(380c) Diffusion of Polymer-Grafted Nanoparticles In Polymer Melts

A considerable amount of work has gone into developing scaling theories and computational models to determine the stability of polymer-grafted nanoparticles in polymer melts. As the number of polymer chains on the surface (i.e. graft density) increases the entropic penalty for interpenetration between the graft chains and the surrounding melt increases. As the graft layer becomes increasingly dense the surface chains are predicted to expel the melt, the particles are then expected to aggregate through graft layer attraction. In this study polystyrene-grafted silica nanoparticles with graft densities ranging from 0.25 to 0.70 chains/nm² are placed into polystyrene melts with molecular weights ranging from much-less-than to much-greater-than the graft molecular weight. Additionally, the impact of particle curvature was evaluated by varying the radius, R, of the silica cores between R = 10 nm - 400 nm. Particle stability was evaluated with a combination of small-angle x-ray scattering, rheology and TEM. Our results show that R = 10 nm particles at graft densities of 0.25 chains/nm² are stable independent of melt molecular weight. Further, increasing the graft density causes nanoparticle aggregation to occur at increasingly smaller melt molecular weights. While similar trends are observed for larger particles at R = 40 - 400 nm, the smaller nanoparticles remained stable over a wider range of graft densities. This was attributed to greater brush penetration with higher particle curvature, which reduces the effective graft density as the chains move away from the particle surface.