4 - Dichroic Measurements of Polymer Films Using Infrared Spectrometry

Abstract

Infrared dichroism is the use of infrared polarized light as p-(90°) and s-(0°) polarized light to measure the molecular orientation of polymer films and other highly oriented molecules. The capability to generate p- and s-polarized infrared energy is required to complete these measurements. The difference between a spectrum of a polymer film taken with s-polarized light and the spectrum of a film illuminated with p-polarized light reveals the molecular orientation of the polymer backbone and attached side groups. When the electric field vector of the infrared-active molecule (dipole) is perpendicular to the field direction vector of the infrared energy, there is no interaction of the incident infrared energy with the dipole and thus no infrared absorption. When the electronic field vector of the infrared-active molecule (dipole) is parallel to the field direction vector of the infrared energy, there is a large interaction and significant infrared absorption. The molecular orientation of the various molecules in a polymer film can be measured by making dichroic (s- and p-polarized light) measurements before and after stretching of the film sample.