Characterization of biaxial orientation in poly(ethylene terephthalate) by means of refractive index measurements and Raman and infra-red spectroscopies

Abstract

The three principal refractive indices of a film of poly(ethylene terephthalate), prepared by drawing an amorphous melt-extruded sheet at constant width between rollers whose speeds were set to give a nominal draw ratio of 3.5:1, were found to be 1.645, 1.573 and 1.541. Thirty-six measurements of the scattered intensity for the 1616 cm −1 Raman line were made for various combinations of orientations of the electric vectors of the incident and scattered light with respect to the sample and six absorbances were determined for each of the 1017 and 875 cm −1 infra-red peaks with the polarization vector of the radiation in different orientations with respect to the sample. From the Raman measurements five parameters P r 200, P r 220, P r 400, P r 420, and P r 440 which characterize the distribution of orientations of the benzene rings were deduced. Values of P r 200 and P r 220, in good agreement with the Raman values, were also deduced from the infra-red measurements, and a further two parameters, P r 202 and P r 222 were deduced from infra-red and Raman results combined. The four quantities P r 200, P r 220, P r 202 and P r 222 are shown to be consistent with the refractive index measurements using either of two alternative models for the chain conformation. The results indicate that there is slight preferential orientation of the chain axes out of the plane of the film, an unexpected result, but that there is also some preferential orientation of the planes of the benzene rings towards the plane of the film, in agreement with earlier X-ray measurements on similar films.