Aging phenomena in poly(methyl methacrylate) by dielectric spectroscopy and temperature modulated DSC

Aging dynamics have been investigated in glassy states of poly(methyl methacrylate) using dielectric relaxation spectroscopy and temperature modulated differential scanning calorimetry. Both the dielectric constant and the heat capacity were found to decrease with increasing aging time during an isothermal aging process at a given temperature below the glass transition temperature. The aging at a temperature for a few hours after an intermittent stop of the cooling process is memorized as a downward shift of the dielectric constant and the heat capacity. The memory of the thermal history is recalled as a minimum in the dielectric constant and the heat capacity during the subsequent heating process. A temperature cycling measurement on the dielectric constant clearly shows that there are rejuvenation and memory effects in the aging dynamics. The observed results are discussed on the basis of a hierarchical model.