Structure and Diffusion in Cross-Linked and Sulfonated Poly(1,3-cyclohexadiene)/Polyethylene Glycol-Based Proton Exchange Membranes

Abstract

The nanoscale structure and water and charge diffusion in cross-linked and sulfonated poly(1,3-cyclohexadiene) (xsPCHD) hydrated membranes based on xsPCHD homopolymer, xsPCHD/polyethylene glycol (xsPCHD/PEG) copolymer, and xsPCHD/PEG blend were studied through molecular dynamics (MD) simulations, confined random walk (CRW) simulations, and percolation theory. MD simulation results show nanophase segregation into hydrophobic and aqueous domains for all three membranes at λ = 10 H2O/HSO3. The presence of PEG and the manner in which it is incorporated, either as a copolymer or as a blend, has a significant impact on both structure and transport properties of the membrane. The PEG–H3O+ pair correlation functions (PCFs) show that the OH end group of PEG has a much stronger affinity for H3O+, while the backbone of PEG has a weaker affinity for H3O+ in the copolymer membrane; the reverse is true in the blend. The PEG–water PCFs show PEG has greater affinity to water in the blend than that in the copolymer membrane, suggesting PEG is well mixed with water. The features of the PCFs suggest very different relative importance of proton transport mechanisms in the copolymer and blend membranes. MD and CRW simulation results show that the existence of PEG slows down water diffusion in the blend membrane. In the blend, PEG can be seen as an additive that slows water diffusion but enhances proton conductivity. The same is true for the xsPCHD/PEG copolymer, but to a lesser extent, due to the poorer distribution of PEG within the aqueous domain. The results can be generalized in terms of understanding the role of a polar cosolvent that reduces water mobility but enhances proton conductivity.