Heterogeneity-Driven Chain Reorganization and Color Evolution in Quaternized Block Copolymer Microparticles under pH Stimuli

Abstract

Stimuli-responsive block copolymer (BCP) particles offer a promising platform for tunable photonic materials; however, most structural transformations originate from uniform lamellar templates that reorganize only under strong thermal or solvent-mediated activation. Here, we report a distinct pH-driven chain reorganization behavior in partially quaternized poly(styrene-block-2-vinylpyridine) (PS-b-P2VP) microparticles that initially possess heterogeneous internal morphologies, comprising PS-encapsulated P2VP domains and stacked lamellar domains. Upon acid exposure, protonation of unquaternized P2VP establishes a hydration-induced swelling gradient: less-constrained lamellae laterally expand and redistribute along interfaces, whereas PS-encapsulated lamellar regions act as rigid anchors. This anisotropic response progressively redistributes chain stress and solvation, transforming the stacked lamellae into an irregular, laterally dilated morphology with thin, hydrated P2VP layers. This progressive chain reorganization gives rise to a steady blue-shift in structural color from 622 to 478 nm, in line with the gradual contraction of domain periodicity. These findings reveal that structural heterogeneity can serve as an intrinsic driving force for topological reconstruction in vitrified BCP particles, enabling programmable, history-dependent photonic responses under mild aqueous conditions.