The effects of stress relaxation on the structure and orientation of tensile drawn poly(ethylene terephthalate)

Tensile drawing experiments on poly(ethylene terephthalate) (PET) have show n that there is no significant strain-induced crystallisation until draw ra tios around 2.3. The actual onset of strain-induced crystallisation depends on the deformation rate, and it occurs at lower draw ratios for higher dra w rates. The development of strain-induced crystallisation has a significan t impact on the relaxation behaviour of PET. Online birefringence measureme nts, during the relaxation of PET drawn to different draw ratios showed tha t: (a) at low draw ratios, the orientation relaxes over long periods of tim e and Fourier transform infrared (FTIR) spectroscopy showed that the overal l orientation falls after relaxation; (b) at higher draw ratios, when signi ficant strain-induced crystallisation has occurred, the orientation decreas es over short times, of the order of 10 s and remains constant thereafter. This behaviour is probably due to the crystallites that lock in the extensi on of the chains in the amorphous material. Post relaxation FTIR measuremen ts showed that the orientation increases due to annealing effect. Different ial scanning calorimetry (DSC) showed that there is an increase in the crys talline fraction after relaxation for all draw ratios, which is probably du e to the conversion of oriented amorphous material (trans conformers) into the crystalline phase. (C) 2000 Elsevier Science Ltd. All rights reserved.