Crystallinity in poly (ethylene terephthalate): A comparison of X-ray, infra-red and density measurements

Abstract

Crystallinities have been measured in oriented poly(ethylene terephthalate) fibres by methods based on density, infra-red spectra and X-ray diffraction. The results have been compared and no correlation exists between the three; the values from infra-red spectra are high, those from X-ray measurement low, and those from the density fall roughly between the two.The crystallinity of a polymer fibre is by definition the relative amount of three-dimensional ordered material present in a sample and this is most directly measured by X-ray diffraction. The results reported here show that in poly(ethylene terephthalate) the infra-red measurements are only correlated with configurational changes affecting the order within individual molecules. Using the X-ray measurements to define crystallinity it is found that the density of non-crystalline material increases with orientation so that density measurements based on the concept of constant non-crystalline density (equal to the density of amorphous material) are inevitably in error when applied to oriented samples.The results also show that highly oriented pin-drawn yarns of low draw ratio have no appreciable crystallinity which is in contradiction to the view that the limiting stress built up in the drawing process is caused by the onset of crystallinity. Although the onset of crystallinity may well increase the reinforcement, it is clear that it is not essential to it. Dynamic loss data which had previously been correlated with crystallinity measured by the infra-red method may also be interpreted in a different light.