A new approach to the characterization of molecular orientation in uniaxially and biaxially oriented samples of Poly(Ethylene Terephthalate)

The usual approach that is used to characterize the molecular orientation in biaxially oriented samples by infrared spectroscopy is to measure spectra with polarization in all three directions: machine, transverse and normal (or thickness). However, the latter measurement is rather difficult to make experimentally. In the present work we propose a new approach to characterizing the molecular orientation in both uniaxially and biaxially oriented samples of PET, based on the use of front-surface reflection spectra. It makes use of the ratio of the absorption bands at 1330–1240 and 1729 cm−1, the first of which shows parallel dichroism and the second perpendicular dichroism. An equation is developed that relates this ratio to the molecular orientation with respect to the direction of measurement. Thus, it is possible to determine individually the orientation functions with respect to the machine and transverse directions. The validity of functions determined in this way is confirmed by comparison with birefringence results.