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

Polarization-modulated laser Raman scattering was used to study the or ientation dynamics of an entangled poly(isobutylene) melt during step uniaxial extension. Anisotropies in the second and fourth moments of t he orientation distribution of chain segment vectors were recovered fr om Raman-scattered light associated with the nu(s) stretching vibratio n modes of carbon-carbon and carbon-hydrogen groups present in the spe cimen. The respective second and fourth moments recovered from the car bon-carbon and carbon-hydrogen vibrations agreed with each other reaso nably well. Moreover, the strain-dependent anisotropies in the second moment compared quite well with those recovered from simultaneous bire fringence measurements. Constitutive theories such as the ''temporary network'' model and the ''partially extending strand convection'' mode l that use strong strain measures, predict second and fourth moment an isotropies that are of similar magnitude to those determined experimen tally. On the other hand, constitutive theories like the Doi-Edwards m odel with the independent alignment approximation, which use weaker st rain measures, predict moment anisotropies that are consistently lower than those observed experimentally for the entire range of strains in vestigated. In addition, starting with a general expression for the Ra man 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 t he fourth moments of the orientation distribution of polymer strand en d-to-end vectors.