It is known that necking and temperature rise occur simultaneously during tensile tests of Poly (butylene terephthalate) (PBT). In this paper, injection moldings, sliced samples, melt pressed films and uniaxially stretched films were used to clarify the effects of various structures in PBT on tensile properties and exothermic behaviors. These samples have different structures which depend on the nature of the particular processing conditions. Exothermic behavior was observed by using an infrared video camera during tensile testing. Density measurements, infrared spectroscopic analysis and infrared dichroism measurements were carried out. The relationship of structures such as crystallinity, crystal modification and molecular orientation to necking and exothermic behaviors were discussed. It was confirmed that a temperature rise occurred at the necking boundary and increased with increasing crystallinity. It was indicated that necking progressed slowly and the temperature rise became gradual in the presence of β-form crystal compared to only α-form crystal. For the uniaxially stretched films, the MD specimens did not show visible necking, thus it can be inferred that necking does not occur in tension along the orientation direction. It is considered that the necking phenomenon is related to an orientation-induced crystallization due to molecular re-orientation accompanied by an exotherm.

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