Effect of the hydrophobic component on the properties of sulfonated poly(arylene ether sulfone)s

Four kinds of sulfonated poly(arylene ether sulfone)s containing hydrophobic component of different size was synthesized by the copolymerization of disodium 3,3′-disulfo-4,4′-difluorophenyl sulfone and 4,4′-difluorophenyl sulfone with 2,2-bis(2-hydroxy-5-biphenylyl)propane (SPE1), 4,4′-dihydroxytetraphenylmethane (SPE2), 9,9-bis(4-hydroxyphenyl) fluorene (SPE3), or 2,7-dihydroxynaphthalene (SPE4) under nucleophilic aromatic substitution conditions. The copolymer composition was set at 45-70 mol % of 3,3′-disulfo-4,4′-difluorophenyl sulfone in order to achieve similar ion exchange capacity (ca. 2.0 mequiv/g) for all the sulfonated copolymers SPE1-4. The copolymers were of high molecular weight (M_n = 150-220 kDa, M_w = 310-695 kDa) to give tough and flexible membranes by solution casting. STEM observation revealed that small hydrophobic components (SPE3 and SPE4) induced larger water cluster than bulky hydrophobic ones (SPE1 and SPE2). The small hydrophobic components induced high proton conductivities and proton diffusion coefficients as well as low water swelling. SPE4 membrane showed the highest proton conductivity at 50-80 °C and 10-90% RH among the four SPEs membranes. The smaller hydrophobic component was also effective in terms of gas permeation and mechanical properties for fuel cell applications.