Linear Dichroism of Polymer Films Using a Polarization-Modulation Fourier Transform Infrared Technique

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.