A rapid and convenient oxidation of secondary alcohols

A rapid (normally 20 min to 2 h) and selective oxidation of secondary alcohols to ketones can be achieved using 0.4 equivalents trichloroisocyanuric acid and 1.2 equivalents pyridine at room temperature in ethyl acetate. A likely mechanism for the reaction is proposed.     

Modulation of the internal structure and surface properties of natural and synthetic polymer matrices by graphene oxide doping

Controlling the surface properties and structure of thin nanosized coatings is of primary importance in diverse engineering and medical applications. Here we report on how the nanostructure, growth mechanism, thickness, roughness, and hydrophilicity of nanocomposites composed of weak natural or strong synthetic polyelectrolytes (PE) can be tailored by graphene oxide (GO) doping. GO reverses the build‐up mechanism affecting the internal structure and the hydrophilicity in a way depending on the type of the PE‐matrix. The extent of GO‐adsorption and its impact on the surface morphology was found to be independent on the type of the underlying PE‐matrix. The nanostructure of the hybrid films is not significantly altered when a single surface‐exposed GO‐layer is deposited, while increasing the number of embedded GO‐layers leads to pronounced surface heterogeneity. These results are expected to have valuable impact on the construction strategies of coatings with tunable surface properties.     

Stability of filled poly(dimethylsiloxane) and poly(diphenylsiloxane-co-dimethylsiloxane) elastomers to cyclic stress at elevated temperature

The response of aluminum oxide-filled poly(dimethyl siloxane) and poly(diphenylsiloxane-co-dimethylsiloxane) elastomers, containing 3–24 mol % diphenylsiloxane, to cyclic stress at elevated temperatures (dynamic creep) was evaluated. The materials could be divided into two classes, based on their response to the application of cyclic stress: no or low-diphenylsiloxane content elastomers in which substantial creep and a decrease in crosslink density were observed, and high diphenylsiloxane content (16–24 mol %) elastomers that showed decreased creep with increasing diphenylsiloxane content and an increase in crosslink density. It was suggested that the phenyl groups stabilize the siloxane bond in the polymer backbone, decreasing the rate of chain scission reactions as the diphenylsiloxane content increases and stabilizing the elastomer against creep. The balance of chain scission, chemical crosslinking, and cyclic formation reactions varies depending on diphenylsiloxane content, giving rise to the differences in dynamic creep behavior. An activation energy of 12.9 kcal/mol was measured for dynamic creep of poly(16% diphenylsiloxane/84% dimethyl siloxane), suggesting that a catalyzed degradation mechanism was responsible. The primary catalysts of the degradation reactions are postulated to be the filler particles. © 1996 John Wiley & Sons, Inc.     

Large deformations during the coalescence of fluid interfaces

Surface forces and shape changes were simultaneously measured during the approach and coalescence of two liquid-liquid and liquid-air interfaces. Large normal and lateral deformations were observed that are nevertheless consistent with a simple theoretical analysis of the long-range effects of short-range attractive van der Waals forces. The results imply that two fluidlike structures such as liquid droplets and soft biological cells can sense each other at much larger separations than previously assumed based on criteria taken from the interactions of hard particles.     

Impact of membrane fluidity on steric stabilization by lipopolymers

In this work, the impact of lipid lateral mobility on the steric interaction between membranes containing poly(ethylene glycol) (PEG) functionalized lipids was investigated using the surface force apparatus. The force-distance profiles show the presence of electrostatic and steric repulsion that arise from the presence of negatively charged PEG functionalized lipids. Fluid-phase bilayers have high lateral diffusion relative to gel-phase bilayers; however, a quantitative comparison of the interaction forces between membranes in these two different phase states demonstrates a reduced rate of diffusion in the fluid phase for the PEG-lipids under constrained geometries. Thus, the amount of polymer in the contact zone can be modulated and is reduced with fluid membranes; however, complete exclusion was not achieved. As a result, the steric repulsion afforded by PEG chains or binding affinity of ligated PEG chains can only be modestly tailored by the phase state of the liposome.     

