Correlation Between Segmental Order Parameter and Entanglement Length in a Monodisperse Comb Polymer

Abstract

The motion of a polymer chain within a hypothetical confining tube leads to arise a segmental order parameter Sb. This parameter is assessed via multiple-quantum (MQ) NMR experiments, providing a valuable molecule-level rheological observable. In both polymer networks and entangled melts, the order parameter is proportional to the inverse of the number of segments between two covalent crosslinks or physical entanglements. In entangled polymer networks, the entanglements have usually been considered as additional but temporary crosslinks and the contribution of the physical and chemical constraints are assumed additive. Recent computer simulation results challenged this assumption for lowly crosslinked polymer networks; instead, Sb was shown to scale with (NeNc) 1/2 , 𝑁 and 𝑁 being the number of segments between crosslinks and entanglements, respectively [M. Lang and J.-U. Sommer, Phys. Rev. Lett. 2010, 104, 177801]. An experimental confirmation remains elusive due to challenges in distinguishing the contributions of entanglements and crosslinks, as well as the long averaging timescales involved. In this study, we assess this correlation by examining chain dynamics in a monodisperse polyisoprene comb, utilized as a model system.To model chain dynamics in this system, the dynamic dilution model, originally designed for predicting the rheological behavior of star and branched polymers, has been modified to facilitate its application in the analysis of MQ NMR signals. We also address some of its shortcomings.