Network Constraint Density and Structure–Property Relationships in Branched Sulfonated Poly( para -phenylene) Membranes for Water Electrolysis

Sulfonated poly(p-phenylene) (SPP) ionomers exhibit excellent chemical stability due to their fully aromatic, ether-free backbone; however, their rigid structure induces dense two-dimensional stacking and limited chain mobility, restricting proton transport. To address this limitation, molecular branching units were introduced during the SPP synthesis. SPP ionomers containing 0–5 mol % of a trifunctional branching unit were synthesized, and the effects of branching on network constraint density (N), swelling behavior, proton conductivity, and oxidative stability were systematically investigated. Interestingly, the resulting membranes showed non-monotonic dependence with a maximum at intermediate branching fractions. At low branching contents, a decrease in the swelling-derived N accompanies enhanced hydration and proton conductivity, consistent with the proposed relaxation of physical constraints. At higher branching contents, a slight increase in N accompanies reduced water uptake and proton conductivity, consistent with increasing branching-induced constraints, while improving oxidative stability, as confirmed by Fenton tests. An optimal balance is achieved at intermediate branching, where membrane exhibits a proton conductivity of 104 mS/cm at 80 °C, approximately 30% higher than that of linear SPP, while maintaining low hydrogen permeability. These results highlight the value of combining swelling-derived network constraint analysis with structural characterization to understand structure–property relationships in branched aromatic ionomers and demonstrate molecular branching as a promising strategy for membranes in water electrolysis.