S., Synthesis of Sulfonated Polysulfone-block-
PVDF Copolymers: Enhancement of Proton Conductivity in Low Ion Exchange
Capacity Membranes. Macromolecules 2004, 37, 1678-1681.
30. Ghassemi, H.; Ndip, G.; McGrath, J. E., New Multiblock Copolymers of
Sulfonated Poly(4'- phenyl-2,5-benzophenone) and Poly(arylene ether sulfone)
for Proton Exchange Membranes. II. . Polymer 2004, 45, 5855-5862.
31. Xing, D.; Kerres, J., Improved Performance of Sulfonated Polyarylene Ethers for
Proton Exchange Membrane Fuel Cells. Polym. Adv. Technol. 2006, 17, 591-597.
32. Tan, A. R.; Magno de Carvalho, L.; de Souza Gomez, A., Nanostructured Proton-
Conducting Membranes for Fuel Cell Applications. Macromol. Symp. 2005, 229,
168-178.
33. Sumner, M. J.; Harrison, W. L.; Weyers, R. M.; Kim, Y. S.; McGrath, J. E.;
Riffle, J. S.; Brink, A.; Brink, M. H., Novel Proton Conducting Sulfonated
Poly(arylene ether) Copolymers Containing Aromatic Nitriles. J. Membr. Sci.
2004, 239, 199-211.
34. Taeger, A.; Vogel, C.; Lehmann, D.; Lenk, W.; Schlenstedt, L.; Meier-Haack, J.,
Sulfonated Multiblock Copoly(ether sulfone)s as Membrane Materials for Fuel
Cell Applications. Macromol. Symp. 2004, 210, 175-184.
35. Harrison, W. L.; Wang, F.; Mecham, J. B.; Bhanu, V. A.; Hill, M.; Kim, Y. S.;
McGrath, J. E., Influence of the Bisphenol Structure on the Direct Synthesis of
Sulfonated Poly(arylene ether) Copolymers. I. J. Polym. Sci.: Part A: Polym.
Chem. 2003, 41, 2264-2276.
48 36. Wang, F.; Hickner, M.; Kim, Y. S.; Zawodzinski, T. A.; McGrath, J. E., Direct
Polymerization of Sulfonated Poly(arylene ether sulfone) Random (statistical)
Copolymers: Candidates for New Proton Exchange Membranes. J. Membr. Sci.
2002, 197, 231-242.
37. Sankir, M.; Bhanu, V. A.; Harrison, W. L.; Ghassemi, H.; Wiles, K. B.; Glass, T.
E.; Brink, A. E.; Brink, M. H.; McGrath, J. E., Synthesis and Characterization of
3,3'-Disulfonated-4,4'- dichlorodiphenyl Sulfone (SDCDPS) Monomer for Proton
Exchange Membranes (PEM) in Fuel Cell Applications. J. Appl. Polym. Sci.
2006, 100, 4595-4602.
38. Chikashige, Y.; Chikyu, Y.; Miyatake, K.; Watanabe, M., Branched and Cross-
linked Proton Conductive Poly(arylene ether sulfone) Ionomers: Synthesis and
Properties. Macromol. Chem. Phys. 2006, 207, 1334-1343.
39. Takeuchi, M.; JIkei, M.; Kakimoto, M.-A., Preparation of Composites of
Hyperbranched Aromatic Poly(ether sulfone)s Possessing Sulfonic Acid Terminal
Groups and Epoxy Resin for Polymer Electrolyte. High Perf. Polym. 2003, 15,
219-228.
40. Mecham, J.B.; Wang, F.; Glass, T.E.; Xu, J.; Wilkes, G.L.; McGrath, J.E.,
Polymeric Materials: Science and Engineering 2001, 84, 105.
41. Kim, D. S.; Shin, K. H.; Park, H. B.; Chung, Y. S.; Nam, S. Y.; Lee, Y. M. J.
Membr. Sci. 2006, 278, 428.
42. Xing, P.; Robertson, G. P.; Guiver, M. D.; Mikhailenko, S. D.; Kaliaguine, S.,
Sulfonated Poly(aryl ether ketone)s Containing Naphthalene Moieties Obtained
by Direct Copolymerization as Novel Polymers for Proton Exchange Membranes.
49 J. Polym. Sci.: Part A: Polym. Chem. 2004, 42, 2866-2876.
43. Shang, X.; Tian, S.; Kong, L., and Y. M., Synthesis and Characterization of
Sulfonated Fluorene-Containing Poly(arylene ether ketone) for Proton Exchange
Membrane. J. Membr. Sci. 2005, 266, 94-101.
