Diffusion in Polypropylene Melts: Role of Stereochemistry.

We have performed numerical simulations and experiments at 180$^{o}$C to study the effect of stereochemical composition on the diffusion (D) of linear polypropylene melts of moderate polydispersity. The coarse-grained Monte-Carlo (MC) simulations were based on the rotational isomeric state model and repulsive Lennard-Jones potentials. For the pulsed-gradient $^{1}$H NMR diffusion measurements the three specimens used had probabilities of meso diad P$_{m}$ = 0.02, 0.23, and 0.89. The conversion factor between MC steps and real time was obtained by comparison with the measured D; no dependence on stereochemistry was evident. Using a molecular-weight (M)-scaling known from earlier work on n-alkanes, results were normalized to a common M after accounting for differences in experimental polydispersity. Results agreed closely with the monodisperse simulations. D at high P$_{m}$ was found to be several times faster than at low P$_{m}$, but the simulation also showed a maximum in D at P$_{m}$ near 0.75, an effect attributed to quenched randomness. Likely for similar reasons the experimental D-distribution for the P$_{m}$ = 0.89 sample greatly exceeded that expected from the known polydispersity.