Effects of Hydration/Dehydration on Interfacial Polymer Fraction and Dynamics in Nanocomposites Based on Metal–Oxides and Physically Adsorbed Polymer

Abstract

Effects of hydration on the interfacial dynamics of polydimethylsiloxane (PDMS) adsorbed on silica and titania nanoparticles in the form of aggregates of different specific surface area (SBET, 25–342 m2/g) were studied employing equilibrium water sorption/desorption isotherms (ESI/EDI) and broad-band dielectric spectroscopy (BDS). ESI revealed a systematic increase of the water content, h, of initial oxides (varying up to 60 wt %) with increasing SBET, whereas changes in h in nanocomposites (PNCs) are very weak (0.1–0.3 wt %) and practically independent from SBET. BDS allowed for the detection of an increase in the interfacial polymer fraction with hydration, accompanied by a significant enhancement of dynamics and cooperativity of the corresponding segmental relaxation in the interfacial layer (αint relaxation), changes being reversible as recorded by hydration and subsequent dehydration of the samples. At the same time, bulk PDMS dynamics in PNCs (α relaxation) was not significantly affected. Effects of hydration on the characteristics of αint in PNCs were found to be qualitatively similar here with changes imposed by thermal annealing (resulting in increasing crystallinity) and surface modifications (resulting in changes in SBET) recorded recently in similar PNCs. Hydration effects were additionally followed via monitoring the dielectric relaxation process attributed to hydrated hydroxyls at the surfaces of the oxides (S relaxation) in the PNCs. The results may suggest that with increasing h the concentration of contact points between the solid surface (oxides) and the polymer chains increases, while the polymer chain conformations at the interface change toward an increase in looplike interfacial polymer segments.