Concentration fluctuations, local order, and the collective structure of polymer nanocomposites

A combined theory-experiment analysis of adsorbing polymer mediated structural reorganization of silica nanoparticles in equilibrated and miscible poly(ethylene oxide) (PEO) and polytetrahydrofuran (PTHF) nanocomposites is presented. Quantitative comparison of microscopic liquid state theory calculations with small-angle X-ray scattering experiments demonstrate the theoretical approach properly accounts for the effects of adsorbed polymer layers on nanoparticle concentration fluctuations over all length scales for a wide range of volume fractions and interfacial cohesion strengths. The mixture total packing fraction is increased as particles are added to the polymer melt in order to account for equation-of-state effects which are important at very high filler loadings. A distinctive microphase separation like peak in the collective polymer structure factor is predicted. Nanoparticle potential of mean force calculations suggest a criterion for the onset of depletion or bridging induced kinetic gelation which is consistent with the observation of complete miscibility for the PEO system but nonequilibrium behavior in the PTHF nanocomposite.