Reentrant Nematic Phase Research Articles

Investigation on the critical behaviour in the vicinity of the smectic-A to re-entrant nematic-phase transition

ABSTRACT The study focused on the binary system 6OCB and 8OCB, which is well-suited for investigating stable re-entrant nematic-phase transitions. High-resolution measurements of heat capacity and birefringence are used to analyse critical behaviours near the nematic – smectic-A (N-SmA) and the smectic-A – re-entrant nematic (SmA-) phase transitions. The critical behaviour of these transitions has been explored with the aid of a differential quotient extracted from the birefringencevalues. Simultaneously, critical exponents extracted from the specific heat capacity anomaly are also used to analyse the critical behaviour of the phase transition. The results indicate that the SmA to phase transition exhibits non-universal behaviour, with effective exponents falling between the tricritical (TCP) and 3D-XY values, similar to the N-SmA phase transition. The study reveals the existence of two tricritical points: one associated with the N-SmA transition and the other corresponding to the SmA- transition. This is supported by plotting the McMillan ratio against the critical exponents obtained from both birefringence and specific heat capacity anomaly. Again, with an increase in the SmA range, the SmA- transition tends towards the tricritical point (TCP), suggesting a relationship between the SmA range and the transitional behaviour.

Photo-driven effects in twist-bend nematic phases: Dynamic and memory response of liquid crystalline dimers

We apply UV light to control the stability of twist-bend nematic phases, NTB, and conventional nematic phases, N, in three new liquid crystalline nonsymmetric dimers containing cyanobiphenyl and light-responsive azobenzene fragments connected by a hexyloxy spacer. The dynamic and static (5 kHz) dielectric responses of (E)-4′-(6-(4-((4-(ethoxy)phenyl)diazenyl)phenoxy)hexyl)-[1,1′-biphenyl]-4-carbonitrile, CB6OAzO2, (E)-4′-(6-(4-((4-(hexyloxy)phenyl)diazenyl)phenoxy)hexyl)-[1,1′-biphenyl]-4-carbonitrile, CB6OAzO6, and (E)-4′-(6-(4-((4-hexylphenyl)diazenyl)phenoxy)hexyl)-[1,1′-biphenyl]-4-carbonitrile, CB6OAz6, are measured at different temperatures, in planar and homeotropic alignments, by the application of bias and alternating electrical fields. The three CB6OAz(O)n dimers show high values of the dielectric mean, ε¯, and dielectric anisotropy, Δε, due to the dipole moment associated with the cyanobiphenyl groups. The absence of the ether linkage to the terminal chain in CB6OAz6 reduces the NTB - N transition temperature and increases the bend elastic constant, K3, compared to those seen for CB6OAzO6. Isothermal phase transitions are promoted by light-irradiation, via trans-to-cis photoisomerisation of the azobenzene units, resulting in the disruption of the liquid crystal order and appearance of re-entrant nematic and isotropic phases at low temperatures. It is possible to control the dynamics and reversibility of these transitions by varying the chemical structure of the new dimers, the temperature and UV light dosage. This may be exploited in devices using multiple external stimuli.

DFT-based numerical study of the re-entrant phase and optical parameters of the homologous series of MBC liquid crystal molecules studied under the influence of an electric field

The homologous series of MBC liquid crystals (LCs) show re-entrant nematic phase (chiral nature or adverse order) under the external electric field in the THz frequency range. The MBC LCs show the phase conversion from isotropic to nematic and subsequently, nematic to isotropic with the variation in electric field and length of alkyl chain in the molecular structure. The negative order parameter is decreased and the positive order parameter of the MBC series is decreased with the length of the alkyl chain of the molecular structure. The positive values of the birefringence of MBC series are reduced; however, adverse birefringence approximately remains constant. On account of the director angle, a short alkyl chain has smaller values, but after 5 MBC, all the molecules are parallel to the applied electric field and maintain a large perspective θ = 90. The refractive index of MBC increases with the extension of the alkyl chain length in the structure. The abstract image of the MBC LC series indicates the phase variation of the MBC series and the band gap of the nematic phase MBC is also affected by the applied electric field.

