Structural analysis of poly(trimethylene terephthalate) fibers and films using polarized Raman spectroscopy

The evolution of the morphology of poly(trimethylene terephthalate) (PTT) samples subjected to different processing conditions was studied using polarized Raman spectroscopy. Three films (amorphous, hot drawn, and crystalline) were used to determine the structural origin of the vibrational bands in the Raman spectra. The findings were applied to a series of melt-spun PTT fibers. A unique combination of quantitative and spectral analyses was successfully used to gain a better understanding of the structural changes in the samples as the birefringence or density of the sample increases. The determination of the average molecular orientation and the distribution of molecular orientations were combined with a detailed analysis of the evolution of the spectral components of the carbonyl stretching vibration. It was determined that an oriented noncrystalline phase was formed in the samples having low densities (low birefringence). Once a certain degree of molecular order is attained, sudden crystallization can occur for a significant fraction of the oriented molecules. Finally, the density gradually increases due to a simultaneous growth of the oriented noncrystalline and crystalline phases. The findings yield valuable information with respect to the evolution of the sample morphology during spinning of PTT fibers and contribute to the fundamental understanding of fiber processing.