Advanced Materials for Membrane Separations - American

Chapter 23

Ion-Exchange Membranes from Blends of Sulfonated Polyphosphazene and Kynar FLEX PVDF 1,3

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Ryszard Wycisk , Roy Carter , Peter N. Pintauro , and Catherine Byrne

Downloaded by UNIV OF GUELPH LIBRARY on September 12, 2012 | http://pubs.acs.org Publication Date: April 20, 2004 | doi: 10.1021/bk-2004-0876.ch023

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Department of Chemical Engineering, Tulane University at New Orleans, New Orleans, L A 70181 Science Research Laboratory, Inc., 15 Ward Street, Somerville, M A 02143-4241 Current address: Department of Chemical Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, O H 44106 2

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Ion-exchange membranes were prepared by blending sulfonated poly[bis(3-methylphenoxy)]phosphazene (SPOP) with Kynar FLEX, a copolymer o f vinylidene fluoride and ®

hexafluoropropylene, which was followed by crosslinking with use of ultraviolet or e-beam radiation. It was found that, in general, Kynar FLEX was immiscible on a molecular level with the SPOP. The polymers had, however, acceptable degree of compatibility and no sign of macroscopic phase separation was observed for a relatively wide composition range. The degree of compatibility increased with increasing IEC of the SPOP. Also, when sulfonic groups o f the SPOP were converted to the tetrabutylammonium ( T B A ) form, highly homogeneous, transparent blends were obtained. These, however, could not be crosslinked effectively. The Na-form of SPOP was then used in further studies. A linear correlation was observed between the conductivity and swelling o f the blended and crosslinked membranes. A conductivity increase from 0.01 to 0.075 S/cm was accompanied by an increase in swelling from 20 to 80%. Additionally, membranes with the

© 2004 American Chemical Society

In Advanced Materials for Membrane Separations; Pinnau, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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same swelling but prepared from SPOP with higher IEC showed higher conductivity. No significant difference in the swelling versus conductivity dependence was observed between membranes crosslinked with benzophenone and U V light and those crosslinked with electron-beam radiation. Preliminary DMFC tests showed methanol crossover rates to be 3 times lower (6 times lower if thickness corrected) for the blended membranes as compared to Nafion 117.

Polyorganophosphazenes offer new possibilities as potential membrane materials. These polymers belong to a class of hybrid polymers; their backbone is inorganic while the side groups are of organic nature. Numerous papers have been published on various aspects of polyphospazene synthesis, properties, and applications but there are only a few reports on their use as ion-exchange or proton-exchange membranes (/-5). Our previous studies (2-6) have shown that ion-exchange membranes composed of sulfonated and crosslinked poly[bis(3-methylphenoxy) phosphazene] possess high proton conductivity (0.01-0.1 S/cm) and low methanol diffusivity (10" -10 cm /s). These properties make this polymers a candidate materials for use in direct methanol fiiel cells. Additionally, at low and moderate sulfonation degrees the polymer had acceptable mechanical properties and it could be crosslinked using benzophenone (BP) and U V irradiation. However, at ion-exchange capacities greater than 1.2 mmol/g the polymer was brittle which rendered the membranes difficult to handle and electrodes for fuel cell testing could not be hot-pressed to them. The search for a solution to this problem was directed in two different approaches. The first one was the synthesis of a new, more sophisticated polyphosphazene, and the second path was focused on blending of SPOP with some other, non-polyphosphazene polymer. The present paper deals with the second path. Several different polymers were tested for their blending capability with sulfonated poly[bis(3-methylphenoxy)phospahzene (SPOP): polyimides, polysulfone, polyphenyleneoxide, polyacrylonitrile, and various fluoropolymers. Recently, membranes have been prepared by blending SPOP with Kynar F L E X , a copolymer of vinylidene fluoride and hexafluoropropylene, manufactured by Elf Atochem North America, Inc. The formulation used in the present work contained ca. 10 % of hexafluoropropylene units. Proton conducting membranes containing Kynar FLEX can be attractive for direct methanol fuel cells because: (i) the mechanical properties of the blended film are better compared to pure SPOP films, (ii) the thermoplastic properties of the Kynar FLEX polymer should make hot-pressing electrodes easier during MEA fabrication, (iii) the blend is expected to have even lower methanol crossover because the fluoropolymer restricts SPOP swelling and is virtually impermeable to water and methanol, and (iv) the amount of expensive 8

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337 polyphosphazene in the film is lowered, thus reducing the overall cost of the membrane. In this paper we present preliminary data on the properties and morphology of blended SPOP/Kynar FLEX ion-exchange membranes.


