Rheology, Microstructure, and Diffusion of Polymer-Grafted Nanoparticles in Polymer Melts

In this work we seek to fundamentally relate how nanoparticle interactions control the bulk properties of polymer nanocomposites. Rheological data show that particles behave similar to hard spheres for particle softness L* ≤ 0.29 and strong deviations are observed as brush height L approaches the order of core radius R (L* ≥ 0.88) for soft spheres. At higher particle loadings, the onset of the overlap of the graft layers results in a liquid-solid transition, where linear viscoelastic measurements show up to a 1000-fold increase in the storage modulus in melts of decreasing molecular weight.;The rheological studies were complemented by investigations into particle microstructure and nanoscale dynamics, which were probed through ultra-small angle x-ray scattering (USAXS) and x-ray photon correlation spectroscopy (XPCS), respectively. The USAXS studies reveal stronger particle interactions in lower molecular weight melts through extraction of the structure factor and radial distribution function. Analysis of particle dynamics through XPCS indicate slower relative diffusivity was observed in a lower molecular weight melt due to the stretching of the graft layer. Finally, the rheological response to varying temperature and melt molecular weight was probed for dispersions of polymer-grafted particles in which the melt de-wets from the graft layer. From fundamental considerations, a particle interaction potential whose depth is related to the ratio of melt polymer molecular weight to that of the graft, was developed. This potential scaled linear viscoelastic and shear dependent rheological data; such master curves of scaled shear viscosities and moduli from known physical parameters are useful in the prediction of bulk properties for processing and materials development.;Ultimately, our studies indicate that the bulk properties of polymer nanocomposites can be fine-tuned through control of nanoparticle interactions on the molecular scale by modulating the stretching of the graft polymer layer through varying the molecular weight of the polymer melt. The results of our studies augment the understanding of the factors that govern nanoparticle dispersion in polymer melts, which is essential to the design of well-controlled polymer nanocomposites for applications including automotive and aerospace parts, packaging, biomaterials, and other advanced plastics.