Aging of polycarbonate studied by temperature modulated differential scanning calorimetry

The enthalpy relaxation behaviour of polycarbonate has been studied by alte rnating differential scanning calorimetry (ADSC). Samples have been anneale d at 125 degrees C, about 20 degrees C below their glass transition tempera ture, for periods up to 2000 h, and then scanned in the ADSC using the modu lation conditions: heating rate=1 K min(-1); temperature amplitude=1 K; per iod=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-ph ase 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 agreem ent between the experimental results and the theoretical predictions, the m ost important aspects being the following. The total heat flow closely corr esponds to conventional DSC in respect of both peak endotherm temperature a nd enthalpy loss (derived from the area under the peak). In contrast, the n on-reversing heat flow peak area does not provide a good measure of the ent halpy loss because the reversing heat flow (and complex specific heat capac ity) depends significantly on aging, the transition region becoming much sh arper as the aging time increases. Likewise, the phase angle (when appropri ately corrected for the problem of heat transfer) also becomes sharper on a ging, 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 m odel that this area provides no information about the enthalpy loss that oc curs during the aging process. (C) 1999 Elsevier Science B.V. All rights re served.