Experimental Sulfonation of the polyphosphazene. A known weight (2g) of poly[bis(3methylphenoxy) phosphazene] (MW=700,000 g/mol) was dissolved in 80 ml of 1,2-dichloroethane (DCE). An appropriate amount (0.3-2.0 ml) of S0 in DCE was then added to the polymer solution at 0°C in a dry nitrogen atmosphere. The resulting precipitate was stirred for about 3 hours, followed by the addition of NaOH dissolved in a water/methanol mixture to terminate the reaction. After solvent evaporation, the sulfonated polymer was soaked in water and then treated sequentially with 0.01 M NaOH, water, 0.1 M HC1, and water. Next, the sulfonic groups were converted into the appropriate salt form using a 1M aqueous solutions of either NaCl or TBAC1 (tetrabutylammonium chloride) followed by through washing with water. In the final step the SPOP membrane was dried at 60°C. 3

Membrane preparation with UV crosslinking. SPOP, Kynar FLEX 2821 and benzophenone (BP) were dissolved in dimethylacetamide (DMAc). The solution was cast into a PTFE dish and kept in an oven at 60°C overnight. The dry membrane was removedfromthe dish and placed in a vacuum oven at 60°C for 24 hours to remove traces of DMAc. The SPOP was then crosslinked by exposing the membrane to U V radiation of 365 nm wavelength (15 mW/cm ) for a period of 12 h on each side. Membrane preparation with e-beam crosslinking. SPOP and Kynar FLEX 2821 were dissolved in dimethylacetamide (DMAc). The solution was cast into a PTFE dish and kept in an oven at 60°C overnight. The dry membrane was removed from the dish and placed in a vacuum oven at 60°C for 24 hours to remove traces of DMAc. The SPOP was then crosslinked by exposing the membrane to a specified dose of electron beam (e-beam) radiation using Science Research Laboratory's EB-10 RF accelerator running at a power of 650 watts and an energy of approximately 4 MeV. The sample chamber consisted of an aluminum base and a thin (1.5 mm) aluminum cover with inside dimensions of 200x300 mm sealed with Viton o-rings. Zero grade nitrogen was passed through the chamber constantly at a rate 280 cmVmin. The total applied dosage ranged from 40 to 160Mrad. Ion-exchange capacity measurements. A known weight of dry polymer sample (0.2-0.4 g, with sulfonic groups in S O 3 H form) was placed into 50 ml of


338 2.0 M NaCl solution at 25°C and shaken occasionally for 48 h. Three 10 ml samples were then removed and the amount of H+ released by the polymer was determined by titration with 0.01 M NaOH. The ion-exchange capacity (IEC) of the sample was calculated according to the following equation: IEC [mmol/g] = 0.05*v/ m

d

(1)


where v [ml] was the endpoint volume of 0.01 M NaOH (average of the three titrations), and ma [g] was the dry weight of the sample. Conductivity Measurements. Proton conductivity of the water swollen membranes, with sulfonic groups in S O 3 H form, was measured by an A C impedance method (real axis intercept of the impedance spectrum) in the frequency range 1 Hz to 100 KHz using a lock-in amplifier (EG&G Model 5210) and potentiostat (EG&G Model 273). The cell employed was a two-probe type, similar to that described in (7). Measurements were taken at 25°C. Swelling Measurements. Membrane sample (sulfonic groups in S O 3 H form) was equilibrated in water at 25°C for 24 h. After removalfromthe water bath, the membrane surface was blotted dry with filter paper and the sample was weighted on an electronic balance (m ). After thorough drying under vacuum and over P2O5 for 24 h, the membrane was re-weighed (ntd). The equilibrium water swelling (S) was calculated according to the following formula: w

S[g/g] = (mw-m )/m d

d

(2)

Tensile Tests. Tensile .measurements were performed on an Instron 5567. The specimens were tested in water-swollen state at 25°C and the extension rate was 50 mm/s. Based on the stress-strain curve, the ultimate strength and elongation to break were determined. SEM micrographs. Dry membranes were manuallyfracturedafter reaching thermal equilibrium in liquid nitrogen. Specimens were sputter-coated with gold to a final thickness of approximately 2 nm and imaged on a JEOL JSM-820 scanning electron microscope at 15 kV. X-ray Measurements Wide-angle x-ray scattering (WAXS) spectra were obtained using Scintag XDS 2000 diffractometer operated at 45 K V and 40 mA, with Cu anode. The scan range was 5-40 deg. and the scan rate was 2.00 deg./min. Membrane samples in the dry state and swollen with water were placed horizontally on zero background quartz plates. The measurements were taken at 25°C.


