Estimation of Crystallinity in Isotropic Isotactic Polypropylene with Raman Spectroscopy

Abstract

The Raman spectrum of isotactic polypropylene (iPP) has been found to exhibit vibrational peaks in the region of 750 to 880 cm⁻¹ that are sensitive to the degree of crystallinity. These features are broadly assigned to various modes of methyl group rocking, ρ(CH₂), and there have been various attempts to assess crystallinity based on the integrated intensities of these bands. Various vibrational analyses performed in the past in combination with experimental studies have concluded that the presence of crystalline order with trans-gauche conformation gives rise to a peak at 809 cm⁻¹, which is assigned to a ρ(CH₂) mode coupled with the skeletal stretching mode. However, the presence of additional peaks at 830 cm⁻¹, 841 cm⁻¹, and 854 cm⁻¹, within the same envelope, have been the subject of controversy. In this work isotropic films of iPP derived from the same precursor of identical tacticity have been subjected to various degrees of annealing and the integrated intensities of the Raman bands were measured. The results showed that true 3d crystallinity in isotropic iPP can only be expressed by the 809 cm⁻¹ band whereas the band at 841 cm⁻¹ corresponds to an uncoupled ρ(CH₂) fundamental mode and thus is a measure of the amorphous content. The less intense satellite bands at 830 cm⁻¹ and 854 cm⁻¹ of solid iPP cannot be distinguished from the 841 cm⁻¹ band in the melt and are generally considered as intermediate phases possibly related to non-crystalline components with 3₁-helical conformations. Independent differential scanning calorimetry (DSC) crystallinity measurements were in broad agreement with the Raman measurements based on the normalized intensity of the 809 cm⁻¹ Raman band. By comparing the Raman with the DSC data a new value for the theoretical heat of fusion for the 100% crystalline iPP has been proposed.