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

Abstract

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.