Kinetics Of Rearrangement Research Articles

Thermadapt shape memory AB-copolymer networks exhibiting tunable properties and kinetics of topological rearrangement

Polycaprolactone (PCL) based thermadapt shape memory polymers (SMPs) have exhibited superiority over traditional analogues by allowing shape-editing to reconfigure permanent shapes into more complex geometrical shapes, driven by the pursuit of advanced functionalities for high-value applications. Nevertheless, although transesterification-based PCL networks exhibit impressive shape reconfigurability as prototypical thermadapt SMPs, there are still constraints in terms of the need for significant flexibility in architectural adjustment and property modulation without compromising reconfigurability, which would be relevant for practical applications. Here, we present a simple yet efficient strategy to create PCL-based thermadapt SMPs that possess a highly tunable structure and properties, as well as exceptional reconfigurability. The family of SMPs, called thermadapt shape memory AB copolymer networks (AB-CPNs), was synthesized by UV-initiated free radical polymerization between PCL diacrylate as crosslinker and 4-hydroxybutyl acrylate (HBA) as comonomer. The thermadapt AB-CPNs allow for significant structural alterations with 0 wt% to 70 wt% HBA while maintaining excellent shape memory and shape reconfigurability effects. The insertion of the polymer segment containing free hydroxyls not only promises tunable material properties but also makes network rearrangement easier since dynamic hydroxy-ester bonds are more active than dynamic ester-ester bonds. It is envisioned that the thermadapt shape memory AB-CPNs with widely tunable macroscopic properties will drive progress in the real-world applications of SMP devices with complicated geometric structures.

Integration atoms in octa- and tetrahedrical internodes of BCC crystals with a free surface

The physical properties of crystals are determined by their chemical nature, the structure of both the entire volume and the surface layer. The surface of a metal, ionic or covalent crystal and semiconductor can be considered as a special state of matter with its own chemistry and physics. The interest in the problem of solid state surfaces, in the science of surface, in the physics of thin films is stimulated by advances in solid state physics, technical innovations, opportunities to create new film materials with unique properties, the challenges of modern technology, the requirements of technology to create devices for computing technology and microelectronics. In this work the processes of redistribution of infiltration atoms along the surface and bulk inter-nodes are considered, the equilibrium distribution of infiltrated atoms on the surface and in the crystal volume, as well as the effect of external pressure on the surface and bulk distribution of infiltration atoms, are studied. Knowledge of the processes of redistribution of infiltration atoms along the inter-nodules of a crystal and of the reasons causing them may allow the purposeful formation of these processes for the purpose of obtaining certain physical characteristics of materials. The kinetics of inter-nodal rearrangement of infiltration atoms has been studied for metals and ordering alloys of various structures.

Thermo-Viscoelastic Response of Protein-Based Hydrogels.

Because of the bioactivity and biocompatibility of protein-based gels and the reversible nature of bonds between associating coiled coils, these materials demonstrate a wide spectrum of potential applications in targeted drug delivery, tissue engineering, and regenerative medicine. The kinetics of rearrangement (association and dissociation) of the physical bonds between chains has been traditionally studied in shear relaxation tests and small-amplitude oscillatory tests. A characteristic feature of recombinant protein gels is that chains in the polymer network are connected by temporary bonds between the coiled coil complexes and permanent cross-links between functional groups of amino acids. A simple model is developed for the linear viscoelastic behavior of protein-based gels. Its advantage is that, on the one hand, the model only involves five material parameters with transparent physical meaning and, on the other, it correctly reproduces experimental data in shear relaxation and oscillatory tests. The model is applied to study the effects of temperature, the concentration of proteins, and their structure on the viscoelastic response of hydrogels.

