Differential scanning calorimetry and thermostimulated current spectroscopy for the study of molecular orientation in polymers

Abstract

The influence of uniaxial and biaxial orientation on the transition/relaxation of poly(ethylene) terephthalate (PET) has been investigated. The transitions were studied by differential scanning calorimetry; the corresponding relaxations were studied by thermostimulated current (TSC) spectroscopy. At around 50°C, a particularly intense TCS peak was observed in the uniaxially oriented sample; this mode is a precursor of the glass transition T g. The relaxation mode associated with the glass transition is observed at 82°C in unoriented PET, at 85°C in uniaxially oriented PET and at 100°C in biaxially oriented PET. An analysis of the fine structure has shown that it is made up of elementary processes that obey a compensation law: at the compensation temperature T c, all the relaxation times become equivalent. It is important to note that for amorphous films (unoriented and uniaxially oriented) T c - T g ≈ 5°C, whereas for the semi-crystalline film (biaxially oriented) T c - T g ≈ 15°C. This result shows that crystallites are strongly coupled to the amorphous phase.