Liquid Crystal Phase Transition Research Articles

Gel–liquid crystal phase transition in dry and hydrated SOPC phospholipid studied by differential scanning calorimetry

ABSTRACTWe study the thermodynamic properties of pure and hydrated samples of SOPC (stearoyl-2-oleoyl-sn-glycero-3-phosphocholine) membrane via Differential Scanning Calorimetry. We estimate the fundamental thermodynamic quantities, such as enthalpy and entropy, of the bilayer phosphatidylcholine. It is found that the gel-liquid crystal phase transition is driven by the van’t Hoff enthalpy, revealing the occurrence of an intermediate phase transition. We discuss the influence of the heating rate on the enthalpy and on the gel ↔ liquid crystal phase transition temperature by introducing the adequate thermodynamic Gibbs potential. The effect of hydrogen bonding of the water molecules with the polar head and polar-apolar interface on the energetics of the bilayer membrane matrix is analysed. The obtained transition temperature was found to vary between 3 and 4°C depending on the hydration level. A result corroborated by the behaviour of the heat capacity of SOPC computed via Molecular dynamics simulation.

Ferroelectric metallomesogens composed of achiral spin crossover molecules.

Ferroelectric liquid crystals (FLCs) are fascinating functional materials that have a remnant and electrically invertible polarization. To date, typical FLCs have been mainly realized by molecular design such as the incorporation of chirality into a given molecular structure. Here, we report for the first time ferroelectricity induced by spin transition associated with a crystal - liquid crystal phase transition in achiral molecules. Iron(ii) metallomesogens incorporating alkyl chains of type [Fe(3C n -bzimpy)2](BF4)2 (n = 8 (1), 10 (2), 12 (3), 14 (4), 16 (5) and 18 (6); bzimpy = 2,6-bis(benzimidazol-2'-yl)pyridine) that exhibit spin crossover (SCO) phenomena have been synthesized. Compounds 5 and 6 were each demonstrated to show SCO behaviour along with the occurrence of a phase transition between the crystalline (Cr) state and corresponding chiral smectic C (SmC*) state. The distortion of the coordination sphere in the high spin state is seen to trigger the generation of the SmC* state. The liquid crystalline compounds do not display ferroelectric behaviour in their Cr state but do exhibit ferroelectric hysteresis loops in their SmC* state. Ferroelectric switching was clearly confirmed by second harmonic generation (SHG) experiments involving the respective phases. These findings will undoubtedly lead to new strategies for the design of new FLCs based on metal-centred spin transitions.

Topological defects and geometric memory across the nematic-smectic A liquid crystal phase transition.

We study transformations of self-organised defect arrays at the nematic-smectic A liquid crystal phase transition, and show that these defect configurations are correlated, or "remembered", across the phase transition. A thin film of thermotropic liquid crystal is subjected to hybrid anchoring by an air interface and a water substrate, and viewed under polarised optical microscopy. Upon heating from smectic-A to nematic, a packing of focal conic domains melts into a dense array of boojums-nematic surface defects-which then coarsens by pair-annihilation. With the aid of Landau-de Gennes numerical modeling, we elucidate the topological and geometrical rules underlying this transformation. In the transition from nematic to smectic-A, we show that focal conic domain packings are organised over large scales in patterns that retain a geometric memory of the nematic boojum configuration, which can be recovered with remarkable fidelity.

Investigation of Phase Transitions in Nematic Liquid Crystals by Fractional Calculation

In this study, we investigate nematic-isotropic phase transitions in liquid crystals using fractionally generalized form of the Maier-Saupe Theory (MST). MST is one of the mean-field theories commonly used in the nematic liquid crystals which proved to be extremely useful in explaining nematic-isotropic phase transitions. Fractionally obtained results compared with those of the experimental data for p-azoxyanisole (PAA) in the literature. In this context, the dependence of fourth rank order parameters on second rank order parameters is handled by being a measure of fractality of space. It is observed that the variation of second-rank and fourth rank order parameters versus temperature are in accordance with some values of fractal dimensions. As a result, we can conclude that there is a close relationship between temperature and fractional derivative order parameters.

