UV-Excited Raman and Resonance Raman Spectra of Synthetic Polymers

Abstract

High-quality, UV-excited (218–242 nm), fluorescence-free conventional Raman spectra have been generated from UV-transparent polymers such as polytetra-fluoroethylene (Teflon®), polyethylene, polypropylene, and polyoxymethylene (Delrin®). Spectra can be generated with the use of low exciting power (<1 mW), but even at low power, precautions have to be taken to prevent photo-oxidation and thermal degradation of samples. Polyvinylchloride polymers especially have been found to be prone to degradation. Weakly absorbing polymers such as nylon and polybutadiene show special promise for Raman analysis, because they exhibit significant preresonance enhancement of structurally significant modes, and because they still retain information derived from conventional Raman spectra. Strongly preresonance-enhanced ethylenic modes of polybutadiene isomers can be resolved, allowing the easy identification of cis and trans isomers. In contrast, strongly absorbing polymers such as polystyrene, polycarbonate, and polyphenolics show only intense, resonance-enhanced Raman spectra. Unfortunately, Raman spectra of these materials are limited to phenyl ring modes, and all valuable conventional Raman spectra are totally lost due to internal absorption.