Thermoresponsive hydrogel with rapid response dynamics

Intelligent hydrogels, particularly poly(N-isopropylacrylamide) (PNIPAAm)-based hydrogels, have attracted extensive interest because the soft wet hydrogels can change their shapes in response to the small changes of environmental factors like temperature. In order to fully make use of this unique property of PNIPAAm-based hydrogels, the response rates of the PNIPAAm hydrogels have to be improved since the dynamics property is critical to certain applications of this material. In this paper, the thermo-sensitive PNIPAAm hydrogels were successfully synthesized by carrying out the polymerization of N-isopropylacrylamide monomer in vacuum (–100 kPa) at room temperature (22 °C). The resultant hydrogel has tremendously improved shrinking rate as well as the large volume changes upon temperature stimulation when comparing with the normal PNIPAAm hydrogel. The SEM micrographs revealed that the improved properties were attributed to the macroporous network structure generated during the synthesis under vacuum.     

混凝土的裂缝与控制

在分析混凝土开裂的危害和产生原因的基础上,提出裂缝控制新技术。采用双亲性单分子膜,减少混凝土早期的水分蒸发,抑制混凝土早期因失水太快而产生的塑性收缩裂缝;采用减水与减缩共同作用,控制混凝土壳体及薄壁结构的早期开裂;采用调控水泥水化过程中的放热量,提高大体积混凝土的温控能力;采用遥爪聚合型增韧剂,增加混凝土的韧性,减少混凝土的脆性开裂。     

Dislocation dynamics theory for fatigue crack growth

Dislocation group dynamics theory is used to deduce a power type expression for fatigue crack growth. In general, the results reflect only a single rate process, and thus one activation energy which is small compared to those in usual rate processes. The frequency dependence for this power function type fatigue crack growth rate was also obtained, and yield reasonable agreement with experimental results from the literature.     

Scaling and dynamics of an interfacial crack front

We report on in situ observations with a high speed CCD camera of an in-plane crack propagating through a transparent heterogeneous Plexiglas block. The toughness is controlled artificially and fluctuates spatially as a random noise. A stable crack in mode I was monitored by loading the system by an imposed displacement. We show that the movement of the fracture front is controlled by local instabilities triggered by the depinning of asperities even for very slow loading. Development of crack roughness is described in terms of a Family-Vicsek scaling with a roughness exponent =0.60 and a dynamical exponent =1.2. We also study the system numerically using a Green function technique. We find =0.6 and =1.5 in contrast to earlier numerical studies that reported a much smaller roughness exponent 1/3. We discuss this discrepancy.     

A method of seamlessly combining a crack tip molecular dynamics enclave with a linear elastic outer domain in simulating elastic–plastic crack advance

Molecular dynamics is applicable only to an extremely small region of simulation. In order to simulate a large region, it is necessary to combine molecular dynamics with continuum mechanics. Therefore, we propose a new model where molecular dynamics is combined with micromechanics. In this model, we apply molecular dynamics to the crack tip region and apply micromechanics to the surrounding region. Serious problems exist at the boundary between the two regions. In this study, we manage to solve these problems, and make possible the simulation of the process of crack propagation at the atomic level. In order to examine the validity of this model, we use α-iron for simulation. If the present model is valid, stress and displacement should vary continuously across the boundary between the molecular dynamics region and the micromechanics region. Our model exhibits just such behavior. This revised version was published online in August 2006 with corrections to the Cover Date.     

