Effect of silica nanoparticles on the local segmental dynamics in poly(vinyl acetate)

The effect of nanosized silica particles on the properties of poly(vinyl acetate) (PVAc) was investigated for a range of silica concentrations encompassing the percolation threshold. The quantity of polymer adsorbed to the particles ("bound rubber") increased systematically with silica content and was roughly equal to the quantity shielded from shear stresses ("occluded rubber"). This bound and occluded polymer attained a level of similar to 12% at a silica volume content of 28%; nevertheless, the glass transition properties of the PVAc, including the glass transition temperature, local segmental relaxation function and relaxation times, and the changes in thermal expansion coefficient and heat capacity at T-g, were unaffected by the interfacial material. That is, there is no indication that the local segmental dynamics of the chains adjacent to silica particles differ from the motions of the bulk chains. Interestingly, the volume sensitivity of the segmental dynamics, as determined from the scaling exponent gamma in the relation T-g similar to V-g(-gamma) in which V-g is the specific volume at the glass transition, becomes stronger with increasing silica concentration. Moreover, this dependence of gamma increases abruptly at the filler percolation threshold. The implication of this result and possible directions for new research are considered.