Optical measurements of phase transitions in difluorophenylazophenyl benzoate thermotropic liquid crystal with specific orientated fluorine atoms

ABSTRACTPhase transition temperature and optical properties of the special oriented fluorine atoms in difluorophenylazophenyl Benzoate Thermotropic Liquid Crystal nominated (III) were studied and measured. Absorbance and transmittance in the visible region at room temperature for the compound III dissolved in methylene chloride of different concentrations were determined. The maximum absorbance for the compound III was in the blue region (447 nm) and the molar absorption coefficient was found to be 0.93 (L. mMol−1.cm−1). The transition temperature was determined for the thermotropic LC compound III by using DSC, POM and modified spectrophotometer techniques. Nematic and smectic phases during the cooling process were observed. Ordinary and extraordinary refractive indices, birefringence, order parameter and the molecular polarizability were determined with different temperatures for the LC phases of compound III.

Highly improved dielectric behaviour of ferronematic nanocomposite for display application

ABSTRACTWe report the investigation of influence of nickel zinc ferrite magnetic nanoparticles (NZFO (Ni0.5Zn0.5Fe2O4)) on phase transition, optical and dielectric properties in a nematic liquid crystal (NLC). The interaction of NZFO nanoparticles with NLC was confirmed by the formation of ferronematic droplets due to the transfer of magnetic orientational effect onto the underlying NLC matrix. The doping results in shift of nematic to isotropic transition to low-temperature region. An enhancement in the value of refractive index is observed in the nematic region after the addition of NZFO nanoparticles. The dielectric constant of NLC was remarkably enhanced by 10 times after doping, which is found to be maximum at 0.1 wt% concentration of NZFO nanoparticles. The decrease in the value of dissipation factor in low-frequency region shows that the magnetic nanoparticles are able to trap ionic impurities effectively. The obtained results suggest that the optimum amount of doping concentration is 0.1 wt% of NZFO nanoparticles in NLC due to high dielectric constant with low dissipation factor and high refractive index with high dispersive power at room temperature.

Counterion-Induced Nanosheet-to-Nanofilament Transition of Lyotropic Bent-Core Liquid Crystals.

The smart flexibility of phase transitions in liquid crystals (LCs) makes them suitable for various applications and is an important research field in contemporary science, engineering, and technology. Unlike most reports focused on bent-core LCs in the thermotropic situation, in our present study, we designed and synthesized a fully rigid bent-core molecule with the sulfonic acid group replacing conventional flexible chains. A rich variety of counterion-induced supramolecular LC phase behaviors have been systematically investigated. It was found that the smectic phase with nanosheets tends to transform to the hexagonal phase with nanofilaments when the protons of the sulfonic acid group are partially replaced by alkali metal ions. The experimental results show that the nanoaggregate and phase transition are controlled by the displacing ratio of alkali metal ions rather than the molecular concentration. Another interesting feature is that the achiral bent-core molecules self-assemble into columns by helical stacking and present macroscopic chirality, indicating that spontaneous chiral symmetry breaking occurs in the columnar phase. The fully rigid bent-core molecules reveal surprisingly hierarchical molecular self-assemblies with the smectic-to-hexagonal phase transition, which was not previously observed in supramolecular complexes. The findings will provide new possibilities for applications in LC-based photonic devices, biosystem switches, and supramolecular actuators.

Driving a Liquid Crystal Phase Transition Using a Photochromic Hydrazone.

The dynamic manipulation of the properties of soft matter can lead to adaptive functional materials that can be used in advanced applications. Here we report on a new chiral dopant, built on an isosorbide scaffold attached to two bistable hydrazone-based light switches that can be used to control the self-assembly, and hence photophysical properties, of nematic liquid crystals (LCs). The bistability of the switch allows kinetic trapping of various helical assemblies as a function of the photostationary states, resulting in the reflection of different wavelengths of light. Surprisingly, doping 5CB with the chiral switch, followed by irradiation with blue light, triggers an isothermal phase change from the helical cholesteric phase to the untwisted lamellar smectic A* phase. This transition was used to modulate the transparency of a LC film, resulting in a light-gated optical window.

Communication: Optically induced decoupling of the smectic-A and nematic order parameters at the AN phase transition in phototropic liquid crystals.

In the present work, by means of the photopyroelectric calorimetric investigations in liquid crystal/naphtopyran (NP) molecule mixtures, the effect of the photoisomerization of NP by UV light on the critical behavior of specific heat could be studied over the smectic-nematic phase transition. It is shown that the UV light irradiation of the sample can shift the critical exponent from a non-universal value typical of the non-irradiated mixture to one close to that of the 3D-XY universality class, predicted for such a transition. This witnesses the optically triggered reduction of the coupling between the smectic and nematic order parameters responsible for the deviation from the XY-like behavior. A tentative explanation is provided based on the weakening of the order parameter coupling caused by the NP molecules entering the smectic layers following the UV light illumination.

