A dynamic Monte Carlo algorithm has been designed to reflect the true randomness of local molecular torsion movements. Combined with NMR experiments, our study confirmed there were finite chain length effects. The reptation-like slowdown can be clearly observed when M is above 2400. A bimodal mixture of PE with one entangled component and one unentangled component has also been studied. Good agreements have been reached when Hess theory and Pearson-von Meerwall model were used to fit our data. Additional bimodal mixtures of PE studied by us involved a small amount of short N-alkane in an entangled PE matrix. The commonly seen excessive M dependence of diffusion coefficients of short polymer chains results from two contributions. One is due to the non-iso-friction environment. Another one, having much less to do with the environment, is still mysterious. It may link to the non-Gaussian statistics of the short polymer chain itself.

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