Stress-induced shifts and intensity changes in the 1616 cm −1 Raman line for isotropic and uniaxially oriented poly(ethylene terephthalate)

Abstract

Measurements of the shifts induced in the 1616 cm −1 Raman line by applied stress have been made for an isotropic undrawn sample and several uniaxially oriented samples of poly(ethylene terephthalate) using various polarization directions of incident and scattered light with respect to the sample and to the direction of the stress, which was applied either parallel or perpendicular to the draw direction for the uniaxial samples. The corresponding changes in intensity with applied stress were also determined. The experiments and analysis are described in detail. The observed shifts for different conditions have previously been shown to be in accord with interrelationships predicted by a formal theoretical treatment, but no physical interpretation was attempted. This is now done, but it is not found possible to interpret the shifts satisfactorily in terms of a simple physical model. A model in which the average shift contributed by chains at a specific angle to the draw direction is assumed to be determined by the macroscopic short-time strain resolved with respect to the chain axes fits the data slightly better than one where the macroscopic stress plays the corresponding role, though neither model fits the data well. In spite of this, the observed shifts depend only on the applied stress and do not change as the sample creeps. The changes of intensity observed with applied stress could not be explained fully in terms of changes of molecular orientation, and it is concluded that the application of stress also changes the magnitude and form of the Raman tensor.