Orientation and Relaxation in Thick Poly(ethylene Terephthalate) Films by Transmission Infrared Linear Dichroism

Abstract

Infrared spectroscopy is a powerful tool for the study of the orientation in amorphous and semi-crystalline polymers, but it is generally limited to thin samples. In this study, we have used transmission infrared linear dichroism to study the orientation of thick poly(ethylene terephthalate) (PET) films. To overcome the saturation problem of the intense bands of PET, overtones and combination bands in the high-frequency region of the mid-infrared spectrum were used. Using polarization-modulation infrared linear dichroism (PM-IRLD), it was possible to follow in real-time the relaxation of orientation of uniaxially oriented PET films up to 500 μm thick. It was observed that between 30 and 500 μm, the thickness of the films has no effect on the orientation relaxation dynamics. It should, therefore, be possible to use thick films, which are much easier to prepare than thin films, for future infrared studies of the deformation of PET. A very good correlation was also observed for the relaxation curves obtained using high- and low-frequency bands related to gauche and trans conformers in thin and thick films. An example of the application of these high-frequency bands to obtain orientation and structural information is also given in the case of a commercial PET bottle showing a biaxial orientation.