Effect of Silica Nanoparticles on the Local Segmental Dynamics in Poly(vinyl acetate)

Abstract

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 ∼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 γ in the relation T g ∼ V g -γ in which V g is the specific volume at the glass transition, becomes stronger with increasing silica concentration. Moreover, this dependence of γ increases abruptly at the filler percolation threshold. The implication of this result and possible directions for new research are considered.

Glass Transition and Interfacial Segmental Dynamics in Polymer-Particle Composites

“…67,68 Another difference between the two studies is that while both groups used silica filler, Fragiadakis et al [60][61][62] synthesized the SiO 2 particles in situ by hydrolysis and condensation of tetraethoxysilane, yielding particles having a mean diameter of ~ 10 nm. This is an order of magnitude smaller than the silica used by Bogoslovov et al 64 Whether the appearance of a new dielectric peak in the silicafilled PDMS represents an immobilized phase arising due to the small reinforcing particles or is an artifact of crystallization is an open question. V. CALORIMETRY Differential scanning calorimetry (DSC) yields a direct measure of T g from the abrupt change in the heat capacity upon heating or cooling a sample.…”

“…They reported similar results for an epoxy with added diamond particles. 63 On the other hand, Bogoslovov et al 64 found no change in the shape of the loss peak upon addition of silica particles to polyvinylacetate ( Figure 7). There are differences in these works, which might account for the contrary results.…”

“…A more important difference is that while the PVAc used in Ref. 64 was completely amorphous, PDMS crystallizes, with the degree of crystallization influenced by the filler. 62 It is well-known that in semicrystalline polymers, segments near crystallites have slower dynamics and a broadened loss peak.…”

Challenges in Multiscale Modeling of Polymer Dynamics

“…On the one hand, both experimental and computational results have suggested that a mobility gradient exists near the surface of nanoparticles (NPs) on a small, sub-chain-length scale [370,376,377]. On the other hand, silica NPs have been found to have no influence on the local segmental dynamics of poly(vinyl acetate) chains adjacent to them, when compared with the chains in the bulk [383]. Very recently, Richter and his co-workers carried out extensive neutron spin echo experiments on the dynamics of entangled polymer chains in PNCs [374,375,384].…”

Enthalpic relaxation of silica–polyvinyl acetate nanocomposites

“…Likewise, for polymer nanocomposites their glass transition temperatures has been observed to increase, 14 decrease, 15 disappear, 14 or remain unaltered. 16 Although the effects of physical size, confinement, and incorporation of fillers on glass transition temperature have received major attention, physical ageing of these materials has not received adequate investigation. All polymers exhibit significant physical ageing in their glassy state.…”