Electro-optical effect of the nCOOCB liquid crystal molecules under the terahertz frequency range: A theoretical approach

The homologous series of 4-cyano-4'-phenyl-phenol-alkanoates (nCOOCB) was studied under the influence of terahertz (THz) frequency range. The nCOOCB series has a re-entrant nematic phase, which is suitable for electro-optical properties under the THz frequency. The birefringence and order parameter expresses the twisting of the nematic phase at the higher frequency range. The director angle has fluctuated at a higher frequency range. The refractive index has remained constant at a higher frequency. The ionisation potential, electron affinity and Homo-Lumo energy gap continuously decrease with an extension of alkyl chain length; however, the dipole moment increases. The Homo-Lumo energy bandgap is reciprocal to the dipole moment.

NCB/8CB (n = 1-6) 二成分系におけるリエントラントネマティック相

nCB/8CB (n = 1-6) 二成分系におけるリエントラントネマティック相

Reentrant lyotropic nematic phase in a ternary mixture

ABSTRACTWe report the results of our studies on optical and thermal properties of ternary mixture of three compounds viz., 4′-n-octyl-4-cyanobiphenyl (8CB), didodecyl dimethyl ammonium bromide, and glacial acetic acid. Higher concentrations of the given mixture show a very interesting reentrant nematic phase, sequentially when the specimen is cooled from its isotropic melt at different temperatures. The temperature variations of optical anisotropy, optical textures, and electrical conductivity have also been discussed. Nanoaggregated size of the molecules has been confirmed by X-ray studies.

Structure-Property Correlations in Cyanobiphenyl-Based Dimer-Like Mesogens.

In this contribution, we present the results of our extensive investigations on the synthesis and phase behavior of five series of dimer-like compounds formed by covalently linking a promesogenic, cyanobiphenyl core, with a nonmesogenic N-(n-alkyl)-salicylaldimine segment via an oxy(oligomethylene)oxy spacer of varying length and parity. This work is a continuation of our previous short study where the occurrence of re-entrant nematic (Nre) phase seemed to be vitally dependent on the parity of the spacer and the length of the terminal chain. Thus, working along these lines the effects on the thermal (Nre phase) behavior of the various spacers and the terminal tails have been investigated comprehensively. In particular, with the aim of exploring the fundamental relationships between molecular structural features and thermal properties, five spacers, namely, oxy(hexyl)oxy, oxy(heptyl)oxy, oxy(octyl)oxy, oxy(nonyl)oxy, and oxy(decyl)oxy spacers, have been used. Each series consists of six mesogens, as the terminal tail attached to the salicylaldimine core has been varied from n-pentyl to n-decyl. For the sake of comparison and completeness, previously reported materials have also been studied in detail, and data are included here. The characterization results derived from the optical and calorimetric studies clearly illustrate a pronounced odd-even effect; however, the effect attenuates as the length of the spacer is elongated. The even-members show higher clearing temperatures and enthalpies of transition than those of adjoining odd-members. Three series of mesogens possessing an even-parity spacer show thermodynamically stable nematic (N) and/or smectic A (SmA) phase(s); the lower homologues display only the N phase, and the middle ones exhibit both N and SmA phases while the higher members show the SmA phase solely. In contrast, the vast majority of the odd-spaced compounds of two series show a monotropic N phase. X-ray diffraction study confirms the partial bilayer order of the SmA (SmAd) phase with L < d < 2L where d is the layer spacing and L is the molecular length. Contrary to what was previously noted, none of the new compounds synthesized favor the stabilization of the Nre phase. At the present time, it is extremely difficult to point out the exact cause for the origin of re-entrant behavior in some compounds exclusively, as they hardly differ in their structure. It was also found that the phase transitional properties of even-members are comparable to that observed for 4'-n-alkoxy-4-cyanobiphenyls (nOCBs) and 4-n-alkyl-4-cyanobiphenyls (nCBs). In essence, our study validates the point that dimer-like compounds resemble liquid crystal (LC) dimers, as well as monomers to some degree, in the context of their thermal behavior; thus, they can be regarded as the bridging structures between LC monomers and dimers.

