New approach to the characterisation of molecular orientation in uniaxially and biaxially stretched samples of PET

The usual approach that is used to characterize the molecular orientation in biaxially oriented samples by infrared spectroscopy is to measure spectra with polar- ization 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 use a new approach to characterizing the molecular orientation in both uniaxially and biaxially oriented samples of PET. It makes use of the ratio of the absorption bands near 1250 and 1725 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.