Solvent induced phase behavior of binary polymer-grafted nanoparticle blends*

Polymer-grafted nanoparticles (PGNPs) have attracted much attention in recent years for their potential applications, from nanoelectronics to tough materials with tunable enhanced mechanical properties. However, recent study of poly (methyl methacrylate) silica (PMMA-SiO₂) and poly(styrene) silica (PS-SiO₂) blends reveal that thermal annealing cannot coarsen the phase separation of large PGNPs, due to the lack of mobility of the large masses involved. This can be resolved through liquid-enabled phase-separation as a facile approach. To this end, we developed a method to coarsen the in-plane and out-of-plane surface patterns, to over 3 and 20 times respectively, by using a direct solvent immersion annealing (DIA) method. By varying the solvents in the DIA solution, interchangeable phase-separated and homogeneous blend morphologies are formed, and the magnitude of the phase separated surface patterns are in the order of the interfacial tension relative to surface tension, similar to homopolymer melt mixtures. Depth profiling by ToF-SIMS reveals that the switchable phase separation state persists within the films. Such homopolymer matrix free PGNP-only blend systems are novel, and these switchable transitions are not readily obtainable by thermal annealing.