Preparation And Properties Of Sulfonated Polyimides As Proton Exchange Membranes

With the rapid development of technology and industry, fossil fuels areexhausting and pollution is worsening day by day. Fuel cells, as high performanceenvironmental friendly energy, calls increasing attention by scholars and scientists allaround the world. After several years’ research, direct methanol fuel cells (DMFCs)and proton exchange membrane fuel cells (PEMFCs) have been systematicallystudied and relating applications have been tested. Polymer electrolyte membranes(PEMs) works as the core part of fuel cells and determines the performance of fuelcells directly. Nowadays, commercial products such as Nafion membranes, which areproduced by Dupont Ltd., are perfluorosulfonic acid type membranes. Although theyhave good proton conductivity, excellent oxidative and dimensional stability andother competitive performances, perfluorosulonic acid membranes are limited infurther applications owning to their high cost, limited working conditions andundesired methanol resistance. Finding substitutions of perfluorosulonic acidmembranes becomes a hot topic in this field. It is found that sulfonated poly aromaticether (SPAE) polymers have good thermal and chemical stability. Among which,sulfonated polyimides (SPIs) are very competitive to be an alternative kind ofmaterials due to their great film-forming property. Also, many of their properties canbe improved by modifying the structures of the polymer chains.Based on many accumulations of previous scholars, it is found that the propertiesof materials are closely related to their structures. Properties can be effectivelyimproved by constructing the structures of polymers. For this purpose, Diaminemonomers with different structures are designed and synthesized, by polymerizingwith sulfonated diamine and1,4,5,8-naphthalenetetracarboxylic dianhydride, differentpolymides are obtained. Some interesting structure-property relationships are revealed after complete comparison of the performances of those materials.In the first part, two isomeric diamine monomers,2,5-bis(4-amino-2-trifluoromethylphenoxy)biphenyl (PPDA) and4,4’-bis(4-amino-2-trifluoromethylphenoxy)biphenyl (DPDA), were prepared afternucleophilic substitution and hydrogenation reduction. Sulfonated polyimides namedPP-SPI and DP-SPI respectively, were synthesized through a typical “one pot”polymerization reaction. Hybrid membranes were constructed by incorporating amultiple-acid functionalized polysilsesquioxane into above polymers via a sol-gelprocess. After characterization, both series of membranes exhibited good performancein thermal, oxidative stability. PP-SPI and DP-SPI showed0.119S/cm and0.113S/cmof proton conductivity in high temperature, PP-POSS even reached0.138S/cm at100oC, which is near to Nafion117. Meanwhile, they all have much lower methabolpermeatibilty than Nafion117, arounding3.87~6.94×10-7cm2s-1. It was also foundthat isomeric structure could affect the properties of both the pristine and hybridmembranes. The microstructures featured with phase-separation morphology could beobserved in the hybrid membranes using scanning electronic microscopy.In the second part, novel multiple cyano groups containing diamine monomers,2,5-bis(4-amino-2-cyanophenoxy)-1-cyano-benzene (CNDA) was constructed bynearly the same method mentioned in part one, some multiple cyano groupsfunctionalized sulfonated polyimides (CN-SPIs) are prepared derived from thoseCNDA. Meanwhile, some similar SPIs but without cyano groups (Ph-SPIs) areprepared as counterparts for comparison purposes. It was of interest to find aftercomparison that cyano-functionalized SPIs could possess an excellent combination oflow swelling ratios (lower than20%at all desire temperatures), better tensile strength(62.7~70.4MPa) and elongation at break (15.6~22.4%).