Physical aging of polystyrene/gold nanocomposites and its relation to the calorimetric Tg depression

The aim of this work is to study the effect of gold nanoparticles on the segmental dynamics, glass transition (T-g) and physical aging of polystyrene (PS). To do so, PS/gold nanocomposite samples containing 5 and 15 wt% of 60 nm spherical gold nanoparticles, surface-treated with thiolated-PS, were prepared. The segmental dynamics of PS, as measured by means of broadband dielectric spectroscopy (BDS), was found to be unchanged in the presence of gold nanoparticles. Conversely, the calorimetric T-g of PS was shown to decrease with increasing the amount of gold nanoparticles in the samples. Furthermore, by measuring the amount of recovered enthalpy of PS-by means of DSC-after annealing at temperatures below T-g for various aging times, the physical aging was shown to speed up with increasing the nanoparticles weight fraction, i.e. the amount of PS/gold interface in the hybrid material. Thus, the main conclusion of our work is that PS molecular mobility and the out-of-equilibrium dynamics are markedly decoupled in these nanocomposites. The significant effect of the amount of PS/gold interface on both the physical aging rate of PS and the depression of the calorimetric T-g in the presence of nanoparticles is quantitatively accounted for by a model based on the diffusion of free volume holes towards polymer interfaces, with a diffusion coefficient depending only on the molecular mobility.