(694d) Molecular Simulation of Homogeneous Crystal Nucleation and Growth in Polyethylene Melts

The crystallization of polymers, like that of small molecules, can be described by a two-stage process of homogeneous nucleation and crystal growth. Yet it is also different from that of small molecules in having chain folding phenomenon and the formation of lamellae structure due to the interplay between long, flexible chain molecules. Chain folding and the lamellar crystalline morphology is essential to the structure and properties of the majority of commodity and high performance plastics, but the understand of their development on the microscopic level remains incomplete.

To study this process, using a realistic united-atom force field, molecular dynamics simulations were performed at about 30% supercooling from the melts of n-pentacontahectane (C150) and a linear polyethylene (C1000), both of which are long enough to exhibit the chain folding that is characteristic of polymer crystallization. The nucleation rate was calculated and the critical nuclei were identified using a mean first-passage time analysis. The nucleation rate was found to be insensitive to the chain length in this range of molecular weight. The critical nucleus contains about 150 carbons on average and is significantly smaller than the radius of gyration of the chains, at this supercooling. Using the survival probability method, the critical nucleus size was determined as a function of temperature. A cylinder model was used to characterize the shape of the crystal nuclei and to calculate the interfacial free energies. The interfacial free energies were found to be temperature-dependent. The nucleation free energy barrier and nucleation rate as functions of temperature were also calculated and compare favorably with experiments. At the end of the crystal growth stage, stable crystal lamella forms. A chain segment analysis was performed to characterize the topology of the lamella-melt interface in terms of loops, bridges and tails. The length distribution of loops is broad, supporting the "switchboard model" for the crystals formed at deep supercooling.

References: [1] P. Yi, C.R. Locker and G.C. Rutledge, Macromolecules 2013 (in press).