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 (∼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 α-relaxation assigned to polymer segments away from the NP surfaces, the nanocomposites also exhibit a slower interfacial α′-relaxation by 3−4 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 α-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.

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Establishing the relationship between interfacial layer chain packing and dynamics remains a continuing challenge in polymer nanocomposites (PNCs). This issue is expected to be significant in our understanding of the mechanism of the dynamic response of such materials and the manner in which these parameters affect the macroscopic properties of PNCs. In this study, we report the dynamics of free polystyrene (PS) and poly(methyl methacrylate) (PMMA) matrix chains, as well as those of polymer chains surrounding the spherical silica nanoparticles (NPs) where silica NPs are either bare or PS grafted, to discriminate the role of grafted chains and interfacial interactions between grafted NPs and the matrix. The α-relaxation dynamics of the PS matrix is unaffected by silica NP loadings, it slows down in PMMA nanocomposites because of polymer-NP interfacial interactions and steric hindrance. More interestingly, we probe the enhanced mobility of the interfacial layer (α'-relaxation) in PNCs filled with grafted NPs, and this phenomenon is further corroborated by the accelerated Maxwell-Wagner-Sillars polarization process in the presence of grafted silica NPs. Moreover, the α'-relaxation time in the vicinity of glass transition temperature of the polymer matrix unexpectedly increases with increasing temperature. Such an anomalous temperature-dependent behavior can be attributed to the influence exerted by slow α-relaxation dynamics. Considering these phenomena and the mechanical properties, we propose a three-layer model to explain the observed behavior of grafted silica NP-filled nanocomposites. These findings provide new insight into the mechanisms responsible for mechanical reinforcement and therefore provide guidance in designing PNCs with tunable macroscopic properties.