Tuning the wetting-dewetting and dispersion-aggregation transitions in polymer nanocomposites using composition of graft and matrix polymers

Recent simulation and experimental work on polymer nanocomposites composed of polymer grafted particles and free matrix polymers, where the graft and matrix homopolymers are chemically dissimilar and exhibit lower critical solution temperature behavior with temperature, has shown that wetting to dewetting is a gradual and distinct transition from the sharp particle dispersion-aggregation transition. In this study, using coarse-grained molecular simulations, we demonstrate that the extent of wetting of the grafted polymer layer and the particle dispersion-aggregation transition are tuned using the composition of graft and matrix polymers. Specifically, we study composites where the graft and matrix chains are random copolymers composed of attractive and athermal monomers. We maintain a dense grafting density on the spherical particles of diameter five times the monomer diameter and study matrix lengths five times that of the graft chain length or equal graft and matrix chain lengths. Wevary the fraction of attractive monomers in the graft (f(G)) and matrix (f(M)) chains, graft-matrix chain composition ratio (f(G)/f(M)) and the graft-matrix interaction strength, as characterized by the Flory-Huggins interaction parameter between graft and matrix attractive monomers: chi(GM). When chi(GM) is negative, decreasing f(G) and/or f(M) decreases the extent of grafted layer wetting by matrix chains because the enthalpic driving force for wetting is reduced. As the chi(GM) increases and becomes positive, the extent of wetting decreases gradually till it reaches the wetting of analogous athermal composites. That value of chi(GM) where the extent of wetting is the same as that of an analogous athermal polymer nanocomposite marks the onset of dispersion-aggregation transition. For symmetric graft and matrix chain compositions (f(G) = f(M)), the magnitude of f(G) and f(M) tunes the overall extent of wetting of the grafted particles in the dispersed state but not the dispersion-aggregation transition. Varying the asymmetry of the graft and matrix chain composition (i.e. f(G)/f(M)) tunes both the extent of wetting of the grafted layer and the dispersion-aggregation transition.