Charge Separation Length of Zwitterions Dictates Coacervation-Mediated Microphase Separation of Zwitterionic Double Hydrophilic Block Copolymers in Water

Abstract

Aqueous mesoscopic compartments produced by the microphase separation of double hydrophilic block copolymers serve as adaptive molecular compartments driven by thermodynamic imbalances. The impact of the inter-zwitterion interaction strength, modulated by the charge separation length (CSL) in zwitterions, on the microphase separation behavior of poly(carboxybetaine methacrylate)-block-poly(sulfobetaine methacrylate) (PCB2-b-PSBX) aqueous solutions was investigated. We demonstrate that longer CSLs significantly enhance the cohesive energy between sulfobetaine pairs, effectively deepening the thermodynamic potential well for phase separation. This enhancement reduces the threshold degree of polymerization required for ordering and promotes the formation of an ordered lattice at lower concentrations. Conversely, the resulting increase in the effective transient crosslink density led to the suppression of the water-uptake capacity of the PSBX-rich phase and the promotion of selective water partitioning into the PCB2-rich phase. The associated increase in interfacial stiffness and elastic constraint facilitates phase separation but can also lead to a kinetically arrested disordered state by inhibiting structural relaxation towards ordered lattices at high polymer concentrations. These findings highlight the importance of precise control of the inter-zwitterion interaction energies and the resulting interfacial mechanics for designing molecular systems with programmable, adaptive mesoscopic architectures.