Thermal and Rheological Behavior of Polymer Grafted Nanoparticles

Composite materials consisting of nanopartides dispersed in a polymer matrix have potential applications in a wide range of fields. Grafting polymer chains to nanopartides is a key strategy for controlling the distribution of those nanopartides throughout the matrix material. The particle distribution is controlled by the relative length of the matrix chains (1) to the graft chains (N) and the density of the graft chains (sigma) for particles of a given radius (R). 6 Using differential scanning calorimetry (DSC) and bulk rheological measurements, we examine the interactions between the graft and matrix chains as a function of the three key parameters: sigma, P, and N. The results of these measurements are compared to ultrasmall-angle X-ray scattering (USAXS) measurements [Macromolecules 2012, 45, 4007-4011], which were used to determine the phase behavior of the particles. DSC results indicate that the matrix chains must be completely expelled from the graft layer for the autophobic transition to occur. Similar trends are observed from the rheological measurements, where the extracted thickness of the graft layer correlates to changes in the glass transition temperature from DSC. Additional rheological measurements demonstrate the differences in the flow behavior between sparsely and densely grafted particles over a range of shear rates. In particular, as the graft density increases to high levels (0.5-0.7 chains/nm(2)), the flow behavior of well-dispersed and aggregated particles becomes increasingly similar, as the particle particle interactions are screened by the thick graft layer.