Design And Properties Of Poly (Aryiene Ether)s Bearing Aliphatic-chain-linked Sulfophenyl Pendants For Proton Exchange Membranes

Fuel cells are regarded as one of the promising clean future power sources for theirhigh conversion efficiency and low emissions. Proton exchange membranes （PEMs）are attracting increasing attention due to being key components ofenvironmental-benign fuel cells. Among all kinds of proton conducting polymerelectrolytes that have been reported, perfluorinated sulfonic acid ionomers such asNafion （Dupont） and Dowmembrane （DOW） brought a major breakthrough in protonexchange membrane fuel cells （PEMFCs） and direct methanol fuel cells （DMFCs）.However, high cost, high methanol crossover, low operation temperature （<80°C）,and environmental recycling uncertainties of Nafion and other similar perfluorinatedmembranes are limiting their widespread commercial application in Fuel cells.Recently, great efforts have been made to design and synthesize alternative PEMmaterials to the perfluorosulfonic acid ones. Sulfonated aromatic polymers, such assulfonated poly（arylene ether ketone）s, sulfonated polyimides, sulfonatedpoly（arylene ether nitrile）s and sulfonated poly（benzimidazole）s, are among the mostpromising candidates as alternative PEM materials due to their high thermal andchemical stability, high proton conductivity and low methanol permeability. However,in comparison with perfluorosulfuric acid membranes, sulfonated aromatic PEMsoften have lower proton conductivity, which are highly related with their weak acidityand poor hydrophilic and hydrophobic phase separation. A successful way to separatethe hydrophilic sulfonic acid units and the hydrophobic polymer main chain is tolocate the sulfonic acid units at the chain ends of short side chains on the polymermain chain.In this study, two series of polyarylether-type polymers having sulfophenyl pendantconnected by flexible–O-（CH₂）₃-O-linkage were synthesized through a direct polymerization of a new sulfonated difluoro-monomer. Different our previouslyreported mainchain-acid and side-group-acid PEMs, these kinds of polymerscontained flexible side chains terminated by sulfophenyl pendant as an attempt toimprove the formability of hydrophobic/hydrophilic phase separation. Meanwhile, theproperties relative to PEMs applications were fully investigated.The generatedmembranes exhibited improved thermo-oxidative stability and dimensional stability.Some of them had high proton conductivities and low permeability. For example,PAEK-3with IEC¹.5meq.g^-1showed a proton conductivity of0.12S.cm^-1at100oC,and its swelling ratio in water at100oC was only about26%. It was interesting toobserve obvious hydrophobic/hydrophilic phase separation morphology, which maybe highly related with their improved proton transport ability. All the data illustratedthat both PAEK-3and PAEN-3might be potential in the possible future usage. Basedon above results,the more hydrophobic mainchain and the more flexible sulfonicpendants favor the properties of sulfonated polymers.