Molecular simulation of crystal growth in long alkanes

We report crystal growth rate data from the melt for C50 and C100 obtained from non-equilibrium molecular dynamics simulations. This extends our previous results for n-eicosane (C20) [Waheed et al. J Chem Phys 2002;116:2301]. We also construct a crystal growth model that accounts for the thermodynamic driving force and relaxation time, using WLF theory and a small number of chemically specific quantities that can be estimated from molecular dynamics simulations. Our model can predict growth rates as a function of temperature and molecular weight, up to the entanglement molecular weight. Qualitatively, we see frequent adsorption and desorption of chain segments on the surface in both C50 and C100 systems. We find evidence for a surface nucleus involving 4-5 chain segments that are approximately 20 beads long, shorter than the ultimate thickness of the chain stem in the crystal, and involving segments from multiple chains. Treatment of relaxation dynamics using the Rouse model and the reptation model does not yield a statistically significant difference within the limits of our data, but the Rouse-based fit yields thermodynamic parameters that are in closer accord with those found from fits to experiments.