An analysis of the Raman spectrum of syndiotactic polypropylene. 1. Conformational defects

Spectroscopic features in the Raman spectrum of both crystalline and amorphous syndiotactic polypropylenes are reported which characterize different chain conformations. A normal coordinate analysis was conducted to assist in identifying conformationally sensitive bands in the experimental spectra. Based on a force field transferred from smaller molecules, and on a rotational isomeric state model, the conformational distribution associated with the amorphous phase is obtained. Changes in the spectrum due to sample deformation are identified. In the calculation of simulated Raman spectra, only two polarizability parameters associated with C-C stretching and C-C-C angle bending were needed to duplicate the spectroscopic features observed. Conformationally sensitive bands occur in the 300 and 800 cm(-1) regions. The bands at 865, 845 and 826 cm(-1) are assigned to all-trans (tttt), amorphous and helical (ggtt) structures in syndiotactic polypropylene. The band near 300 cm(-1) shifts from 290 cm(-1) for a short helix to 310 cm(-1) for a long helix, and is highly dependent on chain configuration. These features have been correlated with changing chain conformation distributions. (C) 2001 Elsevier Science Ltd. All rights reserved.