Physical aging in PMMA/silica nanocomposites: Enthalpy and dielectric relaxation

We have investigated the physical aging below the glass transition temperature, namely the slow evolution occurring in non-equilibrium glasses, of poly(methyl methacrylate)/silica (PMMA/silica) nanocomposites. To do so we have followed the time evolution of the enthalpy and that of the dielectric strength of PMMA β process during isothermal annealing. The results indicate that physical aging is generally accelerated in all nanocomposites in comparison to pure PMMA, despite the lack of effect of the nanoparticles on PMMA molecular dynamics. Furthermore, the shorter is the interparticle distance, and hence the higher is the area/volume of silica in PMMA, the more pronounced is such acceleration. The acceleration of the physical aging together with the invariance of PMMA dynamics in the nanocomposites in comparison to pure PMMA poses serious questions on the idea that the molecular mobility is the only responsible parameter for the rapidity of physical aging. Thus, an interpretation based on the free volume holes diffusion towards the external surface, in this case represented by the polymer/silica interface, is provided.