Counterion Entropy Impacts on Bulk Phase Behavior in Ether-Based Charge-Neutral Block Copolymers*

The impact of electrostatics on block copolymer (BCP) thermodynamics has been an elusive inquiry for several decades. While significant progress has been made in unravelling the effects of lithium salt doping in ether-based BCPs, much less is known about pendant charges wherein a critical gap between theory and experiment exists. The largely reduced entropic screening penalty of pendant charges relative to mobile salts forces us to consider enthalpic correlations in much finer detail which can be greatly convoluted by molecular architecture. To help decouple these equilibrium factors, a fundamental understanding of counterion entropy effects is needed. In this work, we use an ether-based platform to compare the effects on phase behavior and dielectric response between two charged species: one where the counterion is mobile, and the other bound as a zwitterion. To our knowledge, this is the first time that backbone-tethered zwitterions have been used to study BCP thermodynamics in the bulk. The BCP of interest was designed specifically to minimize both electrostatic cohesion and preferential ion solvation between phases, thus allowing counterion translational entropy to become the dominant influence on phase behavior. Using dielectric relaxation spectroscopy, small angle X-ray scattering, and rheology, we seek to understand how counterion entropy impacts BCP thermodynamics by coupling microphase morphology and segregation strength to ion dynamics and interactions.