Tetrafunctional Prepolymer Effects on the Properties of Sulfonated Poly(p-phenylene)-Based Multiblock Ionomers for Polymer Electrolyte Membranes

Sulfonated poly(p-phenylene) ionomers (SPP) have emerged as promising replacement candidates for perfluorinated sulfonic acid ionomers due to their excellent chemical stability. However, their practical application is hindered by relatively poor physical properties, despite having a high Young's modulus resulting from their rigid structure. To address this, we employed a tetrafunctional prepolymer (t-PSK, 5.4 kg/mol) to enhance the molecular weight of the ionomer (SPP-PSK) and improve its mechanical characteristics as a polymer electrolyte membrane, especially for water electrolysis. While introducing t-PSK increased the molecular weight of the ionomer, it led to a slight decrease in the modulus and tensile strength in the hydrated state, primarily attributed to higher water uptake at elevated t-PSK content. To gain insights into this unexpected water uptake behavior, we applied the modified Flory-Rehner equation to evaluate the effective number of physical and chemical cross-links in the water-swollen equilibrium state. The analysis revealed a reduction in the number of cross-links and entanglements with an increasing t-PSK content. Our findings indicate that ionomer chain entanglements are more critical than chemical cross-links in controlling water uptake in poly(p-phenylene) copolymers. Remarkably, the linear SPP-PSK showed the highest proton/hydrogen selectivity among the SPP-PSK ionomers with tPSK, despite absorbing less water than the other SPP-PSK ionomers with t-PSK. These insights shed light on water uptake behavior and emphasize the importance of ionomer chain entanglements in controlling water uptake, providing valuable guidance for further advancements in this field.