Rutledge Research Group, Chemical Engineering, MIT: Polymer Science and Engineering, Statistical Thermodynamics, Molecular Simulation

Modeling of Crystallization

Personnel:

Peng Yi, Alex Bourque, David Nicholson

We use molecular simulation to study the crystallization of polymers, starting with the prototypical polyethylene. Using a variety of techniques, we observe crystallization behavior under different conditions including quiescent, flow-induced, and heterogeneous crystallization. The goal of this research is to gain a theoretical understanding of how the crystallization process occurs, allowing connections to be drawn to the behavior observed in experimental studies. This knowledge will aid in the design of better polymer resins, processes, and additives in the polymer processing industry.

Simulations of crystallization are carried out predominantly using the molecular dynamics (MD) method. The polyethylene molecules are modeled at the united atom level, where one "bead" corresponds to a CH₂ or CH₃ group. Crystallization is induced by quenching below the polymer melting temperature in the NPT ensemble. By monitoring the formation of clusters in the melt for a large number of trajectories we can connect the process to a stochastic description of crystallization using the mean-first passage time (MFPT) formalism. This process has been performed to characterize crystallization in systems ranging from n-octane, which exhibits an extended-chain crystal structure, up to 1000-mer chains that form a polyethylene-like folded chain crystalline morphology.

Heterogeneous crystallization is studied by introducing a surface to the MD simulations. In this case, crystal nucleation can be studied on a layer-by-layer basis. In the spirit of the Materials Genome Initiative, both real and hypothetical surfaces can be scanned quickly and efficiently, thereby accelerating the process of discovery for new additives. The effect of flow is studied in MD simulations by using the SLLOD algorithm to deform the polymer melt. Methods are being developed to illuminate the effects of different flow fields, strain rates, and total strains on crystal nucleation and growth, to relate to the variety of flow conditions employed in polymer processing operations.

Figure: Snapshot of the critical nucleus from an MD simulation of the crystallization of C₁₅₀.