174. ANISOTROPIC INTERACTIONS BETWEEN ISOTROPIC, GRAFTED SPHERICAL NPS WITHIN A POLYMER MATRIX

Department: NanoEngineering
Research Institute Affiliation: Graduate Program in Materials Science and Engineering
Faculty Advisor(s): Gaurav Arya

Primary Student
Name: Tsungyeh Tang
Email: tytang@ucsd.edu
Phone: 718-419-2250
Grad Year: 2018

Abstract
Understanding how nanoparticles (NPs) assemble to form higher-order structures is critical to the fabrication of advanced composite materials. Recent observations indicate that under certain conditions isotropic NPs grafted with polymer chains can spontaneously assemble into highly anisotropic string-like aggregates. This anisotropy has been suggested to arise from the uneven density distribution of polymer chains surrounding NP clusters. However, this hypothesis has not been fully tested and the related mechanism has not been explored further. Here we investigate this hypothesis further by carrying out molecular dynamics simulations of polymer-grafted NPs in a polymer matrix treated using coarse-grained models. Specifically, we compute the potential of mean force (PMF) between a polymer-grafted NP and a dimer of polymer-grafted NPs held stationary within the polymer matrix with their NP cores touching each other; the PMFs are computed for the single NP approaching the NP dimer along the longitudinal direction and two lateral directions. Our PMFs show that the interaction free energy is more favorable along the longitudinal direction, compared to the two lateral directions, confirming the formation of string-like aggregates. The anisotropic interactions between NPs is found to arise from the redistribution of grafted polymer chains around the NPs without any change in the net polymer density from grafted and matrix chains. Decomposition of PMFs into its various components indicates that the net interaction between grafted NPs within polymers is dictated by a delicate balance between steric and depletion interactions. Parametric analysis reveals that the anisotropy is particularly strong for large NP cores, long polymer grafts, and high grafting density and that our results consistent with the experimentally-observed transition from dispersed to string-like to isotropic morphologies of NP aggregates with decreasing graft chain length and/or decreasing grafting density.

Industry Application Area(s)
Life Sciences/Medical Devices & Instruments | Materials

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