DSC study of polyethylene terephtalate´s physical ageing

When we keep an amorphous polymer at a temperature below its glass transition temperature T_g or when we vary the cooling rate of the polymer from its melting temperature to a temperature below T_g, it is not in a true thermodynamical equilibrium state and many of its properties change such as the free volume, the enthalpy and other properties. This behavior is called Physical ageing. In this paper, the study of this phenomenon in partially crystallized poly (ethylene terephtalate) (PET) (14%) is done by differential scanning calorimetry (DSC). The sample of PET is aged in two different ways: the first one consists on annealing the material at temperatures T_a below the glass transition temperature T_g for different ageing times Δt_a. The second one consists of cooling the polymer from a temperature higher than T_g (95°C) to the room temperature following various cooling rates b. An endothermic peak appears at a temperature situated just above T_g. We notice that for a slow cooling rate and higher annealing time, the area´s peak increases and the temperature shifts toward a higher value. Examining the results reveals that the influence of the polymer´s thermal history is important for a slow cooling rate and a significant annealing time.