Infrared Spectroscopy and Polymer Structure

INTRODUCTION The power of infrared spectroscopy as a tool in studying the structure of small molecules is well known. This insight into structure results largely from our ability to analyse the spectrum theoretically by means of a normal coordinate analysis. This technique consists of calculating the normal vibration frequencies of a molecule from information on its structure and internal force field. From such a calculation we obtain the values of the frequencies and the forms of the vibrations of the molecule which can be observed in infrared absorption and in Raman scattering. By comparing such calculated frequencies with the observed spectrum it is possible to evaluate our assumptions concerning the structure and force field of the molecule. A review article published in 1960k stressed the importance of extending such theoretical methods of analysis to the study of the infrared spectra of high polymers. Only in this way could the full power of spectroscopy be brought to bear on answering questions related to the structure of, and interactions between, macromolecules. Prior to 1960 the few normal coordinate calculations which had been done (see 1 for references to such studies) were limited in their detailed predictive capability by the lack of a sufficiently complete force field for the molecule and by simplifying approximations made about the structure. The derivation of satisfactory force fields for polymers, which is still the most basic problem associated with a normal coordinate analysis, began in the early 1 960s, with the use of extended Urey—Bradley force fields2' and the development of a detailed valence force field for hydrocarbons4' 5. Since then the technique of normal coordinate analysis has been applied to the interpretation of the infrared spectra of many regular polymer structures (see, for example, polyethylene4' 69; polypropylene, both isotactic1014 and syndiotactic'5' 16; isotactic polybutene-1'7; syndiotactic 1 ,2-polybutadiene'8; polyoxymethylene19; polyethylene oxide20' 21; polytetrahydrofuran22; poly(vinyl chloride)23' 24; polyacrylonitrile25' 26; poly-3,3-bis(chloromethyl) oxacyclobutane27). In some of these cases the chain structure was already known from x-ray diffraction studies; in other cases vibrational analysis shed light on aspects of the structure which were not obtainable from x-ray studies. In this paper the author wishes to illustrate some of the facets of polymer structure which can be illuminated by means of detailed vibrational analysis. These aspects include not only the structure of the individual regular chain in the crystalline phase, but also features of the arrangement of chains in