(418bw) Structure Dynamics and Rheology of Silica-Peg Nanocomposites

Polymer composites are typically composed of inorganic fillers dispersed in a polymeric matrix and have applications in coatings, tires, airplane wings and windmill blades. Composites in which fillers have at least one dimension less than 100 nm are known as nano-composites. Shrinking filler size is associated with enhancements in mechanical, electrical, optical and flow properties. The dispersion of these fillers in the polymer is key to optimizing the benefits derived from the small particle size.  Particle dispersion is surprisingly difficult and requires enthalpic gains of polymer adsorption to particle surface to balance the increases in entropy associated with how the particle surface alters polymer configuration.  At the same time there is evidence that as the particle size shrinks and the strength of attraction between polymer segments and the particle surface diminishes, the mechanical properties of nanocomposites can be dramatically altered.

In this work we study the structure, dynamics and rheology of bare and surface modified silica particles suspended in polyethylene glycol (PEG) melts.  We alter polymer particle interactions by reacting the particle surface with isobutyltrimethoxysilane which will replace some of the surface hydroxyl groups with isobutyl groups in increasing amounts and study changes in microstructure, flow and relaxation properties up to high volume fractions. The reaction technique used here is expected to create silane monolayers on the surface and the coverage is expected to increase monotonically with degree of silanization. We have chosen to work here with PEG with a molecular weight of 400 that corresponds to a degree of polymerization of 8. This molecular weight was chosen because of the success of PRISM (polymer reference interaction site model) at predicting numerous structural properties at low molecular weights. Detailed studies indicate that the polymer adsorbs to bare silica surface producing a layer which increases the particle’s hydrodynamic diameter by ~ 2.9Rg (or ~ 2.3 nm). Our studies suggest that at the levels investigated, silanization has a small effect on suspension microstructure and flow properties at the low volume fractions where the composite mechanics are essentially those of particles suspended in a Newtonian fluid and that particle’s hydrodynamic diameter is not impacted by surface treatment. At elevated volume fractions, where h < 6Rg, the impact of surface treatment becomes evident with higher levels of surface treatment giving rise to faster relaxation of density fluctuations and lower composite viscosities. These studies suggest polymer dynamics near the particle surface are sensitive to particle surface chemistry where increasing the degree of surface silanization results in more rapid polymer relaxation rates.