Probabilistic Approach to Low Strain Rate Atomistic Simulations of Ultimate Tensile Strength of Polymer Crystals.

Molecular dynamics simulations of the tensile ultimate properties of polymer crystals require the use of empirical potentials that model bond dissociation. However, fully reactive potentials are computationally expensive such that reactive simulations cannot reach the low strain rates of typical experiments. Here, we present a hybrid approach that uses the simplicity of a classical, nonreactive potential, information from bond dissociation energy calculations, and a probabilistic expression that mimics bond breaking. The approach is demonstrated for poly(p-phenylene terephthalamide) and, with one tunable parameter, the calculated tensile ultimate stress matches that obtained using a fully reactive simulation at high strain rates. Then, the hybrid simulations are run at much lower strain rates where the ultimate tensile stress is strain rate-independent and consistent with the expected experimental range.

Simulations of Subnanometer Scale Image Contrast in Atomic Force Microscopy of Self-Assembled Monolayers in Water.

Achieving high-resolution images using dynamic atomic force microscopy (AFM) requires understanding how chemical and structural features of the surface affect image contrast. This understanding is particularly challenging when imaging samples in water. An initial step is to determine how well-characterized surface features interact with the AFM tip in wet environments. Here, we use molecular dynamics simulations of a model AFM tip apex oscillating in water above self-assembled monolayers (SAMs) with different chain lengths and functional groups. The amplitude response of the tip is characterized across a range of vertical distances and amplitude set points. Then relative image contrast is quantified as the difference of the amplitude response of the tip when it is positioned directly above a SAM functional group vs positioned between two functional groups. Differences in contrast between SAMs with different lengths and functional groups are explained in terms of the vertical deflection of the SAMs due to interactions with the tip and water during dynamic imaging. The knowledge gained from simulations of these simple model systems may ultimately be used to guide selection of imaging parameters for more complex surfaces.

Rheology of water in small nanotubes.

The properties of water in confinement are very different from those under bulk conditions. In some cases the melting point of ice may be shifted and one may find either ice, icelike water, or a state in which freezing is completely inhibited. Understanding the dynamics and rheology of water in confined media, such as small nanotubes, is of fundamental importance to the biological properties of micro-organisms at low temperatures, to the development of new devices for preserving DNA samples, and for other biological materials and fluids, lubrication, and development of nanostructured materials. We study rheology and dynamics of water in small nanotubes using extensive equilibrium and nonequilibrium molecular dynamics simulations. The results demonstrate that in strong confinement in nanotubes at temperatures significantly below and above bulk freezing temperature water behaves as a shear-thinning fluid at shear rates smaller than the inverse of the relaxation time in the confined medium. In addition, our results indicate the presence of regions in which the local density of water varies significantly over the same range of temperature in the nanotube. These findings may also have important implications for the design of nanofluidic systems.

Complex Behavior of Ordered and Icelike Water in Carbon Nanotubes near Its Bulk Boiling Point.

We report the results of extensive molecular dynamics (MD) simulation of water in a carbon nanotube (CNT) with a specific diameter over a wide range of temperatures from 343 to 423 K. In order to characterize the nature of water, we have computed the Kirkwood g-factor, the ten Wolde parameter, the radial distribution, the cage correlation, the intermediate scattering functions, the mean-square displacements of the water molecules, and the connectivity of the oxygen atoms. The computed properties provide evidence for complex behavior. Some of the properties indicate an icelike structure, while others point to ordered (but not necessarily frozen) water. The connectivity is close to 9. The ordered water exists both below and above its bulk boiling point. The order is identified based on the ten Wolde parameter and may explain, along with the dynamic slow down, the recent discovery of "ice" in CNTs near the bulk boiling point in a certain range of CNT diameters, not seen in tubes of other sizes.