Phase behavior and dynamics of charge-neutral AB, ABA, and BAB block ionomer melts

Block copolymers composed of ionic (A) and neutral (B) segments exhibit distinctive self-assembly behavior arising from the interplay between interfacial tension and strong electrostatic correlations. We investigate AB, ABA, and BAB copolymers containing ~18 mol% ionic functionality with total a degree of polymerization (N) of 200. Small-angle X-ray scattering and rheology reveal that the AB diblocks form an inverse hexagonally packed cylindrical morphology (iHEX), where the minority ionic block constitutes the continuous matrix. This unusual microstructure yields high modulus and elasticity over a broad temperature range. Even at T ≫ T_g,B, chain exchange across domains remains hindered due to the large segregation strength (χN ≫ 1) and the strong Coulombic cohesive energy of the ionic matrix. By contrast, triblocks with identical charge density exhibit markedly different thermomechanical responses. The ABA architecture behaves as a conventional thermoplastic elastomer and flows at elevated temperature, whereas the BAB analogue maintains network integrity and mechanical stability across the same temperature range. These differences arise from distinct connectivity of ionic junctions and the corresponding nanodomains. Our results delineate the interplay between electrostatics, architecture, and segmental dynamics in dictating phase behavior and viscoelasticity, providing quantitative insights into structure–mechanics relationships in ion-containing block copolymer melts.