Is the distribution of entanglements homogeneous in polymer melts?

Abstract

Neutron-scattering experiments have been performed on elongated polystyrene melts consisting of relatively short labelled chains (M<100 000) dispersed in a matrix of very long ones (M>10⁶), which were not labelled. Important anomalies with respect to the theoretical predications have been observed. In particular, the iso-intensity lines exhibit, in a large range of relaxation times, striking butterfly shapes and an unexpected orientation: the long axis of these patterns is oriented parallel to the stretching direction. As a working assumption, we propose to relate this phenomenon to a non-homogeneous arrangement of chain entanglements in the melts. As a matter of facts, it has been proposed that elongated gels should exhibit butterfly patterns (The whole solvent being labelled) as a result of a sort of anisotropic unscreening of cross-linking heterogeneities. Experiments were performed on samples synthesized specially in order to match the assumptions of the model and butterfly patterns have effectively been observed. The same type of mechanism can be driven by an heterogeneity of chain entanglement in the case of assymetrical melts: the short labelled chains may behave as a kind of polymeric solvent and accumulate, during the relaxation, in the less entangled regions (which should be also regions of lower constraint).