A continuum description for cholesteric and nematic twist-bend phases based on symmetry considerations

ABSTRACT

The recently discovered twist-bend nematic phase, N_tb, is a non-uniform equilibrium nematic phase that presents a spontaneous bend with a precession of the nematic director, n, on a conical helix with a tilt angle θ and helical pitch P. The stability of the N_tb phase has been recently demonstrated from the elastic point of view by extending the Frank elastic energy density of the nematic phase to include the symmetry element of the helical axis, t. In the present article, we investigate the influence of an external bulk field (magnetic or electric) on the N_tb phase. Using symmetry arguments we derive the expression for the flexoelectric polarisation in twist-bend nematic phases. We show that, besides the standard contribution related to the spatial variation of the nematic director, two new contributions connected with the existence of the helical axis appear. In the ground state, where the nematic deformation is a pure heliconical deformation, the new contribution vanishes identically, and the total flexoelectric polarisation is perpendicular to the nematic director. Furthermore, as an example, we study the role of an external magnetic field applied parallel to the helical axis for a material with positive magnetic susceptibility anisotropy. We show that the field modifies the range of values of the coupling parameter between the director and the helical axis, thus shifting the interval of values for which this coupling results in the N_tb phase.     

Stress–Strain Dependence of Segmented Polyurethanes and Polyurethane Ureas

The stress–strain dependences of segmented elastomers based on polyetherurethanes, polyesterurethane ureas, and polybutadieneurethane ureas were investigated. These dependences were adequately described using the generalized approach proposed earlier by the authors for amorphous elastomers. The experimental data obtained for four series of samples with chemical cross-linking confirmed an applicability of this approach for segmented elastomers. Unlike amorphous elastomers, the effective network density value was affected not only by the density of the chemical network, but the contribution of the network formed by the hard domains also needs be taken into account.     

CONFORMATIONS OF 4,5,6-TRIPHENYL-TETRAHYDRO-1,3-THIAZINE-2-THIONES AND THEIR N-ALKYL DERIVATIVES. UNUSUAL THIAZINETHIONE OR AZETIDINE FORMATION UPON REACTION OF 3-AMINO- AND 3-METHYLAMINO-1,2,3-TRIPHENYLPROPYL CHLORIDES WITH POTASSIUM ETHYLXANTHATE

Abstract

Reaction of 3-amino- or 3-methylamino-1,2,3-triphenylpropyl chlorides (6 or 7) with potassium ethylxanthate leads to 4.5.6-triphenyl-tetrahydro-,3-thiazine-2-thione 1 or 1-methyl-2,3,4-triphenylazetidine 8 depending on the N-substitution. Conformational distribution of all possible diastereoisomeric thiazinethiones 1 and their N-alkyl derivatives 2–5 is determined by means of ¹H NMR spectroscopy. Allylic strain caused by N- substituents in the trans,cis-isomers strongly shifts the equilibrium between conformations with a,a,e or e,e,a phenyl groups towards the conformer with an axial neighbouring Ph-4 group. Vicinal couplings data for the diastereoisomeric azetidine 8 show different ring geometry depending on the configuration.     

A Tensile Strain Hardening Test Indicator of Environmental Stress Cracking Resistance

Environmental stress cracking resistance (ESCR) is an important indicator of performance for high density polyethylene (HDPE) in structural and polymer pipe applications. The commonly used test for determining ESCR of HDPE can be time consuming and rather imprecise. A tensile strain hardening test was recently proposed to offer a faster way to characterize ESCR of polyethylene. In this paper, a practical approach is adopted whereby the test is extended to room temperature and shown to relate reliably to the ESCR of HDPE. Several HDPE resins (including pipe‐grade resins) are analyzed at strain rates of 0.5 mm/min and 7 mm/min to compare the effect of strain rate. Comparisons between the conventional ESCR test method and the strain hardening test show that strain hardening can be used to rank ESCR of HDPE in a reliable fashion. In our study the more direct measure of “hardening stiffness” is used to compare resins instead of strain hardening modulus. Because no true stress‐strain measurement is needed, this is a much simpler test method than other methods previously suggested. In addition, the use of the natural drawing ratio (NDR) as ESCR ranking indicator is examined. Results show that NDR can also be employed as a strain rate‐independent indicator of ESCR of HDPE. The test proposed herein is practical, simple and precise, and hence a more reliable indicator of ESCR performance of HDPE.     

Rheological Properties of Model and Crude Oil Systems when Wax Precipitate under Quiescent and Flowing Conditions

Wax precipitation and deposition is a recurring challenge in transportation of crude oil, and increased knowledge about the behavior of such systems is necessary. Microscopy and differential scanning calorimetry were used to follow the crystallisation of wax for two model systems. The amount of solid was also determined by the latter method as well. The flow and viscoelastic behavior were investigated around the wax precipitation temperature, and the yield stress was determined both after dynamic and static cooling. Interpretation of the results was carried out in view of crystal growth and microstructure of the wax crystals. The variables that were studied were wax composition, amount of wax and thermal and shear history.     

