Effect of tacticity on the molecular dynamics of polypropylene melts

In this work we present the results of molecular dynamics simulations of polypropylene melts of different tacticities, based on a recently published molecular model. Volumetric properties of isotactic, syndiotactic, and atactic polypropylene exhibit no significant differences and are in good agreement with experimental data. The simulated X-ray diffraction patterns agree well with experimental scattering results, suggesting that the simulated structure is realistic. Methyl-methyl intramolecular distribution functions exhibit several peaks characteristic of the conformations adopted by each tacticity. Conformational distributions of dyads reveal a preference for tg conformers in isotactic polypropylene (iPP) and for tt in syndiotactic polypropylene (sPP), which is closely related to the type of helices observed in the crystalline form. Time autocorrelation functions have been computed for pendant (C-CH3) bonds and skeletal torsion angles. Predicted correlation times are found to be shortest for the iPP melt and longest for the sPP melt; a simple estimation of barriers to t --> g conformational transitions explains this difference in the dynamics.