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.