Importance of End-block Contributions in the Single Chain Dynamics of Unentangled Polymer Melts

Many properties of polymers are well-described by a few simple concepts, such as a coarse-grained chain of N beads. Here, we study the motion of the two end-blocks. To observe end-block relaxation, we chose a molecular weight high enough to avoid effects by chemical end-groups but still avoid entanglement effects due to the entanglement molecular weight. We take advantage of molecular interactions, molecular weight, isotopic sensitivity, and length-scale information encoded in the momentum transfer of neutron spin echo spectroscopy to arrive at a more holistic picture of the contributions of the end-blocks to the polymer dynamics of unentangled chains. Our model polymer melt, poly(dimethyl siloxane), PDMS, has weak intermolecular interactions, leading to a lower sub-diffusion contribution. The experimental results are in excellent agreement with the theory and indicate the appearance of two regions that show terminal bead relaxation in addition to the well-known sub-diffusion of the center of mass, the center of mass diffusion, and (Rouse) segmental relaxation. The quantitative analysis shows terminal beads with a length equivalent to 1-2 Kuhn segments that relax twice as fast as beads in the middle of the polymer chain. The time-dependence of the mean square displacement is in quantitative agreement with simulations with terminal bead segmental relaxation (t^0.65) and sub-Rouse relaxation of end-beads (t^0.57) in the intermediate range between sub-diffusion (t^0.8) and center of mass diffusion (t^1). For the specific example of a polymer with 143 segments, 11 end-segments define one end-block with faster dynamics, whereas the remaining 121 belong to the middle block. Segments in the end block exhibit faster dynamics compared to those in the middle block. Both the end block and the middle block consist of multiple segments. Therefore, analyzing the data requires considering both concepts to effectively describe the dynamics of polymer chains.