Amphiphile-Induced Phase Transition of Liquid Crystals at Aqueous Interfaces.

Monolayer assemblies of amphiphiles at planar interfaces between thermotropic liquid crystals (LCs) and an aqueous phase can give rise to configurational transitions of the underlying LCs. A common assumption has been that a reconfiguration of the LC phase is caused by an interdigitation of the hydrophobic tails of amphiphiles with the molecules of the LC at the interface. A different mechanism is discovered here, whereby reorientation of the LC systems is shown to occur through lowering of the orientation-dependent surface energy of the LC due to formation of a thin isotropic layer at the aqueous interface. Using a combination of atomistic molecular dynamics simulations and experiments, we demonstrate that a monolayer of specific amphiphiles at an aqueous interface can cause a local nematic-to-isotropic phase transition of the LC by disturbing the antiparallel configuration of the LC molecules. These results provide new insights into the interfacial, molecular-level organization of LCs that can be exploited for rational design of biological sensors and responsive systems.

On-Demand Control of Phase Transition and Orientation of Organic-Inorganic Complex Lyotropic Liquid Crystals

On-Demand Control of Phase Transition and Orientation of Organic-Inorganic Complex Lyotropic Liquid Crystals

Unique Supramolecular Liquid-Crystal Phases with Different Two-Dimensional Crystal Layers.

We report herein a series of tetrablock-mimic azobenzene-containing [60]fullerene dyads that form supramolecular liquid crystals (LCs) from phase-segregated two-dimensional (2D) crystals. The unique double-, triple-, and quadruple-layer packing structure of fullerenes in the 2D crystals leads to different smectic supramolecular LC phases, and novel LC phase transitions were observed upon changes in the fullerene packing layer number in the 2D crystals. Interestingly, by combining the LC properties with 2D crystals, these materials show excellent electron mobility in the order of 10-3 cm2 V-1 s-1 , despite their relatively low fullerene content. Our results provide a novel method to manipulate 2D crystal layer thickness, with promising applications in optoelectronic devices.

Unique Supramolecular Liquid‐Crystal Phases with Different Two‐Dimensional Crystal Layers

AbstractWe report herein a series of tetrablock‐mimic azobenzene‐containing [60]fullerene dyads that form supramolecular liquid crystals (LCs) from phase‐segregated two‐dimensional (2D) crystals. The unique double‐, triple‐, and quadruple‐layer packing structure of fullerenes in the 2D crystals leads to different smectic supramolecular LC phases, and novel LC phase transitions were observed upon changes in the fullerene packing layer number in the 2D crystals. Interestingly, by combining the LC properties with 2D crystals, these materials show excellent electron mobility in the order of 10−3 cm2 V−1 s−1, despite their relatively low fullerene content. Our results provide a novel method to manipulate 2D crystal layer thickness, with promising applications in optoelectronic devices.

Liquid Crystals under Confinement in Submicrometer Capsules.

Liquid crystal (LC) ordering and phase transition behavior under confined conditions have attracted extensive attention and enabled many applications. However, the ordering and phase transition behavior of LCs in submicrometer capsules have seldom been studied, primarily due to the lack of proper capsulizing and visualization approaches to such small LC microcapsules. Herein, we achieve submicrometer LC capsules with the sizes down to 100 nm by using emulsion-based interfacial sol-gel reaction. The behavior of LCs under the submicrometer confinement conditions is investigated while the sizes and chemical composition of the microcapsule shell surface are tuned in a controllable way. The phase transition temperatures of LCs in the submicrometer capsules shift from those of bulk LCs due to the surface-induced ordering of LCs under the strong confinement conditions, which causes formation of topological defects and alters the order parameter. Using nonlinear optical imaging technology, we explore the structures of director field of LCs that arise as a result of the competition between the surface boundary conditions and LC elasticity. The results show that the nanoscale encapsulation can significantly influence the structural configurations of the director and phase transitions of LCs under various confinement conditions.

Emission ellipsometry as a tool for luminescent liquid crystal phase transition identification.

In the present work, we have studied the liquid crystalline properties of a luminescent molecule, which presents smectic mesophases, using the emission ellipsometry technique. A methodology with photoselection of the excitation light to improve the experiment is presented. An empirical model is proposed to explain the experimental results. The Stokes parameters of the emitted light allowed the correlation of changes in the polarization degree, ellipticity angle, and rotation angle with phase transitions.

