Effect of Extreme Nanoconfinement and Polymer-Nanoparticle Interactions on the Thermodynamics of Multiphasic Nanocomposite Films

Polymer blends are often immiscible, leading to the formation of chemically dissimilar phases. Novel compatibilization techniques are required to maximize the synergistic properties of these blends. In this work, we evaluate the effect of confinement and polymer-nanoparticle interactions on the phase behavior of polystyrene (PS) and polymethyl methacrylate (PMMA) confined within the interstitial pores of a dense SiO₂ nanoparticle (NP) packing. A blend of PS and PMMA with SiO₂ NPs is annealed at 150-200°C for up to 24hr. The confinement ratio (Γ), which is the ratio of a polymer’s radius of gyration to the NP packing’s pore radius, is varied between 0.25 and 8 by changing the molecular weight of the polymers between 7 and 99 kg/mol, and the size of the NPs from 7 to 61nm. Systems with Γ<1.5 display macroscopic phase separation, with globular PMMA domain sizes around 10 microns as characterized by both optical and scanning electron microscopy. When the system is confined further to Γ>2, macroscopic phase separation is suppressed down to a length scale of at least 500nm. Passivation of SiO₂ NPs with chlorotrimethylsilane, which weakens the interactions between PMMA and SiO₂, leads to macroscopic phase separation at all tested Γ up to 5. These results provide new insights into the interplay between confinement and polymer-NP interactions on the miscibility of two polymers, which could be useful for designing blend-based nanocomposite films with novel properties.