Synthesis of azopolymers with controlled structure and photoinduced birefringence in their LB films

The molecular architecture of azopolymers may be controlled via chemical synthesis and with selection of a suitable film-forming method, which is important for improving their properties for practical uses. Here we address the main challenge of combining the photoinduced birefringence features of azopolymers with the higher thermal and mechanical stabilities of poly(methyl methacrylate) (PMMA) using Atom Transfer Radical Polymerization (ATRP) to synthesize diblock- and triblock-copolymers of an azomonomer and the monomer methyl methacrylate. Langmuir-Blodgett (LB) films made with the copolymers mixed with cadmium stearate displayed essentially the same optically induced birefringence characteristics, in terms of maximum and residual birefringence and time for writing, as the mixed LB films with the homopolymer poly[4-(N-ethyl-N-(2-methacryloxyethyl))amino-2′-chloro-4′-nitroazobenzene] (HPDR13), also synthesized via ATRP. In fact, the controlled architecture of HPDR13 chains led to Langmuir films that could be more closely packed and reach higher collapse pressures than the corresponding films obtained with HPDR13-conv synthesized via conventional radicalar polymerization. This allowed LB films to be fabricated from neat HPDR13, which was not possible with HPDR13-conv. The enhanced organization in the LB films produced with controlled azopolymer chains, however, led to a smaller free volume available for isomerization of the azochromophores, thus yielding a lower photoinduced birefringence than in the HPDR13-conv films. The combination of ATRP synthesis and LB technology is then promising to obtain optical storage in films with improved thermal and mechanical processabilities, though a further degree of control must be sought to exploit film organization while maintaining the necessary free volume in the films.     

Force control of a very lightweight single-link flexible arm based on coupling torque feedback

In this paper, a feedback control law is proposed for regulating the contact force exerted by a very lightweight single-link flexible manipulator when it comes into contact with a motionless object. This control law is based on a lumped-parameter model. The tracking of the desired contact force is obtained by using a feedback loop control of the coupling torque between the motor and the flexible arm. To achieve a good performance on the force control it is only necessary to measure the coupling torque at the root of the arm. Neither the contact force sensor nor the angular position sensor of the motor are used in the control method. A modified PID controller is proposed for this control law. In this work the force control problem is studied for both free and constrained motions of the flexible manipulator, and a collision detection algorithm is also described.     

分子动力学模拟研究直链淀粉与α-亚油酸的包合行为

α-亚油酸和直链淀粉可以形成热力学稳定复合物。为了研究α-亚油酸影响直链淀粉构象的机理、复合物的构象趋势及复合作用过程中氢键的变化情况,在373 K条件下进行了长时间(500 ns)的全原子分子动力学模拟。模拟发现在有/无α-亚油酸存在的情况下,在500 ns内都可以观察到大量有序到无序的构象转变,这表明直链淀粉和α-亚油酸之间结合作用微弱。原子均方根偏差(RMSD)、α-亚油酸与直链淀粉轴心间距、氢键增加都证明直链淀粉-α-亚油酸复合物的形成是热力学自发行为。     

Biosynthesis of liver glycerolipids from normal and essential fatty acid-deficient rats:3H-glycerol and 1-14C linoleic acid incorporation

Normal and essential fatty acid (EFA)-deficient rats were injected via the portal vein with a labeled solution containing³H-glycerol and 1-¹⁴C-linoleic acid during a 1 min period. Livers were immediately frozen, pulverized, and the lipids extracted and fractioned by thin layer chromatography. The incorporation of³H-glycerol and 1-¹⁴C-linoleic acid into the different lipid fractions was measured, and the per cent distribution and specific radioactivity determined. A parallel increase was found between the specific activity and the amount of³H-glycerol incorporated into 1,2-diglycerides, triglycerides, lecithin and cephalin from EFA-deficient and normal rats. Since the amount of glycerol in each fraction studied was quite similar in both groups of rats, these findings can explain the increase in the specific activity observed in the EFA-deficient rats. Nevertheless these facts do not necessarily imply an increased turnover rate of these molecules, since we do not know the specific radioactivity of the 1,2-diacylglycerol precursors. A remarkable increase in the specific radioactivity of the¹⁴C-linoleic acid incorporated into lipid fractions from EFA-deficient rats compared with control rats was observed. While the amount of 1-¹⁴C-linoleic acid incorporated into neutral lipids was similar in both groups of rats, a statistically significant increase in the amount of the label incorporated into phospholipids from EFA-deficient rats was observed. These facts suggest an increased turnover rate of the radiolinoleic acid into phospholipid molecules from EFA-deficient rats via deacylation-reacylation pathway.     