Influence of Neutron Irradiation on the Characteristics of Phase Transitions in Multifunctional Materials with a Perovskite Structure (A Review)

Exposure to neutron radiation is an efficient way to enter dosed amounts of defects into the crystal and real structures of advanced inorganic materials. Thus-gained experimental data are of importance for both fundamental radiation materials science and for finding ways of usefully modifying the effect of radiation on the properties of practically important multifunctional materials with the perovskite structure. The review considers the specificity of the effect of exposure to neutron radiation on the parameters of crystal and real structures and on the multifunctional (ferroelectric, superconducting, nonlinear optical, magnetic, and other) properties of crystals of the perovskite family that experience various structural phase transitions. The review dwells on the effect of radiation defects on variations of the local and macroscopic properties of crystals, reflected in the anomalies of these properties in phase-transition regions. Ferroelectric perovskite crystals are shown to be particularly susceptible to defects. The main types of radiation-induced structural states are considered, including their formation mechanisms and patterns. Correlation relations between the structural alteration upon radiation exposure and the evolution of phase-transformation temperatures are considered. It is shown that variable irradiation parameters (dose, radiation annealing, impurity content, and concentration of a certain type of initial defects) offer a means to control the kinetics of radiation rearrangement and, ultimately, the radiation resistance of the structure. In addition, taking these factors into account, one can provide conditions for the generation of almost any metastable structural state, close to those formed in intact (non-irradiated) crystals at the pre-transition stage, that would be stable at room temperature. The study of effects of neutron irradiation by precision X-ray structural analysis shows that the structural states that occur under radiation exposure are stable over a wide temperature range, including room temperature, and the stability of metastable states, as a rule, increases with the radiation fluence. These radiation-induced states are actually new structural states, intermediate between low-temperature and high-temperature polymorphs of perovskite crystals. The analysis of literature data implies that the radiation-induced structural rearrangement, amorphization, and decomposition of the initial compound are in many cases competing processes, the activity of which depends on the neutron irradiation fluence and temperature. A decrease in exposure temperature often slows down the decomposition and suppresses the attendant deformation of the crystal, which is essential for the implementation and identification of structural alterations in irradiated crystals.

Acid-catalyzed rearrangement of tetrahydrotricyclopentadiene for synthesis of high density alkyl-diamondoid fuel

Alkyl-diamondoid fuels are attractive for aerospace vehicles due to their high energy density and supreme low-temperature performance. Here we reported a facile route to synthesize alkyl-diamondoids via solvent-free rearrangement of tetrahydrotricyclopentadiene simply catalyzed by acid. The reaction conditions including catalysts, temperature, catalyst dosage, and solvent were optimized. Notably, under the optimal conditions (10.7 wt% cat., 180 °C, 20 h) an extremely high content of alkyl-diamondoids (85.6%) was found in liquid phase, with high carbon yield of alkyl-diamondoids (60.0%) was obtained. The reaction kinetics of rearrangement has been investigated, and the reaction rate constants and apparent activation energies were calculated based on the experimental kinetic data. The resultant diamondoid-based product shows excellent low-temperature performance and a higher energy content than reported diamondoids and widely used JP-10 fuel, thus is superior as a high-energy-density fuel.

Kinetics and mechanistic study of the Bamberger rearrangement of N-phenylhydroxylamine to 4-aminophenol in acetonitrile-trifluoroacetic acid: A substrate acid complex as para selectivity driver

The paper deals with the kinetics of Bamberger rearrangement of N-phenylhydroxylamine to 4-aminophenol in acetonitrile as a solvent catalyzed by TFA. The influence of the water is also investigated being the rearranging moiety in the mechanism. It is evident that the presence of water depresses the rate of the rearrangement suggesting the rate-determining step does not involve water as the key reagent. In acetonitrile, at lower temperature than that of reaction, we evidence the formation of a PHA-TFA complex, and its equilibrium has been measured between 288K and 298K. We observe also the addition of water destroys this complex but the ternary equilibrium PHA, water and TFA cannot easily measured because of the complex solvent effect on the Uv–vis and NMR signals. Starting from this evidence, we have proposed a kinetic model, which takes into account all the equilibria, and the results of the fitting gives reliable thermodynamic and kinetic parameters of the entire process. Finally, we have suggested that the PHA-TFA complex is the key intermediate, which determines the regioselectivity of the rearrangement. In fact, preliminary quantum chemistry calculation have shown that the TFA interaction with the NHOH group causes the hindering of the ortho position, thus favoring the attack of water to the para one.

