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

Spectroscopic features in the Raman spectrum of both crystalline and amorph ous syndiotactic polypropylenes are reported which characterize different c hain 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 rotatio nal isomeric state model, the conformational distribution associated with t he amorphous phase is obtained. Changes in the spectrum due to sample defor mation are identified. In the calculation of simulated Raman spectra, only two polarizability parameters associated with C-C stretching and C-C-C angl e bending were needed to duplicate the spectroscopic features observed. Con formationally 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), amorphou s and helical (ggtt) structures in syndiotactic polypropylene. The band nea r 300 cm(-1) shifts from 290 cm(-1) for a short helix to 310 cm(-1) for a l ong helix, and is highly dependent on chain configuration. These features h ave been correlated with changing chain conformation distributions. (C) 200 1 Elsevier Science Ltd. All rights reserved.