Analysis of the low-frequency isotropic Raman spectrum of molten isotactic polypropylene

• Published in: Macromolecules 1991-07, Vol.24 (14), p.4025-4032
• Main Authors: Hallmark, V. M, Bohan, S. P, Strauss, H. L, Snyder, R. G
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Properties and characterization; Structure, morphology and analysis
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma00014a009

The feasibility of numerically calculating the low-frequency isotropic Raman spectrum of molten isotactic polypropylene has been explored 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 conformers 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 conformers generated from an isomeric three-state model and a second based on those from a five-state model. The two isomeric state models 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 exp --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 exp --1 , has two components rather than the one seen for molten polyethylene. The two components are associated with the two Raman-active zone-center modes of conformationally ordered (gt) infinity polypropylene. Graphs, Spectra. 21 ref.--AA