Pressure-induced phase transitions in liquid crystals: A molecular field approach

A rigorous microscopic treatment of a nematic fluid system based on a pairwise interaction potential is immensely complex. For studying such systems molecular field theories are often the standard method of choice. In this paper we have chosen a simple effective potential U=u_{4}/v^{4}-u_{2}/v^{2}-Au_{2}/v^{2}〈P_{2}〉P_{2}(cosϑ) to study an isothermal-isobaric ensemble describing a liquid crystalline system. Using this we have studied in particular the pressure dependence of liquid crystalline phase transitions.

On the Calculation of the Debye Temperature and Crystal–Liquid Phase Transition Temperature of a Binary Substitution Alloy

A method of estimating the interatomic pair interaction potential parameters for a binary substitution alloy with consideration for the deviation of its lattice parameter from the Vegard law is proposed. This method is used as a basis to calculate the Debye temperature and Gruneisen parameters of a SiGe alloy. It is shown that all these function nonlinearly variate with a change in the germanium concentration. Based on this technique and Lindemann's melting criterion, a method for calculating the liquidus and solidus temperatures of a disordered substitution alloy is proposed. The method is tested on the SiGe alloy and demonstrates good agreement with experimental data. It is shown that when the size of a nanocrystal of a solid substitution solution decreases, the difference between the liquidus and solidus temperatures decreases the more, the more noticeably the nanocrystal shape is deflected from the most energetically optimal shape.

Laser-Induced Phase Transition in a Monolayer of Polymer Particles Levitating in a Low-Pressure Gas-Discharge Plasma

We report on the results of analysis of the mean kinetic energy and the pair correlation function of polymer particles in a plasma–dust structure under the action of laser radiation. We have observed experimentally the crystal–liquid phase transition in the monolayer of particles levitating in the near-electrode layer of a capacitive high-frequency discharge. The coupling parameter of the dust system has been estimated. The results of analysis of the modification of the polymer dust particle surface after holding in the plasma are considered. We propose an explanation of the phase transition taking into account the role of the photophoretic force in the motion of macroparticles. The effect of the photophoretic force is associated with the modification of the dust particle surface in the plasma, as a result of which the particles can effectively absorb laser radiation.

1H Nuclear Magnetic Resonance Study of an Incommensurate-Commensurate Phase Transition in a Dimesogenic Liquid Crystal

1H nuclear magnetic resonance (NMR) was employed in order to probe an incommensuratecommensurate phase transition in a dimesogenic liquid crystal, N-[4-(6-cholesteryloxycarbonyl) pentyloxy]-4-n-butylazobenzene (KI5A). The recrystallization and the isotropic phase transitions exhibited a discontinuous first-order nature in the 1H NMR spin-lattice (T1) and in the spin-spin relaxation time (T2E) measurements, and an incommensurate smectic A (SAinc) to commensurate helical smectic C (S* C ) liquid crystalline phase transition was marked by a discontinuity in the NMR line splitting representing the orientational order. Furthermore, T1 and T2E showed distinct temperature dependencies in the SAinc phase and in the S* C phase.

Influence of flexible chain, polar substitution and hydrogen bonding on phase stability in Shiff based (4)PyBD(4I)BrA-nOBA series of liquid crystals

ABSTRACTShiff based Linear Hydrogen Bonded Liquid Crystalline homologues, viz., (4)PyBD(4I)BrA-nOBAs for n = 2,3,4,5,6,7,8,10,11 and 12 are synthesized. 1H-NMR and 13C-NMR studies confirm the formation of HBLC and IR confirms the linear hydrogen bonding. Liquid crystal phase abundance, transition temperatures, enthalpy and order of transition are investigated by Polarized Optical Microscopy textural studies and Differential Scanning Calorimetry. Nematic, Smectic-A, -D, -C, Bhexatic, -BCrystal, -F, -I and –G phases are identified. Large LC phase variances, viz., NABhexFG and NACFG are exhibited by intermediate chain length with n = 5 and 6. A-C transition exhibited unique II-order nature. II-order nature for G-Solid transition explained. Phase diagram drawn with flexible chain length infers INA, and ACF and CIF MultiCritical Points and Lifshitz behaviour. NA TriCiritical Point predicted in [3+7] binary. Influences of chain length, linear HB and end polar(-Br) substitution for LC phase stability are is discussed. Results of Phase stability are discussed in the wake of the body of the data and reports in other LCs. DSC enthalpy suggests for utility of intermediate and higher homologues for device savvy tilted Smectic LC phases.