TiS2 and ZrS2 single‐ and double‐wall nanotubes: First‐principles study

Hybrid density functional theory has been applied for investigations of the electronic and atomic structure of bulk phases, nanolayers, and nanotubes based on titanium and zirconium disulfides. Calculations have been performed on the basis of the localized atomic functions by means of the CRYSTAL‐2009 computer code. The full optimization of all atomic positions in the regarded systems has been made to study the atomic relaxation and to determine the most favorable structures. The different layered and isotropic bulk phases have been considered as the possible precursors of the nanotubes. Calculations on single‐walled TiS₂ and ZrS₂ nanotubes confirmed that the nanotubes obtained by rolling up the hexagonal crystalline layers with octahedral 1T morphology are the most stable. The strain energy of TiS₂ and ZrS₂ nanotubes is small, does not depend on the tube chirality, and approximately obeys to D^–2 law (D is nanotube diameter) of the classical elasticity theory. It is greater than the strain energy of the similar TiO₂ and ZrO₂ nanotubes; however, the formation energy of the disulfide nanotubes is considerably less than the formation energy of the dioxide nanotubes. The distance and interaction energy between the single‐wall components of the double‐wall nanotubes is proved to be close to the distance and interaction energy between layers in the layered crystals. Analysis of the relaxed nanotube shape using radial coordinate of the metal atoms demonstrates a small but noticeable deviation from completely cylindrical cross‐section of the external walls in the armchair‐like double‐wall nanotubes. © 2013 Wiley Periodicals, Inc.     

Spatially‐resolved and polarized Raman scattering from a single Si nanowire

We report spatially‐resolved and polarized Raman scattering results from a single Si nanowire (NW). Transmission electron microscope images show that the surface morphology of the Si NW varies from smooth to rough along the long axis. As the NW grows, the smooth surface becomes rough because of Au diffusion to the surface, resulting in the formation of facets and stacking faults. Spatially‐resolved Raman spectra along the NW long axis reveal variations in tensile strain related to the morphological changes in NW surface. The tensile strain in the top segment of the NW with a smooth surface is greater than that in the bottom segment with a rough surface. Despite the formation of facets and stacking faults, polarized Raman scattering results both from the top and bottom segments of the NW are consistent with the Raman polarization selection rules expected for a cubic crystal. Copyright © 2015 John Wiley & Sons, Ltd.     

Rational Tuning of the Reactivity of Three-Membered Heterocycle Ring Openings via SN2 Reactions

The development of small-molecule covalent inhibitors and probes continuously pushes the rapidly evolving field of chemical biology forward. A key element in these molecular tool compounds is the “electrophilic trap” that allows a covalent linkage with the target enzyme. The reactivity of this entity needs to be well balanced to effectively trap the desired enzyme, while not being attacked by off-target nucleophiles. Here we investigate the intrinsic reactivity of substrates containing a class of widely used electrophilic traps, the three-membered heterocycles with a nitrogen (aziridine), phosphorus (phosphirane), oxygen (epoxide) or sulfur atom (thiirane) as heteroatom. Using quantum chemical approaches, we studied the conformational flexibility and nucleophilic ring opening of a series of model substrates, in which these electrophilic traps are mounted on a cyclohexene scaffold (C₆H₁₀Y with Y=NH, PH, O, S). It was revealed that the activation energy of the ring opening does not necessarily follow the trend that is expected from C−Y leaving-group bond strength, but steeply decreases from Y=NH, to PH, to O, to S. We illustrate that the HOMO_Nu–LUMO_Substrate interaction is an all-important factor for the observed reactivity. In addition, we show that the activation energy of aziridines and phosphiranes can be tuned far below that of the corresponding epoxides and thiiranes by the addition of proper electron-withdrawing ring substituents. Our results provide mechanistic insights to rationally tune the reactivity of this class of popular electrophilic traps and can guide the experimental design of covalent inhibitors and probes for enzymatic activity.     

Flexible strain sensor with ridge-like microstructures for wearable applications

The flexible stretchable sensors have great potential for implementation in various applications, such as intelligent soft robots, health monitoring, and motion detection. However, most of the flexible stretchable sensors with microstructures and high sensitivities are fabricated by expensive templates and complex processes. In consideration of large-scale fabrication, a low cost and efficient way is in great demand. Herein, electroless plating on Nafion films with decent swelling ratios are proposed to fabricate stretchable sensors with wrinkle-structured electrodes. By adding isopropanol (IPA) to the electroless plating process, the H₂O-IPA sensor with larger swelling ratio shows deeper surface wrinkles, higher surface roughness, and better sensitivity to strain. At the same time, the H₂O-IPA sensor exhibit good durability (500 cycles). By mounting the sensor on the joint of human finger, the motion of the finger bending and even the bending degree can be accurately detected, indicating the potential use in the field of wearable devices and soft electronic skins.     

Switch From Pauli-Lowering to LUMO-Lowering Catalysis in Brønsted Acid-Catalyzed Aza-Diels-Alder Reactions

Brønsted acid-catalyzed inverse-electron demand (IED) aza-Diels-Alder reactions between 2-aza-dienes and ethylene were studied using quantum chemical calculations. The computed activation energy systematically decreases as the basic sites of the diene progressively become protonated. Our activation strain and Kohn-Sham molecular orbital analyses traced the origin of this enhanced reactivity to i) “Pauli-lowering catalysis” for mono-protonated 2-aza-dienes due to the induction of an asynchronous, but still concerted, reaction pathway that reduces the Pauli repulsion between the reactants; and ii) “LUMO-lowering catalysis” for multi-protonated 2-aza-dienes due to their highly stabilized LUMO(s) and more concerted synchronous reaction path that facilitates more efficient orbital overlaps in IED interactions. In all, we illustrate how the novel concept of “Pauli-lowering catalysis” can be overruled by the traditional concept of “LUMO-lowering catalysis” when the degree of LUMO stabilization is extreme as in the case of multi-protonated 2-aza-dienes.