Controlling crystallization process and thermal stability of a binary Cu–Zr bulk metallic glass via minor element addition

In this paper, the effect of minor element addition on the initial structural evolution during crystallization in a simple binary Cu–Zr bulk metallic glass (BMG) forming liquid has been investigated by using differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Despite no changes in the completely crystallized products, the remarkable opposite impacts on the supercooled liquid region (SLR) and crystallization reaction rate constant [Formula: see text] are observed as a result of minor selective additions of an affine element, i.e., Sn and an immiscible element, i.e., Nb into the Cu–Zr BMG alloy, respectively. Furthermore, it is demonstrated that the primary devitrification pathway and crystalline phases are simultaneously modified, which leads to significant changes in kinetics of atomic rearrangement and thus thermal stability of this material. Such a finding offers a promising way to control the type of primary crystalline phases of BMG-forming metallic supercooled liquids to synthesize novel BMGs or BMG matrix composites for structural or functional applications.

Predicting the Agitation-Induced Aggregation of Monoclonal Antibodies Using Surface Tensiometry.

Adsorption of antibody therapeutics to air-liquid interfaces can enhance aggregation, particularly when the solution does not contain protective surfactant or when the surfactant is diluted as occurs during preparation of intravenous infusion bags. The ability to predict an antibody's propensity for interfacially mediated aggregation is particularly useful during product development to ensure the quality, potency, and safety of the therapeutic. To develop a predictive tool, we investigated the surface pressure and surface excess of a panel of 16 antibodies as well as determined their aggregation propensity at the air-liquid interface in an agitation stress model. Our data demonstrated that the initial rate of surface pressure increase upon antibody adsorption to the air-liquid interface strongly predicted the extent of agitation-induced aggregation. Other factors, including the hydrophobicity, equilibrium surface pressure, and interfacial concentration of an antibody, were not adequate predictors of its susceptibility to aggregation. In addition to developing a predictive tool, we extended the interfacial characterization to better understand the mechanisms of antibody aggregation at an air-liquid interface during agitation stress. We believe that the kinetics of antibody rearrangement and conformational change after adsorbing to the interface, leading to the development of attractive antibody-antibody interactions, dictated the extent of aggregation. Overall, our results demonstrate how surface pressure measurements can be implemented as a rapid screening tool for the identification of antibodies with a high propensity to aggregate upon adsorption to an air-liquid interface while also furthering our understanding of interfacially mediated protein aggregation.

A nanoscale-modified LaMer model for particle synthesis from inorganic tin–platinum complexes

The size-tunable structure and properties of Pt nanoparticles at the atomic length scale have attracted significant attention across a wide variety of fields including magnetics, electrocatalysis, optics, and gas-phase synthesis. Mechanisms responsible for the formation Pt nanoparticles remain unclear because of the difficulty generating in situ data for the time-evolution of size, shape, distribution, volume fraction, particle number density, and oxidation state from the starting complexes. We here demonstrate the use of simultaneous small- and wide-angle X-ray scattering combined with UV-vis spectroscopy to measure these key synthesis metrics for the reduction of Pt(IV) by Sn(II) in aqueous solution. This synthesis approach has been previously shown to permit continuous control over Pt nanoparticle size from 0.9 to 2.6 nm to within 10% standard deviation. Such fine control led to the discovery of densely packed amorphous structures at ca. 1.7 nm with substantially enhanced electrocatalytic oxygen reduction relative to nanocrystals and commercial electrocatalysts. Ex situ UV-vis and in situ X-ray scattering are here shown to reveal four distinct stages during synthesis: (1) autoreduction of a ligand/noble metal complex with a unique structure that depends on the Sn(II)/Pt(II) ratio, (2) generation of Pt primary particles and the formation of Pt nuclei at a rate that depends on the structure of the initial complex, (3) nanoparticle growth via LaMer's diffusion of these primary particles to the nuclei, and (4) growth termination due to capping from a stabilizing, two-layer ligand shell. We derive a set of consecutive rate equations and associated kinetic parameters that describe each step. The kinetics of ligand rearrangement has been previously found to limit the rate of nanoparticle growth. We incorporate this phenomenon into LaMer's classic diffusion-limited growth scheme to extend it to the nanoscale regime. This new model provides detailed understanding of how metal ligands serve as both reducing and stabilizing agents and allow for unprecedented, continuous control over both size and distribution. Systematic variation of temperature permits detailed time resolution at the very onset of Pt primary particle formation, as well as a means to determine temperature sensitivity of nanoparticle growth.

