Structural relaxation in polyethylene in the presence of silver oxide investigated by positron-lifetime spectroscopy

Structural relaxation in high-density virgin polyethylene and silver oxide doped polyethylene has been investigated using positron annihilation lifetime technique at 30 °C and 100 °C ageing temperatures. The ortho-positronium (o-Ps) pick-off lifetime and its intensity show no changes with isothermal ageing time for the virgin sample at 30 °C. In the case of the doped sample at 30 °C, the o-Ps intensity shows exponential relaxation in the short-time range. At 100 °C, the o-Ps lifetime remains almost constant, whereas the o-Ps intensity exhibits an exponential character in the virgin and doped samples, which can be fitted with exponential decay curves. The relaxation times have been evaluated from the structural relaxation function constructed using o-Ps intensity values, and the stretching exponent has been estimated using the Kohlrausch–Williams–Watts (KWW) function. Positron results indicate that the stretching exponent seems to be temperature dependent. PACS 78.70.Bj     

Simplified analytical expressions for classical turning points of molecular oscillators

Simplified analytical expressions for classical turning ppoints (r_1,2) are derived by expanding the empirical potential function of Dunham and Morse in series forms. These expressions are equivalent to those obtained by Rees when the energy (E) is quadratic in the vibrational quantum number. Our first approximation approach is also found to be equivalent to Jarmain's factor f/r_e.     

Frequency distribution function and critical points for phonons in graphite

A molecular force field for the study of the dynamics of graphite has been reported earlier (Mani and Ramani 1974). In the present paper we present calculations of the single phonon density of states and the lattice specific heat. Most of the peaks in the frequency distribution function have been identified and classified using symmetry-required critical points. The validity of the force field used has also been discussed.     

Computation of scattering from N spheres using multipole reexpansion

A computational technique for the solution of problems of wave scattering from multiple spheres is developed. This technique, based on the T-matrix method, uses the theory for the translation and reexpansion of multipole solutions of the Helmholtz equation for fast and exact recursive computation of the matrix elements. The spheres can have prescribed radii, impedances, and locations. Results are validated by comparison with boundary element calculations, and by convergence analyses. The method is much faster than numerical methods based on discretization of space, or of the sphere surfaces. An even faster method is presented for the case when the spheres are aligned coaxially.     

Precise estimate of fundamental in-vivo MT parameters in human brain in clinically feasible times

A methodology is presented for extracting precise quantitative MT parameters using a magnetisation-prepared spoiled gradient echo sequence. This method, based on a new mathematical model, provides relaxation parameters for human brain in-vitro and in-vivo. The in-vivo parameters have been obtained from three different regions of normal white matter: occipital white matter, frontal white matter and centrum semiovale; two regions of normal grey matter: cerebral cortex and cerebellum, and from five regions with MS lesions. All this has been achieved using MT images collected within a timeframe that is clinically feasible. We hope that this new technique will shed light on the properties and dynamics of water compartments within the brain.     

On Discrete Painlevé Equations Associated with the Lattice KdV Systems and the Painlevé VI Equation

A new integrable nonautonomous nonlinear ordinary difference equation is presented that can be considered to be a discrete analogue of the Painlevé V equation. Its derivation is based on the similarity reduction on the two-dimensional lattice of integrable partial differential equations of Korteweg–de Vries (KdV) type. The new equation, which is referred to as generalized discrete Painlevé equation (GDP), contains various "discrete Painlevé equations" as subcases for special values/limits of the parameters, some of which have already been given in the literature. The general solution of the GDP can be expressed in terms of Painlevé VI (PVI) transcendents. In fact, continuous PVI emerges as the equation obeyed by the solutions of the discrete equation in terms of the lattice parameters rather than the lattice variables that label the lattice sites. We show that the bilinear form of PVI is embedded naturally in the lattice systems leading to the GDP. Further results include the establishment of Bäcklund and Schlesinger transformations for the GDP, the corresponding isomonodromic deformation problem, and the self-duality of its bilinear scheme.     

Methods for the synthesis of chiral sulfur heterocycles and their application in the asymmetric Baylis–Hillman reactions

Enantiomerically pure (2S,6S)-2,6-diphenyltetrahydro-2H-thiopyran, (2S)-2-phenyltetrahydro thiophene, and (2S)-2-phenyltetrahydro-2H-thiopyran were prepared in 70–72% yields and with 86–99% ee via cyclization of the corresponding dimesylate in an S_N2 cyclization reaction using sodium sulfide nonahydrate. The results on the application of various chiral sulfides in asymmetric Baylis–Hillman reactions are also described.     

Bacterial cellulose-reinforced hydroxyapatite functionalized graphene oxide: a potential osteoinductive composite

Bacterial cellulose (BC) is a nanofibrous biocompatible scaffold used in bone tissue engineering. Graphene oxide/hydroxyapatite (GOHA) composite supports the adhesion of osteoblast cells with good viability. In the present study, GOHA was prepared by the wet chemical precipitation method, and BC obtained from Acetobacter aceti was added to the aqueous suspension of GOHA to prepare GOHABC. The scanning electron microscopic image of GOHABC showed uniform adsorption of GOHA on the surface of BC. The osteoinductive potential of the GOHABC scaffold was analyzed by alkaline phosphatase (ALP) activity assay using MG-63 cells, and its biocompatibility nature was studied by using both MG-63 and NIH-3T3 cells. The ALP activity and biocompatibility studies showed that GOHABC is a potential osteoinductive material in vitro and may be tried in the future for in vivo studies.     

ZrB2/Pt/Au Ohmic contacts on bulk, single-crystal ZnO

There is a strong interest in developing thermally stable metallization schemes for ZnO and boride-based contact stacks are expected to have potential because of their thermodynamic stability. The contact characteristics on bulk single-crystal n-ZnO of a ZrB₂/Pt/Au metallization scheme deposited by sputtering are reported as a function of annealing temperature in the range 300-800°C. The contacts were rectifying for anneal temperatures <500 °C but exhibited Ohmic behavior at higher temperatures and exhibit a minimum specific contact resistivity of 9 × 10⁻³ Ω cm after 700 °C anneals. The contact stack reverts to rectifying behavior after annealing above 800 °C, coincident with a degraded surface morphology and intermixing of the Au, Pt and ZrB₂. The boride-based contacts exhibit higher thermal stability but poorer specific contact resistivity than conventional Ti/Au metal stacks on ZnO.