Effect of electric field on reentrance transition in a binary mixture of liquid crystals

Employing a phenomenological mean field theory, we analyze the effect of an electric field on the N − SmA phase transition for pure liquid crystal and on the reentrant nematic phase in a binary mixture of liquid crystals exhibiting the phase sequence I − N − SmA − NR on cooling. The basic idea of the work is to explain the phase transition behavior of the system by assuming that certain Landau coefficients associated with the order parameters coupling terms of the free-energy density expansion are field dependent. These parameters play a crucial role and show a rapid variation at the SmA − NR transition as compared to the SmA − N transition.

Relationship between molecular association and re-entrant phenomena in polar calamitic liquid crystals.

The relationship between molecular association and re-entrant phase behavior in polar calamitic liquid crystals has been explored in two families of materials: the 4'-alkoxy-4-cyanobiphenyls (6OCB and 8OCB) and the 4'-alkoxy-4-nitrobiphenyls. Although re-entrant nematic phase behavior has previously been observed in the phase diagram of 6OCB/8OCB, this is not observed in mixtures of the analogous nitro materials. As there is no stabilization of the smectic A phase in mixture studies, it was conjectured that the degree of association for the nitro systems is greater than that for the cyano analogues. This hypothesis was tested by using measured dielectric anisotropies and computed molecular properties to obtain a value of the Kirkwood factor, g, which describes the degree of association of dipoles in a liquid. These computed values of g confirm that the degree of association for nitro materials is greater than that for cyano and offer a useful method for quantifying molecular association in systems exhibiting a re-entrant polymorphism.

Induced Reentrant Nematic and Smectic— A Phase in Mixture of Mesogenic and Nonmesogenic Compounds

We report the results of our studies on the optical and thermal properties of the mixture of 4-cyano-4′-octyloxybiphenyl (80CB) and N-Cetyl-N, N, N, trimethyl ammonium bromide (CTAB) + Glacial acetic acid (GAA), which exhibits a very interesting lower temperature reentrant nematic and smectic-A phases, sequentially when the specimen is cooled from its isotropic liquid phase. The different phases of these mixtures are identified using optical and microscopic techniques. The temperature variations of optical anisotropy, Ultrasonic velocity, molar compressibility, and adiabatic compressibility have been also discussed.

Reentrant nematic and tricritical behavior of the nematic to smectic-A phase transition in liquid crystal mixture

We present an improved analysis of the reentrant nematic and tricritical behavior of the nematic to smectic A phase transition in liquid crystal mixture within phenomenological theory. To describe the nematic–smectic A–reentrant nematic phase sequence in binary mixture, nematic and smectic order parameters have been coupled with the concentration. We show that reentrant nematic and the tricritical point on the N–SmA phase transition line can be achieved in the same phase diagram under certain conditions. We predict two tricritical points. One corresponds to nematic to smectic A phase transition and other corresponds to reentrant nematic to smectic A phase transition. A comparison of the results with experimental phase diagrams is described in great details. The predictions of the theory are in good agreement with available experimental data.

