Rheo‐optical Fourier‐transform infrared spectroscopy of polymers. VII. Uniaxial deformation of heat‐set poly‐(ethylene terephthalate) film

Poly(ethylene terephthalate) (PET) film was uniaxially stretched to a draw ratio of 2.5. Two samples were prepared from this oriented film by heat‐setting it at 493 K while free to relax and when held at constant length. The structural changes occurring during uniaxial elongation up to fracture of these two oriented crystalline samples and their stress–strain characteristics were simultaneously monitored by rapid‐scanning Fourier‐transform infrared (FTIR) spectroscopy. In the free‐annealed sample, the orientation of the molecules in the amorphous phase shows a gradual improvement throughout the test, while chain unfolding occurs above 20% strain. This indicates that the predominant mechanism of deformation in this sample could be chain uncoiling in the amorphous phase followed by longitudinal slip processes within the sample. In the taut‐annealed sample, chain unfolding occurs at low strains accompanied by slight improvements in amorphous and crystalline orientation. Thus, in this sample longitudinal slip would appear to be the main deformation mechanism. The results of the FTIR measurements are discussed with reference to the dependence of the deformation mechanisms on the initial structure of the samples.