Marc S. Lavine

Molecular dynamics simulation of orientation and crystallization of polyethylene during uniaxial extension

Abstract Molecular dynamics simulations of realistic, united atom models of polyethylene undergoing uniaxial extension are described. Systems composed of chains ranging from 25 to 400 carbons have been studied, under a variety of processing histories, including isothermal deformation at constant applied stress below the melt temperature Tm, isothermal deformation below Tm followed by annealing, isothermal deformation above Tm followed by crystallization at a quench temperature below Tm, and non-isothermal crystallization during simultaneous deformation and cooling through Tm. Extension and orientation of large segments of flexible chains by uniaxial deformation accelerates the primary nucleation rate to a time scale accessible by molecular dynamics simulation. Entanglements operative during active deformation promote extension and orientation without nucleation of a crystal phase, while relaxation of stress at constant strain is sufficient to allow slippage of chains past pinning points and rapid nucleation and growth of crystallites as neighboring oriented chains come into registry. Isothermal crystallization of pre-oriented systems shows an apparent increase in nucleation density at lower temperatures; the resulting ordered regions are smaller and more closely aligned in the direction of orientation. During non-isothermal deformation, where stretching and cooling occur simultaneously, a first order transition is observed, with discontinuities in the volume and global order parameter, when the system crystallizes.

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

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.

Molecular simulation of crystal growth in n-eicosane

Molecular dynamics is used to obtain crystal growth rates for a model n-alkane. A united atom model of bulk n-eicosane exhibits an observable phase change from an amorphous phase to a close-packed hexagonal phase, in the presence of a crystal surface. Rates are calculated from the translation of the order–disorder transition in the simulation cell as a function of time. The temperature dependence of crystallization is analyzed in terms of Ziabicki’s rate model, and behavior is considered in light of more coarse-grained models.

Trapping polymers to improve flexibility

Flexible Electronics Polymer molecules at a free surface or trapped in thin layers or tubes will show different properties from those of the bulk. Confinement can prevent crystallization and oddly can sometimes give the chains more scope for motion. Xu et al. found that a conducting polymer confined

Fine-tuned for image formation

Biological Optics![Figure][1] Scallops have multiple reflective eyes (blue) in their mantle. CREDIT: DAVID LIITTSCHWAGER/NATIONAL GEOGRAPHIC CREATIVE We typically think of eyes as having one or more lenses for focusing incoming light onto a surface such as our retina. However, light can

Recycle, recycle, recycle

Polymers Some polymers, such as polyethylene terephthalate in soft drink bottles, can be depolymerized back to the starting monomers. This makes it possible to repolymerize true virgin material for repeated use. Zhu et al. developed a polymer based on a five-membered ring cyclic monomer derived from

Mimicking the designs found in proteins

Polymers Natural proteins combine a range of useful features, including chemical diversity, the ability to rapidly switch between preprogrammed shapes, and a hierarchy of structures. Panganiban et al. designed random copolymers with polar and nonpolar groups, using many of the features found in proteins (see the Perspective by Alexander-Katz and Van Lehn). Their structures could serve as “broad spectrum” surfactants, able to promote the solubilization of proteins in organic solvents and help preserve the functionality of proteins in aqueous environments. Science , this issue p. [1239][1]; see also p. [1216][2] [1]: /lookup/doi/10.1126/science.aao0335 [2]: /lookup/doi/10.1126/science.aat0155

Simple routes to self-healing

Polymers Biology provides many routes for self-healing or repair, but this trait is hard to endow into engineering materials. Although self-repair has been demonstrated for some polymers, it usually required specialized monomers. Urban et al. demonstrate that for a very narrow range of compositions, simple vinyl polymers based on methyl methacrylate and n -butyl acrylate show repeatable self-healing properties (see the Perspective by Sumerlin). A key characteristic of this system is that it relies on van der Waals interactions rather than the reformation of hydrogen or covalent bonds for repair. Science , this issue p. [220][1]; see also p. [150][2] [1]: /lookup/doi/10.1126/science.aat2975 [2]: /lookup/doi/10.1126/science.aau6453

A longer exciton pathway

Organic Electronics Organic semiconductors typically exhibit exciton diffusion lengths on the order of tens of nanometers. Jin et al. prepared nanofibers from block polymers consisting of emissive polyfluorene cores surrounded by coronas of polyethylene glycol and polythiophene (see the Perspective

Oscillating one-dimensional chains

Nanomaterials The confinement of materials to nanoscale dimensions often reveals properties not seen in bulk materials. Pham et al. confined NbSe3 within carbon nanotubes (a conductor) or boron nitride nanotubes (an insulator). Transmission electron microscopy revealed an oscillatory motion of the confined chains not observed in bulk crystals. Electronic structure calculations showed that charge transfer drives the torsional wave instability, and the limited covalent bonding between the chains and the nanotube sheath allows unhindered dynamics. Application of an external potential applied to the nanotube should directly affect the torsion and thus lead to different optical and electron transport properties. Science , this issue p. [263][1] [1]: /lookup/doi/10.1126/science.aat4749