Using a realistic united-atom force field, molecular dynamics simulations were performed to study homogeneous nucleation of the crystal phase 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. A cylinder model was used to characterize the shape of the crystal nuclei and to calculate the interfacial free energies. A chain segment analysis was performed to characterize the topology of the crystal surface in terms of loops (including folds) and tails. The length distribution of loops is broad, supporting the "switchboard model" for the early stage crystals formed at deep supercooling. Using the survival probability method, the critical nucleus size was determined as a function of temperature. The interfacial free energies were found to be temperature-dependent. The free energy barrier and nucleation rate as functions of temperature were also calculated and compare favorably with experiments.

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Crystal growth from the melt of n-pentacontane (C50) was studied by molecular dynamics simulation.Quenching below the melting temperature gives rise to propagation of the crystal growth front into the C50 melt from a crystalline polyethylene surface. By tracking the location of the crystal-melt interface, crystal growth rates between 0.02-0.05 m/s were observed, for quench depths of 10-70 K below the melting point. These growth rates compare favorably with those from a previous study by Waheed et al (2005). Next, surface nucleation was identified with the formation of two-dimensional clusters of crystalline sites within layers parallel to the propagating growth front. Critical nucleus sizes, waiting times and rates for surface nucleation were estimated by a mean first passage time analysis. A surface nucleation rate of ~0.05 nm -2 ns -1 was observed, and it was nearly temperature-independent. Post-critical "spreading" of the surface nuclei to form a completely crystallized layer slowed with deeper supercooling.

Heterogeneous nucleation refers to the propensity for phase transformations to initiate preferentially on foreign surfaces, such as vessel walls, dust particles or formulation additives. In crystallization, the form of the initial nucleus has ramifications for the crystallographic form, morphology and properties of the resulting solid. Nevertheless, the discovery and design of nucleating agents remains a matter of trial and error, due to the very small spatio-temporal scales over which formation of the critical nucleus occurs, and the extreme difficulty of examining such events empirically. Using molecular dynamics simulations, we demonstrate a method for the rapid screening of entire families of materials for activity as nucleating agents, and for characterizing their mechanism of action. The method is applied to the crystallization of n-pentacontane, a model surrogate for polyethylene, on the family of tetrahedrally coordinated crystals including diamond and silicon. Systematic variation of parameters in the interaction potential permits a comprehensive, physically-based screening of nucleating agents in this class of materials, including both real and hypothetical candidates. The induction time for heterogeneous nucleation is shown to depend strongly on crystallographic registry between the nucleating agent and the critical nucleus, indicative of an epitaxial mechanism in this class of materials. Importantly, the severity of this registry requirement weakens with decreasing rigidity of the substrate and increasing strength of attraction to the nucleating agent surface. Employing this method, high throughput computational screening of nucleating agents becomes possible, facilitating the discovery of novel nucleating agents within a broad, "materials genome" of possible additives.3

We use molecular simulations with a united atom force field to examine the effect of short chain branching (SCB) on the noncrystalline, interlamellar structure typical of linear low density polyethylene (LLDPE). The model is predicated on a metastable thermodynamic equilibrium within the interlamellar space of the crystal stack and accounts explicitly for the various chain topologies (loops, tails, and bridges) therein. We examine three branched systems containing methyl, ethyl, and butyl side branches, and compare our results to high density polyethylene (HDPE), without branches. We also compare results for two united atom force fields, PYS and TraPPE-UA, within the context of these simulations. In contrast to conventional wisdom, our simulations indicate that the thicknesses of the interfacial regions in systems with SCB are smaller than those observed for a linear polyethylene without branches, and that branches are uniformly distributed throughout the interlamellar region. We find a prevalence of gauche states along the backbone due to the presence of branches, and an abrupt decrease in the orientational order in the region immediately adjacent to the crystallite.3 Introduction: