Determination of density and birefringence of poly(ethylene terephthalate) fibers using Raman microscopy

Multiple regression analysis has been used to calibrate polarized Raman spectra of poly(ethylene terephthalate) fibers in terms of density and birefringence. The calibration spans PET fibers having a wide range of density and birefringence values. The calibration required the Raman spectrum in only one polarization direction, that is, with the polarization directions of the incident and scattered light parallel to each other and to the fiber axis. The peak at 631 cm⁻¹, which has been used previously as an internal standard band, could be used for the prediction of density, but not for the prediction of birefringence. The peak at 702 cm⁻¹ was found to be a good internal standard band for both density and birefringence. Density could be predicted with a standard error of prediction of 0.003 g/cc using only the ratio of the intensity of the band at 996 cm⁻¹ to that of 702 cm⁻¹ and the full width at half maximum of the 1725-cm⁻¹ band. Birefringence was predicted with a standard error of 0.01 using the ratios of the intensities of the bands at 996 and 1616 cm⁻¹ to that of the 702-cm⁻¹ band. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 943–952, 1999