Characterization of polyethylene crystallization from an oriented melt by molecular dynamics simulation

Abstract

Molecular dynamics is used to characterize the process of crystallization for a united atom model of polyethylene. An oriented melt is produced by uniaxial deformation under constant load, followed by quenching below the melting temperature at zero load. The development of crystallinity is monitored simultaneously using molecular-based order parameters for density, energy, and orientation. For crystallization temperatures ranging from 325 to 375 K, these simulations clearly show the hallmarks of crystal nucleation and growth. We can identify multiple nucleation events, lamellar growth up to the limit imposed by periodic boundaries of the simulation cell, and lamellar thickening. We observe a competition between the rate of nucleation, which results in multiple crystallites, the rate of chain extension, which results in thicker lamellae, and the rate of chain conformational relaxation, which is manifested in lower degrees of residual order in the noncrystalline portion of the simulation. The temperature dependence of lamellar thickness is in accord with experimental data. At the higher temperatures, tilted chain lamellae are observed to form with lamellar interfaces corresponding approximately to the [201] facet, indicative of the influence of interfacial energy.

Crystal shapes and crystallization in continuum modeling

“…[35][36][37][38] However, we shall not primarily examine the thermodynamic background of these rates here. Rather, we use a procedure analogous to the one described above to model different particle shapes and different growth dimensionalities.…”

Structure Formation under Steady-State Isothermal Planar Elongational Flow ofn-Eicosane: A Comparison between Simulation and Experiment

“…Phenomena such as shear thickening, associated with crystalline structure formation under extreme shear rates, have also been reported [6,17]. In the past several years, NEMD algorithms have also been developed for simulating the rheological and structural properties of liquids under planar elongational flow (PEF).Some recent studies focused on characterizing the crystallization behavior of oriented n-alkane melts [7,14,15] by means of uniaxial stretching. It should be noted, however, that no rigorous NEMD algorithm capable of steady-state simulation was employed in any of these studies.…”

“…Even for the relatively short alkane chains, the simulation times needed to observe ordered phase formation are on the order of tens of nanoseconds, which is prohibitive on most supercomputers today. To this end, alternative methods have been proposed in order to enhance the crystallization rates, which include crystallization in the presence of a surface [2] or increasing the melting point by driving the system away from equilibrium via uniaxial stretching [7,14,15] or shear flow [6,16,17].…”

Molecular simulation of flow-enhanced nucleation in n-eicosane melts under steady shear and uniaxial extension

“…5,11 Their model exhibited good agreement with experimental data, in support of the viewpoint that FEN can be explained by flow-induced entropy reduction in the melt; however, it must be considered to be a phenomenological model due to its rigid assumptions about the nucleus shape and its reliance on unknown thermodynamic parameters. At the molecular level, there have been simulation efforts using both Monte Carlo (MC) [13][14][15] and Molecular Dynamics (MD) [16][17][18][19][20][21] to study FIC; however, relatively few focused specifically on the kinetics of nucleation. 22 Given that nucleation is a transient event, a MC simulation cannot accurately capture its kinetics without an enormous amount of prior knowledge regarding the dynamic processes in the melt and their rates.…”

“…They observed rapid nucleation of oriented crystals, but were unable to distinguish nucleation and growth regimes. In a similar study by Ko et al, 19 oriented samples were prepared by drawing under a constant, uniaxial load above T m , then quenching below T m in order to observe crystallization in the absence of an applied load. In these simulations, the nucleation and growth mechanism was observed, and found to result in rapid crystallization.…”