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 ǻt 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.

In order to optimize the use of some materials in electronic and microelectronic fields, the study of some of their properties is very important. Among these materials, we cite poly (ethylene naphtalate), PEN, which the description with a three-phase model is necessary to explain some of its dielectric behaviors, such as, for example, the relaxation peak. A thermal treatment by quenching the material in ice water at different temperatures has allowed us to obtain various partially crystalline poly ethylene naphtalate. DSC measurements have been carried out from 40 to 290 C in order to characterize the glass transition, melting point, and crystallinity of the obtained semicrystalline PEN. The results show the presence of a crystalline phase and a part of the amorphous phase that does not participate in glass transition, called the rigid amorphous phase. This later is more important in PEN than the one present in PET.

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