Thermally Responsive Selenide-containing Materials Based on Transalkylation of Selenonium Salts

Covalent adaptable networks (CANs) possess unique properties as a result of their internal dynamic bonds, such as self-healing and reprocessing abilities. In this study, we report a thermally responsive C−Se dynamic covalent chemistry (DCC) that relies on the transalkylation exchange between selenonium salts and selenides, which undergo a fast transalkylation reaction in the absence of any catalyst. Additionally, we demonstrate the presence of a dissociative mechanism in the absence of selenide groups. After incorporation of this DCC into selenide-containing polymer materials, it was observed that the cross-linked networks display varying dynamic exchange rates when using different alkylation reagents, suggesting that the reprocessing capacity of selenide-containing materials can be regulated. Also, by incorporating selenonium salts into polymer materials, we observed that the materials exhibited good healing ability at elevated temperatures as well as excellent solvent resistance at ambient temperature. This novel dynamic covalent chemistry thus provides a straightforward method for the healing and reprocessing of selenide-containing materials.     

Dynamic scaling for gelation of a thermosensitive chitosan-β-glycerophosphate hydrogel

This paper investigates the thermal gelation of a chitosan-β-glycerophosphate system using rheometry. The gelation, performed for two polymer concentrations, was achieved in isothermal conditions to examine the effect of various temperatures (between 35 and 65 °C) on the sol-gel transition. The oscillatory shear data was used to verify the scaling behavior (G′ ~ G″ ~ ω ⁿ ) at the gel point, and the power-law index n was found to be dependent to some extent on chitosan concentration and temperature. The solutions zero shear viscosity (η ₀) and the gels low frequency modulus (G _e) were analyzed in terms of the relative distance to gelation time (ɛ). The two properties showed power–law relationships with ɛ, i.e., η ₀ ~ ɛ ^−s and G _e ~ ɛ ^z, and the indexes slightly increased with temperature. The dynamic scaling theory predicts a relationship between the indexes, i.e., n  =  z/(z  +  s), and this was in reasonably good agreement with the experimental findings.     

Separation of casein micelles from whey proteins by high shear microfiltration of skim milk using rotating ceramic membranes and organic membranes in a rotating disk module

This paper investigates the microfiltration of skim milk in order to separate caseins micelles from two whey proteins, α-lactalbumin (α-La) and β-lactoglobulin (β-Lg), using a modified dynamic filtration pilot (MSD) consisting in 6 ceramic 9-cm diameter membrane disks of 0.2 μm pores, rotating around a shaft inside cylindrical housing. A comparison was made with another dynamic filtration module consisting in a disk rotating near a fixed PVDF 15.5 cm diameter membrane with 0.15 μm pores. Maximum permeate fluxes were 120 L h⁻¹ m⁻² with the MSD module at 1930 rpm and at 40 °C, and 210 L h⁻¹ m⁻² at 2500 rpm and 45 °C, with the rotating disk module. Casein rejection was around 99% at high speed for both membranes. α-La transmission decreased with increasing transmembrane pressure (TMP) from 75% to 60% for ceramic membranes and from 25% to 10% for the PVDF one. β-Lg transmissions were lower, ranging from 23% to 15% for ceramic membranes and from 20% to 5% for the PVDF one. In a concentration test with the PVDF membrane at 2000 rpm, the flux decayed from 200 L h⁻¹ m⁻² at initial concentration to 80 L h⁻¹ m⁻² at VRR = 3.2 and 22.1% of the initial α-La mass was recovered in the permeate, against 8.1% for β-Lg. Permeate fluxes in the mass transfer limited regime (J_lim) of the MSD and rotating disk module operated at various speeds were well correlated by the equation J_lim = 17.13 V_av where V_av denoted the disk azimuthal velocity averaged over the membrane area. Measurements of J_lim, taken from Ref. [G. Samuelsson, P. Dejlmek, G. Tragardh, M. Paulsson, Minimizing whey protein retention in crossflow microfiltration of skim milk. Int. Dairy J. 7 (1997) 237–242] during MF of skim milk using tubular ceramic membranes at velocities from 1.5 to 8 m s⁻¹ with permeate co-current recirculation were found to obey the same correlation.     

