ORIENTATION DYNAMICS OF A POLYMER MELT STUDIED BY POLARIZATION-MODULATED LASER RAMAN-SCATTERING

Polarization-modulated laser Raman scattering was used to study the orientation dynamics of an entangled poly(isobutylene) melt during step uniaxial extension. Anisotropies in the second and fourth moments of the orientation distribution of chain segment vectors were recovered from Raman-scattered light associated with the nu(s) stretching vibration modes of carbon-carbon and carbon-hydrogen groups present in the specimen. The respective second and fourth moments recovered from the carbon-carbon and carbon-hydrogen vibrations agreed with each other reasonably well. Moreover, the strain-dependent anisotropies in the second moment compared quite well with those recovered from simultaneous birefringence measurements. Constitutive theories such as the ''temporary network'' model and the ''partially extending strand convection'' model that use strong strain measures, predict second and fourth moment anisotropies that are of similar magnitude to those determined experimentally. On the other hand, constitutive theories like the Doi-Edwards model with the independent alignment approximation, which use weaker strain measures, predict moment anisotropies that are consistently lower than those observed experimentally for the entire range of strains investigated. In addition, starting with a general expression for the Raman tensor in a principal molecule-fixed coordinate frame, it is shown that the polarization components of the Raman-scattered intensity are functions of the birefringence retardation, the second moments, and the fourth moments of the orientation distribution of polymer strand end-to-end vectors.