Polymer Electrolyte Membranes Based on Multiblock Poly(phenylene ether ketone)s with Pendant Alkylsulfonic Acids: Effects on the Isomeric Configuration and Ion Transport Mechanism

Two structural variations of multiblock poly­(phenylene ether ketone)­s (bSPPEKs) with pendant alkylsulfonic acids were prepared by a polycondensation reaction between oligomeric difluoro and diphenoxide precursors, followed by successive demethylation and sulfonation processes. Two isomers, bis­[4-fluoro-3-(4′-methoxylbenzoyl)]­biphenyl (p-BFMBP) and bis­[5-fluoro-2-(4′-methoxylbenzoyl)]­biphenyl (m-BFMBP), were prepared as the difluoro starting monomers. The corresponding polymer membranes were obtained with good mechanical stability to facilitate the further characterizations. The morphology of bSPPEKs was confirmed by atomic force microscopy, from which the distinct phase separation could be identified with the average hydrophilic domain size of ca. 15–20 nm for the m-bSPPEKs. It is worth noting that both m- and p-bSPPEKs show quite comparable, or even better, ion conduction capacities than commercially available Nafion membrane, especially under low relative humidity conditions (30–50%), suggesting their promising future as polymer electrolyte membrane candidates. Meanwhile, the proton conductivities of all of the m-bSPPEKs were found to be greater than those of the p-bSPPEKs over the entire relative humidity range, indicating better water-retention capacity and lower resistance to the ion transport. In addition, molecular dynamics simulation was employed to extensively explore the structure–property relationship between the two polymers. The length, angle, and torsion distribution results clearly reveal their steric configurations, that is, a longer length and smaller angle in the side chain, together with a smaller torsion angle in the main chain for m-bSPPEKs, which produces slightly more free volume inside the polymer matrix. Analysis of the diffusion coefficients and coordination numbers shows that there is more water clustering around the sulfonic acid groups in the meta-polymers than in the para-polymers.