Effect of Stereochemistry on Diffusion of Polypropylene Melts: Comparison of Simulation and Experiment

• Published in: Macromolecules Vol. 42; no. 22; pp. 8864 - 8869
• Main Authors: von Meerwall, Ernst, Waheed, Numan, Mattice, Wayne L
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 24.11.2009
• Subjects: Applied sciences; Exact sciences and technology; Miscellaneous; Organic polymers; Physicochemistry of polymers; Properties and characterization; Monte Carlo method; Molten state; Computer simulation; Propylene polymer; Spin echo; NMR spectrometry; Experimental study; Syndiotactic polymer; Isotactic polymer; Proton; Diffusion coefficient; Comparative study; Atactic polymer
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma9016983

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 P m = 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 P m several times faster than at low P m. The constant-M simulation also showed a maximum near P m = 0.8 due to quenched randomness. To permit a more precise comparison with experiment, new simulations tracked the samples’ P m, 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 P m-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.