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Ligand structure and mechanical properties of single-nanoparticle-thick membranes

Physical Review E - Statistical, Nonlinear, and Soft Matter Physics

Salerno, Kenneth M.; Bolintineanu, Dan S.; Lane, James M.D.; Grest, Gary S.

The high mechanical stiffness of single-nanoparticle-thick membranes is believed to result from the local structure of ligand coatings that mediate interactions between nanoparticles. These ligand structures are not directly observable experimentally. We use molecular dynamics simulations to observe variations in ligand structure and simultaneously measure variations in membrane mechanical properties. We have shown previously that ligand end group has a large impact on ligand structure and membrane mechanical properties. Here we introduce and apply quantitative molecular structure measures to these membranes and extend analysis to multiple nanoparticle core sizes and ligand lengths. Simulations of nanoparticle membranes with a nanoparticle core diameter of 4 or 6 nm, a ligand length of 11 or 17 methylenes, and either carboxyl (COOH) or methyl (CH3) ligand end groups are presented. In carboxyl-terminated ligand systems, structure and interactions are dominated by an end-to-end orientation of ligands. In methyl-terminated ligand systems large ordered ligand structures form, but nanoparticle interactions are dominated by disordered, partially interdigitated ligands. Core size and ligand length also affect both ligand arrangement within the membrane and the membrane's macroscopic mechanical response, but are secondary to the role of the ligand end group. Moreover, the particular end group (COOH or CH3) alters the nature of how ligand length, in turn, affects the membrane properties. The effect of core size does not depend on the ligand end group, with larger cores always leading to stiffer membranes. Asymmetry in the stress and ligand density is observed in membranes during preparation at a water-vapor interface, with the stress asymmetry persisting in all membranes after drying.

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TYPE Journal Article YEAR 2015
DOIOSTIScopus

Enhanced densification under shock compression in porous silicon

Physical Review B - Condensed Matter and Materials Physics

Lane, James M.D.; Thompson, A.P.; Vogler, Tracy J.

Under shock compression, most porous materials exhibit lower densities for a given pressure than that of a full-dense sample of the same material. However, some porous materials exhibit an anomalous, or enhanced, densification under shock compression. We demonstrate a molecular mechanism that drives this behavior. We also present evidence from atomistic simulation that silicon belongs to this anomalous class of materials. Atomistic simulations indicate that local shear strain in the neighborhood of collapsing pores nucleates a local solid-solid phase transformation even when bulk pressures are below the thermodynamic phase transformation pressure. This metastable, local, and partial, solid-solid phase transformation, which accounts for the enhanced densification in silicon, is driven by the local stress state near the void, not equilibrium thermodynamics. This mechanism may also explain the phenomenon in other covalently bonded materials.

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TYPE Journal Article YEAR 2014
DOIOSTIScopus

Hot spot and temperature analysis of shocked hydrocarbon polymer foams using molecular dynamics simulation

Computational Materials Science

Lane, James M.D.; Grest, Gary S.; Mattsson, Thomas

Hydrocarbon polymers, foams and nanocomposites are increasingly being subjected to extreme environments. Molecular scale modeling of these materials offers insight into failure mechanisms and complex response. Prior classical molecular dynamics (MD) simulations of the principal shock Hugoniot for two hydrocarbon polymers, polyethylene (PE) and poly (4-methyl-1-pentene) (PMP) have shown good agreement with density functional theory (DFT) calculations and experiments conducted at Sandia National Laboratories. We extended these results to include low-density polymer foams using nonequilibrium MD techniques and found good quantitative agreement with experiment. Here, we have measured the local temperature during void collapse to investigate the formation of hot spots and their relationship to polymer dissociation in foams. © 2013 Elsevier B.V. All rights reserved.

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TYPE Journal Article YEAR 2013
DOIOSTIScopus
Results 76–100 of 122
Results 76–100 of 122