Glass transition of atactic polystyrene probed at the submolecular level by dynamic infrared linear dichroism (DIRLD) spectroscopy

Dynamic infrared linear dichroism (DIRLD) spectroscopy is a rheo-optical characterization technique developed specifically to probe the submolecular dynamics of polymer segments. The technique combines the measurement of submolecular orientation based on the directionally selective absorption of polarized IR light with a small-amplitude oscillatory tensile deformation used in dynamic mechanical analysis. A DIRLD spectroscopic study of atactic polystyrene reveals that a dramatic change in the reorientation behavior of aromatic side groups is observed around the glass transition temperature of 100 °C. The transition point for the main chain backbone, on the other hand, is observed at a much higher temperature around 125 °C. Thus, the macroscopically observable glass transition of polystyrene seems to be dominated by the dynamics of side groups rather than that of the coordinated motions of polymer segments along the backbone. This result suggests a fundamental similarity between the glass transition phenomena of polymers and those of small-molecule inorganic glasses.