44. Xing, P.; Robertson, G. P.; Guiver, M. D.; Mikhailenko, S. D.; Wang, K.;
Kaliaguine, S., Synthesis and Characterization of Sulfonated Poly(ether ether
ketone) for Proton Exchange Membranes. J. Membr. Sci. 2004, 229, 95-106.
45. Liu, B.; Robertson, G. P; Kim, D. S; Guiver, M. D; Hu, W; Jiang, Z.
Macromolecules 2007, 40, 1934.
46. Gao, Y.; Robertson, G. P.; Guiver, M. D.; Mikhailenko, S. D.; Li, X; Kaliaguine,
S. Macromolecules 2005, 38, 3237.
47. Gao, Y.; Robertson, G. P.; Guiver, M. D.; Mikhailenko, S. D.; Li, X.; Kaliaguine,
S. Polymer. 2006, 3, 808.
48. Kim, D. S.; Robertson, G. P.; Guiver, M. D.; Lee, Y. M. J. Membr. Sci. 2006,
281, 111.
49. Bai, Z.; Dang, T. D., Direct Synthesis of Fully Sulfonated Polyarylenethioether
Sulfones as Proton-Conducting Polymers for Fuel Cells. Macromol. Rapid
Commun. 2006, 27, 1271- 1277.
50. Miyatake, K.; Oyaizu, K.; Tscuchida, E.; Hay, A. S., Synthesis and Properties of
Novel Sulfonated Arylene Ether/Fluorinated Alkane Copolymers.
Macromolecules 2001, 34, 2065- 2071.
51. Miyatake, K.; Hay, A. S., Synthesis and Properties of Poly(arylene ether)s
Bearing Sulfonic Acid Groups on Pendant Phenyl Rings. J. Polym. Sci.: Part A:
50 Polym. Chem. 2001, 39, 3211- 3217.
52. Miyatake, K.; Chikashige, Y.; Watanabe, M., Novel Sulfonated Poly(arylene
ether): A Proton Conductive Polymer Electrolyte Designed for Fuel Cells.
Macromolecules 2003, 36, 9691-9693.
53. Wang, L.; Meng, Y. Z.; Wang, S. J.; Hay, A. S., Synthesis and Sulfonation of
Poly(arylene ether)s Containing Tetraphenyl Methane Moieties. J. Polym.
Sci.:Part A: Polym. Chem. 2004, 42, 1779-1788.
54. Wang, L.; Meng, Y. Z.; Wang, S. J.; Shang, X. Y.; Li, L.; Hay, A. S., Synthesis
and Sulfonation of Poly(aryl ethers) Containing Triphenyl Methane and
Tetraphenyl Methane Moieties from Isocyanate-Masked Bisphenols.
Macromolecules 2004, 37, 3151-3158.
55. Lafitte, B.; Puchner, M.; Jannasch, P. Macromol. Rapid Commun. 2005, 26,
1464.
56. Liu, B.; Robertson, G. P.; Guiver, M. D.; Shi, Z.; Navessin, T.; Holdcroft, S.,
Fluorinated Poly(aryl ether) Containing a 4-Bromophenyl Pendant Group and its
Phosphonated Derivative. Macromol. Rapid Commun. 2006, 27, 1411-1417.
57. Doe, S. K.; Gilles, P. R.; Michael, D. G. Comb-Shape Poly(arylene ether
sulfone)s as Proton Exchange Membranes Macromolecules. 2008, 41, 2126-2134.
58. Kaiti, S.; Himmelberg, P.; Williams, J.; Abdellatif, M.; Fossum, E., Linear
Poly(arylene ether)s with Pendant Phenylsulfonyl Groups: Nucleophilic Aromatic
Substitution Activated from the Meta Position. Macromolecules 2006, 39, 7909-
7914.
59. Kerres, J. A.; Ullrich, A.; Hein, M., Preparation and Characterization of Novel
51 Basic Polysulfone Polymers. J. Polym. Sci.: Part A: Polym. Chem. 2001,
39,2874-2888.
52 VI. Appendix
Figure 17a: 300MHz 1H NMR diphenyl ether polymer
Figure 17b: 75.5MHz 13C NMR diphenyl ether polymer
53
Figure 18a: 300MHz 1H NMR BisAF polymer
Figure 18b: Zoomed in 300MHz 1H NMR BisAF polymer
54
Figure 18c: 75.5 MHz 13C NMR BisAF polymer
Figure 18d: Zoomed in 75.5 MHz 13C NMR BisAF polymer
55
Figure 19a: 300MHz 1H NMR BisA polymer
Figure 18c: 75.5 MHz 13C NMR BisA polymer
56