Poly(para-phenylene) ionomer membranes: effect of methyl and trifluoromethyl substituents†
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 (lp, 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 (Mn = 28–30 kDa and Mw = 88–100 kDa for SPP-BP-CH3 and Mn = 49–149 kDa and Mw = 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.).