New insights into the structure of polypropylene polymorphs and propylene copolymers probed by low-frequency Raman spectroscopy

Abstract

We present a low-frequency (or low-wavenumber) Raman spectroscopy study of isotactic polypropylene, namely the different modifications - α, γ, and smectic, with a degree of crystallinity ranging from 0 to 70 %, as well as of random propylene/1-butene and propylene/1-octene copolymers with the incorporated monomer contents of 5.3 and 19.5 mol % for 1-butene and 4.5 and 24.0 mol % for 1-octene. In the Raman spectra of the propylene/1-octene copolymers and the neat isotactic polypropylene, we have found that the band at 105 cm−1, which is connected to the torsion vibrations of CH2-CHCH3 groups in the polymer backbone, can be assigned to the vibrations of helical macromolecules in the α and γ crystallites and the smectic phase. If the smectic phase is absent in the corresponding polymer system, then the intensity of the 105 cm−1 band is related to the molecular vibrations in the crystallites. We have also observed a broadening of the band at 398 cm−1 accompanied by the appearance of a high-wavenumber shoulder in the Raman spectra of the copolymers with the increase in the incorporated monomer content. This experimental finding can be explained by the reduction of the degree of crystallinity, conformational disordering or by the increase in the dimensions of the crystal unit cell.