Membrane post-sulfonated polymers

Sakaguchi, Y, Kaji, A., Kitamura, K., Takase, S., Omote, K., Asako, Y, Kimura, K., Polymer electrolyte membranes derived from novel fluorine-containing polyfarylene ether ketone)s by controlled post-sulfonation. Polymer, 2012,53,4388-4398. 

Y. Sakaguchi, A. Kaji, K. Kitamura, S. Takase, K. Omote, Y. Asako, K. Kimura, Polymer electrolyte membranes derived from novel fluorine-containing poly(arylene ether ketone)s by controlled post-sulfonation.Po/ymer2012,53(20), 4388-4398. 

Polymer membranes have low proton conductivity until they contain enough proton conductive functional groups, e.g., sulfonic acid, in polymer backbones or side chains. Sulfonic acid in polymer chains shows a high dissociation constant, resulting in high proton conductivity in aqueous environments compared with other acids therefore, most DMFC membranes use the sulfonic moiety as the proton transfer medium. Sulfonation methods of polymer membranes are generally classified as post-sulfonation in the presence of polymers and in situ sulfonation through co-polymerization of sulfonated monomers and nonsulfonated monomers. 

The basic concept to use block co-polymer for the application to the DMFC is that ordered hydrophilic/hydrophobic phase separations offer a route for the selective transport of proton ions with reduced methanol crossover in the hydrophilic domains, because block co-polymers can be selectively sulfonated using post-sulfonation methods, and the block co-polymers can be verified over a wide range of structures during anionic polymerization. For example, methanol transport behaviors of a triblock co-polymer ionomer, sulfonated poly(styrene-isobutylene-styrene) (S-SIBS), were compared with Nafion to determine whether the sulfonated block co-polymer could serve as a viable alternative membrane for application to the DMFC [62]. The S-SIBS membranes showed approximately 5-10 times more methanol selectivity than that of Nafionll , although the S-SIBS membranes exhibited low conductivity compared with Nafion 117. 

Other groups have investigated additional inorganic additives, such as zir-conivun tricarboxybutylphosphonate (ZrPBTC), for use in a direct methanol fuel cell [35-37]. They also applied a post-sulfonation thermal treatment to the m-PBI/ZrPBTC membrane to increase the conductivity. ZrPBTC was introduced into the polymer by dispersing ZrPBTC powder in a DMAc solution of m-PBI [37]. The solvent was evaporated, leaving behind a 50 wt% ZrPBTC/m-PBI composite membrane. The membrane was then soaked in hydrochloric acid to introduce protons. Further immersion in either phosphoric acid or sulfuric acid produced a doped membrane. The sulfuric acid (SA)-doped membrane was then thermally treated at 480 °C for 60 s. 

Ion-permeable membranes are also made by swelling existing films with styrene and DVB, which can then be post-treated to add functional groups, or by grafting of ion-exchange functional groups directly onto the polymer matrix of existing films. For example, free radicals formed by radiation of polyethylene or fluoropolymer films become sites for addition of vinyl sulfonic acid, acrylic acid or vinyl amines [30]. 

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