The role of functionality on branch point motion of symmetric star polymers studied with MD simulations and NSE spectroscopy

Resumen del trabajo presentado al 8th International Discussion Meeting on Relaxations in Complex Systems: "New results, Directions and Opportunities", celebrado en Wisla (Polonia) del 23 al 28 de julio de 2017.Large-scale molecular dynamics simulations and neutron spin echo (NSE) spectroscopy experiments of center labeled star polymers with different number of arms (functionality) and different arm length have been performed in order to directly observe the effect of functionality on the mobility of the branch point (BP). Different scenarios can arise by increasing the functionality, including early tube dilution through more "diving modes", or tube-narrowing by increasing the topological constraints around the branch point. By varying the arm length the influence of dynamic tube dilution (DTD) is investigated. Using a standard bead spring model three-, four- and five-arm star polymers were simulated with a focus on the branch point motion. NSE spectroscopy experiments with a labeling scheme unique to neutron scattering allow us to directly observe the BP motion on a molecular scale by measuring the dynamic structure factor S(Q,t). We investigated center labeled polyethylene three and four-arm stars of different lengths in a pure melt. The change in mobility induced by a change in functionality and different arm length manifests itself e.g in different time dependencies of S(Q,t) in the longer time limit relating to an reduced mean square displacement MSD of the BP for higher functionality and/or longer arms. By rescaling the time dependence of the simulated S(Q,t) we can directly compare the simulations with our experiments. We observed a decrease in mobility with higher functionality with both techniques, which is suggesting dominant tube narrowing. This is also in accordance with our BP trajectory analysis from our simulations. The influence of DTD can be described using a model that considers the tube survival probability to describe the dilution of the branch point confinement over time for the different arm lengths.Peer reviewe