Aging of polycarbonate studied by temperature modulated differential scanning calorimetry

Abstract

The enthalpy relaxation behaviour of polycarbonate has been studied by alternating differential scanning calorimetry (ADSC). Samples have been annealed at 125°C, about 20°C below their glass transition temperature, for periods up to 2000 h, and then scanned in the ADSC using the modulation conditions: heating rate=1 K min −1; temperature amplitude=1 K; period=1 min. The data have been analysed in terms of total, reversing and non-reversing heat flows, and also in terms of complex, in-phase and out-of-phase specific heat capacities and a phase angle. The effect of aging time on each of these parameters is illustrated and compared with the predictions of an earlier theoretical model. It is shown that there is very good agreement between the experimental results and the theoretical predictions, the most important aspects being the following. The total heat flow closely corresponds to conventional DSC in respect of both peak endotherm temperature and enthalpy loss (derived from the area under the peak). In contrast, the non-reversing heat flow peak area does not provide a good measure of the enthalpy loss because the reversing heat flow (and complex specific heat capacity) depends significantly on aging, the transition region becoming much sharper as the aging time increases. Likewise, the phase angle (when appropriately corrected for the problem of heat transfer) also becomes sharper on aging, and the (negative) peak moves towards higher temperatures. The out-of-phase specific heat capacity is calculated using the corrected phase angle, and it is shown that the area under this peak is essentially independent of aging time, confirming another prediction from the earlier theoretical model that this area provides no information about the enthalpy loss that occurs during the aging process.