In this paper we explore the effects of polymer molecular weight and particle volume fraction on particle interactions and particle suspension microstructure when nanoparticles are suspended in polymer melts. These studies were conducted as part of an effort to understand the role of particle/polymer segment interactions on the degree of aggregation in filled polymer systems. These systems are an important class of materials where colloidal particulate matter is dispersed in a polymer matrix. They have considerable commercial importance in the manufacture of rubber, nanocomposites, biomaterials, insulating and conductive polymers, and in coatings. Fillers are used in polymers to improve and alter mechanical, electrical, rheological, optical, and/or thermal properties of the composite. Many of these improvements are believed to be governed by the interaction between the particle surface and the polymer matrix.

The phase behavior of filled polymers will be governed by enthalpic and entropic contributions. A variety of phases are expected as particle volume fraction, polymer molecular weight, and segment-surface interactions are varied: homogeneous fluid, phase separation, or nonequilibrium gel. The competition between enthalpic and entropic contributions will drive the organization of the particles in the polymer matrix. Recent studies suggest several types of particle organization: entropic depletion flocculation, steric stabilization, local bridging flocculation, and tele-bridging flocculation. The ability of a given polymer/particle mixture to fall into one of these classes is predicted to be governed by the polymer segment/particle size ratio, particle volume fraction, and the strength and range of attraction between polymer segments and the particle surface.

The development of systematic studies aimed at understanding the microscopic physio-chemical effects between polymer and particle is difficult. Relaxation times are long for high molecular weight polymers. This makes attaining equilibrium questionable. To circumvent this issue, we have investigated silica nanoparticle dispersions in low molecular weight polyethylene glycol of increasing molecular weight. By performing ultra-small angle x-ray scattering experiments (USAXS), we have assessed the thermodynamic stability of our particles in the polymer matrix through calculation of second virial coefficients and the particle structure through the calculation of structure factors. We find the second virial coefficient to be greater than that of hard spheres for all molecular weights explored indicating that the particles feel a net repulsion in relation to each other and are thus thermodynamically stable in our polymer. This implies steric stabilization due to a favorable attraction between the particle surface and polymer segments. We also find the polymer to adsorb to the particle surface giving a larger effective particle diameter and nonmonotonic ability of the polymer to disrupt suspension microstructures as molecular weight is increased at fixed particle volume fractions.