Rheology and Microstructure of an Unentangled Polymer Nanocomposite Melt

The rheology and microstructure of 44 nm diameter silica particles suspended in unentangled polyethylene oxide (PEO) melts are studied through measurement of filled melt viscosity and X-ray scattering measurement of interparticle structure factors, S(q,phi(c)), where q is the scattering vector and phi(c) is the silica volume fraction. The neat polymer melts are Newtonian over the probed shear range. Filled melts have a constant viscosity at low particle concentrations and shear thin at high concentrations. At the same particle volume fraction, filled melt viscosities increase with polymer molecular weight. By defining an effective particle volume, assuming polymer adsorption adds 2.9R(g) to the particle diameter, suspension viscosities of both molecular weights at all volume fractions resemble that of hard sphere suspensions. The particle osmotic compressibility and position and coherence of the first nearest neighbor shelf suggest that the particles have radii larger than the core size due to polymer structuring near the particle surface. These results are interpreted as resulting from a modest strength of attraction between polymer segments and particle surfaces.