Dynamic Mechanical Measurements on Polyethylene Terephthalate

Measurements of dynamic extensional and torsional moduli have been made for polyethylene terephthalate fibres of different physical properties. These include fibre in the undrawn state (i.e. amorphous and unoriented) and fibre in the highly drawn state (i.e. crystalline and highly oriented). The modulus measurements have been made over a range of temperature from room temperature to about 30 degc above the glass/rubber transition temperature. Similar extensional measurements have been carried out on uniaxially oriented polyethylene terephthalate films of varying degrees of orientation. At room temperature the fibres may to a good approximation be considered elastic. The measured extensional and torsional moduli, E u and G u respectively, for the unoriented fibre have been compared with those calculated assuming that the unoriented material can be regarded as an isotropic aggregate of units whose elastic constants are identical with those of the oriented fibre. The measured values of E u and G u are 2.3 and 0.77 × 10 10 dyn cm -2 , which compare favourably with the calculated values of 1.8 and 0.61 × 10 10 dyn cm -2 respectively. It has been shown that for all the fibres examined at constant frequency the maximum energy loss and the maximum rate of decrease of modulus both occur at the same temperature (the `transition temperature') whereas the maximum in tanδ occurs at a higher temperature. This fact has been shown to be consistent with linear viscoelastic behaviour assuming to a first approximation that a change in temperature produces a uniform shift of a logarithmic time scale. The transition temperature as defined here, has been determined for samples of different crystallinity and orientation. Both crystallinity and orientation produce appreciable changes in viscoelastic behaviour, and it is noted particularly that the oriented samples show different behaviour in extension and torsion.