Enhanced Diffusion at Intermediate Stereochemical Composition in Polypropylene by Dynamical Monte Carlo

• Published in: Macromolecules Vol. 40; no. 5; pp. 1504 - 1511
• Main Authors: Waheed, Numan, Mattice, Wayne L., von Meerwall, Ernst D.
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 06.03.2007
• Subjects: Applied sciences; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Properties and characterization; Structure, morphology and analysis; Structure effect; Monte Carlo method; Tacticity; Computer simulation; Isotactic polymer; Propylene polymer; Theoretical study; Diffusion coefficient; Syndiotactic polymer; Comparative study; Atactic polymer
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma062285r

We conduct a comprehensive study of the effect of stereochemistry on the diffusion of isotactic, atactic, and syndiotactic polypropylene (PP), with dynamical Monte Carlo. We simulate realistic two-bead Monte Carlo moves on a stereochemistry-dependent, coarse-grained model of PP based on the rotational isomeric state (RIS) model and repulsive Lennard-Jones interactions. We find that isotactic PP (iPP) diffuses faster than syndiotactic PP (sPP) as expected, but a maximum in the diffusion rate at an intermediate stereochemical composition (aPP) is also found. To determine the origin of this effect, we compare pair correlation functions of the slowest-moving beads to those of all beads, identifying conformations that limit mobility. We find a few small, long-lived clusters with chain conformations reminiscent of those in crystals in iPP and sPP; although slow-moving beads also cluster in aPP, they are short-lived with no dominant conformations. We look at the effects of local stereochemistry through Monte Carlo acceptance rates, categorized by the stereochemistry of the local tetrad and overall stereochemical composition. Sequences of meso and racemo diads diffuse faster in aPP due to quenched randomness of stereochemical sequences than in iPP and sPP. Comparison to isolated chain acceptance rates suggests that intermolecular cohesiveness of stereochemically pure sequences lowers mobility but can be overcome by the randomness of atactic stereochemistry. The mechanisms described here for PP may affect the mobility of other polymers in which stereochemical composition is an issue.