Crystallization mechanism in melts of short n-alkane chains

The Journal of chemical physics, 2014

We present a molecular dynamics simulation study of crystal nucleation from undercooled melts of n-alkanes, and we identify the molecular mechanism of homogeneous crystal nucleation under quiescent conditions and under shear flow. We compare results for n-eicosane (C20) and n-pentacontahectane (C150), i.e., one system below the entanglement length and one above, at 20%-30% undercooling. Under quiescent conditions, we observe that entanglement does not have an effect on the nucleation mechanism. For both chain lengths, the chains first align and then straighten locally, then the local density increases and finally positional ordering sets in. At low shear rates the nucleation mechanism is the same as under quiescent conditions, while at high shear rates the chains align and straighten at the same time. We report on the effects of shear rate and temperature on the nucleation rates and estimate the critical shear rates, beyond which the nucleation rates increase with the shear rate. In...

Polymer, 2015

We have performed molecular dynamics simulations to study the mechanism of crystallization from an undercooled polyethylene (C500) melt. We observe that crystal nucleation is initiated by the alignment of chain segments, which is followed by straightening of the chains and densification. Growth procedes via alignment of segments, which are in the vicinity of the growth front, with the chains in the crystalline lamella. Once chains are attached, the lamella thickens by sliding of the segments along the long axis of the chain from the amorphous regions into the crystalline regions. We do not observe the formation of any folded precursors.

While the process by which a polymer crystal nucleates from the melt has been extensively studied via molecular simulation, differences in polymer models and simulated crystallization conditions have led to contradictory results. We make steps to resolve this controversy by computing low-temperature phase diagrams of oligomer melts using Wang Landau Monte Carlo simulations. Two qualitatively different crystallization mechanisms are possible depending on the local bending stiffness potential. Polymers with a discrete bending potential crystallize via a single-step mechanism, whereas polymers with a continuous bending potential can crystallize via a two-step mechanism that includes an intermediate nematic phase. Other model differences can be quantitatively accounted for using an effective volume fraction and a temperature scaled by the bending stiffness. These results suggest that at least two universality classes of nucleation exist for melts and that local chain stiffness is a key ...

Polyolefins Journal, 2021

The influence of long branches on crystallization behavior has been studied by means of molecular dynamics simulations. Using two systems: polyethylene (PE) with long branches (LCB-PE) and PE without long branches (linear-PE) with the same molecular weight, we have examined the crystallization behavior of the two systems by molecular dynamics simulation. This paper explains the influence of long branches on the isothermal crystallization process and the non-isothermal crystallization process with similar initial interchain contact fraction (ICF) in terms of final ICF, crystal regions, crystallinity, concentration of tie chains and energy. It is found that the crystallization process is classified as two stages: the nucleation stage and the crystal growth stage. The existence of long branches is favorable for the first stage while unfavorable for the second stage. Knots that act as crystalline defects are excluded from the lamella, resulting in decreasing in regularity and crystallinity of molecular chains. From the perspective of potential energy and non-bond energy, LCB-PE has lower energy than linear-PE in the nucleation stage while the energy of linear-PE is lower than that of LCB-PE in the second stage. In short, the long branched chains inhibit the crystallization process.

2011

We have carried out a quantitative analysis of the chain packing in polymeric melts using molecular dynamics simulations. The analysis involves constructing Voronoi tessellations in the equilibrated configurations of the polymeric melts. In this work, we focus on the effects of temperature and polymer backbone rigidity on the packing. We found that the Voronoi polyhedra near the chain ends are of higher volumes than those constructed around the other sites along the backbone. Furthermore, we demonstrated that the backbone rigidity (tuned by fixing the bond angles) affect the Voronoi cell distribution in a significant manner, especially at lower temperatures. For the melts consisting of chains with fixed bond angles, the Voronoi cell distribution was found to be wider than that for the freely jointed chains without any angular restrictions. As the temperature is increased, the effect of backbone rigidity on the Voronoi cell distributions diminishes and becomes similar to that of the freely jointed chains. Demonstrated dependencies of the distribution of the Voronoi cell volumes on the nature of the polymers are argued to be important for efficiently designing the polymeric materials for various energy applications.

Macromolecules, 2003

We report a numerical study of the free energy barrier for crystallization and melting of a single homopolymer chain. The simulations show that the free energy barrier separating the crystalline and molten states at the same free energy level strongly depends on the chain length. However, at a fixed temperature the barrier for single-chain crystallization is independent of chain length. This observation is in agreement with recent experiments on multichain bulk-polymer systems and can be understood theoretically if we assume that the primary nucleation of polymer crystals is determined by intramolecular nucleation. If we further assume that the subsequent growth of polymer crystals is controlled by two-dimensional intramolecular nucleation on the growth front, we can even account for the experimentally observed molecular segregation during crystal growth as well as the chain-length independence of the free energy barrier for secondary nucleation.

Physical review, 1990

Molecular-dynamics simulations of 15000 and 10 particles have been performed to study the onset of crystallization in supercooled Lennard-Jones liquids. The calculations were performed by suddenly cooling an equilibrated liquid and calculating the subsequent time evolution of the system (at constant energy and volume with periodic boundary conditions). The configurations at evenly spaced times along the trajectory were subjected to an analysis that consisted of a short steepestdescents energy minimization toward an inherent structure followed by a Voronoi analysis to identify crystalline regions. The sequence of these quenched configurations was analyzed to study the time evolution of the solidlike regions. Several observations are consistent with the existence of a free-energy barrier to crystallization, as described by classical nucleation theory, including an identification of a critical nucleus size. Critical nuclei by our analysis and under the conditions of this simulation consist of 10 to 20 particles in face-centered cubic and hexagonal close-packed environments. A steady-state distribution of sizes of precritical clusters is observed at intermediate times, but the first critical and postcritical nuclei form and there is a significant amount of crystallization before this steady-state distribution is achieved.

arXiv (Cornell University), 2012

We study the effect of bond length fluctuations on the nucleation rate and crystal morphology of linear and cyclic chains of flexibly connected hard spheres using extensive molecular dynamics simulations. For bond length fluctuations as small as a tenth of the bead diameter, the relaxation and crystallization resemble that of disconnected spheres. For shorter bond lengths and chains with < 10 beads the nucleation rates depend sensitively on bond length, number of beads per chain, and chain topology, while for longer chains the nucleation rates are rather independent of chain length. Surprisingly, we find a nice data collapse of the nucleation rate as a function of number of bonds per sphere in the system, independent of bead chain composition, chain length and topology. Hence, the crystal nucleation rate of bead chains can be enhanced by adding monomers to the system. We also find that the resulting crystal morphologies of bead chains with bond fluctuations resemble closely those for hard spheres including structures with five-fold symmetry.

Macromolecules, 1990

Molecular dynamics is used to study the melting on the surface of a polyethylene-like crystal. The rate constant for melting of a crystalline molecule without and with one to four folds is determined at several different temperatures and molecular lengths. The results show a strong dependence of the transition rate on the number of folds. For a constant lamellar thickness, the transition rate decreases with increasing number of folds for temperatures near the equilibrium melting temperature, as expected from analogy with experimental melting temperatures. In contrast, the transition rate increases with increasing number of folds for temperaturesthat exceed the equilibrium melting temperature by more than 100 K. Two melting paths are suggested to explain the simulation data. One pathway involves a competition between melting and crystallization. This pathway leads to a decreasing transition rate as a function of increasing folding. The second pathway exhibits dominating melting. In this case, the rate of transition tends to increase with increasing number of folds. Diffusion coefficients of segments at different locations along the chain show that motion of the ends of a polymer chain or of the folds is faster than in the center of the stem. The overall effect of increasing temperature is to increase the diffusion coefficients.

The Journal of Chemical Physics, 2011

We performed molecular dynamics (MD) simulations of nucleation from vapor at temperatures below the triple point for systems consisting of 10 4 -10 5 Lennard-Jones (L-J) type molecules in order to test nucleation theories at relatively low temperatures. Simulations are performed for a wide range of initial supersaturation ratio (S 0 10 − 10 8 ) and temperature (kT = 0.2 − 0.6ε), where ε and k are the depth of the L-J potential and the Boltzmann constant, respectively. Clusters are nucleated as supercooled liquid droplets because of their small size. Crystallization of the supercooled liquid nuclei is observed after their growth slows. The classical nucleation theory (CNT) significantly underestimates the nucleation rates (or the number density of critical clusters) in the low-T region. The semi-phenomenological (SP) model, which corrects the CNT prediction of the formation energy of clusters using the second virial coefficient of a vapor, reproduces the nucleation rate and the cluster size distributions with good accuracy in the low-T region, as well as in the higher-T cases considered in our previous study. The sticking probability of vapor molecules onto the clusters is also obtained in the present MD simulations. Using the obtained values of sticking probability in the SP model, we can further refine the accuracy of the SP model.