Modulated differential scanning calorimetry observation of physical ageing in polymers

Modulated-temperature differential scanning calorimetry (M-TDSC) is becoming a useful tool in the characterisation of thermal behaviour of polymers. In this paper, we discuss whether the non-reversing M-TDSC signal can be used, quantitatively and directly, to study the process of physical ageing in polymers. Difference exists between the values of relaxation enthalpy determined by using average heat capacity, <C-p>, signal as for conventional DSC and non-reversing heat capacity, C-p(n), signal. When the signal of reversing heat capacity of unaged sample is considered as baseline for <C-p> and C-p(n) signals, the difference disappears. It is concluded that non-reversing M-TDSC signal can be used to observe the process of physical ageing semi-qualitatively and directly. With increasing annealing time, the peak of the imaginary part, C-p", of the complex heat capacity becomes narrow, but peak area changes little. This indicated that C-p" is not correlated with relaxation enthalpy. It may be related to entropy change during the modulation. However, the entropy change is quite small.