Supersaturated α-iron in vapour-deposited Fe–C thin films

While the conventional Fe–C phase diagram predicts negligible solubility of carbon in α-Fe at room temperature, there are many examples where substantial carbon supersaturations have been reported. Non-equilibrium processing routes, such as physical vapour deposition (PVD), appear to be particularly well suited to generating very high levels of supersaturation. Few descriptions of the stability and microstructure of the supersaturated state exist, however. In this study, experiments have been performed using two PVD techniques allowing for the production of nanocrystalline films containing between 11 and 16 at% C. Transmission electron microscopy and electron energy loss spectroscopy reveal no phase other than a nearly body-centred cubic iron in the as-deposited films. This result is discussed with respect to the kinetics of carbon rearrangement and the possibility of the stabilization of the supersaturated state via defects.

Cytotoxic 1,2-Dialkynylimidazole-Based Aza-Enediynes: Aza-Bergman Rearrangement Rates Do Not Predict Cytotoxicity

A new class of potential antitumor agents inspired by the enediyne antitumor antibiotics has been synthesized: the 1,2-dialkynylimidazoles. The aza-Bergman rearrangement of these 1,2-dialkynylimidazoles has been investigated theoretically at the B3LYP/6-31G(d,p) level and experimentally by measuring the kinetics of rearrangement in 1,4-cyclohexadiene. There is a good correlation between the theoretical and experimental results; subtle substituent effects on the initial aza-Bergman cyclization barrier predicted by theory are confirmed by experiment. Yet, despite the ability of these 1,2-dialkynylimidazoles to undergo Bergman rearrangement to diradical/carbene intermediates under relatively mild conditions, there is no correlation between the rate of Bergman cyclization and cytotoxicity to A459 cells. In addition, cytotoxic 1,2-dialkynylimidazoles do not cause nicking of supercoiled plasmid DNA or cleavage of bovine serum albumin. An alternative mechanism for cytotoxicity involving the unexpected selective thiol addition to the N-ethynyl group of certain 1,2-dialkynylimidazoles is proposed.

1H NMR‐based kinetic and mechanistic study of unusual skeletal rearrangements of a spirobornyl tosylate derivative

Abstract1H NMR analysis of the kinetics of skeletal rearrangement of optically pure 3,3‐xylyl‐2‐exo‐bornyl tosylate in CDCl3 indicates the operation of tandem autocatalytic and pseudo‐first‐order transformations, leading sequentially to a pair of isomeric camphene derivatives and involving partial racemization. Changing the solvent system has been shown to permit the chemoselective isolation of either of the isomeric camphenes. Copyright © 2010 John Wiley & Sons, Ltd.

Thermally Induced Changes in Amphiphilicity Drive Reversible Restructuring of Assemblies of ABC Triblock Copolymers with Statistical Polyether Blocks

ABC triblock copolymers in which a block with stimulus-dependent solvophilicity resides between solvophilic and solvophobic end blocks can undergo reversible transitions between different thermodynamically stable assemblies in the presence or absence of stimulus. As a new example of such a copolymer system, thermoresponsive poly(ethylene oxide)-b-poly(ethylene oxide-stat-butylene oxide)-b-poly(isoprene) (E-BE-I) triblock copolymers with narrow molecular weight distributions (M(w)/M(n): 1.05-1.18) were prepared by sequential living anionic and nitroxide-mediated radical polymerizations. The specific copolymers examined (9.0 ≤ M(n) ≤ 14.4 kg/mol, 14% ≤ wt % isoprene ≤35%) form near-spherical aggregates with narrow size distributions at 25 °C. The thermoresponsive behavior of these polymers was studied by applying cloud point, DLS, and TEM measurements to a representative polymer, E(2.3)BE(5.3)I(2.3). The transformation of polymer aggregates from spherical micelles to vesicles (polymersomes) at elevated temperatures was detected by DLS and TEM studies, both with and without cross-linking of polymer assemblies. The rate of transformation with E-BE-I systems is more rapid than that observed for poly(ethylene oxide)-b-poly(N-isopropylacrylamide)-b-poly(isoprene) assemblies, suggesting that interchain hydrogen bonding of responsive blocks after dehydration plays an important role in the kinetics of aggregate rearrangement.