Squeezing flow viscometry for nonelastic semiliquid foods--theory and applications

In most conventional rheometers, notably the coaxial cylinders and capillary viscometers, the food specimen is pressed into a narrow gap and its structure is altered by uncontrolled shear. Also, most semiliquid foods exhibit slip, and consequently the measurements do not always reflect their true rheological properties. A feasible solution to these two problems is squeezing flow viscometry where the specimen, practically intact and with or without suspended particles, is squeezed between parallel plates. The outward flow pattern mainly depends on the friction between the fluid and plates or its absence ("lubricated squeezing flow"). Among the possible test geometries, the one of constant area and changing volume is the most practical for foods. The test can be performed at a constant displacement rate using common Universal Testing Machines or under constant loads (creep array). The tests output is in the form of a force-height, force-time, or height-time relationship, from which several rheological parameters can be derived. With the current state of the art, the method can only be applied at small displacement rates. Despite the method's crudeness, its results are remarkably reproducible and sensitive to textural differences among semiliquid food products. The flow patterns observed in foods do not always follow the predictions of rheological models originally developed for polymer melts because of the foods' unique microstructures. The implications of these discrepancies and the role that artifacts may play are evaluated in light of theoretical and practical considerations. The use of squeezing flow viscometry to quantify rheological changes that occur during a product's handling and to determine whether they are perceived sensorily is suggested.     

Prenatal protein depletion and Δ9, Δ6 and Δ5 desaturases in the rat

Pregnant rats were kept throughout gestation on a control diet (i.e., 25% protein), on a low protein diet (i.e., 5% protein) or on a fat-free diet. At 20–21 days of gestation, the rate of 9-, 6-, and 5-desaturation was measured, using microsomes from maternal and fetal livers and placenta microsomes. The effect of protein malnutrition was more evident upon Δ6-desaturase activity from maternal liver, while a less severe reduction in the activities of Δ9- and Δ5-desaturases was observed. No measurable activities of Δ5- and Δ6-desaturases were observed in fetal liver and placenta, while a low activity of Δ9-desaturase was detected in both tissues from the three groups under study. We concluded that Δ6-desaturation is greatly affected by maternal protein deprivation, and this fact could affect the normal supply of polyunsaturated fatty acids for the normal fetus growth and tissue development.     

Statistical characterization of morphological features of layer-by-layer polymer films by image analysis

This article presents an image analysis-based approach to the statistical characterization of morphological features in layer-by-layer (LBL) polymer films, which depend on the experimental conditions of fabrication. The method is based on the recently introduced concept of exact dilations. This concept allows the precise determination of all possible parallel contours around any general object and, as a consequence, the respective fully accurate Voronoi partition defined by the objects. With the use of a fast wavefront propagation scheme, such diagrams can be obtained at high speed. The method has been applied to statistically characterize features such as grain size and distribution on LBL polymer films. To do this, the polymer surface is tracked from its highest z values downward to its bottom, and each new connected component is detected as a Voronoi seed. Through determination of equivalent radii of the Voronoi cells, the probability density function characterizing the distribution of the domain radius can then be obtained.     

Non‐Heme Imine‐Based Iron Complexes as Catalysts for Oxidative Processes

Non‐heme iron complexes are emerging as powerful and versatile catalysts in several oxidative transformations. The most investigated iron complex structures are based on aminopyridine ligands, but a number of imine‐based ligands have been also tested. In this review a collection of recent results obtained in oxidation catalysis with non‐heme imine‐based iron complexes is presented. Their catalytic performances in C H, CC and  S oxidation are spread over a wide range of efficiency, going from very low to quite high. Such performances are discussed, whenever possible, in light of the operating reaction mechanisms and of catalyst stability. In order to facilitate the discussion, an initial survey of the most useful mechanistic tools widely applied to distinguish a metal‐based oxidation from a radical‐chain process is also reported. Imine‐based catalysts are divided into two classes: (i) salen‐Fe complexes, and (ii) imine‐Fe complexes. In some cases clues for free‐radical oxidation mechanisms have been reported while in other cases evidence for metal‐based mechanisms has been collected. The preferred mechanistic pathway is shown to be a function of catalyst structure and features. Interestingly, some imine‐based iron complexes are able to perform stereospecific oxidation reactions, demonstrating that the imine functionality can be incorporated in ligands designed for oxidation catalysis.

     

Analysis of Operating Variables in the Catalytic Purification of Butene-1 in a Trickle Bed

A mathematical model for analyzing and simulating selective hydrogenation of butyne-1 and 1,3-bu-tadiene in industrial trickle bed reactors operated to obtain high purity butene-1 is presented in this work.Fromthe analysis of typical operating conditions,only the relevant phenomena are finally retained in the model formu-lation.The model is then employed to analyze the effect of operating variables and catalyst selectivity in processperformance.In particular,the search for the minimum hydrogen flow rate necessary to fulfil butene-1 purityspecifications is undertaken.