Effect of heat treatment on the properties of polyethyleneterephthalate fibres

Linear dilatometry and X-ray diffraction have been used to investigate the thermomechanical properties at various stresses of oriented and unoriented fibres of polyethyleneterephthalate that had been subjected to various heat treatments. It has been shown that linear dilatometry, being a static thermomechanical method, is not less informative than other dynamic thermomechanical methods in investigating the nature of the temperature transitions in fibres. The new experimental results that have been obtained confirm the non-linear relationship between the glass temperature Tg (melting) on the imposed stresses. It has been shown that the maxima on that Tg (melting)-stress curves are found at stresses equal to the internal stresses caused by crystallization processes and by processes involving loss of orientation in oriented fibres. It has been observed experimentally that a tendency to the formation of a minimum appears on the melting point-stress curve at high stresses.