The development of structure and mechanical properties in poly(ethylene terephthalate) fibres during heat treatment under stress

The development of microstructure during the heat treatment of poly(ethylene terephthalate) fibres has been investigated. Specimens representing different durations (down to a few milliseconds) of isothermal treatment were prepared through interrupted annealing and rapid quenching. Characterization of the structure and properties of such specimens demonstrated that the transformation occurred in two stages. In the first stage, microfibrillar crystals develop. The shrinkage behaviour of the material is set at this stage. In a second stage, the morphology transforms from fibrillar to lamellar. During this process, crystals become increasingly perfect and the crystallinity increases. The stiffness of the fibre increases continuously through these stages.