Nanoparticle Diffusion in Miscible Polymer Nanocomposite Melts

X-ray photon correlation spectroscopy measurements wereused toquantify the dynamics of bare and bimodal grafted silica nanoparticlesmixed with PEO melts of different molecular weights. In dilute polymernanocomposite (PNC) samples, we find diffusive NP behavior as describedby the Stokes-Einstein relationship so long as the adsorbedPEO polymer layer is taken into account in determining both the effectiveNP size and its role on composite viscosity. The size of this boundlayer was found to be approximately 2R (g), where R (g) is the chain radius of gyration.We also expanded our system to investigate how the dynamics of graftedNPs differ from bare NPs with an adsorbed layer. We showed that thedynamics again can be determined by an effective NP radius at a scalesmaller than the effective interparticle spacing; however, at largerlength scales, the morphology and grafting parameters play a majorrole in the system dynamics. These results allow us to quantify NPordering driven by polymer crystallization. It has previously beenspeculated that behavior is controlled by the relative ratio of timescale of crystal growth and the diffusive time scale of the NPs, aPeclet number. When the former time scale is longer, then the NPsare expected to be segregated into the interlamellar amorphous zones,while the NPs are too slow to be reorganized in the opposite case.We show here that this conjecture is quantitatively correct and thedemarcation in behavior occurs for Pe = 1. Thus,we provide a way to estimate a critical spherulite growth rate forany semicrystalline PNC, at which a given NP can be ordered. Together,the results of this study permit us to tunably design PNCs throughdirected dispersion of NPs in a polymer matrix.