Accelerated physical aging in PMMA/silica nanocomposites

We have monitored the physical aging process below the glass transition temperature (T(g)) of poly(methyl methacrylate) PMMA/silica nanocomposites by means of broadband dielectric spectroscopy (BDS). To do so, we have followed the evolution with time of the dielectric strength of the PMMA secondary relaxation process that dominates the dielectric response overall below T(g). The employed silica particles are spherical and present a diameter of several hundred nanometres. We have investigated polymer nanocomposites with silica concentration of about 10% wt. This results in an interparticle distance of the order of several hundred nanometers. Despite the general similarity between the segmental dynamics of the nanocomposites and that of pure PMMA as evidenced by both differential scanning calorimetry (DSC) and BDS experiments, the former systems display markedly accelerated physical aging in comparison to the pure polymer. This striking result suggests that the relevant length scale of the system under investigation plays a crucial role in affecting the mechanism of the physical aging process. As a natural consequence of such evidence, the diffusion of free volume holes-annihilating at the "external surface" of the polymer being aged-has been invoked to explain the strong mismatch between the physical aging in the nanocomposite and that of pure PMMA. Such an interpretation is discussed in light of the recent results on physical aging of polymer nanocomposites.