Simulations of the Dynamics of Entangled Polymer Melts

entanglement;polymer;tube model;molecular dynamics simulation;hard chain model;reptation

McCormick, Julie Ann ; Carol K. Hall, Committee Chair,Saad A. Khan, Committee Co-Chair,Benny D. Freeman, Committee Member,Donald W. Brenner, Committee Member,McCormick, Julie Ann ; Carol K. Hall ; Committee Chair ; Saad A. Khan ; Committee Co-Chair ; Benny D. Freeman ; Committee Member ; Donald W. Brenner ; Committee Member

University:North Carolina State University

关键词: entanglement;    polymer;    tube model;    molecular dynamics simulation;    hard chain model;    reptation;   

Others  :  https://repository.lib.ncsu.edu/bitstream/handle/1840.16/5197/etd.pdf?sequence=1&isAllowed=y

美国|英语

来源: null

【 摘 要 】

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 systems containing 32 hard chains of length 192 at three volume fractions, 0.40, 0.45, and 0.50, to investigate their dynamic properties. The mean squared displacement of the chain center of mass, the mean squared displacements of 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, we study the relaxation and release of entanglements and compare 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, different methods for predicting the longest relaxation time provide inconsistent results.An analysis of the mean squared displacement behavior of chain segments at various positions along the chain suggests that the final relaxation is occurring at the chain ends, inconsistent with the tube model but compatible with the release of interchain entanglements or knots.A combined analysis of the end-to-end vector autocorrelation function, the outer segment mean squared displacement, and the apparent diffusion coefficient suggests that knot release behavior is occurring in the systems.The results provide support for a proposed mechanism of interchain entanglement relaxation consisting of initial relaxation, followed by memory and final release from a chain end; however, the uncertainty is large at these long times.Next, we investigate the effect of position along the chain on the segmental mean squared displacement.The mean squared displacements of various sized blocks of segments at different positions along the chain are calculated.An investigation of the effect of block size on the dynamics reveals 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 at different positions along the chain starts at the chain ends and progresses toward the chain middle with time, causing the minor chain length, the portion of the chain that has relaxed, to follow a power law with time.Finally, we extend the time scale of the simulations and explore the diffusive and stress relaxation behavior of individual chains within the system.Increased time-averaging causes the anomalous relaxation-memory-release behavior that was observed previously in the system to smooth out; however, anomalous behavior can still be observed when studying the dynamics of the individual chains.Although the apparent diffusion coefficient curve averaged over all chains displays the predicted long-time diffusive behavior, the curves for the individual chains differ both qualitatively and quantitatively.They display super-diffusive, diffusive, and sub-diffusive behavior, with the largest percentage of chains exhibiting super-diffusive behavior and the smallest percentage exhibiting the predicted diffusive behavior.For some chains, the end-to-end vector autocorrelation function relaxes smoothly toward zero similar to the system average; however, almost half of the chains exhibit the anomalous behavior in the end-to-end vector autocorrelation function.This causes the longest relaxation times to span almost an order of magnitude in reduced time.The fact that anomalous diffusive and stress relaxation behaviors are exhibited by a large portion of chains suggests that localized entanglements are present in the system.