Toward a physical definition of entanglements

To examine the role of chain entanglements on polymer melt properties, the interaction potential energy between a loop and chain at its centre is evaluated. The interaction potential energy between binary contacts of two adjacent Kuhn monomers is also evaluated. The elasticity in polymer melts and the evaluation of the different contributions to flow activation energy of linear polymer chains are used to demonstrate that chain loops, or any other binary interactions between polymer chain segments, cannot justify the properties assigned to entanglements. These properties may be understood if interactions between several parallel chain segments are considered instead, which implies the assumption of the existence of short-range local ordered regions in polymer melts. Their existence is demonstrated from abundant literature experiments, molecular dynamics simulation results, and also from a detailed discussion on the interaction potential energy values. Because more than 80% of conformational states in one chain are random sequences of chain segments, the remaining states being in short-range ordered regions, we conclude that the random coil model is not an exact model for the morphology of polymer melts, but it is still a very good description of chain conformations at the molten state.This work was carried out under the projects POCTI / CTM / 46270 / 2002 and Project 3599 / PPCDT - PIDDAC- PTDC / CTM / 68614 / 2006 of the Portuguese Foundation for the Science and Technology (FCT) with funding from the POCTI and FEDER programs