Self-Assembly of Polymer-Grafted Nanoparticles in Solvent-free Conditions
Alexandros Chremos, Jack F. Douglas
The grafting of polymer chains onto the surface of spherical nanoparticles leads to a hybrid type of fluid that exhibits properties of both particle suspensions and melts of star polymers these properties being controlled by the relative dimensions of the grafted polymer chains to the nanoparticle diameter, D, and the number of the number of chains grafted on the nanoparticle surface, f . While polymer-grafted nanoparticles (GNP) of this kind typically have a spherical average shape after grafting even a moderate number of chains, their instantaneous molecular shape can fluctuate significantly as in homopolymer conformations in the melt fluid. Both simulations and measurements have previously revealed that these conformationally polarizable particles can exhibit self-assembly into large scale polymeric structures in both solution and in polymer melts and we simulate polymer- grafted nanoparticles with D and temperature (T ) variations without a dispersing solvent to better understand the nature of this unanticipated self-assembly process. We observe a reversible self- assembly into linear and branched dynamic GNP structures where the extent of the assembly and geometry depend on D and T. Since the shape of the GNPs appeared to be correlated with the occurrence of the GNP self-assembly, we quantified the average shape and a measure of shape fluctuations to better understand how molecular shape influence their propensity to self-assemble into different structural forms. Based on this framework, we quantify the clustering process of the GPN as an equilibrium polymerization phenomenon and we calculate the order parameter governing the of GNP in dynamic clusters, the average mass of the clusters, size distribution, and the apparent fractal dimension of the clusters.