(423c) Effects of Matrix Chain Length on Miscibility of Nanoparticles

Authors: 
Koh, C., Columbia University
Kumar, S. K., Columbia University
Recent work in nanocomposites have studied ways to control the dispersion of nanofillers in
polymer matrices/melts. The grafting of chemically similar polymer chains to the nanofiller has
been a commonly proposed solution. However, aggregation of filler particles remains a problem.
Furthermore, the particle aggregation observed experimentally by decreasing the graft/matrix
chain ratio (N/M) remains unobservable in simulations of a single nanoparticle (NP) in a ho-
mopolymer matrix. Here, molecular dynamics simulations were used to study the miscibility
effects of decreasing the ratio N/M on a polymer grafted nanoparticle in a polymer melt. The
polymer matrix chain length was varied systematically in a series of simulations , while NP
radius, grafting density, and grafted chain length were held constant. We first show that com-
position profiles of graft/matrix chains do not reveal any signs of brush collapse, observed
experimentally, with increasing matrix chain length. The brush heights were then calculated
using a second moment of the segment density and only highlighted a good to theta solvent
transition. As most of the differences in segment density are evident closer to the surface of the
nanoparticle, we thus conjecture that the first solvation shell, defined as the region bounding
the first peak of the radial distribution function of monomers around the NP, might be more
illuminating. Indeed, an analysis of the monomer fluctuations in the first solvation shell reveals
several findings: (1) Miscibility effects by the matrix on the polymer grafted NP can be observed
in the first solvation shell, (2) The matrix chains rather than the graft chains might give more
insight into any conformational changes in the brush, (3) there is a subtle hint of a second
transition at a N/M ratio of ∼ 0.3 . These results imply that the interactions at
the surface, perhaps between the NP itself and the melt, are significant in studying changes in
miscibility of the polymer grafted NP and might be driving the morphology of the entire brush.