Preparation and mechanical properties of bacterial cellulose nanocomposites loaded with silica nanoparticles

Bacterial cellulose (BC), which is produced by Gluconacetobacter xylinus (Ga. xylinus) in culture, is made up of a three-dimensional network of ribbon-shaped bundles of cellulose microfibrils. In the current studies, we used two processes to prepare nanocomposites of BC filled with silica particles. In Process I, Ga. xylinus was incubated in medium containing silica sol Snowtex 0 (ST 0, pH 2–4) or Snowtex 20 (ST 20, pH 9.5–10.0). The elastic modulus at 20 °C was improved by keeping the amount of silica in the nanocomposites below 4% when ST 20 was used and below 8.7% when ST 0 was used. This process allowed incorporation of 50% silica in BC. Inclusion of higher amounts of silica reduced the modulus at 20 °C and the strength of the nanocomposites below that of BC. X-ray diffraction measurements revealed that the silica particles disturb the formation of ribbon-shaped fibrils and affect the preferential orientation of the ( ) plane. We also produced BC-silica nanocomposites by Process II, wherein the BC hydrogel was immersed in different concentrations of silica sols, allowing silica particles to diffuse into the BC hydrogel and lodge in the spaces between the ribbon-shaped fibrils. This method increased the modulus at 20°C and the strength compared to the BC matrix, but it was difficult to load the BC with more than 10% silica in this way.     

Relative quantification of long chain branching in essentially linear polyethylenes

The aim of this work is to describe a method whereby low levels of long chain branching, LCB, can be quantified on a relative basis for whole, unfractionated, and essentially linear ethylene/α-olefin copolymers. The method is based on a well established, relatively fast and robust experiment, namely the measurement of the linear viscoelastic properties by a single, isothermal, small amplitude oscillatory shear experiment. The analysis of the data is predicated on the use of the so-called van Gurp-Palmen plots (the phase angle, δ (=tan⁻¹(G″/G′)), plotted against the absolute value of the dynamic complex modulus, |G∗| = (G′²+G″²)^1/2). From this plot, the value of δ at |G∗| = 10 kPa is recorded, and it is demonstrated that the amount of LCB inversely correlates with such value of the phase angle, δ. Depending on the desired frequency range, the experiment duration varies between 15 and 60 min rendering this technique well suited for high throughput parallel testing. Its applicability is critically examined with a wide variety of commercial ethylene/α-olefin copolymers. Moreover, we have improved on the long chain branching index (LCBI) proposed by Shroff and Mavridis [Shroff RN, Mavridis H. Long-chain-branching index for essentially linear polyethylenes. Macromolecules 1999;32:8454-64] by basing it on data of truly linear polyethylenes (hydrogenated anionically synthesized polybutadienes) instead of apparently linear commercial polyethylenes.     

Optimization strategy based on genetic algorithms and neural networks applied to a polymerization process

The article presents a simple and general methodology, especially destined to the optimization of complex, strongly nonlinear systems, for which no extensive knowledge or precise models are available. The optimization problem is solved by means of a simple genetic algorithm, and the results are interpreted both from the mathematical point of view (the minimization of the objective function) and technological (the estimation of the achievement of individual objectives in multiobjective optimization). The use of a scalar objective function is supported by the fact that the genetic algorithm also computes the weights attached to the individual objectives along with the optimal values of the decision variables. The optimization strategy is accomplished in three stages: (1) the design and training of the neural model by a new method based on a genetic algorithm where information about the network is coded into the chromosomes; (2) the actual optimization based on genetic algorithms, which implies testing different values for parameters and different variants of the algorithm, computing the weights of the individual objectives and determining the optimal values for the decision variables; (3) the user's decision, who chooses a solution based on technological criteria. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Optimal replacement and allocation of multi‐state elements in k‐within‐m‐from‐r/n sliding window systems

