Morphology and molecular dynamics investigation of PDMS adsorbed on titania nanoparticles: Effects of polymer molecular weight

Abstract

Morphology, thermal transitions and molecular dynamics of polydimethylsiloxane (PDMS) adsorbed onto low specific surface area (25m2/g) spherical-like titania (TiO2) nanoparticles were studied employing adsorption–desorption nitrogen isotherms, morphology (SEM), calorimetry (DSC) and dielectric (BDS) techniques. The initial titania particles (crystallites of 40–140nm in diameter) were found to aggregate forming voids of a broad range. PDMS was adsorbed in the interior of voids in aggregates and onto external surfaces of the aggregates. With nanoparticles content increasing an increase in the glass transition temperature, Tg, is observed in DSC, accompanied by a broadening of the glass transition step and a decrease in the heat capacity step, ΔCp. According to BDS the interfacial polymer fraction is larger and exhibits slower dynamics (αint relaxation) in the case of shorter PDMS chains (∼22 monomers/chain) as compared to longer chains (∼105 monomers/chain). Changes in the interfacial polymer fraction correlate well with changes in the specific surface area of the aggregates, as recorded by gas molecules adsorption–desorption, and with morphology (SEM). Comparison with previous results obtained with PDMS-based nanocomposites suggests that interfacial polymer fraction and dynamics are dominated by the concentration of polymer–particle contact points (increased for shorter chains due to more free chain ends), rather than by the strength of polymer–filler physical bonds.