Chain retraction,length fluctuations,and small-angle neutron scattering in strained polymer melts

We propose a possible explanation for an apparent contradiction between the Doi–Edwards (DE) theory and time-resolved small-angle neutron scattering experiments. The original DE theory predicts that a chain in a melt subjected to a step-elongation undergoes a reduction of its dimensions in both the parallel and perpendicular directions to the strain axis, before returning to an isotropic conformation by reptation. In the present paper, we propose a crude model to compare the relative effect of retraction and of the chain length fluctuations, introduced later by Doi to explain the M power law of viscosity of polymer melts. We show that length fluctuations are able to screen retraction in most experimental situations available to small-angle neutron scattering, whereas that is not the case for viscoelasticity. Quantitative predictions are consistent with the experimental data presently available.