聚对苯二甲酸乙二酯非晶态膜片单轴热——拉伸中的结晶与松弛

用动态力学损耗温度谱作为测试手段,研究了非晶态PET膜片在78—112℃温度范围内的单轴拉伸。实验结果说明,在较低温度下所得结晶的拉伸试样,完全由于应变诱发结晶,发生在应力-应变曲线的屈服后应力开始上升的阶段。在较高温度下(90℃或更高)拉伸可得非晶态而且光学各向同性的试样,是由于分子链的小尺度取向在拉伸过程中已完全热松弛所致,而分子链的大尺度取向要通过高弹态流动而松弛,其速率较慢,用拉伸后试样两端固定时的应力松弛进行了观察。在较低温度下应力松弛后仍为非晶态,在较高温度下应力松弛到起始应力的1O％下才开始结晶。FTIR研究表明在这种状态下的结晶有一结晶诱导期,其时间尺度与应力松弛阶段相当。

CRYSTALLIZATION AND RELAXATION DURING UNI-AXIAL STRETCHING OF AMORPHOUS POLY(ETHYLENE TEREPHTHALATE) FILM

The uni-axial stretching of amorphous poly (ethylene terephthalate) (PET) film has been studied in the temperature range of 78-112℃ by means of the temperature scanned dynamical mechanical loss curve of the stretched film. Amorphous PET films stretched at lower temperatures were crystalline, caused solely by straininced crystallization which took place when the stress-strain curve stated to show an incease (strain hardening) after yielding. Amorphous tilrns stretched at higher temperatures, arround 90℃ or higher, remained amorphous and optically isotropic. The latter resulted from complete relaxation of the local chain orientation during hot streching, while the global orientation of the chain could only be relaxed by rubbery flow which would be rather slow. The thermal relaxation of the global chain orientation have been studied by stress relaxation of the stretched amorphous film with fixed ends. At lower temperatures the films after almost complete relaxation remained amorphous. At higher temperatures the film started to crystallize during stress relaxation after the stress had relaxed to a value below 10% of the mitial stress. FTIR studies showed that this stressrelation time corresponds to the induction period of crystallization under such conditions.