Poly(para-phenylene) ionomer membranes: effect of methyl and trifluoromethyl substituents

Abstract

Sulfonated poly(para-phenylene)s with a high molecular weight and membrane forming capability were obtained by using methyl and trifluoromethyl substituents. The linearity of the polymer main chain decreased by introducing these substituents; the persistence length (l_p, index of linearity, distance required for a polymer chain to bend by 90° on average) of homopolymers for 2,2′-dimethyl-1,1′-biphenyl (BP-CH₃) and 2,2′-bis(trifluoromethyl)-1,1′-biphenyl (BP-CF₃) was ca.350.6 nm and 87.7 nm, respectively, estimated by numerically averaging backbone conformations. Copolymers with sulfo-para-phenylene, SPP-BP-CH₃ and SPP-BP-CF₃, were obtained with a high molecular weight (M_n = 28–30 kDa and M_w = 88–100 kDa for SPP-BP-CH₃ and M_n = 49–149 kDa and M_w = 161–316 kDa for SPP-BP-CF₃, respectively) to provide flexible membranes by casting from the solution. Despite the more hydrophobic nature of the substituents, SPP-BP-CF₃ membranes showed higher water uptake and proton conductivity than SPP-BP-CH₃ membranes with comparable ion exchange capacity (IEC). SPP-BP-CF₃ membranes showed slightly higher maximum strain (2.9–5.2%) than SPP-BP-CH₃ membranes (1.1–2.1%), leading to a higher rupture energy as expected from the smaller persistence length of BP-CF₃ homopolymers. While SPP-BP-CH₃ decomposed under harsh oxidative conditions, SPP-BP-CF₃ was more oxidatively stable and exhibited negligible changes in the weight, molecular weight, molecular structure and membrane properties (proton conductivity, mechanical properties, etc.).