Infrared spectra of poly(ethylene terephthalate) yarns. Fitting of spectra, evaluation of parameters, and applications

Abstract

A quantitative model for fitting infrared spectra of polyethyleneterephthalate samples over the region from 1100 to 720 cm⁻¹ has been evaluated. The absorbance was described as the sum of a linear base line and a set of symmetrical bell‐shaped Pearson VII curves. Using this model, experimental transmission spectra of yarns, measured with the electric vector parallel and perpendicular to the fiber axis, were fitted. It was found that each of the trans bands near 972 and 845 cm⁻¹ consists of two components, a narrow and a broad one. These two components represent the crystalline and amorphous phases respectively. The band due to the out of plane benzene ring CH deformation vibration at about 875 cm⁻¹ appeared also to be composed of two contributions. In this case, the narrow component was found to be due to molecules having interactions with direct neighbors. So this narrow component not only contains the molecules in the crystalline phase but also those forming part of bundles which are too small to be detected by x‐ray diffraction. For a detailed insight into the molecular arrangement of the amorphous regions, the infrared detection of trans‐guache transitions, fold content, detection of bundlelike structures, and molecular stress on tie molecules turned out to be of great practical importance. The structural details, which can be revealed by the infrared technique, cannot be obtained by other means. The potential of the method is illustrated with some practical examples.