This paper proposes a new model that generalizes the linear sliding window system to the case of multiple failures. The considered k ‐within‐ m ‐from‐ r / n sliding window system consists of n linearly ordered multi‐state elements and fails if at least k groups out of m consecutive groups of r consecutive multi‐state elements have cumulative performance lower than the demand W . A reliability evaluation algorithm is suggested for the proposed system. In order to increase the system availability, maintenance actions can be performed, and the elements can be optimally allocated. A joint element allocation and maintenance optimization model is formulated with the objective of minimizing the total maintenance cost subjected to the pre‐specified system availability requirement. Basic procedures of genetic algorithms are adapted to solve the optimization problem. Numerical experiments are presented to illustrate the applications. Copyright © 2015 John Wiley & Sons, Ltd.     

SHAH: SHape-Adaptive Haar Wavelets for Image Processing

We propose the shape-adaptive Haar (SHAH) transform for images, which results in an orthonormal, adaptive decomposition of the image into Haar-wavelet-like components, arranged hierarchically according to decreasing importance, whose shapes reflect the features present in the image. The decomposition is as sparse as it can be for piecewise-constant images. It is performed via a stepwise bottom-up algorithm with quadratic computational complexity; however, nearly linear variants also exist. SHAH is rapidly invertible. We show how to use SHAH for image denoising. Having performed the SHAH transform, the coefficients are hard- or soft-thresholded, and the inverse transform taken. The SHAH image denoising algorithm compares favorably to the state of the art for piecewise-constant images. A clear asset of the methodology is its very general scope: it can be used with any images or more generally with any data that can be represented as graphs or networks.     

Determination of chlorpyrifos in soils using multi-throughput dynamic microwave-assisted extraction coupled online with salting-out-assisted liquid–liquid extraction followed by LC–MS/MS

A simple and rapid method based on multi-throughput dynamic microwave-assisted extraction coupled online with salting-out-assisted liquid–liquid extraction was developed for the analysis of chlorpyrifos in soil. First, the chlorpyrifos was extracted with acetonitrile aqueous (50%, v/v) under the action of microwave energy. Then the obtained extract was separated clearly and easily into an acetonitrile phase and an aqueous phase with the assistance of ammonium acetate. The acetonitrile phase containing chlorpyrifos was concentrated and determined by liquid chromatography–tandem mass spectrometry. The effects of parameters on extraction efficiency including microwave power, extraction solvent, volumes and flow rate of extraction solvent, sample pH, types and amount of salt were studied and optimised. To eliminate the matrix effect, validation of the method was carried out using the matrix-based calibration curve. The limits of detection and quantification for chlorpyrifos were 0.17 and 0.5 ng g^?1, respectively. The proposed method was applied to analyse chlorpyrifos in five soil samples and verified by the recovery test. The recoveries of chlorpyrifos at three spiked levels (5, 50, 200 ng g^?1) were in the range of 90.0–100.5%, with relative standard deviations varying from 1.3% to 5.7%. Compared with the methods reported previously, the proposed method can simplify the operation procedure and reduce solvent consumption in sample pretreatment.     

城市区域性人口承载力动态综合测度

对马尔萨斯人口增长模型进行研究,设计人口承载力发展路径和基于增长模型的隶属函数,从概率角度构建人口承载力动态综合测度模型,同时,以城市区域为研究对象,以经济作为人口承载力主导影响因素,辅之以自然、环境、资源与科技因素,构建人口承载力综合评价指标体系,多角度、多因素分析区域人口承载力,寻找人口承载短板,为人口承载力研究提供新的研究方法和视角.实证研究结论显示,研究区域人口承载力当前处于承载力强阶段,并开始向适度阶段过渡,其内部指标和子系统发展不平衡,人口系统自身发展稳定,经济和环境人口承载力强,资源和科技承载力较弱,随着区域经济的发展,在相当长一段时间内可进入适度人口阶段.