The heat of neck formation during the stretching of polyethylene terephthalate (PETP)

Abstract

The heat of neck formation was studied by calorimetry during the stretching of the amorphous and crystallizing PETP under various conditions. Most of the attention was given to cold-stretching and to the heats associated with the spontaneous vibration mechanism of stretching. Small deformations (not exceeding 1–2%) in a temperature range below the glass temperature ( T g) were found to cause the polymer to behave like an elastic material. A constant thermal energy is absorbed during stretching and the heat of stressing is the same as that of stress release; there is no distinct elastic after effect. At larger deformations (around 50%), forced elasticity will appear in this temperature range and the polymer behaviour is that of an ideal rubber. The spontaneous vibration mechanism of cold stretching is purely entropic and any internal energy changes are zero within the error limits of the experiments. The heat liberation is of the periodic type under spontaneous stretching conditions and corresponds to the periodic change of the stress, the rate of neck formation, and the external shape of the sample.