Applications Of Fourier Transform Raman Spectroscopy To Polymeric Systems

Raman spectroscopy is an extremely sensitive technique used to characterize structure and local order in polymeric systems. Unfortunately it has never attained the routine analytical utility which other techniques such as infrared spectroscopy have due to the presence, in most long chain polymers, of trace amounts of impurities (chemical initiators, catalyst residues, stabilizers, etc.). These impurities give rise to strong fluorescence in the visible which interferes with the recording of a Raman spectrum in this region. Recently, however, with the advent of cw Nd:YAG lasers and low temperature InGaAs detectors, Raman spectroscopy in the near infrared has become possible. Near infrared photons do not give rise to fluorescence and hence Raman scattering from polymeric materials containing chromophores either as an inherent part of their molecular structure or as an impurity can be routinely obtained. The spectral distribution of the scattered light is obtained using a commercially available Fourier transform IR (FTIR) spectrometer thus giving name to the technique. Since the inception of this technique in 1985, Fourier transform Raman spectroscopy has been utilized to investigate the structure of biomolecules, the nature of order-disorder transitions in polysilanes and the orientation of side chain chromophores in carbon backbone polymers useful for nonlinear optical applications. Several examples will be used to explicitly illustrate the utility of this new spectroscopic technique.