Defects in smectic C* liquid crystals

We consider the structure and properties of various topological defects that can occur in smectic C* liquid crystals. The polarization field associated with disclinations, the effect of incommensuration on the structure of dispirations, some interesting situations in the interaction between dispiration and disclination and between dispirations themselves have been discussed in detail. The properties of cholesteric type disclinations and a possible model for the core structure of a wedge disclination have also been dealt with. The author felicitates Prof. D S Kothari on his eightieth birthday and dedicates this paper to him on this occasion.     

Observation of edge dislocations in helical smectic liquid crystals

We report the observation of elementary edge dislocations in smectic liquid crystals possessing helical structure. The dislocations were observed in the entire temperature range of helical phases, including ferroelectric, ferrielectric and antiferroelectric phases. The mechanism for visualizing the dislocations is based on the phenomenon of selective reflection of circularly polarised light. The performed observations of dislocations deliver not only information on the mechanisms of defect creation in various chiral smectic phases, but also on the structure and properties of the investigated smectics, often inaccessible using standard methods.     

Synthesis and mesomorphic behaviors of a series of heterocyclic pyridine-imine-ester based liquid crystals

ABSTRACT

A series of new calamitic liquid crystals, 4-{[(5-methylpyridin-2-yl)imino]methyl}phenyl 4-alkoxybenzoates were synthesized. This series consists of nine members wherein they are differed by the length of alkoxy chain. Spectral analysis results were in accordance with the expected structure. Their thermotropic behaviors were analyzed with Differential Scanning Calorimetry (DSC), Polarizing Optical Microscopy (POM) and powder X-ray diffraction techniques. First member with the shortest alkoxy chain (n?=?2) is a non-mesogen. As the alkoxy chain increased to n?=?4, the monotropic nematic phase appeared. An enantiotropic mesophase (nematic) was observed for the following three members (n?=?6, 8, 10). As the alkoxy chain increased to n?=?12, enantiotropic nematic phase exhibited together with monotropic smectic A (SmA) phase. As the alkoxy chain continuously increased to n?=?14 and 16, enantiotropic phases were observed for both N and SmA. When moving to n?=?18, the nematic phase disappeared and this compound only exhibited a single mesophase (SmA).     

Field-induced phase transitions in chiral smectic liquid crystals studied by the constant current method

In ferroelectric liquid crystals, phase transitions can be induced by an electric field. The current constant method allows these transition to be quickly localized and thus the(E, T) phase diagram of the studied product can be obtained.In this work, we make a slight modification to the measurement principles based on this method. This modification allows the characteristic parameters of ferroelectric liquid crystal to be quantitatively measured. The use of a current square signal highlights a phenomenon of ferroelectric hysteresis with remnant polarization at null field, which points out an effect of memory in this compound.     

Effect of defects and surface anchoring on the phase behavior of chiral smectic liquid crystals

The influence of defects and surface effects on the stability of phenomena accompanying phase transitions, mainly coexistence phenomenon, has been studied. Experimental measurements were carried out by varying the cell thickness and using simple experimental on chiral smectic liquid crystal product. The analysis of the results with different cell thickness leads us to interpret the anomaly shown by our product related to impact of defects and surface effects. Moreover, the phase transition is affected by the trace of defects for thick cell and by the surface effect for thin cell. These effects have the same role as an electric local field which can induce phase transition.     

Mesomorphic behaviors of a series of heterocyclic benzothiazole-imine-ester-based liquid crystals

A series of new calamitic liquid crystals, 2-[4-(4-alkoxybenzoyloxy)benzylidenamino]benzothiazoles comprising a heterocyclic (benzothiazole) and two phenyl rings core system, terminal alkoxy chain, imine and ester linkers were prepared and characterized. This series comprises eight members wherein the members vary by the length of alkoxy chain (C_nH_2n+1O-, where n?=?4, 6, 8, 10, 12, 14, 16, 18). Spectral analysis results were in conformity with the expected structure. Their thermotropic behaviors were studied by using differential scanning calorimetry, optical polarizing microscopy and powder X-ray diffraction techniques. A single mesophase (nematic) was observed for the first member of the series (n?=?4). As the alkoxy chain increased to n?=?6, 8, 10, 12, 14, the nematic phase appeared together with an additional SmA phase. When moving from n?=?16 until the highest member (n?=?18), the nematic phase disappeared and these compounds only exhibited a single mesophase (SmA).     

