The relationship between the $T_{g}$ depression and the speeding up of physical aging in polystyrene/gold nanocomposites

The effect of gold nanoparticles on the segmental dynamics, glass transition ($T_{g})$ and physical aging of polystyrene (PS) was studied in PS/Gold nanocomposites samples containing 5 and 15 wt.{\%} of 60 nm spherical gold nanoparticles, surface-treated with thiolated-PS. While the segmental dynamics of PS, as assessed by broadband dielectric spectroscopy (BDS), was found to be unchanged in presence of gold nanoparticles, the calorimetric $T_{g}$ of PS was shown to decrease with increasing the amount of nanoparticles in the samples. Furthermore, the physical aging of PS, monitored by measuring the enthalpy relaxation below $T_{g }$ by means of DSC, 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 out-of-equilibrium dynamics are decoupled in these nanocomposites. The significant effect of the amount of PS/Gold interface on both the physical aging rate of PS and the calorimetric $T_{g}$ depression are 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.