Stereochemical Effects on Diffusion in Polypropylenes: Simulation and Experiment

In support of our dynamic Monte-Carlo (MC) simulations we performed pulsed-gradient NMR diffusion (D) measurements at 180$^{o}$C on moderately disperse polypropylene (PP) melts to study the effects of stereochemical composition. 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, with probabilities of a meso dyad P$_{m}$ = 0.02 (syndiotactic), 0.23 (atactic), and 0.89 (nearly isotactic). After accounting for differences in polydispersity, results were normalized to a common molecular weight M, using a scaling law taken from our earlier work with n-alkanes. Conversion between MC steps and real time was derived from experiment; no dependence on P$_{m}$ was expected. Both experiment and simulation found D at high P$_{m}$ several times faster than at low P$_{m}$, but the simulation also showed a maximum in D near P$_{m}$ = 0.8. The latter effect is attributed to quenched randomness; no specimen was available for experimental verification. For reasons not well understood, the experimental D-distribution on the P$_{m}$ = 0.89 sample exceeded that expected from the known M-distribution.