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.

The rheology and microstructure of 44 nm diameter silica particles suspended in entangled poly(ethylene oxide) (PEO) melts are studied through measurement of filled melt viscosity and X-ray scattering measurement of interparticle structure factors, S(q,φ c ), where q is the scattering vector and φ c is the silica volume fraction. The particles have a similar refractive index to PEO which minimizes van der Waals attractions acting between particles. The introduction of particles causes an elevation in the viscosity of the nanocomposite melt more than would be expected of particles merely interacting with hard core repulsions. Further addition of particles causes a rise in the elastic and viscous moduli. The rheological characterization of these nanocomposite melts is discussed in terms of several critical particle volume fractions that result from confinement of polymer, adsorption of polymer segments to the particle surface, and overlap and entanglement of adsorbed polymer as the particle volume fraction is increased. Characterization of the particle microstructure shows that the association of the polymer with the particles drives the particles to structure more than would be expected of particles with interactions governed merely by hard core repulsions. Particles show signs of instability in the polymer melt at a common elevated volume fraction independent of polymer molecular weight.

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,φ c ), where q is the scattering vector and φ 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 shell 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.

The stability of 44 nm diameter silica particles in poly(ethylene glycol) (PEG) melts is assessed through the experimental measurement of particle second virial coefficients, B 2 = B 2,cc/ , where B 2,cc is the particle second virial coefficient and is the excluded volume second virial coefficient of the particles. Measurements are made using side bounce ultra-small-angle X-ray scattering (SBUSAXS) of dilute filled PEG melts of variable molecular weight. Results show B 2 values to be equal to or greater than one for the molecular weights investigated. Experimental results are compared to recent PRISM theory adaptations to filled polymer melts.

The linear and non-linear rheology of a high volume fraction particle filled unentangled polymer melt is measured. The particles in the polymer melt behave like hard spheres as the particle volume fraction is raised. At high volume fractions, the suspension develops a plateau elastic modulus. Over the frequency range of the elastic modulus plateau, the viscous modulus develops a minimum and a maximum. The frequencies of the two local extrema initially have critical power law scaling, suggesting the approach of a singular glass transition. At higher volume fractions in excess of the glass transition, the viscous modulus continues to show a well defined minimum and a well defined maximum. The non-linear moduli show a single perturbative yield point beyond which the suspension softens. The yielding behavior of the nanocomposite is shown to be sensitive to the strain frequency and the proximity of the strain frequency to the maximum frequency for the linear viscous modulus from linear rheology which characterizes thermal relaxation of glassy particle clusters in the zero strain limit. The linear and non-linear measurements are compared against a recently developed mechanical theory for colloidal glasses.