Nanoscale Phase Separation of Sulfonated Poly(arylene ether sulfone)/Poly(ether sulfone) Semi-IPNs for DMFC Membrane Applications

Abstract

Unprecedented improvement in the selectivity (the ratio of proton conductivity to methanol permeability) of DMFC (direct methanol fuel cell) membranes has been demonstrated with a value roughly 16 times higher than that of Nafion 117 having been achieved. The novel morphology of semi-interpenetrating polymer network (semi-IPN) membranes characterized by manometer-sized domains as well as well-developed phase cocontinuity is a key factor in enabling such notable progress, which has not been seen in conventional microscale phase separation. The semi-IPN membranes (sIPN-100) consisted of a hydrophilic component acting as a proton conductor, that is, acrylate-terminated fully sulfonated poly(arylene ether sulfone) oligomers (acSPAES-100, degree of sulfonation = 100%), and a hydrophobic component functioning as a methanol barrier, that is, poly(ether sulfone) copolymers (RH-2000). We determined the nanoscale phase separation of sIPN-100 by deliberately controlling the kinetics (the change of solvent-evaporation conditions) as well as the thermodynamics (shift of the phase separation boundary to the lower concentration of solvent in the phase diagram, mostly driven by the low molecular weight and the low hydrophilicity of acSPAES-100). Finally, the influence of this unique morphology on the membrane transport properties including the proton conductivity, the methanol permeability, and, more notably, the selectivity, was systematically investigated.