The constraint release (CR) effect in entangled branched polymers is generally described by the widely accepted Dynamic Tube Dilation (DTD) theory based on the tube model, which predicts the stress relaxation function reasonably well, but not the dielectric or arm end-to-end vector relaxation. The microscopic picture of entanglement dynamics even in the simple case of star polymers is still not fully resolved. In this work, we first perform molecular dynamics simulations of symmetric star polymer melts using the Kremer-Grest bead-spring model. The entanglement events are analysed microscopically using the persistent close-contacts between mean paths of neighboring polymer strands. The resulted survival probability function of these entanglements or close-contacts show reasonably good agreement with the stress relaxation function, which provides qualitative evidence for the binary picture of entanglements. Based on this understanding we further investigate the star arm retraction and CR effects using the coarse-grained single-chain slip-spring model originally developed by Likhtman and also a simplified single-chain stochastic model. Our simulations revealed that, for entanglements sitting on a target star arm, only those destroyed by the arm free end dominate the arm end-to-end vector relaxation, while the constraint release events produce an accelerated drift of the mean positions of these specific entanglements towards the arm-end, which is an essential mechanism for understanding the relaxation of star polymers in concentrated solutions or melts. Our findings call for an examination of the microscopic foundation of conventional DTD picture and inspire the development of quantitative theories with consideration of more microscopic details.
展开▼
