Molecular orientation in polyester films using polarized laser Raman and Fourier transform infrared spectroscopies and X-ray diffraction

Polarized laser Raman and FTIR spectroscopies, as well as wide angle X-ray diffraction (WAXD), have been employed in order to investigate the distribution of molecular orientation in uniaxially drawn solution-cast films of well-characterized polyesters, bearing hexyl side chains, at different draw ratios. Both the second ([P-2(cos theta)]) and fourth ([P-4(cos theta)], only with Raman) moments of the segment orientation distribution function have been determined. Results reveal physically meaningful trends of both P-2 and P-4 with draw ratio. A critical comparison among the three techniques confirms the sensitivity of Raman and FTIR to order at molecular level, when detecting the orientation of a particular segment, compared to similar WAXD results that provide information on the larger scale liquid crystalline domain orientation only and thus correspond to higher values of P-2. Therefore, the corresponding dependencies of P-2 on draw ratio are also different. A simplified approach for the analysis of the Raman spectra, based on the cylindrical symmetry of the Raman tensors at a specific vibrational normal mode, demonstrates the effectiveness and usefulness of this technique for accurately and fully determining the molecular orientation in rodlike polymers.