Molecular Vibrations of Polymers

Abstract

Vibrational spectroscopy provides information on three important aspects of polymers, that is, their chemical composition, the geometric arrangement of atoms in space (physical structure), and the interatomic forces associated either with valence bonding or intermolecular interactions. The theory and experimental procedures developed so far for application of vibrational spectroscopy to small molecules and their crystals carry over to polymeric systems, with important differences that have to do with the unique structures exhibited by polymers. In this review, the theory of molecular vibrations of polymeric molecules is presented in a manner that emphasizes the basic structural differences between these molecules and their low-molecular-weight analogues. In particular, recent approaches to calculations of the frequencies of polymers with irregular conformational defects are reviewed. On the experimental side, the focus is on those experimental tech­ niques which, because of recent developments in instrumentation, per­ mit new or improved measurements. The most significant recent devel­ opments involve digital data acquisition and analysis, and the recent availability and use of Fourier transform infrared spectroscopy (FTIR). The improved specificity of infrared spectroscopy, achieved mainly through application of spectral subtraction techniques to high signal-to­ noise digital data, has led to a renewed interest in determinations of the chemical composition of macromolecular systems, including the nature of the bonding of chromophores in the polymer chain. Chemical com­ position descriptions rely on correlations between frequencies and specific chemical groups as, for example, the C = 0 stretching frequency of the carbonyl group. Although developing such correlations is a rather old topic, and one in which much progress and payoff have been