Characterization of molecular orientation in polyethylene by Raman spectroscopy

Abstract

The polarization dependences of several Raman bands of roll-drawn high-density polyethylene of uniaxial symmetry were investigated at different draw ratios between 7.0 and 11.7. When the right-angle scattering geometry was used, it was possible to determine quantitatively the principal components of the Raman tensors of the 1080-, 1130-, and 1170-cm-' vibrations. The orientation coefficients (P*(cos e)) and (P4(cos e)) of the C-C bonds with trans conformation and gauche defects were also calculated by using these bands. The results obtained from the 1080-cm-' band show that the orientation of gauche structures does not vary much with the draw ratio (between 7.0 and 11.7). On the other hand, the polarization behavior of the 1130-cm-' band indicates that trans conformers in the amorphous phase orient more readily in the draw direction. When the components of the Raman tensors obtained from right-angle scattering were used, it was found that backscattering measurements could yield good estimates of (P&os e) ) and (P~(cos e)) orientation averages. Finally, a method is proposed for the rapid evaluation of these parameters from calibration curves obtained from backscattering spectra measured with the polarization direction of both the incident and scattered light parallel to the draw direction. This method should be particularly useful for the determi- nation of the distribution of the orientation of trans bonds in thick processed samples of polyethylene.