Determination of elastic constants of a binary system (7CPB+9.CN) showing nematic, induced smectic Ad and re-entrant nematic phases

The temperature variation of the splay and bend elastic constants of a binary system exhibiting nematic-induced smectic Ad and re-entrant nematic phases measured by electric field-induced Freedericksz transition method has been reported. As bend deformation is not permitted in the smectic A phase, bend elastic constant (K33) could only be determined in the nematic and re-entrant nematic phases. In both the nematic phases, the splay elastic constant has the same order of magnitude and does not show any pretransitional effect. However, in the induced smectic Ad phase, the value of K11 is about one to two orders higher than that in the nematic phases. The bend elastic constant shows a strong pretransitional effect near the nematic–smectic and smectic–re-entrant nematic phase transitions. The influence of the presence of the induced smectic phase is observed even in those mixtures which have no induced smectic phases. As the smectic phase is approached, the ratio K33/K11 increases rapidly and diverges to infinity.

Reentrant nematic phase in 4-alkyl-4′-cyanobiphenyl (nCB) binary mixtures

The appearance of reentrant nematic phase was identified in nCB/8CB (n = 1–6) binary mixtures using differential scanning calorimetry, small-angle X-ray diffraction and polarising microscopy with photomonitor. Phase diagrams can roughly be unified if plotted against the average number of alkyl chain length. Distinction was recognised for the phase boundaries between nematic and smectic A phases of even and odd n of the minor component nCB. The character of 8CB as ‘SmA former’ is briefly discussed.

Calculation of the nematic entropy using digital images

In this work we will use digital images to compute the entropy dependence on temperature of a nematic lyotropic sample. The set of images comprehend the entire temperature range between a reentrant nematic isotropic phase transition, at a low temperature, and a usual nematic isotropic phase transition at a higher temperature. We will show that, inside the nematic phase, the image entropy profile agrees accurately with the entropy given by the Maier-Saupe model. As far as we know, this is the first time that the entropy of a lyotropic nematic phase is evaluated by this method, which introduces a way to measure their macroscopic variables. Namely, being that the entropy is a thermodynamical potential, this result implies that digital images can be used to compute mean values of nematic random variables.

NMR of Short-Chain Hydrocarbons in Nematic and Smectic A Liquid Crystals

The proton NMR spectra of ethane, propane, and n-butane codissolved and orientationally ordered in two liquid-crystal solvents that exhibit both nematic (N) and smectic A (SmA) phases (one of which also has a reentrant nematic (RN) phase) are analyzed using CMA-ES (covariance-matrix adaptation evolution strategy). To deal with problems arising from the broad liquid-crystal background signal, a smoothed experimental spectrum is fitted to a smoothed calculated spectrum. The ethane and propane dipolar couplings and anisotropic energy parameters scale with each other, and the n-butane couplings are assumed to behave likewise. This restriction on the relative values of the n-butane energy parameters facilitates a fit to the temperature dependence of the n-butane dipolar couplings, in which the six n-butane energy parameters (three for trans and three for gauche), the isotropic trans-gauche energy difference E(tg), and its temperature coefficient E(tg)', and the methyl CCH angle decrease are obtained. Unlike earlier studies, the fit does not employ a model for the anisotropic intermolecular potential. The nematic potential in the SmA phase of the liquid-crystal mixture 6OCB/8OCB is estimated by interpolated values obtained from the ethane spectra for the N and RN regions. Analysis of results for the SmA phase involves adding a nematic-smectic coupling prefactor and a smectic prefactor. Spectra obtained in the liquid-crystal 8OCB are calculated with no adjustable parameters, scaling the potentials using the ethane values, and the agreement between experiment and calculation is outstanding. Conformer populations are affected by the environment, and the isotropic solute-solvent interactions result in increased gauche populations, whereas anisotropic interactions increase the trans probability. The trans probability in the SmA phase is slightly lower than expected from the nematic results.

