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CROSSOVER FROM UNENTANGLED TO ENTANGLED DYNAMICS: MONTE CARLO SIMULATION OF POLYETHYLENE, SUPPORTED BY NMR EXPERIMENTS

Lin, Heng
http://rave.ohiolink.edu/etdc/view?acc_num=akron1142028839

Abstract Details

2006, Doctor of Philosophy, University of Akron, Polymer Science.
A two-bead move algorithm for dynamic Monte Carlo simulation has been designed to reflect the true randomness of local torsion dynamics on a high coordination lattice (2nnd lattice). All possible configurations of two consecutive beads of a chain on this high coordination lattice have been included in a library by this algorithm. The moves were implemented by randomly choosing one of the possible configurations from the library. Thus there is no artificial rule for the moves. The algorithm is capable of introducing new bond vectors to local configurations without going through chain ends. The chain-cross has been eliminated in this two-bead move algorithm. The algorithm is fast enough for simulating polyethylene (PE) melts ranging from C40 to C324 on a regular desktop computer. The results of our simulation confirmed there were finite chain length effects, e.g. chain length dependent friction coefficients and non-Gaussian statistics for short PE chains. A detailed comparison has been made among the experiments, prior simulations by other groups, and the results of our new algorithm. The diffusion coefficients scale with molecular weight (M) to the -1.7 power for short chains and -2.2 for longer chains at 180°C, which coincides very well with experimental results. Due to the finite chain length effect, no pure Rouse scaling in diffusion has been observed. The reptation-like slowdown can be clearly observed when M is above 2400 according to the mean square displacements of middle beads. The slope 0.25 predicted by the reptation theory was missing for the intermediate regime of diffusion; instead a slope close to 0.4 appeared, indicating that additional relaxation mechanisms exist in this transition region. The relaxation times extracted by fitting the autocorrelation function of the end-to-end vectors scale with M to 2.5 and 2.7 power using the reptation model and KWW equation, respectively, for entangled chains. The dynamic Monte Carlo algorithm has also been used for simulating a bimodal mixture of PE with one entangled component (M~4.5k) and one unentangled component (M~1k). Detailed normal mode analyses have been presented. Rouse dynamics have not been observed at the short time and distance scale. The KWW equation generally provides better fits than does the pure exponential decay for the data of normal mode relaxation. The KWW index â varies with the concentration and increases with the decrease of mode numbers until a plateau is reached. Tube dilation was observed before the gradual disappearance of a reptation-like slowdown with increasing short chain content in mixtures. Reasonable agreements have been reached when Hess theory and Pearson-von Meerwall model were used to fit the concentration-dependent diffusion coefficients of PE in the mixture. Additional bimodal mixtures of PE studied by our simulation involved a small amount of short N-alkane in an entangled PE matrix. Most of our data agree with the NMR experimental measurements. Extrapolating the concentration-dependent diffusion coefficients of N-alkanes to the trace limit did not lead to the complete restoration of Rouse scaling of the M dependence of the trace diffusion coefficients (Dtr). Although the M dependence of Dtr was found to be obviously weaker than that of the diffusion coefficients in monodisperse melts (Dmelt), a residue of the slope still exists. Thus, the commonly seen excessive M dependence of short polymer chains may result from two contributions. One is the non-iso-friction environment of the polymers at the different chain lengths, which can largely be eliminated by the trace diffusion measurements. Another one, which is related to the underlying physics of the residue of the slope in trace diffusion, is still mysterious. It may link to the non-Gaussian statistics of short polymer chains.
Wayne Mattice (Advisor)
181 p.
Polymer Dynamics; Monte Carlo Simulation; Spin Echo Pulse Gradient NMR; Diffusion; N-Alkanes; Rouse Theory; Reptation Theory

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Citations

  • Lin, H. (2006). CROSSOVER FROM UNENTANGLED TO ENTANGLED DYNAMICS: MONTE CARLO SIMULATION OF POLYETHYLENE, SUPPORTED BY NMR EXPERIMENTS [Doctoral dissertation, University of Akron]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=akron1142028839

    APA Style (7th edition)

  • Lin, Heng. CROSSOVER FROM UNENTANGLED TO ENTANGLED DYNAMICS: MONTE CARLO SIMULATION OF POLYETHYLENE, SUPPORTED BY NMR EXPERIMENTS. 2006. University of Akron, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=akron1142028839.

    MLA Style (8th edition)

  • Lin, Heng. "CROSSOVER FROM UNENTANGLED TO ENTANGLED DYNAMICS: MONTE CARLO SIMULATION OF POLYETHYLENE, SUPPORTED BY NMR EXPERIMENTS." Doctoral dissertation, University of Akron, 2006. http://rave.ohiolink.edu/etdc/view?acc_num=akron1142028839

    Chicago Manual of Style (17th edition)

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This open access ETD is published by University of Akron and OhioLINK.