Simulations of the dynamics of entangled polymer melts

The objective of this thesis is to increase our understanding of the basic physical principles governing the dynamics of entangled polymer melts. Discontinuous molecular dynamics simulations are performed on dense systems containing 32 hard chains of length 192 to investigate their dynamic properties. The mean squared displacements of the chain center of mass, the inner, outer, and intermediate segments along the chain, the end-to-end vector autocorrelation function, and the apparent self-diffusion coefficient are calculated over the course of the simulations.; First, the relaxation and release of entanglements is compared with that predicted by the tube model and that associated with the release of interchain entanglements, or knots. The initial relaxation of chain segments occurs from the ends toward the middle as the tube model predicts; however, the final relaxation occurs at the chain ends, compatible with interchain entanglement release. The results provide evidence for a proposed mechanism of interchain entanglement relaxation consisting of initial relaxation, memory, and final release from a chain end.; Next, the effect of position along the chain on the segmental mean squared displacement is investigated. The results reveal that small blocks provide a greater difference between the mean squared displacements of middle blocks, end blocks, and the whole chain than larger equal-sized blocks. A large portion of the chain displays middle behavior, while a small portion displays end behavior. The relaxation of small blocks of segments along the chain starts at the chain ends and progresses toward the chain middle.; Finally, the time scale is extended, and the diffusive and stress relaxation behaviors of individual chains are explored. Increased time averaging causes the anomalous relaxation-memory-release behavior to smooth out of the system results; however, anomalous behavior is still exhibited by individual chains. They display super-diffusive, diffusive, and sub-diffusive behavior, with the largest percentage exhibiting super-diffusive behavior and the smallest percentage exhibiting the predicted diffusive behavior. Almost half of the chains exhibit anomalous behavior in the end-to-end vector autocorrelation function. The observed anomalous behaviors suggest that localized entanglements are present in the system.