Enthalpy-Driven Stabilization of Dispersions of Polymer-Grafted Nanoparticles in High-Molecular-Weight Polymer Melts

Phase stability of polymer nanocomposites (PNCs) composed of polymer-grafted SiO₂ nanoparticles (NPs) blended with high-molar-mass host polymer chains is investigated. We focus on blends in which the particle-grafted polymer, polyethylene glycol (PEG), and the host-atactic poly(methylmethacrylate) (PMMA) or PMMA/oligo-PEG blends-exhibit favorable enthalpic interactions. Small-angle X-ray scattering measurements are used to evaluate the phase stability of the blends and to report on the structure of the materials at intermediate and long length scales. By exploring SiO₂-PEG/PMMA and SiO₂-PEG/PMMA-PEG systems covering a wide range of molecular weights (M_w) of PMMA (1.1 kDa ≤ M_w,PMMA ≤ 1.1 × 10³ kDa) and tethered PEG (0.5 kDa ≤ M_w, PEG ≤ 2 kDa), we are able to develop a comprehensive stability map for PNCs based on hairy NPs. At low M_w,PEG, the phase behavior is dominated by entropic effects and the negative Flory-Huggins χ parameter between PEG and PMMA plays no role in phase stability. For higher M_w,PEO and intermediate M_w,PMMA, a crossover from entropy- to enthalpy-dominated behavior is observed, which leads to the phase stability in PNCs well beyond the conventional limits reported for SiO₂-PEG/PEG mixtures. This enhanced mixing ceases above a critical M_w,PMMA, where it is found that PMMA chains wet a sufficiently large number of SiO₂-PEG particles to bridge and thereby destabilize the composites.