Assembly of Polymer-Grafted Nanoparticles in Polymer Matrices
C Koh and GS Grest and SK Kumar, ACS NANO, 14, 13491-13499 (2020).
Leibler pioneered the idea that long enough matrix polymers of length P will spontaneously dewet a chemically identical polymer layer, comprising chains of length N, densely end-grafted to a flat surface ("brush"). This entropically driven idea is routinely used to explain experiments in which 10-20 nm diameter narmparticles (NPs) densely grafted with polymer chains are found to phase separate from chemically identical melts for P/N greater than or similar to 4. At lower grafting densities, these effects are also thought to underpin the selfassembly of grafted NPs into a variety of structures. To explore the validity of this picture, we conducted large-scale molecular dynamics simulations of grafted NPs in a chemically identical polymer melt. For the NPs we consider, in the approximate to 5 nm diameter range, we find no phase separation even for P/N = 10 in the absence of attractions. Instead, we find behavior that more closely parallels experiments when all of the chain monomers are equally attractive to each other but repel the NPs. Our results thus imply that experimental situations investigated to date are dominatedby the surfactancy of hN, which driven by the chemical mismatchb h inorganic d the organic ligands (the graft and free chains are chemically identical). Entropic effects, that is, the translational entropy of the NPs and the matrix, the entropy of mixing of the grafts and the matrix, and the conformational entropy of the chains appear to thus have a second-order effect even in the context of these model systems.
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