Effect of Stereochemistry on Diffusion of Polypropylene Melts: Comparison of Simulation and Experiment
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- 作者：Ernst von Meerwall ; Numan Waheed ; Wayne L. Mattice
- 刊名：Macromolecules
- 出版年：2009
- 出版时间：November 24, 2009
- 年：2009
- 卷：42
- 期：22
- 页码：8864-8869
- 全文大小：732K
- 年卷期：v.42,no.22(November 24, 2009)
- ISSN：1520-5835
文摘
We have performed dynamic Monte Carlo (MC) simulations and pulsed-gradient diffusion (D) experiments to study the effect of stereochemical composition on diffusion in linear polypropylene (PP) melts. The coarse-grained simulations were based on the rotational isomeric state model and Lennard-Jones potentials. For the proton NMR diffusion measurements we obtained three PP specimens of differing molecular weight M and dispersity, with the probability of a meso diad Pm = 0.02 (syndiotactic), 0.23 (atactic), and 0.89 (nearly isotactic). The experiment supplied the conversion factor K between MC steps and real time. Both simulation and M-scaled experiment found D at high Pm several times faster than at low Pm. The constant-M simulation also showed a maximum near Pm = 0.8 due to quenched randomness. To permit a more precise comparison with experiment, new simulations tracked the samples’ Pm, mean M, and polydispersity, but K was found to vary significantly between samples. The GPC determination of M and its distribution, based on linear polyethylene calibration, may be dependent on PP stereochemistry (via D), generating misleading results. However, dilute PP diffusion measurements in a chlorinated solvent, consistent with classical dilute diffusion theory, suggest that the observed Pm-dependence of K may instead be an intrinsic feature of this MC methodology, implying that MC cannot be relied upon to provide microstructure-independent results for self-diffusion.