BENZENE-RING ORIENTATION IN UNIAXIAL-PLANAR POLY(ETHYLENE-TEREPHTHALATE) FILMS

A study is presented of the benzene ring orientation in a series of eight films of poly(ethylene terephthalate) drawn at constant width under constant force above the glass transition temperature. For each sample, seven orientation moment averages, P(lmm)r, for the overall benzene ring distribution were determined from a combination of Raman and infrared spectroscopic measurements. Comparison of orientation moment averages calculated with and without the assumption of a uniaxial Raman tensor, and with and without a refractive index correction to the Raman intensities, leads to the conclusion that the seven P(lmm)r are given, to a good approximation, by the simplest calculation assuming a uniaxial Raman tensor and neglecting the refractive index correction. The seven P(lmm)r were used to calculate the 'most probable' distribution function and the isometric projections of this distribution have been produced. The results indicate that the C1-C4 axis shows an approximately uniaxial overall distribution with a slight tendency to be more oriented out of the plane of the film and that the benzene rings tend to lie in the plane of the film. From a combination of these results with those of earlier X-ray measurements, the amorphous and crystalline contributions to the overall molecular orientation were obtained. The results show that the benzene ring planes are preferentially oriented towards the plane of the film only in the crystalline phase and have an almost uniaxial distribution around the draw direction in the amorphous phase. It has been shown that, in the amorphous phase as well as in the crystalline phase, the benzene rings tend to lie in the plane defined by the draw direction and the C1-C4 axis.