(437g) Increased Mobility in Polypropylene at Intermediate Stereochemical Compositions by Dynamical Monte Carlo
Tacticity of polymer chains play a large role in their fundamental behavior. Relaxation times, glass transition temperatures, and entanglement molecular weights are all dependent on stereochemical composition. While the differences between isotactic and syndiotactic chains are well understood, variations at intermediate stereochemical compositions (0 < pm < 1, where pm denotes the probability of a meso diad) are not as well understood, perhaps because they are all characterized as ?atactic? chains. Because molecular simulation allows for the precise labeling of stereochemical centers, these techniques make a more comprehensive study possible. In this report, we investigate the effect of stereochemical composition on the diffusion of melt polypropylene (PP) using dynamical Monte Carlo.
Melt density PP chains at 473 K are studied on a high-coordination second-nearest-neighbor diamond lattice. Realistic two-bead moves allow for simulation of dynamics using stereochemistry-dependent move probabilities derived from the rotational isomeric state model and repulsive Lennard-Jones interactions.
Increased diffusion of isotactic PP, compared to syndiotactic PP, is reproduced. Higher diffusion at intermediate stereochemical composition is also found, near a probability of meso diad of 0.75. By categorizing the Monte Carlo acceptance rates by the stereochemistry of the beads being moved and the overall stereochemical composition, the origin of the increased diffusion can be studied. Sequences of pure stereochemistry are found to have higher acceptance rates in melts when a mix of stereochemical sequences is present (as in atactic melts), than when surrounded by identical sequences (as in stereochemically pure melts). Sequences that already contain mixed stereochemistry seem to be relatively insensitive to the stereochemical composition. Acceptance rates for sequence of pure stereochemistry in an isolated RIS chain do not display a difference in acceptance rate based on the overall composition of the chain, suggesting that this is an intermolecular effect.
This finding implies that quenched randomness of stereochemical sequences leads to fundamentally different behavior at melt densities. Although stereochemistry affects only the intramolecular RIS probabilities, it is the intermolecular effects between stereochemically-pure segments that are responsible for the change in behavior. Pure stereochemistry creates a regular steric hindrance that limits mobility when surrounded by identical segments. Acceptance rates indicate that the irregularity due to quenched randomness in the stereochemical segments prevent this effect. Intermolecular cohesiveness of stereochemically pure sequences seems to form long-lived crystal-like structures that lower mobility, which can be broken up by atactic stereochemistry.