Anisotropic self-diffusion in nematic, smectic-A, and reentrant nematic phases

The nature of the reentrant nematic phase has been actively investigated both experimentally and theoretically during the past few decades. Most studies concluded that, as concerning molecular dynamics, a reentrant nematic phase is essentially analogous to a conventional nematic one. Recent computer simulations [Mazza et al., Phys. Rev. Lett. 105, 227802 (2010)], however, predicted molecular translational self-diffusion along the phase director that was dominated by a collective transport mode and was, relative to that observed in a conventional nematic phase, enhanced by an order of magnitude. In the present work, the principal components of the diffusion tensor in a reentrant nematic phase are determined experimentally and compared to those in conventional nematic and smectic-A phases. We find that the temperature dependence of the translational diffusion in the two nematic phases, within experimental error, follows a uniform trend and can be adequately described in terms of available diffusion models in nematics. Hence, we find no evidence for enhanced diffusion but confirm instead the similarity of conventional and reentrant nematic phases with respect to molecular translational dynamics.

Diffusivity Maximum in a Reentrant Nematic Phase

We report molecular dynamics simulations of confined liquid crystals using the Gay–Berne–Kihara model. Upon isobaric cooling, the standard sequence of isotropic–nematic–smectic A phase transitions is found. Upon further cooling a reentrant nematic phase occurs. We investigate the temperature dependence of the self-diffusion coefficient of the fluid in the nematic, smectic and reentrant nematic phases. We find a maximum in diffusivity upon isobaric cooling. Diffusion increases dramatically in the reentrant phase due to the high orientational molecular order. As the temperature is lowered, the diffusion coefficient follows an Arrhenius behavior. The activation energy of the reentrant phase is found in reasonable agreement with the reported experimental data. We discuss how repulsive interactions may be the underlying mechanism that could explain the occurrence of reentrant nematic behavior for polar and non-polar molecules.

Structure and Dynamics of Reentrant Nematics: Any Open Questions after Almost 40 Years?

Liquid crystals have attracted enormous interest because of the variety of their phases and richness of their application. The interplay of general physical symmetries and specific molecular features generates a myriad of different phenomena. A surprising behavior of liquid crystals is the reentrancy of phases as temperature, pressure, or concentration are varied. Here, we review the main experimental facts and the different theoretical scenarios that have guided the understanding of bulk reentrant nematics. Recently, some computer simulations of a system confined to nanoscopic scales have found new dynamical features of the reentrant nematic phase. We discuss this prediction in relation with the available experimental evidence on reentrant nematics and with the dynamics of liquids in strongly confined environments.

Separation of nematic and smectic-A potential effects on solute ordering in a series of binary 6OCB–8OCB mixtures

1H NMR and optical microscopy have been used to study four solutes dissolved in samples of a binary mixture of 4-n-hexyloxy-4′-cyanobiphenyl (6OCB) and 4-n-octyloxy-4′-cyanobiphenyl (8OCB) that are close to the region of the phase diagram where the smectic-A (SmA) and re-entrant nematic (RN) phases exist. Optical microscopy clearly indicates that one of the four studied samples shows the phase sequence of isotropic–nematic–SmA–RN. The derived solute order parameters were interpreted by means of two Maier–Saupe mechanisms in conjunction with the Kobayashi–McMillan theory in the case of the SmA phase. The novel feature of this study is that the nematic potential is extrapolated from the nematic to the SmA phase based on concentrations of the 6OCB–8OCB mixtures. The derived smectic order parameters for each of the studied solutes clearly show a maximum absolute magnitude near the centre of the SmA temperature range. The different partitioning of these solutes in the binary mixture is also discussed.

Rotational Viscosity Measurement of a Binary Mixture Showing Both Induced Smectic and Re-Entrant Nematic Phases

The rotational viscosity has been measured in a binary system of two polar nematic compounds, exhibiting nematic – smectic Ad – re-entrant nematic phase sequence. In both the nematic and re-entrant nematic phases, the rotational viscosity values are about 0.5 Pa s and show small temperature dependence. However, in the smectic Ad phase, the rotational viscosity values initially increase with decrease in temperature to attain a maximum near the middle of the smectic phase and then decreases with further decrease in temperature. The activation energies associated with rotational viscosity for the nematic phase are distinctly higher than those obtained for the re-entrant nematic phase.