Molecular orientation distributions in uniaxially oriented poly(L-lactic acid) films determined by polarized Raman spectroscopy

Molecular orientation distributions in the crystalline and amorphous regions were studied for uniaxially oriented poly(L-lactic acid) (PLLA) films using polarized Raman spectroscopy. By combining wide-angle X-ray diffraction and birefringence measurements, the tilt angle between the principal axis of the Raman tensor and the molecular chain axis was determined for Raman bands assigned to the crystalline regions only, and also to those being independent of morphology. The changes of the molecular orientation distribution functions during the drawing were analyzed in detail. The PLLA molecules in the crystalline regions were found to be highly oriented at a draw ratio of 4.5, probably as a result of crystal rotation and crystal slip. In contrast, the development of molecular orientation in the amorphous regions appeared to take place by pseudo-affine deformation. The results suggest that the film needs to be stretched to draw ratios above 4.5 to achieve successful molecular orientation in the amorphous regions.