Contribution to the Study of the Enthalpy Relaxation of Polyesters by DSC Experiments

The differential scanning calorimetry (DSC) technique was used to study the enthalpy relaxation of weakly crystalline polyethylene naphatalate (PEN). The samples were examined under two different thermal histories inside the calorimeter. The first method consists in annealing samples at temperature Ta below the glass transition temperature Tg, for different times Δta. The second one consists in cooling samples from a higher temperature than Tg to the room temperature using following cooling ratesb. In order to highlight the effect of thermal rigidity of molecular chains on the enthalpy relaxation of polyester, a comparative study between the enthalpy relaxation of polyethylene terephtalate (PET) and PEN with very close degrees of crystallinity χc was made. This study shows that the structural relaxation (physical aging) induces a decrease in the molecular mobility of the samples, which prevents the polymer from initiating a rapid return to the equilibrium state. On the other hand, the molecular chains in ...