Hunting for smectic C in calamitic azobenzene ionic liquid crystals with different cationic head groups

The tilted smectic C phase is a rather uncommon phase in ionic liquid crystals (ILCs), whereas the orthogonal smectic A phase is the most common phase in ILCs. We now present 2 new groups of mesogens with an azobenzene core that exhibit smectic C as well as smectic A phases. Their phase sequences and tilt angles were studied by polarizing microscopy, and their temperature‐dependent layer spacings and orientational order parameters were investigated by X‐ray diffraction. We present 1 new amidinium azobenzene mesogen that forms enantiotropic smectic C and A phases and another amidinium as well as 2 new guanidinium azobenzene mesogens that exhibit monotropic smectic C and enantiotropic smectic A phases. With this study, we show that azobenzene is indeed an SmC‐promoting group in ILCs. Comparing these results with our earlier results on azobenzenes with an N‐methylimidazolium head group (N Kapernaum et al, ChemPhysChem 2016, 17, 4116‐4123), we show that the aromaticity of the imidazolium head group plays an important role in the formation of smectic C phases.     

Climb via vacancy diffusion of edge dislocations in 2D dislocation microstructures

The prevention of strength degradation of components is one of the great challenges in solid mechanics. In particular, at high temperatures material may deform even at low stresses, a deformation mode known as deformation creep. One of the microstructural mechanisms that governs deformation creep is dislocation motion due to the absorption or emission of vacancies, which results in motion perpendicular to the glide plane, called dislocation climb. However, the importance of the dislocation network for the deformation creep remains far from being understood. In this study, a climb model that accounts for the dislocation network is developed, by solving the diffusion equation for vacancies in a region with a general dislocation distribution. The definition of the sink strength is extended, to account for the contributions of neighbouring dislocations to the climb rate. The model is then applied to dislocation dipoles and dislocation pile-ups, which are dense dislocation structures and it is found that the sink strength of dislocations in a pile-up is reduced since the vacancy field is distributed between the dislocations. Finally, the importance of the results for modelling deformation creep is discussed.     

The stress field and energy of screw dislocation in smectic A liquid crystals and the mistakes of the classical solution

The mistakes in the classical solution of a screw dislocation in smectic A liquid crystals are pointed out. A serious problem with the well-known theory is pointed, which may be named de Gennes-Kleman-Pershan paradox and has existed for many decades in the scientific community of liquid crystal study. The correct solution is given in this paper by a simplest, elementary, and straight forward method. In connection with this, the stress field and energy of dislocation are discussed in detail. The present article provides the correct stress field and dislocation energy as well.     

Plastic zone size and crack tip opening displacement of a Dugdale crack interacting with a coated circular inclusion

An analytical investigation on the plastic zone size of a crack near a coated circular inclusion under three different loading conditions of uniaxial tension, uniform tension and pure shear was carried out. Both the crack and coated circular inclusion are embedded in an infinite matrix, with the crack oriented along the radial direction of the inclusion. In the solution procedure, the crack is simulated as a continuous distribution of edge dislocations. With the Dugdale model of small-scale yielding [J. Mech. Phys. Solids 8 (1960) p. 100], two thin strips of yielded plastic zones are introduced at both crack tips. Using the solution for a coated circular inclusion interacting with a single dislocation as the Green's function, the physical problem is formulated into a set of singular integral equations. Using the method of Erdogan and Gupta [Q. J. Appl. Math. 29 (1972) p. 525] and iterative numerical procedures, the singular integral equations are solved numerically for the plastic zone sizes and crack tip opening displacement.     

Synthesis and phase transition behaviours of laterally substituted liquid crystals containing methylhydroquinone: emerging of smectic C phase for higher homologues

A series of laterally substituted low-molar-mass liquid crystals with molecular geometry were constructed with three phenyl rings bridged through ester central groups as the rigid core and a lateral methyl group as the flexible part of a molecule, with a view to understanding and establishing the effect of molecular structure on liquid crystal behaviour. Low-molar-mass mesogens known as 1,4-bis[(4′-n-alkyloxybenzoyl)oxy]toluene with different number of carbon (n) at the alkyl chain have been prepared. Their molecular structures were proposed via physical measurements and spectroscopic techniques. Mesomorphic properties were studied by using differential scanning calorimetry, optical polarizing microscopy and powder X-ray diffraction techniques. The results showed that the melting points as well as the clearing temperatures decreased upon lengthening of the terminal alkyloxy chain lengths. Members with the shorter chain (n = 2–10) exhibited nematic phase. As for higher homologues, members with n = 12, 14, 16 and 18 showed polymorphism, whereby these compounds displayed both smectic and nematic properties.     

Shielding effect and emission criterion of a screw dislocation near an interfacial blunt crack

Shielding effect and emission criterion of a screw dislocation near an interfacial blunt crack are dealt with in this paper. Utilizing the conformal mapping technique, the closed-form solutions are derived for complex potentials and stress fields due to a screw dislocation located near the interfacial blunt crack. The stress intensity factor on the crack tips and the critical stress intensity factor for dislocation emission are also calculated. The influence of the orientation of the dislocation and the morphology of the blunt crack as well as the material elastic dissimilarity on the shielding effect and the emission criterion is discussed in detail. The results show that positive screw dislocations can reduce the stress intensity factor of the interfacial blunt crack tip (shielding effect). The shielding effect increases with the increase of the shear modulus of the lower half-plane, but it decreases with the increase of the dislocation azimuth angle. The critical loads at infinity for dislocation emission increases with the increase of emission angle and curvature radius of blunt crack tip, and the most probable angle for screw dislocation emission is zero. The present solutions contain previous results as special cases.     

