Equilibrium Nature of Self-Assembled Polymer-Grafted Nanoparticles in a Homopolymer Matrix

It is commonly believed that in systems of polymer grafted nanoparticles (NPs) in homopolymer matrices, the transition between various NP morphologies, for instance well-dispersed NPs to phase separated NPs, occur spontaneously. To explore this assumption, we conducted large-scale molecular dynamics simulations of grafted NPs in a chemically identical polymer melt. The model is a simple bead sping model with standard Lennard-Jones interactions that were previously proven successful in modeling the self-assembly observed in experiments. By varying the tunable parameters of grafting density, matrix polymer chain length and grafted polymer chain length, we were able to extensively probe the morphology space and arrive at several key findings. Firstly, not only is there a gradual transition between the different self-assembled NP states, regardless of initial configuration states (well-dispersed or aggregated), one seems to be able to arrive at the same final morphology eventually, especially for short length-scale conformations. These results appear to validate the equilibrium nature of these morphologies. Second, we find that while the surfactancy of the NPs drives self-assembly of the diverse array of morphologies observed experimentally, the tuning of NP size is crucial to access the phase-separated morphology.