Effects of orientation and constraint release in the dynamics of entangled polymer melts

The reptation model of de Gennes, as adapted by Doi and Edwards for chains under deformation, has been a very successful theory for describing polymer dynamics. The authors incorporate the effects of orientational couplings on the configurational relaxation of entangled polymer melts by modifying the Doi-Edwards postulate of orientational isotropy of new tube segments. A model for reptation in oriented polymer melts is presented in which local orientational correlations between the segments of the chain molecules are incorporated. The Doi-Edwards assumption of isotropic chain end segments is replaced by a coupling condition between the chain end orientation and the average orientation of the medium. Since the medium is itself the melt composed of these chain segments, this is a self-consistent mean field treatment. The retarded stress relaxation rate and other viscoelastic properties are calculated for monodisperse polymers and polydisperse mixtures. The potential implications of this orientational bias for the dynamic properties of polymers are discussed.