Fluorination of the hydrophilic head accelerates the collapse of the monolayer but stabilizes the bilayer of a long-chain trifluoroethyl ether on water

A comparison of the collapse of Langmuir monolayers of docosyl trifluoroethyl ether (DFEE) and docosyl ethyl ether (DEE) on water shows that in both films the 3D phase is formed layer-by-layer. The substitution of CH3 by a CF3 group in the hydrophilic head yields a more stable bilayer exhibiting lower equilibrium spreading pressure, pi(esp)(DFEE) < pi(esp)(DEE). Upon lateral compression, the DFEE bilayer fractures abruptly as a compact solid body whereas the DEE bilayer breaks down gradually as a polycrystalline material. A comparison of the collapse kinetics of the two films at the same constant supersaturation pi-pi(esp) = 7 mN/m shows that the fluorinated DFEE monolayer transforms more quickly, yielding a stable bilayer of closely packed upright molecules, whereas the DEE film undergoes a continuous monolayer-bilayer-multilayer transition. Brewster angle microscopy allows us to visualize different collapse mechanisms of the DFEE and DEE films; the domains of the fluorinated DFEE bilayer grow laterally at constant thickness and density, and the collapse of the nonfluorinated DEE monolayer occurs through a sequence of disordered stripelike and broken elongated textures. The characteristic molecular areas of the monolayer and bilayer collapse suggest that the 2D-3D transition in the DFEE and DEE films is accompanied by at least partial dehydration of their headgroups. The faster collapse of the fluorinated monolayer could result from a lower energy barrier due to the more hydrophobic CF3 group in the heads. The increased stability of the DFEE bilayer could be associated with the electrostatic attraction between the -C(F delta-)3 versus (H delta+)3C- terminals at the heads-to-tails contact plane of the top and the bottom layer, contrasting with the repulsion between the -C(H delta+)3 versus (H delta+)3C- terminals of the top-layer heads and the bottom-layer tails in the DEE bilayer.     

Fitting a free-form scattering length density profile to reflectivity data using temperature-proportional quenching

A technique for fitting a free-form scattering length density profile to reflectivity data via least-squares minimization is presented. The approach combines aspects of simulated annealing with a parametrized representation of the scattering length density profile. The ability of the algorithm to accurately recover the scattering length density profile from arbitrary initial parameter values is demonstrated for simulated and experimental data.     

Empirical likelihood bivariate nonparametric maximum likelihood estimator with right censored data

This article considers the estimation for bivariate distribution function (d.f.) \(F_0(t, z)\) of survival time \(T\) and covariate variable \(Z\) based on bivariate data where \(T\) is subject to right censoring. We derive the empirical likelihood-based bivariate nonparametric maximum likelihood estimator \(\hat{F}_n(t,z)\) for \(F_0(t,z)\) , which has an explicit expression and is unique in the sense of empirical likelihood. Other nice features of \(\hat{F}_n(t,z)\) include that it has only nonnegative probability masses, thus it is monotone in bivariate sense. We show that under \(\hat{F}_n(t,z)\) , the conditional d.f. of \(T\) given \(Z\) is of the same form as the Kaplan–Meier estimator for the univariate case, and that the marginal d.f. \(\hat{F}_n(\infty ,z)\) coincides with the empirical d.f. of the covariate sample. We also show that when there is no censoring, \(\hat{F}_n(t,z)\) coincides with the bivariate empirical d.f. For discrete covariate \(Z\) , the strong consistency and weak convergence of \(\hat{F}_n(t,z)\) are established. Some simulation results are presented.     

Analyzing refractive index profiles of confined fluids by interferometry part II: Multilayer and asymmetric systems

Methods for determining the substrate properties and the optical thickness of thin films or any variation in the refractive index of a fluid or film near a surface for unknown 5-layer symmetric and 3-layer asymmetric interferometers are presented. Both systems can be fully resolved without any known layer properties and without contact or confining the films. The method was tested using realistic simulated interferometer data, and was found to consistently yield accurate values for all desired properties. The method was experimentally validated through analysis of an asymmetric three layer interferometer system of linear polyethyleneimine (LPEI) adsorbed onto mica substrates of differing thickness and identical refractive index. The results were in excellent agreement with the dry polymer film properties measured using conventional SFA contact measurements. More complicated systems were also evaluated for feasibility, and any additional parameter specifications required for analysis were determined. The utility of this method is broad, as a single experiment in a laboratory setting can independently provide non-contact film properties and the effects of confinement on the film structure, which can be correlated to a simultaneously measured interaction force profile.