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Results and Discussion The membrane preparation procedure consisted of the following four steps: (i) dissolving the components (SPOP in a salt form and Kynar FLEX) in DMAc, (ii) casting a film and evaporating the solvent, (iii) crosslinking the SPOP with benzophenone and U V light or e-beam radiation, and (iv) converting the sulfonic groups back to the acid form. Several batches of sulfonated poly[bis(3-methylphenoxy)phosphazene] (SPOP) in appropriate salt forms were prepared. Their IEC ranged from 1.6 to 3.5 mmol/g. Blends with Kynar FLEX were then prepared. Half of the blends was crosslinked with use of benzophenone and UV light and the other half was crosslinked with e-beam radiation. Blending can be treated as a dilution of the polymers. In order for the blended membrane to show appropriate electrochemical performance it is necessary to use an SPOP of significantly higher ion-exchange capacity than that in the final blend. For example, if the desired IEC of the blended membrane is 1.2 mmol/g and the Kynar FLEX content is to be 50% than the required IEC of the SPOP has to be 2.4 mmol/g. It may occur that the sulfonated polymer of that high IEC is water-soluble and the blended membrane may degrade in aqueous solutions. Therefore, additional crosslinking or grafting may be required to ensure stability of the blended membrane. Figure 1 shows a relationship between the ion-exchange capacity of the SPOP and its water affinity. The starting, unsulfonated material has an IEC of 0 mmol/g. When half of the number of aromatic rings is monosubstituted with S O 3 H , the IEC is 3 mmol/g. Finally, when all the aromatic rings are monosubstituted, an IEC of 4.8 mmol/g is reached. The most important finding was that at an IEC greater than 2.1 mmol/g, the SPOP became water soluble. Below that limit the polyphosphazene only swelled in water but did not dissolve. It may be concluded, that using SPOP of IEC greater than 2.1 will require crosslinking or grafting not only to reduce swelling but, primarily, to prevent the sulfonated componentfrombeing leached outfromthe blend in aqueous solutions. Two crosslinking procedures were used in the present research. The first one was benzophenone-assisted UV-crosslinking ( 2 mmol/g

BLENDING

PROBLEM- phase separation (domain-type blends, limited UV light penetration, low mechanical strength) RECOMMENDATION - highest IEC possible, SPOP in S0 TBA form 3

CROSSUNKING

PROBLEM - SPOP leaching or membrane swellingtoohigh RECOMMENDATION - low IEC, SPOP in S0 Na form, high Kynar FLEX loading, high irradiation dosage 3

Figure 8. The most important conclusions on each of the three steps of membrane preparation.


350 appropriate optimization of the morphology and crosslinking degree of the blended membranes and with improvement of the MEA preparation technique, much better fuel cell performance can be achieved.


Conclusions Blended membranes composed of sulfonated polyphosphazene and Kynar FLEX were synthesized. The membranes had good proton conductivities and mechanical properties. Successful blending and crosslinking was dependent on the ion-exchange capacity and the counter-ion form of the sulfonated polyphosphazene. A summary of the most important challenges in the membrane preparation procedure along with some possible recommendations are presented in Fig. 8. A linear correlation was observed between the swelling and proton conductivity of the membranes. A conductivity increase from 0.01 to 0.075 S/cm was accompanied by an increase in swellingfrom20 to 80%. Additionally, membranes of the same swelling but prepared from SPOP with higher IEC showed a higher conductivity. No significant difference in the swelling versus conductivity behavior was observed between membranes crosslinked with benzophenone, UV light, and those crosslinked with electron-beam radiation. Future research will focus on improving the blended membrane morphology by optimizing membrane casting conditions and on increasing the efficiency of crosslinking.

Acknowledgements This work was funded by the Army Research Office, Grant No. DAAD1900-1-0517, and by an Army Research Office STTR program, through a grant to Science Research Laboratory, Inc. We would like to thank Giner, Inc., Waltham, M A for preparing membrane electrode assemblies and performing the fuel cell tests. We also thank Elf Atochem North America, Inc. for supplying the Kynar FLEX®.

References 1. 2. 3.

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4. Guo, Q.; Pintauro, P.N.; Tang, H.; O'Connor, S. J. Membr. Sci. 1999, 154, 175. 5. Tang, H.; Pintauro, P.N. J. Appl. Polym. Sci. 2000, 79, 49. 6. Carter, R.; Evilia, R.; Pintauro, P.N. J. Phys. Chem. B 2001, 105, 2351. 7. Zawodzinski, T.A., Neeman, M., Sillerud, L.O., Gottesfeld, S. J. Phys. Chem. 1991, 95, 6040. 8. Graves, R.; Pintauro, P.N. J. Appl. Polym. Sci. 1997, 68, 827. 9. Smith, P.; Hara, M.; Eisenberg, A. In Current Topics in Polymer Science, Volume II; Ottenbrite, X.; Utracki, X.; Inoue, X., Eds.; Hanser Publishers: New York, NY, 1992, p 255.


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