Infrared linear dichroism has been a useful tool for studying the conformation and orientation of polymer molecules in deformed samples. When a polymer film is drawn, the macromolecular chains tend to align in a specific direction. The oriented film may then absorb, to different extents, incident infrared radiation polarized parallel and perpendicular to a reference direction usually defined as the draw direction. When a sample film absorbs more parallel than perpendicular light, the dichroic difference, ΔA = A∥ − A⊥, is positive and the dichroic ratio, R =A∥/A⊥, is greater than one. The dichroic ratio has traditionally been used to aid in the assigning of infrared bands to specific vibrational modes and to help determine the molecular chain orientation.
Curtis Marcott, "Linear Dichroism of Polymer Films Using a Polarization-Modulation Fourier Transform Infrared Technique," Appl. Spectrosc. 38, 442-443 (1984)