Structure and molecular motion changes in poly(ethylene terephthalate) induced by annealing under dry and wet conditions

Abstract

Structural changes in poly(ethylene terephthalate) (PET) induced by annealing under dry nitrogen gas flow (dry annealing) and in water (wet annealing) were investigated using small- and wide-angle X-ray diffraction methods, density measurements, differential scanning calorimetry and dynamic viscoelastic measurements. The degree of crystallinity of PET was evaluated by wide-angle X-ray diffraction patterns and by heat of fusion measured using differential scanning calorimetry. Based on the two-phase concept, the amorphous density of PET was calculated from the observed density and degree of crystallinity. The amorphous density decreased as crystallization increased for both the dry and wet conditions. The glass transition temperature and activation energy of the glass transition of PET based on dynamic viscoelastic measurements decreased as crystallization proceeded. Since the amorphous density decreased with annealing in the late phases of crystallization, molecular motion in the amorphous region easily occurred as crystallization proceeded. Water molecules enhanced the structural changes during the crystallization of PET.