When BAB Beats ABA ionomers: Architecture-Dependent Micelle Packing under Limited Intermicellar Connectivity

In conventional ABA triblock copolymers, hard domains (A) often act as physical cross-links connected by a soft midblock (B), leading ABA architecture to be commonly associated with better mechanical performance than BAB analogues. Here, we propose a new design principle using triblock ionomers in which charged A blocks self-assemble into spherical ionic micelles dispersed within a polymer matrix. Surprisingly, BAB triblock ionomer exhibits higher modulus and markedly slower stress relaxation than compositionally analogous ABA triblock. Small-angle X-ray scattering and atomic force microscopy reveal that the BAB architecture produces a much higher density of smaller ionic micelles with substantially reduced intermicellar spacing. With both ABA and BAB show little-to-no intermicellar connectivity, the enhanced mechanical performance cannot be explained by conventional thermoplastic elastomer (TPE) theory. Instead, the dense micellar packing and the resulting cooperative structural relaxation play a critical role in governing the mechanical response. More broadly, this work demonstrates that self-assembled ionic domains can fundamentally alter the structure–property relationships of triblock copolymers and identifies micellar organization as an important design parameter for controlling the mechanics and thermomechanical stability of ionomer materials.