Oscillatory Molecular Driving Force for Protein Folding at High Concentration: A Molecular Simulation

This paper presents a Langevin dynamics simulation that suggests a novel way to fold protein at high concentration, a fundamental issue in neurodegenerative diseases in vivo and the production of recombinant proteins in vitro. The simulation indicates that the folding of a coarse-grained beta-barrel protein at high concentration follows the "collapse-rearrangement" mechanism but it yields products of various forms, including single proteins in the native, misfolded, and uncollapsed forms and protein aggregates. Misfolded and uncollapased proteins are the "nucleus" of the aggregates that also encapsulate some correctly folded proteins (native proteins). An optimum hydrophobic interaction strength (epsilon*(p)) between the hydrophobic beads of the model protein, which results from a compromise between the kinetics of collapse and rearrangement, is identified for use in increasing the rate of folding over aggregating. Increased protein concentration hinders the structural transitions in both collapse and rearrangement and thus favors aggregation. A new method for protein folding at high concentration is proposed, which uses an oscillatory molecular driving force (epsilon*(p)) to promote the dissociation of aggregates in the low epsilon*(p) regime while promoting folding at a high epsilon*(p). The advantage of this method in enhancing protein folding while depressing aggregation is illustrated by a comparison with the methods based on direct dilution or applying a denaturant gradient.

Dynamic characterization of carboxymethyl cellulosic nonwoven material in the environmental scanning electron microscope

Carboxymethyl cellulosic fibers have been increasingly used in health care, agriculture and biomedical areas. The fundamental understanding of the material under different conditions is of importance in these applications. The use of environmental scanning electron microscopy for dynamic characterization of these nonwoven materials under different conditions has been explored in this study. Dynamic tensile testing under different humidity conditions was performed in a Philips XL 30 FEG-ESEM. The relative humidity in the microscope chamber was adjusted from 10% up to 100% by controlling the specimen temperature and the chamber pressure. The tensile testing was carried out on a stage that was placed in the microscope chamber. The studies under dynamic conditions have given new insight into the kinetics of structure formation, rearrangement and breakdown that are important for the processing and product development of these fiber materials.

Morphology and mechanical and dynamic mechanical properties of linear and star poly(styrene-b-isobutylene-b-styrene) block copolymers

The morphologies of solution cast linear and star poly(styrene-b-isobutylene-b-styrene) (SIBS) block copolymers, of ∼0.3–0.33 polystyrene (PS) volume fraction, were investigated using transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) methods. The linear (L) and 4-arm star materials have hexagonal packed cylinder morphologies. TEM indicates more continuous domains for the 3-arm SIBS and SAXS indicates lamellar morphology. The diameters of the PS cylinders are 25–30nm in all cases, but inter-domain distances increase in the order: L→3-arm star →4-arm star SIBS. Dynamic mechanical analysis shows a decrease of plateau modulus in the order L→3-arm →4-arm star. Tg for the PIB phase is independent of star-branching. Tg increases for the PS block phase from L- to 3-SIBS although Tg(PS) for 4-SIBS is not significantly greater than that of L-SIBS. A high temperature shoulder on the PIB block glass transition peak is associated with sub-Rouse motions. Tensile modulus is essentially the same for L- and 3-SIBS but considerably greater for 4-SIBS. Stress levels and energy-to rupture increase in the order: L-SIBS <3-SIBS <4-SIBS and this behavior is rationalized in terms of the covalent junction points, entanglements, and hard block domain reinforcement. The results of cyclic deformation studies of hysteresis vs. maximum percent strain per cycle are interpreted in terms of irreversible morphological rearrangements, chain slippage through entanglements, and the influence of degree of star branching on motional constraints. There is a permanent set that increases while the stress required to extend the sample to a given strain decreases with each cycle. For cycles of lower maximum strain, and before PS block domains are disrupted, microscopic deformation is viewed as somewhat reversible, accompanied by a measure of energy dissipated as a consequence of slow relaxation kinetics of chain rearrangement vs. rate of bulk deformation. At high strain, a major fraction of the energy loss is attributed to irreversible, longer ranged disruption of the PS domains leading to permanent morphological change and permanent set. Percent hysteresis for all three cases decreases—then increases—with increasing maximum percent strain per cycle. The initial decrease is thought of in terms of a progressive decrease in the entropy of stretched chains with consecutive extensions where the chains are increasingly less able to contribute to hysteresis until the PS domains become disrupted on a large scale, after which the curves begin to rise.

