Analysis of the Low-Frequency Isotropic Raman Spectrum of Molten Isotactic Polypropylene

We have explored the feasibility of numerically calculating the low-frequency isotropic Raman spectrum of molten isotactic polypropylene to determine relations between the Raman spectrum and conformational disorder. The spectrum of the disordered polymer was approximated by summing the calculated spectra of a set of random conformera of 2,4,6,8,10,12,14,16,18-nonamethylnonadecane. The main features of the observed spectrum are reproduced in the calculated spectrum. Two spectra were calculated, one based on conformera generated from an isomeric three-state model and a second based on those from a five-state model. The two isomeric state models we employed have been previously described in the literature and used to calculate the colligative properties of molten polypropylene. The two spectra derived differ significantly mainly in the width of the “umbrella” band near 400 cm−1. The observed width of this band can be accounted for only by the five-state model. In addition, the Raman spectrum of disordered isotactic polypropylene has been characterized. The distribution of intensity can be correlated with the frequency distribution of backbone and methyl modes. The D-LAM band, which is observed near 200 cm−1, has two components rather than the one seen for molten polyethylene. We associate the two components with the two Raman-active zone-center modes of conformationally ordered (gt)∞. polypropylene. © 1991, American Chemical Society. All rights reserved.