Preparation of semi-interpenetrating polymer networks with adjustable mesh width and hydrophobicity

Proton exchange membranes (PEMs) with a semi-interpenetrating polymer network (SIPN) structure were prepared by cross-linking brominated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) with different aliphatic α,ω-diamine cross-linkers in the presence of sulfonated PPO (SPPO). The alkylation of the α,ω-diamine with BPPO resulted in a covalently cross-linked BPPO network where SPPO could be immobilized by interlocking. The interlocked structure was also strengthened by ion pair interactions between the sulfonic acid groups of SPPO and the amine moieties formed in cross-linking. The length of the aliphatic α,ω-diamine cross-linker was varied to modify the hydrophobicity and the mesh width (the average distance between two cross-linked polymer segments) of the cross-linked network host. The effects of these adjustments on the structure of the cross-linked network host were evaluated by the morphology of the hydrophilic domains, and transport properties such as proton conductivity and methanol permeability. It was found that the increase in mesh width and hydrophobicity of networks formed by long cross-linkers resulted in more scattered hydrophilic domains and fewer contiguous connections. On the contrary, the hydrophilic domains in networks with short cross-linkers were closer and more extensively-connected to facilitate proton transport. Methanol crossover in the SIPN membranes, on the other hand, could be suppressed by downsizing the size of the hydrophilic domains. Overall water uptake and dimensional swelling of the membranes were also affected by the cross-linker length. Some of the SIPN membranes delivered better performance than the Nafion® 117 membrane in single stack fuel cell tests, demonstrating the potential of SIPNs for the construction of fuel cell PEMs.