Diffusion and Relaxation in Oriented Polymer Media

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 presence of local orientational correlations has been indicated by an array of equilibrium experiments on alkane fluids and on oriented polymer networks. More recent dynamic experiments by Monnerie and co-workers have indicated that the relaxation behavior of polymers can also be affected. Our model to account for the effect of this orientational bias on reptation is a modification of the Doi-Edwards model. 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 the melt of chains itself, 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.