Enthalpy Recovery of PMMA/Silica Nanocomposites

In this work, we have studied the effect of silica particles on the physical aging of nanocomposites based on poly(methyl methacrylate) (PM MA). To do that, we have followed the enthalpy relaxation by means of differential scanning calorimetry (DSC). In agreement with previous results carried out by means of broadband dielectric spectroscopy (BDS), we observe an acceleration of the physical aging process of PM MA nanocomposites in comparison to bulk PM MA. The fitting of the enthalpy relaxation results to the well-known Tool-Narayanaswamy-Moynihan (TNM) model gives rise to equal structural parameters for all investigated samples. including hulk PM MA. This implies that the molecular mechanism for physical aging in PMMA is not affected by the presence of silica particles. The only parameter changing is the pre-exponential factor setting the time scale of physical aging. The values obtained are correlated to the area/volume ratio or silica particles in the polymer and thereby to the silica interparticle distance in the nanocomposites. This latter observation is an indication that the physical aging process is driven by the diffusion of free volume holes toward polymer interfaces, as already proposed in the past.