Interfacial Interactions and Segmental Dynamics of Poly(vinyl acetate)/Silica Nanocomposites

We investigate the interfacial interactions and segmental dynamics of hydrophilic and hydrophobic (calcined) silica nanoparticle (NP) filled poly(vinyl acetate) (PVAc) composites via a combination of Fourier transform infrared spectroscopy (FTIR) and broadband dielectric spectroscopy measurements. For hydrophilic silica NP filled composites, an increase in the amount of bound carbonyl groups with increasing silica loading can be noted due to hydrogen bonding interactions with hydroxyls on the NP surfaces and carbonyl groups of PVAc. It is surprising that the apparent glass transition temperature (T-g) increases very slightly (similar to 1 K) compared to pure PVAc, but the glass transition process becomes much broader, indicating the existence of a slower relaxation mode. In addition to the bulklike alpha-relaxation assigned to polymer segments away from the NP surfaces, the nanocomposites also exhibit a slower interfacial alpha'-relaxation by 34 orders of magnitude compared to bulk polymers. However, in the case of hydrophobic (calcined) silica NP filled composites, T-g shifts to higher temperatures by 4-5 K even if there is the absence of strong polymer-NP interfacial interactions confirmed by the FTIR results. Consequently, the overall alpha-relaxation dynamics are suppressed in the presence of silica NPs, which is attributed to an increase of steric hindrance and decrease of free volume. More importantly, in the nanocomposites with high NP loadings, it is worth noting a weak physical adsorption interfacial layer for which the segmental mobility is on average slower by 1-2 orders of magnitude relative to that for bulk polymers. However, the dielectric strength and interfacial bound fraction is much smaller than that of hydrophilic silica NP filled composites, indicating the fact that physical adsorption is much weaker compared to hydrogen bonding