Vibrational Relaxation in Atactic Polystyrene: An Infrared Spectroscopic Study

Abstract

The shapes and widths of infrared bands and Raman lines are an important source of information on interactions and molecular dynamics and have been used to study vibrational and rotational relaxation processes in the picosecond time range. Most of this work has focused on small molecules, however, and the analysis of bandwidths and band shapes has been a largely neglected topic in polymer vibrational spectroscopy. The characterization of polymers introduces both advantages and new problems. One simplification is a consequence of the large mass and moments of inertia of these molecules. Relaxation times associated with the reorientation of chain segments and many side groups are long compared to vibrational relaxations, and their contribution to band and line contours can be neglected. Unfortunately, this simplification is accompanied by new problems: the multiplicity of overlapping bands observed in many systems and the complications introduced by conformational sensitivity. Some of these experimental problems are addressed in this paper, where an analysis of the ring stretching modes in the 1600 cm-1 region of the infrared spectrum of atactic polystyrene is presented. It is shown that the shape of one of the fundamental modes in this region of the spectrum changes significantly as the polymer is heated through the glass transition temperature. This is interpreted in terms of a coupling of this mode to lattice vibrations through a combination vibration. It is thought that the α-relaxation process in glassy solids succeeds much faster than picosecond scale motions, and vibrational relaxation, as suggested by these results, would appear to be a sensitive indicator of the type of cooperative motions thought to be involved.