Change of Amorphous Structure of Poly (ethylene terephthalate)Induced by Drawing and Subsequent Annealing at Temperatures below Glass Transition Temperature

Abstract

Amorphous poly (ethylene terephthalate), (PET), film was stretched up to v=2, where v is draw ratio, and subsequently annealed at 60.5 and 62.2°C for 3 hours in a silicone oil bath, followed by quenching in ice water. Changes of the amorphous structure of PET during these processes were studied by density measurement, differential scanning calorimetry, IR spectroscopy and dielectric loss measurement and the following results were obtained.(1) In the range of v=1∼1.5, density (d), fraction of trans segment (r), and glass transition temperature (Tg) decreased, while frequency at the maximum dielectric loss(finax) increases with increasing v.(2) In the range v=1.5∼2.0, d, r and Tr, increased and fm decreased with v, i. e., each of these quantities versus v just passed through a maximum or minimum at v=1.5. (3) By annealing, d, r and Tg increased and fmax decreased. (4) A differential scanning calorimetry (DSC) curve exhibited an endothermic peak around 340K°, which became less conspicuous with stretching and recovered the initial quantity by annealing.It was concluded from (1)∼(4) that the free volume of amorphous PET increases in the initial stage of stretching, bbut recovers the initial quantity by annealing. It may be suggested that a weak structure exists within amorphous PET, which is destroyed by stretching but regenerated by annealing at temperatures below Tg.