Raman Vibrational Studies of Syndiotactic Polystyrene. 2. Use of the Fundamental ν₁ Vibrational Mode as a Quantitative Measure of Crystallinity within Isotropic Material

Syndiotactic polystyrene (sPS) has been found to exhibit two distinct vibrational peaks in the Raman spectrum corresponding to the ν 1 vibrational mode. These features have been assigned to the symmetric phenyl ring breathing mode coupled to the polymer backbone. It has been shown that the presence of long all- trans sequences gives rise to a peak at ∼773 cm -1, whereas less well-defined trans / gauche conformations result in a separate peak at ∼798 cm -1 . With increasing levels of crystallinity, the integrated intensity of the former feature grows at the expense of the high-frequency feature. The use of a series of annealed isotropic films of glassy sPS of increasing crystallinity and molten sPS has made it possible to examine the precise relationship between these two features and other neighboring vibrational peaks. The level of crystallinity within each sample has been determined independently using wide-angle X-ray scattering, differential scanning calorimetry, and infrared spectroscopic measurements. It has been found that the relative area under the all- trans ν 1 band can be directly related to the crystalline volume fraction within each sample. It is proposed that this Raman vibrational feature centered around 773 cm -1 can be used as a quantitative measure of crystallinity for sPS once the contribution from the all- trans sequences in the melt is subtracted from the spectrum.