Dispersion-Aggregation and Wetting-Dewetting Phase Transitions in Mixtures of Polymer Grafted Nanoparticles and a Chemically Dissimilar Polymer Matrix

Significant efforts have been focused towards controlling morphology of the nanoscale fillers and matrix polymer in polymer nanocomposites as the composite morphology is directly related to the macroscopic properties of that material. One strategy for controlling filler arrangement in the matrix polymer is to graft the surface of the filler with polymer that mediate the effective filler-filler and filler-matrix interactions, primarily through the graft -matrix interactions. For nanocomposites with chemically identical graft and matrix polymers, it is well understood that the polymer grafted particle dispersion to aggregation transition is directly linked to and synonymous with wetting/dewetting of the graft and matrix polymer. In this talk I will present our recent work exploring composites with chemically different graft and matrix polymers, specifically those with attractive graft-matrix interactions that lead to a dispersed filler state at low temperature and aggregated filler state at high temperatures. We show, using generic coarse-grained models and molecular simulations, that the sharp phase transition from dispersed to aggregated states is distinct from the continuous wetting-dewetting transition. The onset of wetting to dewetting occurs at temperatures lower than the dispersion to aggregation transition, and dewetting continues at temperatures above the dispersion to aggregation temperature in the aggregated state. Experiments of deuterated poly(styrene) grafted silica particles in a poly(vinyl methyl ether) matrix using small angle neutron and x-ray scattering results show remarkable agreement with our simulation results. These results are in contrast to the chemically identical graft-matrix composites where the two transitions have been considered to be synonymous. Additional computational comparison of polymer grafted particles in matrix polymer with free, ungrafted blends of the same, dissimilar graft and matrix homopolymers shows that the free case, unlike the grafted case, shows wetting-dewetting as a sharp transition directly coinciding with the macrophase separation.