On the plastic zone size and crack tip opening displacement of a sub-interface crack in an infinite bi-material plate

Elastic–plastic stress analysis has been carried out for the plastic zone size and crack tip opening displacement of a sub-interface crack with small scale yielding. In our study, the shape of plastic zone is assumed as a long, slim strip at both crack tips. In the plastic zone, both normal stress and shear stress exist and are considered due to the bi-material interface. The values of the plastic zone size, normal stress and shear stress are determined by satisfying the conditions where both Modes I and II stress intensity factors vanish and Von Mises yield criterion is met. In the present paper, the sub-interface crack is simulated by continuously distributed dislocations which will result in singular integral equations. Those singular integral equations can be solved by reducing them to a set of linear equations. The values of the plastic zone size and crack tip opening displacement are obtained through an iterative procedure. Finally, the effect of normalized loading, normalized crack depth (distance to the interface) and Dundurs’ parameters on the normalized plastic zone size and the normalized crack tip opening displacement is discussed.     

A model of a belt chamber cracking based on crack profiles calculations

Abstract

The problem of a Belt chamber matrix cracking is presented. The influence of crack surface quality on the effective values of near crack tip stress is discussed. It is shown that under working conditions of the vessel, the existing shear friction between upper and lower crack surfaces caused by crack surface roughness can prevent the crack surface sliding displacement. Therefore, the control variable for matrix cracking is the value of stress intensity factor K_I corresponding to normal node of loading only. The calculations are performed by finite element method within the range of linear elastic fracture mechanics.     

一维六方准晶中螺形位错与楔形裂纹的相互作用

采用保角映射方法和扰动技巧,研究了一维六方准晶中螺形位错和半无限楔形裂纹的相互作用. 讨论了位错的位置和楔形角对作用在位错上的力的影响,得到了应力强度因子和作用在 位错上的力的解析解.此外,还详细地讨论了位错对裂纹的影响.当楔形角参数λ=1/2时, 半无限楔形裂纹退化成半无限尖裂纹,相应的结果 可以作为特殊情况而直接得到.     

A study of polarization in smectic liquid crystals via statistical-physics methods

The spontaneous polarization processes in synclinic and anticlinic smectic liquid crystals are discussed in the context of molecular statistical theory. The effects of interaction between molecules, external electric fields, and bending of molecules via polarization are elucidated as well.     

A study on polymorphism of hydrogen-bonded thermotropic liquid crystals

A novel mesogenic homologous series comprising of eight hydrogen-bonded liquid crystalline complexes are isolated and analyzed by forming a hydrogen bond between p-n alkyloxy benzoic acids (where n represent alkyloxy carbon number which varies from 5 to 12) and mesaconic acid, respectively. Eight synthesized complexes are subjected to Fourier transform infra-red spectroscopy, polarizing optical microscopy and differential scanning calorimetry studies to meet the basic characterization. The variation of optical tilt angle with respect to temperature in various conventional and smectic X phases are investigated and analyzed. A phase diagram is constructed to elucidate the mesogenic behavior of novel liquid crystalline series. The Cox ratio that reveals the order of various mesophases and the stability factor, to invoke the thermal stability of mesophases, is studied under the results of DSC thermogram.     

Dislocation unpinning model of acoustic emission from alkali halide crystals

The present paper reports the dislocation unpinning model of acoustic emission (AE) from alkali halide crystals. Equations are derived for the strain dependence of the transient AE pulse rate, peak value of the AE pulse rate and the total number of AE pulse emitted. It is found that the AE pulse rate should be maximum for a particular strain of the crystals. The peak value of the AE pulse rate should depend on the volume and strain rate of the crystals, and also on the pinning time of dislocations. Since the pinning time of dislocations decreases with increasing strain rate, the AE pulse rate should be weakly dependent on the strain rate of the crystals. The total number of AE should increase linearly with deformation and then it should attain a saturation value for the large deformation. By measuring the strain dependence of the AE pulse rate at a fixed strain rate, the time constantτ _s for surface annihilation of dislocations and the pinning timeτ _p of the dislocations can be determined. A good agreement is found between the theoretical and experimental results related to the AE from alkali halide crystals.     

Formation of extended compound dislocations and barriers in the course of interdislocation reactions in FCC crystals

The behavior of compound dislocations under stress loading is considered. Dislocation configurations are onsidered for an arbitrary asymmetric intersection of reacting dislocation segments. It is demonstrated that depending on the character of dislocation segment intersection, compound dislocations of two types can be formed, one of which is destructed under increasing stress loading. In the other case, the length of the compound dislocation increases, thereby causing the formation of long extended barriers. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 25–30, March, 2009.