Molecular orientation and conformational changes due to uniaxial-planar deformation of poly(ethylene terephthalate) films

Abstract

Conformational changes and chain axis orientation in poly(ethylene terephthalate) due to uniaxial-planar deformation (at constant width) at constant engineering stress in the range 1.2–5.5 MPa at temperatures in the range 85–105°C have been studied by means of infrared spectroscopy. For all samples the state of orientation was frozen just after the deformation reached the equilibrium value at a draw ratio λ p. For a given temperature there are two stress regimes, a lower one where λ p decreases with increasing stress, corresponding to ‘stretching with flow’, and a higher one where λ p increases with increasing stress, corresponding to ‘crystallizing stretching’. It is shown that the crystallinity of the samples is uniquely related to the content of trans conformers and that the overall chain axis orientation may be determined, to a good approximation, by determining the orientation of the trans conformers; the chain axes associated with gauche conformers are effectively randomly oriented. Under conditions of ‘crystallizing stretching’ the content of trans conformers and the chain axis orientation can be understood in terms of a rubber-like network extension if the drawing temperature is between 80°C (the glass transition temperature) and 95°C. At higher temperatures of drawing, relaxation occurs in the amorphous phase. For all samples there is an excellent correlation between the trans concentrations and the overall orientation, independent of whether relaxation or crystallization occurs.