Measurement of Orientation and Crystallinity in Uniaxially Drawn Poly(Ethylene Terephthalate) Using Polarized Confocal Raman Microscopy

Polarized confocal Raman microscopy has been used to measure molecular orientation in uniaxially drawn poly(ethylene terephthalate) (PET) films, prepared with draw ratios from 1 to 3.5. The P₂₀₀ values obtained agreed well with those measured with the use of refractive index and infrared dichroism, provided that the numerical aperture (NA) of the microscope objective was not too high. For example, a 0.75 NA objective gave good agreement with IR and refractive index results, but a 0.95 NA objective significantly reduced the measured P₂₀₀. Corrections for the influence of sample birefringence on the measured Raman intensities were made and found to have a small (∼5%) effect on P₂₀₀. This result is fortunate, because such corrections demand knowledge of the principal refractive indices of the sample, which may be unavailable. It was also shown that the presence of significant biaxial orientation in these samples did not significantly affect the P₂₀₀ values computed under the assumption of uniaxial orientation. Orientation measurements were also compared with crystallinity data obtained by confocal Raman mapping through the thickness of a PET film. Significant gradients in both crystallinity and orientation were found through the thickness of the film. Surprisingly, these gradients were in the opposite sense, so regions of high crystallinity correlated with low orientation and vice versa. These results illustrate the extra insight obtained by combining multiple morphological measurements on the same sample.