Structural Investigation of Sulfonated Polyphenylene Ionomers for the Design of Better Performing Proton-Conductive Membranes

To achieve high-performance proton-exchange membranes (PEMs), understanding of the polymer structure/property relationship is crucial. In particular, the structure of water clusters (number, size, interdomain distance, interconnectivity, etc.) and hydrophobic domains dominates important membrane properties, such as proton conductivity and mechanical strength, which can be adjusted by the monomer sequence in the polymer chains. In the present paper, we have prepared three sulfonated polyphenylene-based copolymers (SPP-MP, SPP-BP, and SPP-QP) whose main chain components were the same but their sequence differed by the use of different hydrophobic monomers (monophenylene, -MP; biphenylene, -BP; and quinquephenylene, -QP, respectively). Careful investigation of the proton nuclear magnetic resonance (1H NMR) spectra suggested that the randomness of the hydrophilic component (sulfophenylene unit) was dominated by the hydrophobic component: 51% for -MP, 32% for -BP, and 19% for -QP, respectively. Transmission electron microscopy (TEM) observation of the three polyphenylene ionomer membranes revealed that the lower randomness of the hydrophilic component caused a larger hydrophilic domain size in their phase-separated morphology under dry conditions. Small-angle X-ray scattering (SAXS) measurements suggested that SPP-QP, with the lowest randomness of the hydrophilic component, possessed the most pronounced periodic structure under humidified conditions. The connectivity of water clusters, estimated by the small-angle neutron scattering (SANS) measurements, was in the order SPP-QP > SPP-MP > SPP-BP with a minor humidity dependence. The proton conductivity and elongation at break increased with increasing connectivity of the water clusters or decreasing randomness of the hydrophilic component. These results suggest that the sequence of the hydrophobic component strongly affected the hydrophilic component, and accordingly, the membrane morphology and properties.