The control of crack arrays in thin films

Thin-film fracture can be used as a nano-fabrication technique, but generally, it is a stochastic process that results in nonuniform patterns. Crack spacings depend on the interaction between intrinsic flaw populations and the fracture mechanics of crack channeling. Geometrical features can be used to trigger cracks at specific locations to generate controlled crack patterns. However, while this basic idea is intuitive, it is not so obvious how to realize the concept in practice, nor what the limitations are. The control of crack arrays depends on the nature of the intrinsic flaw population. If there is a relatively large density of long flaws, as commonly assumed in fracture mechanics analyses, reliable crack patterns can be obtained fairly robustly using relatively blunt geometrical features to initiate cracks, provided the applied strain is carefully matched to the properties of the system and the desired crack spacing. This process is analyzed both for cracks confined to the thickness of a film and for cracks growing into a substrate. The latter analysis is complicated by the fact that increases in strain can either drive cracks deeper into the substrate or generate new cracks at shallower depths. If the intrinsic flaws are all very short, the geometrical features need to be very sharp to achieve the desired patterns. While careful control of the applied strain is not required, the strain needs to be relatively large compared to that which would be required to propagate a large flaw across the film. This results in an approach that is not robust against the introduction of accidental damage or a few large flaws.     

3-D culture of human umbilical vein endothelial cells with reversible thermosensitive hydroxybutyl chitosan hydrogel

The aim of this study was to present a non-trypsin 3D cell culture method with a reversible thermosensitive HBCS hydrogel. In this study, hydroxybutyl chitosan (HBCS) was synthesized by grafting hydroxybutyl groups on chitosan molecule chains. The prepared HBCS was water-soluble, and the reversible phase transformation temperature was 26 °C. Scanning electron microscope images illuminated the 3-D network of hydrogel formed irregular porous structure which ranged from 50–250 μm. Cell viability assay indicated that HBCS solution could promote the proliferation of human umbilical vein endothelial cells (HUVECs), and the boost of proliferation was enhanced with the increase of HBCS concentration. HBCS had no harm to the nitric oxide (NO) synthesis functionality of HUVECs. HUVECs could grow and reproduce inside the hydrogel, and showed good vitality after 14-days culture. Meanwhile, cells cultured inside the hydrogel could be passaged successively through the reversible phase transformation process of HBCS. The results revealed that HBCS have the potential to be used for 3-D cell culture without the use of trypsin.     

Transient modeling of the reversible response of the hydrogel to the change in the ionic strength of solutions

In this paper, a multi-effect-coupling ionic-strength-stimulus (MECis) model, which couples the mechanical, chemical and electrical effects on the swelling/shrinking of the smart hydrogel responsive to the ionic strength of the environmental solution, is presented for the transient analysis of the reversible kinetics of the ionic-strength-sensitive hydrogel. The MECis model is based on the laws of momentum and mass conservation, and considers the poroelasticity of the ionic-strength-sensitive hydrogel. In order to examine the model, the simulation results are compared with the experimental data with reversible swelling-shrinking kinetics, and they are in quite agreement. The reversible characteristics, including the mobile ions concentrations, the fixed charge density, the electric potential and the hydrogel displacement, are studied in order to fully understand the kinetics behavior of the ionic-strength-sensitive hydrogel.     

Molecular dynamics simulation of crack growth under cyclic loading

The mechanical behaviors around a crack tip for a system including both a crack and two tilt grain boundaries under cyclic loading are examined using a molecular dynamics simulation. Not only a phase transition but also the emission of edge dislocations is observed in order to relax stress concentration around a crack tip during the first loading. Then, a dislocation pile-up is formed near the grain boundary after the edge dislocations reach the grain boundary, because they cannot move beyond the grain boundary. During the first unloading, the edge dislocations emitted from the crack tip return to the crack tip and disappear in the system. We observe several vacancies generated around the crack tip and crack growth corresponding to an atomic scale during cyclic loading. Conclusively, we propose the fatigue crack growth mechanism for the initial phase of the fatigue fracture. That is, a fatigue crack propagates due to coalescence of the crack and the vacancies caused by the emission and absorption of dislocations.     

Scale invariance in brittle fracture and the dynamics of crack fusion

The brittle fracture of solids subject to extreme differential stress is assumed to be a consequence of the sequential fusion of smaller cracks into larger ones. Renormalization methods quantitatively describe the dynamics of the cascade of fusion events that culminates in failure, including the observed time-to-failure versus stress scaling law.

