Suppression of Macroscopic Phase Separation in Polymer Blends Confined within the Interstitial Pores of Dense Nanoparticle Packings

Polymer blends often suffer from macroscopic phase separation due to incompatibility, with conventional compatibilization techniques relying on kinetically trapped, inhomogeneous structures. Here, we show that confining prototypical immiscible polymers, polystyrene (PS) and poly(methyl methacrylate) (PMMA), within the interstices of a nanoparticle packing effectively suppresses phase separation at the macroscopic scale. By varying the confinement ratio (Γ, the ratio of a bulk polymer's radius of gyration to the nanoparticle packing's pore radius) between 0.6 and 2.2 through modulating the polymer molecular weight and nanoparticle diameters (7-61 nm), we establish a confinement-driven morphology transition. Systems with Γ < 0.9 display macroscopic phase separation, akin to bulk blends, as observed via optical and scanning electron microscopy. In contrast, for Γ > 2, macroscopic phase separation is suppressed across all microscopy scales. Passivating SiO₂ nanoparticles with chlorotrimethylsilane, which weakens PMMA-SiO₂ interactions, induces macrophase separation across all tested Γs, underscoring the critical role of polymer-nanoparticle interactions in phase behavior. Self-consistent field theory simulations also show that confinement to the pores between nanoparticles suppresses phase separation, which is further suppressed when the nanoparticles are preferentially wetted by one of the polymers. We propose a pore-scale segregation mechanism in which PMMA preferentially wets the nanoparticle surfaces, while PS localizes to pore centers. Selective solvation experiments indicate the presence of a continuous PMMA layer, consistent with a core-shell morphology validated by resonant soft X-ray scattering. These results demonstrate how confinement within nanoparticle packings can influence polymer blend phase behavior with implications for the design of nanocomposite films with tunable properties.