Suppression of macroscopic phase separation in polystyrene/polymethyl methacrylate blends confined within the interstices of silica nanoparticle packings*

Polymer blends often suffer from incompatibility, leading to the formation of chemically dissimilar phases. New compatibilization methods 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 morphology 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 0.9 and 8.0 by using different-sized NPs from 7 to 61nm. Systems with Γ<1.5 display macroscopic phase separation, with globular PS domain sizes around 10 microns as characterized by both optical and scanning electron microscopy. When confined to Γ>2, macroscopic phase separation is suppressed at all observed microscopy length scales. Small angle neutron scattering is used to identify a pore-scale segregation regime for Γ>2, with PS domains on the order of 3-5nm. Passivation of SiO2 NPs with chlorotrimethylsilane, which weakens PMMA and SiO2 interactions, leads to macroscopic phase separation at all Γ tested up to 5. These results provide a new insight into the interplay between confinement and polymer-NP interactions on the phase morphology of two polymers, which could be useful for designing blend-based nanocomposite films with novel properties.