Computational Materials Chemistry at the Nanoscale

In order to illustrate how atomistic modeling is being used to determine the structure, physical, and chemical properties of materials at the nanoscale, we present here the results of molecular dynamics (MD) simulations on nanoscale assemblies of such materials as carbon nanotubes, diamond surfaces, metal alloy nanowires, and ceramics. We also include here the results of nonequilibrium MD simulations on the nanorheology of a monolayer of wear inhibitor self-assembled on two metal oxide surfaces, separated by hexadecane lubricant, and subjected to steady state shear.We also present recent developments in force fields (FF) required to describe bond breaking and phase transformations in such systems. We apply these to study of plasticity in metal alloy nanowires where we find that depending on the strain rate, the wire may deform plastically (forming twins), neck and fracture, or transition to the amorphous phase.     

The structure of wholly aromatic polyesters with bulky side chains: Poly(p-phenylene phenylthioterephthalate)

Poly(p-phenylene phenylthio-terephthalate) (PPTT) forms nematic melts and is highly crystalline in the solid state, despite the probable random 2- and 3-dispo-sition of the S-phenyl substituents. The X-ray pattern of melt-spun fibers of PPTT contains 24 Bragg reflections that are indexed by a monoclinic unit cell with dimensions a = 28.6 Å, b = 4.81 Å, c = 12.57 Å (fiber axis), and γ= 101.6°. The cell contains monomer units of four chains that are arranged in pairs with the thiophenyl side chains interdigitated; successive pairs of chains are staggered by about c/2. We used molecular mechanics modeling to simulate arrays of chains with random 2- and 3-disposition of the side chains on the terephthalic acid units and compared the results with those for an idealized structure in which all the substituents were at the 2-position. The refined model for random substitution is more distorted, but the average separations of the monomer units are within the experimental errors of the observed unit cell dimensions, and their standard deviations are very similar to those derived from the line-broadening data. The potential energy of the model with random substitution is only about 1.9 kcal/mol of monomer higher than that for the model with all-2-substitution, indicating random substitution is not a major problem to the formation of an ordered structure. Compared to the structure formed by the analogous polyester, poly(p-phenylene phenylterephthalate) (PPT), the additional flexibility due to the thioether linkage between the backbone and phenyl side groups in PPTT allows better chain packing both within and between the layers of stacked chains.     

CFD analysis of an oscillating wing at various reduced frequencies

The effect of various reduced frequencies has been examined for an oscillating aspect ratio 10 NACA 0015 wing. An unsteady, compressible three‐dimensional (3D) Navier–Stokes code based on Beam and Warming algorithm with the Baldwin–Lomax turbulence model has been used. The code is validated for the study against published experimental data. The 3D unsteady flow field is simulated for reduced frequency values of 0.1, 0.2 and 0.3 for a fixed mean angle of attack position and fixed amplitude. The type of motion is sinusoidal harmonic. The force coefficients, pressure distributions and flow visualization show that at the given conditions the flow remains attached to the wing surface even at high angles of attack with no clear separation or typical light‐to‐deep category of dynamic stall. Increased magnitude of hysteresis and higher gradients are seen at higher reduced frequencies. The 3D effects are even found at midspan locations. In addition, the rate of decrease in lift near the wing tips compared with the wing root is not much like in the static cases. Copyright © 2008 John Wiley & Sons, Ltd.     

Lead‐sensitive PNIPAM microgels modified with crown ether groups

A series of lead‐sensitive poly(N‐isopropylacrylamide) microgels with pendant crown ether groups were prepared. Their cation‐sensitive behaviors were studied by dynamic light scattering. When ionic strength is not controlled, adding salts causes the microgel particles to deswell. However, when the salt effect is ruled out by keeping a constant ionic strength, adding Pb²⁺ results in much larger swelling. The Pb²⁺‐induced swelling was explained by the formation of host–guest complex between Pb²⁺ and the pendant crown ether groups, which increases the hydrophilicity of the polymer and accordingly the degree of swelling. The lead sensitivity of the microgels increases with increasing crown ether content. For the modified microgel with the highest crown ether content, it swells to ～430% of its original volume at [Pb²⁺] = 10 mM. Other cations also increase the swelling degree of the modified microgels. The extent of the cation‐induced swelling mainly depends on their affinity to the pendant crown ether groups. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4120–4127, 2010     

New compounds in the reaction of aromatic amines and 5-hydroxy-4-pyrone-2-carboxylate. The influence of a methoxy group

Unlike most of other aromatic amines, 4-methoxyaniline with methyl comenate ( 1 ) in the mixture of acetic acid and methanol (1:1) gave two unexpected products with azomethyne moiety. The structures of new compounds were determined from their characteristic spectroscopic behaviour and were confirmed by X-ray crystallographic measurements.     

On the metric dimension of barycentric subdivision of Cayley graphs

Acta Mathematicae Applicatae Sinica, English Series - In a connected graph G, the distance d(u, v) denotes the distance between two vertices u and v of G. Let W = {w 1, w 2, ···, w...