Infra-red measurements on one-way drawn poly(ethylene terephthalalate) films subjected to constant strain

Abstract

The infra-red spectrum of one-way drawn PET film was measured as a function of further applied strain, the specimens being maintained under fixed strain, while the spectra were obtained for the two polarization situations where the electric vector is, respectively, parallel and perpendicular to the draw direction. The changes in molecular orientation and conformation were determined from the reconstruction procedures outlined as a function of applied strain. The spectra, together with those of poly(ethylene terephthalate) (PET) in several different structural forms were reconstructed over the frequency range 700–1100 cm −1 by a computational procedure using the method of damped least-squares. The minimum number of bands required to reconstruct all the spectra was chosen, and the existence of such bands in the second derivative spectrum was used as a criterion for their validity. With the exception of bands at 726 and 733 cm −1, final fits were obtained on the basis that all the band shapes are Lorentzian. Comparison of the spectra of a number of model compounds showed that in nearly all cases the minor bands revealed by the curve fitting procedures could be identified with bands in the model compounds. It proved possible to relate the observed changes in the infra-red spectrum to the relaxed stress-strain curve of the film. The following picture emerges from the results for frequency shifts and changes in intensity. Up to the yield point there is considerable stress in the trans conformers (as reported by previous workers) and also in most of the gauche conformers. There is also a substantial increase in trans orientation, in contrast to the orientation of the terephthalate residue, which remains at the level observed at zero stress. The yield point marks the region where the stress on the trans and gauche conformers shows a sudden fall. At the same time there is a rapid increase in the trans-gauche ratio, a fall in the trans orientation, but an increase in the orientation of the terephthalate residues. It appears from these results that up to the yield point the elastic strains are concentrated in the glycol residues. After yield, both the stress and orientation decrease because conformational changes take place which allow the overall network structure to rearrange. When this occurs, there is some stress relaxation associated with the permanent flow, and the network as a whole achieves a higher degree of overall orientation, which is shown by changes in the orientation of the terephthalate residues. Subsequent deformation causes systematic changes in conformer content and molecular orientation which are consistent with continuum models for the plastic deformation of polymers.