STRESS-INDUCED SHIFTS AND INTENSITY CHANGES IN THE 1616 CM(-1) RAMAN LINE FOR ISOTROPIC AND UNIAXIALLY ORIENTED POLY(ETHYLENE-TEREPHTHALATE)

Measurements of the shifts induced in the 1616 cm(-1) Raman line by ap plied stress have been made for an isotropic undrawn sample and severa l uniaxially oriented samples of poly(ethylene terephthalate) using va rious polarization directions of incident and scattered light with res pect to the sample and to the direction of the stress, which was appli ed either parallel or perpendicular to the draw direction for the unia xial samples. The corresponding changes in intensity with applied stre ss 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 form al theoretical treatment, but no physical interpretation was attempted . This is now done, but it is not found possible to interpret the shif ts satisfactorily in terms of a simple physical model. A model in whic h the average shift contributed by chains at a specific angle to the d raw direction is assumed to be determined by the macroscopic short-tim e strain resolved with respect to the chain axes fits the data slightl y 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 str ess could not be explained fully in terms of changes of molecular orie ntation, and it is concluded that the application of stress also chang es the magnitude and form of the Raman tenser.