Competition between side chain length and side chain distribution: Searching for optimal polymeric architectures for application in fuel cell membranes

Abstract

Microphase separation within 10 polymeric membranes of similar ion exchange capacity is studied by dissipative particle dynamics (DPD). The polymers consist of hydrophobic A and hydrophilic C fragments. For 8 grafted architectures, the side chains ([C], [AC], [AAC], or [AAAC]) are distributed uniformly or pairwise along the hydrophobic backbone. For the other 2 (block type) architectures the C fragments are uniformly and pairwise distributed within the backbone, respectively. For the water containing pore networks the following trends are found: For the uniform architectures, the pore size is lowest for the block- and increases further for the grafted architectures with increase of side chain length, while for the pairwise architectures the reverse trend is observed. Water diffusion through the hydrophilic pore networks is deduced from Monte Carlo tracer diffusion calculations (through 800 snapshots). Among the uniform architectures diffusion is highest for the grafted architecture with long [AAAC] side chains. Interestingly, for the pairwise architectures diffusion is highest for the grafted polymers with the short ([C]) side chains. Side chain length and side chain distribution are thus predicted to be interesting design parameters in order to optimize proton and or solvent transport within flexible amphiphilic polymeric membranes.