Strain-induced crystallisation in bulk amorphous PET under uni-axial loading

Strain induced crystallisation of poly(ethylene terephthalate) (PET) designed for stretch-blow moulding is studied combining well-controlled tensile tests, different quenching protocols and X-ray diffraction technique. As well known [Polymer, 33 (1992) 3182; Polymer, 33 (1992) 3189], crystallisation begins when a minimum strain has been reached. However, crystallisation is a progressive phenomenon that could involve intermediate stages or phases as suggested by some recent papers [Macromolecules, 31 (1998) 7562; Polymer, 41 (2000) 1217]. In this study, first evidence for the crystallisation is the appearance, parallel to the tensile direction, of zones having the crystal lateral order. The spreading out of the diffraction dots suggests that these zones are small in comparison to usual lamella thickness or that the order is imperfect. Longitudinal order appears later, progressively leading to fibre-texture. At low strain, before this texture is totally developed, cooling influences the final microstructure superposing steps in which crystallisation and relaxation compete illustrating the relative importance of cooling protocol when strain induced crystallisation of PET is studied. Nevertheless, strain hardening can be observed before the crystalline microstructure is totally developed, emphasising the fact that strain hardening is mainly controlled by first stages of crystallisation and that actual crystallisation occur during a following relaxation step.