Unlocking Hidden Miscibility: Entropy Diluent Strategy for Incompatible Polymer Blends

Abstract

Polymer mixtures are generally considered unfavorable for the preparation of homogeneous blend systems from thermodynamic perspectives, and enhancing miscibility remains particularly challenging for polymers lacking specific intermolecular interactions. Here, we demonstrate a simple and generalizable approach to enhance the apparent miscibility of such systems by incorporating benzoic acid (BA) as a removable entropy diluent. Using polystyrene (PS) and poly(methyl methacrylate) (PMMA) as a model pair, we demonstrate that quaternary solutions of PS, PMMA, BA, and solvent yield blend films (PS/PMMA_BA) with markedly suppressed phase separation upon solution casting, in contrast to films prepared without BA (PS/PMMA_SC). Despite BA-induced chain extension in solution, the PS/PMMA_BA blend films exhibit a single glass transition temperature (T_g) across various blend ratios, and the T_g composition behavior follows the Kwei equation, indicative of enhanced, though not ideal, miscibility. Solid-state NMR measurements reveal persistent microheterogeneity with a characteristic length scale estimated to be within tens of nanometers, below the resolution limits of conventional thermal analysis. Consequently, dynamic heterogeneity is indistinguishable by DSC, and the mechanical properties are significantly improved in the PS/PMMA_BA blends compared to BA-free controls. These findings highlight the utility of entropy diluents in compatibilizing classically immiscible polymer systems without relying on chemical modification.