Quantitative Conformational Studies on Poly(vinyl Chloride)

Although curve fitting provides a method for obtaining intensity data for the individual components of overlapping band systems, the number of configurational and conformational bands in the C-Cl stretching region of the vibrational spectrum of poly(vinyl chloride) is such that many parameters have to be optimized. It is therefore desirable to impose constraints in the calculations, and prior knowledge of the number of component bands would be valuable. The potential of derivative spectroscopy for obtaining this information has been examined. It is shown that the superior resolution of second and fourth derivative spectra is partially offset by their inferior signal-to-noise ratio, a discriminatory effect against broader bands, and interference by subsidiary derivative peaks. The method has been used to examine the CH2 deformation and C-Cl stretching modes of three PVC samples of different tacticity. With the former the overlapping system proves to be more complex than hitherto realized; hence, tacticity determinations based on the intensity ratio of two peaks only at 1428 and 1434 cm−1 must be suspect. With the C-Cl stretching bands second derivatives are more useful than fourth derivatives, and the number and positions of the located bands are in broad agreement with the results from curve fitting, so providing confirmatory evidence for the correctness of the latter.