Computer Simulation Studies of Chain Conformations and Dynamics in Polymer Melts: Effects of Topology and Interfaces

Abstract

In this dissertation, the effects of topological constraints and interface on the conformations and dynamic properties of polyethylene ring and linear chains in melts have been investigated by employing atomistic molecular dynamic simulation method. In pure melts, we observe the faster diffusion for rings than linear chains in entangled regime. However, the dynamic similarities between rings and linear chains in both unentangled and entangled regime have been found from the analysis of dynamic structure factors, chain-length dependence of diffusion coefficients and time dependent monomer displacements. The topological interactions between ring and linear chains affect the motion of rings, with minor contribution to linear chains, resulting in the slower diffusion of rings than linear chains. The measured diffusion coefficients and displacements indicate that the motion of rings in the entangled linear matrices is slower than the reptation mechanism of entangled linear chains probably due to the trapping effect. From the analysis of the longest relaxation times, it is found that the reorientation process for rings is decoupled from the diffusion process in the entangled matrix, regardless of the matrix chain architecture. These results strongly point towards the importance of Constraint-release mechanism in general. Interfacial characterization of ring and linear PE chains in melts under confinement shows that geometrical confinement becomes important in the entangled melt dynamics at the intermediate time scale while the attractive surface interactions are dominant factors for interfacial structures and the chain dynamics at short times.