Synthesis of amphiphilic (meth)acrylates with oligo(ethylene glycol) and (or) oligo(propylene glycol) blocks by the esterification of (meth)acrylic acid

Thermoresponsive PEG-based (PEG stands polyethylene glycol) polymers are unique for use in medicine because of their low toxicity, good biocompatibility and biodegradability, but usually more hydrophobic and more toxic comonomers are used to adjust lower critical solution temperature (LCST). A convenient way to overcome this problem and to finely tune LCST is to use alkoxy oligo(ethylene glycol)- or alkoxy oligo(propylene glycol) (meth)acrylates as starting comonomers. Here we report on the conditions for the simple and affordable synthesis of methoxy oligo(propylene glycol) (meth)acrylate- and methoxy oligo(propylene glycol)-block-oligo(ethylene glycol) (meth)acrylate-based macromonomers with high yields (80%–98.7%) by the acid-catalyzed esterification of (meth)acrylic acid with alkoxy oligo(alkylene glycols) containing oligo(ethylene glycol) (OEG) and/or oligo(propylene glycol) (OPG) blocks. p-Toluene sulphonic acid (pTSA), alkyl(C₁₂–C₁₄)benzene sulfonic acid (ABSA) and H₂SO₄ were used as catalysts. It has been shown that pTSA and ABSA are practically the same in catalytic activity and are superior to sulfuric acid. The reaction orders with respect to catalyst was found to be close to 1 in all cases. It has been shown that the reaction is actually insensitive to the lengths of OEG and OPG blocks, as well as to the structure of the terminal alkyl group, while the esterification of acrylic acid (AA) proceeds much faster compared to methacrylic acid (MAA) one under the same conditions. The influence of temperature on the equilibrium conversions of alcohols was determined, which were found to be 89%–93% for the esterification of AA and 61%–86% for MAA in the temperature range of 60–120°C. A further increase in conversion was achieved by introducing an azeotropic agent (toluene), its optimal concentration was found to be 10%–15%.     

Current flow through metal shunts in ohmic contacts to n +-Si

It is experimentally found that an ohmic contact based on Au-Pt-Ti-Pd-n ⁺-Si metallization is formed due to nanoscale metal shunts containing Si, Au, and Pt in the region of the interface with n ⁺-Si, which appears during heat treatment at T = 450°C for 10 min in a vacuum chamber with a residual pressure of 10^?6 Torr. The high density of shunts adjoining dislocations and other imperfections is confirmed by the temperature dependence of the specific contact resistance ρ _c (T). The density of conductive dislocations, calculated from the temperature dependence of ρ _c is ～5 × 10⁹ cm^?2 which correlates with the density of structural defects, determined by the etch pits after removal of the metallization layers.     

Raman scattering of InAs/AlAs quantum dot superlattices grown on (001) and (311)B GaAs surfaces

ABSTRACT: We present a comparative analysis of Raman scattering by acoustic and optical phonons in InAs/AlAs quantum dot superlattices grown on (001) and (311)B GaAs surfaces. Doublets of folded longitudinal acoustic phonons up to the fifth order were observed in the Raman spectra of (001)- and (311)B-oriented quantum dot superlattices measured in the polarized scattering geometries. The energy positions of the folded acoustic phonons are well described by the elastic continuum model. Besides the acoustic phonons, the spectra display features related to confined transverse and longitudinal optical as well as interface phonons in quantum dots and spacer layers. Their frequency positions are discussed in terms of phonon confinement, elastic stress, and atomic intermixing.     

The Rayleigh-Benard convection in an enclosure with walls of finite thickness

Mathematical simulation of transient natural convection in an enclosure having walls of a finite thickness at the presence of a heat source located at the bottom of the cavity has been carried out. Special attention was given to the analysis of the Grashov number (Gr) effect, describing the intensity of a heat source; of the transient factor, defining the formation and development of thermodynamic structures; and also of the heat conductivity ratio. Typical distributions of streamlines and temperature fields have been received. Scales of key parameters (the Grashov number, dimensionless time, the heat conductivity ratio), affecting both local characteristics (streamlines, isotherms) and integral characteristics (the average Nusselt number) of the analyzed process, have been determined.     

A New Tensorial Framework for Single-Shell High Angular Resolution Diffusion Imaging

Single-shell high angular resolution diffusion imaging data (HARDI) may be decomposed into a sum of eigenpolynomials of the Laplace-Beltrami operator on the unit sphere. The resulting representation combines the strengths hitherto offered by higher order tensor decomposition in a tensorial framework and spherical harmonic expansion in an analytical framework, but removes some of the conceptual weaknesses of either. In particular it admits analytically closed form expressions for Tikhonov regularization schemes and estimation of an orientation distribution function via the Funk-Radon Transform in tensorial form, which previously required recourse to spherical harmonic decomposition. As such it provides a natural point of departure for a Riemann-Finsler extension of the geometric approach towards tractography and connectivity analysis as has been stipulated in the context of diffusion tensor imaging (DTI), while at the same time retaining the natural coarse-to-fine hierarchy intrinsic to spherical harmonic decomposition.     

Radiation effects and interphase interactions in ohmic and barrier contacts to indium phosphide as induced by rapid thermal annealing and irradiation with γ-ray 60Co photons

The radiation resistance of Au-Pd-Ti-Pd-n ⁺⁺-InP ohmic contacts and Au-TiB _x -n-n ⁺-n ⁺⁺-InP barrier contacts—both initial and subjected to a rapid thermal annealing and irradiated with ⁶⁰Co γ-ray photons with doses as high as 10⁹ R—has been studied. Before and after external effects, the electrical characteristics of the barrier and ohmic contacts, distribution profiles for components, and phase composition in the metallization layers have been measured. In ohmic Pd-Ti-Pd-Au contacts subjected to rapid thermal annealing and irradiation, a significant distortion of the layered structure of metallization occurs; this distortion is caused by the thermal and irradiation-stimulated transport of Pd over the grain boundaries in polycrystalline Ti and Au films. However, the specific contact resistance ρ _c does not change appreciably, which is related to a comparatively unvaried composition of the contact-forming layer at the Pd-n ⁺-InP interface. In the initial sample and the sample subjected to the rapid thermal annealing at T = 400°C with the Au-TiB _x -n-n ⁺-n ⁺⁺-InP barrier contacts and irradiated with the dose as high as 2 × 10⁸ R, a layered structure of metallization is retained. After irradiation with the dose as high as 10⁹ R, in the samples subjected to a rapid thermal annealing at T = 400°C, the layered structure of metallization becomes completely distorted; however, this structure is retained in the initial sample. The electrical properties of the contact structure appreciably degrade only after irradiation of the sample preliminarily subjected to a rapid thermal annealing at T = 400°C.     

Metrological aspects of measuring resistance of ohmic contacts

Modern methods of measuring resistance of ohmic contacts are considered in the review: Cocks-Strack, transmission line, Kelvin, boundary probing. Examples of their application are provided and errors of measuring the contact resistivity are calculated. The most accurate measuring methods are the Kelvin method and the boundary probing method, while the least accurate one is the Cocks-Strack method. Requirements and possibilities of their application are specified.