Isotopic Substitution in the Analysis of the Infrared Spectra of High Polymers
One of the important methods for identifying the origin of bands in the infrared spectrum of a high polymer, and therefore aiding in their assignment, is isotopic substitution. The most useful of such substitutions is that of deuterium for hydrogen, which leads to a lowering of the frequencies of all modes that involve the motions of the substituted hydrogen atoms. In the past this effect has only been discussed qualitatively. The use of a recently proposed approximate isotopic frequency rule, however, now makes it possible in many cases to identify the particular normal mode associated with a given band from the magnitude of its frequency shift on deuteration. This can be an important aid in the analysis of a spectrum. The applications and limitations of this rule, as well as the general problems associated with the study of deuterated polymers, will be discussed with reference to several polymers, such as polyethylene, polystyrene, and polyvinyl chloride.