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Résumé On pose l'hypothèse que la rupture fragile des solides soumis à des contraintes différentielles est une conséquence de la fusion en série de petites fissures en de grandes.Des méthodes de renormalisation décrivant de manière quantitative la dynamique qui préside à la cascade de fusion conduisant à l'extrême à la rupture, y compris la durée à rupture par rapport à la loi de croissance de la contrainte.

     

Molecular dynamics simulation and in situ TEM study of crack healing

Abstract

In situ TEM observation of crack healing during heating was carried out in an α-Fe crystal, and the results indicated that a crack in α-Fe could completely heal when the temperature increased to a critical value. The molecular dynamics method was used to simulate crack healing during heating and/or under compressive stress in a Cu crystal. The simulation results showed that a centre crack in a Cu crystal would close under a compressive stress or by heating. The roles of compressive stress and heating in crack healing were additive. During crack healing, dislocations generation and motion occurred. If there were pre-existing dislocations around the crack, the critical temperature or compressive stress necessary for crack healing would decrease, and the higher the number of dislocations, the lower the critical temperature or compressive stress. The critical temperature necessary for a crack to heal depended upon the orientation of the crack plane.     

单晶硅预制初裂纹扩展行为的分子动力学研究

采用基于Stillinger-Weber作用势的分子动力学方法研究了单晶硅预制初裂纹前缘方向分别为[100]、[110]、[111]晶向的裂纹扩展行为.模拟结果表明:低温时裂纹尖端形成新的环状结构,温度逐渐升高后,出现母-子裂纹传播机制,裂纹前缘方向为[111]晶向的初裂纹扩展呈现出明显的取向效应.     

Workload control dynamics in practice

Workload control (WLC) is a well established production planning and control concept for make-to-order companies. The main insights into WLC are being gained from stationary simulation studies. Knowledge on its functioning in practice is limited, especially in a dynamic setting. The aim of this paper is to identify and classify the key issues when WLC is being used to control logistical performance in a dynamic setting. For this purpose an empirical research project in a company is carried out, in which both quantitative scanning data and qualitative workshop data are obtained and analysed over one year. Throughput diagrams and order progress diagrams support the analysis of the quantitative data obtained. Various types of dynamics could be discerned, but all implying similar requirements in terms of adaptations to the control concept. Whereas research in stationary settings addresses parameter choices, this study shows that it is issues on timing and magnitude of control adjustments that need to be addressed if WLC is to be used effectively in a dynamic setting. The empirical study shows that if the dynamics are not adequately taken into account, buffers may starve while throughput times become uncontrolled over time, despite the use of WLC.     

Dynamic and reversible control of 2D membrane protein concentration in a droplet interface bilayer

We form an artificial lipid bilayer between a nanolitre aqueous droplet and a supporting hydrogel immersed in an oil/lipid solution. Manipulation of the axial position of the droplet relative to the hydrogel controls the size of the bilayer formed at the interface; this enables the surface density of integral membrane proteins to be controlled. We are able to modulate the surface density of the β-barrel pore-forming toxin α-hemolysin over a range of 4 orders of magnitude within a time frame of a few seconds. The concentration changes are fully reversible. Membrane protein function and diffusion are unaltered, as measured by single molecule microscopy and single channel electrical recording.     

Kinetic theory approach to fatigue crack propagation in terms of dislocation dynamics

International Journal of Fracture -     

Solvent additive control of morphology and crystallization in semiconducting polymer blends

4-bromoanisole is used as a very versatile processing additive to control the phase separation and phase purity of organic photovoltaic devices. Polymer-polymer blends based on P3HT as donor and a wide range of acceptor materials (F8TBT, PCDTBT,…?) are investigated. The additive promotes the aggregation of P3HT which improves the morphology for both initially mixed and demixed blends.