Rearrangement of 2-methyl-2,3-epoxypentane: a kinetic study

The kinetics of the rearrangement of 2-methyl-2,3-epoxypentane over ZnCl2/pumice stone is reported. 2-Methyl-3-pentanone and 2,2-dimethylbutanal are formed in parallel reactions. The reaction scheme and kinetic equations best fitting the experimental data are given.

Dynamic Characterisation of Industrial Textiles Using an Environmental Scanning Electron Microscope

The fundamental understanding of individual fibers and fiber assemblies has driven innovations in industrial textiles. The use of new tools is of importance in this innovation process. The use of Environmental Scanning Electron Microscope (ESEM) for characterization of industrial textiles under dynamic conditions has been explored in this study. We have developed different techniques for examining and imaging industrial textiles under varying conditions. The dynamic experiments of wetting, absorption, loading deformation, and thermal bonding of industrial textiles were performed in the Philips XL 30 FEG-ESEM. ESEM provides new approaches to dynamic studies of industrial textiles. The relative humidity in the ESEM chamber was raised up to 100% by controlling the specimen's temperature and the chamber pressure to produce the condition for dynamic wetting and absorption tests. The tensile testing was carried out on a tensile stage that was placed in the ESEM chamber. The thermal bonding process was imaged on a hot stage in the ESEM. Studies under dynamic conditions have given new insight into the kinetics of structure formation, rearrangement, and breakdown that are important for processing and product development of industrial textiles.

Kinetics of atomic rearrangement in the processes of crystal growth into undercooled melts

Dendrite crystal growth is one of the examples of crystal growth in undercooled liquids. In spite of a number of attempts to improve the widely quoted model of dendritic crystal growth from undercooled melts (Acta Metall. 35 (1987) 957) the agreement between the theory and the experiment, especially at high undercooling, is still poor. In this study, we show that the temperature of crystal/melt interface of a growing crystal into a deeply undercooled liquid can differ considerably from the melting temperature, thus making the kinetic attachment coefficient to depend on the undercooling temperature, a fact ignored by previous models. For deeply undercooled liquids we propose a new fluctuation model of atomic mobility, viscosity and diffusion, and on its basis a phenomenological model for the temperature dependence of the kinetic coefficient is developed. Analysis of the model and the experimental data shows that the slowed attachment kinetics in deeply undercooled liquids considerably decreases the velocity of crystal growth, and can even lead to glass transition.

The kinetics of V–J joining throughout 3.5 megabases of the mouse Igκ locus fit a constrained diffusion model of nuclear organization

To gain insight into the nuclear organization of the mouse Igκ locus and how it may relate to the formation of synapses during recombination, we have studied the kinetics of rearrangement of different Vκ gene families to Jκ gene segments in the pre-B cell line, 103bcl2. Remarkably, Vκ gene families separated by more than 3.5 Mb from Jκ gene segments rearranged with nearly identical kinetics to those as close as 18 kb to Jκ gene segments. These results fit a model of nuclear organization in which the entire VκJκ region resides within a single nuclear subcompartment and is capable of exhibiting multiple reversible contacts through diffusion and Brownian motion.