(374a) Role of Casting Solvent on Nanoparticle Dispersion in Polymer Nanocomposites

We investigate the influence of casting solvent on the final spatial distribution of nanoparticles (NPs) in polymer nanocomposites (PNCs). The nanocomposites formed from bare silica NPs and poly(2-vinylpyridine) (P2VP) were cast from two different solvents: methylethylketone (MEK) and pyridine, which are either theta or good solvents for both the NPs and the polymer. In MEK, we show that P2VP strongly adsorbs onto the silica surface, creating a temporally stable, bound polymer layer. The resulting “hairy” particles are sterically stabilized against agglomeration and thus good NPs dispersion is achieved at a low silica loading. With high silica content, due to the local bridging of silica NPs by P2VP chains, the composites demix into NP-rich and NP-lean phases. On the contrary, in pyridine, P2VP does not adsorb onto the silica surface. As a result, the phase behavior in such a case is governed by a subtle balance among electrostatic repulsion, Van der Waals attraction, polymer-induced depletion forces, and the kinetic slowdown of diffusion-limited NP aggregation. These initial, solvent-driven non-equilibrium agglomerated NP states are difficult to completely anneal away within a reasonable experimental time scale, especially for PNCs containing high molecular weight (MW) matrices. We further show that, in the case of using low MW matrices, the NP structure starts to approach its thermodynamic equilibrium state upon sufficient thermal annealing. These results emphasize the crucial role played by the casting solvent in the spatial dispersion of NPs in a polymer matrix.