Nematic-isotropic Transition Temperature Research Articles

Phase Equilibria of Flexible Polymer and Monomer Liquid Crystal Blends

In this study, we explored the phase equilibria in blends of a nematic liquid crystal monomer and a nematic liquid crystal flexible polymer within the whole composition of the phase diagram. The crystal-nematic and nematic-isotropic transition temperatures, as well as the transition enthalpies and entropies, indicated the total miscibility in this mesogenic blend system. We also extracted an empirical axial ratio of 4.52 for the polymer from the nematic-isotropic transitions of the blends. Further, from the biphasic nematic-isotropic transitions of the blends in low concentration regions, we estimated the entropies of pure monomer and polymer from a theoretical lattice model. The calculated and experimental results showed good agreement. This work will be continued with a more detailed study of phase equilibria and comparison with other theoretical models in nematic flexible polymer and non-mesogenic rigid-rod monomer blends will be made.

Electro-optical, dielectric and optical properties of graphene oxide dispersed nematic liquid crystal composites

ABSTRACT Graphene oxide (GO) in various concentrations (0.04 wt.%, 0.1 wt.% and 0.3 wt.%) was dispersed into nematic liquid crystal (LC) 5CB and studied for electro-optical, dielectric and optical responses. The textural and phase transition studies were investigated by optical polarising microscopy in the nematic phase. The effect of GO concentration on the micro-texture, phase transition temperatures, birefringence, threshold voltage, contrast ratio, dielectric permittivity, dielectric loss and conductivity was also investigated. The nematic–isotropic transition temperature slight increases (~3%) in the 0.04 wt.% and 0.1 wt.% GO dispersed 5CB composites as compared to 5CB. Threshold voltage decreases in the GO–5CB composites with the increase of GO concentration. Additionally, the effect of GO concentration on the optical absorbance of UV–visible light and energy bandgap is also considered.

Influence of SiO2 nanoparticles on electro-optical and thermophysical properties of polyacrylate/liquid crystal composites

ABSTRACTIn this paper, SiO2 nanoparticle-doped polymer dispersed liquid crystal (PDLC) lenses are made from a mixture of prepolymer, 5CB liquid crystal and SiO2 nanoparticles by the polymerisation induced phase separation (PIPS) process. The effect of SiO2 nanoparticles on the electro-optical properties of PDLC films are studied. It is established that SiO2 nanoparticles affect the microstructure of PDLC films significantly because of the formed agglomerates of SiO2 nanoparticles. Results show an improvement in the electro-optical properties and a decrease in the response time for doped systems with small amount of SiO2 nanoparticles. We also observe a shift of nematic–isotropic transition temperature as a function of SiO2 nanoparticle contents. A good agreement between the electro-optical study and thermophysical properties is reached.

Dispersion of multiferroic BiFeO3 nanoparticles in nematic liquid crystals

Dispersion of ferroelectric- and ferromagnetic nanoparticles in liquid crystals (LCs) is investigated in the last decades. Recently, doping multiferroic-BiFeO3 nanoparticles in the LCs has become of interest. These nanoparticles have coupled ferroelectric- and ferromagnetic properties at ambient temperature. In the present experiments, a nematic LC was doped with a low concentration of BiFeO3 nanoparticles, synthesized using a modified Pechini method. The particles were characterized and their dispersions in heptane with an added surfactant were used for doping of the LC. The measured dielectric constants in the doped LCs varied as compared to the pure LC. It was shown that the presence of a magnetic field in the cooling process (cooling down from isotropic to anisotropic phase) of the LC cells leads to a significant increase of dielectric anisotropy. Also, the transition temperature decreased in the nematic-isotropic transition temperature for the doped sample.

Temperature dependence of voltage-controlled polarization plane rotation in a magnetic and electro-optic heterostructure

We propose a theoretical study of the influence of temperature on the state of polarization of a near-infrared light beam reflected from or transmitted through a voltage-controlled nematic liquid crystal (LC) cell deposited on a magnetic film. In both cases, the change in polarization with respect to that of the incoming beam originates in the linear Kerr or Faraday effects taking place in the magnetic layer, but the overall value of the polarization plane rotation depends on the reflectivity and transmittivity of the whole heterostructure. Temperature manifests itself in all layers of the structure through both thermal expansion and the thermo-optic effect when it varies between room temperature and the temperature of nematic-isotropic phase transition in the LC. The thermo-optic effect is strongest in the LC cell, where it is shown to induce noticeable variations of the polarization plane rotation of both reflected and transmitted light beams in that temperature range. Such temperature-dependent changes of the magneto-optical response can be compensated for by adjusting the dc voltage applied to the LC. On the other hand, we show that they may be exploited for sensitive temperature monitoring.

Thermoresponsive liquid crystalline polymer membranes that undergo phase transition at body temperature

Biological membranes regulate their properties and functions by drastic changes in their organized structure and mobility in response to changes in environmental factors such as pH, temperature, and biomolecule concentration. Liquid crystalline polymers (LCPs) are widely used as functional materials in a variety of fields because, similarly to biological membranes, they undergo abrupt changes in their organized structures and mobilities at their phase transition temperatures. In order to design smart membranes that regulate drug permeability as a function of temperature, we focused on well-organized and dynamic LCPs that undergo liquid crystalline-isotropic phase transition around body temperature. In this study, side-chain LCPs composed of a flexible siloxane main chain and pendant mesogens were synthesized and LCP membranes were prepared by casting the LCPs/xylene on porous support membranes. The nematic-isotropic phase transition temperature (TNI) of the resulting LCPs strongly depended on the mesogen content. Although the permeation of drugs through the LCP membranes was hindered at temperatures below TNI, their drug permeability was drastically enhanced over TNI. Thermoresponsive regulation of drug permeation through the LCP membranes is attributed to the drastic changes in their well-organized and dynamic structures that are induced by the nematic-isotropic phase transition.

Fumed alumina-in-nematic liquid crystal suspensions under shear and electric field

We study rheology, microstructure, and response to an applied electric field (E) in suspensions of fumed alumina (Al2O3) nanoparticles in a nematic liquid crystal (NLC) made of N-(4-methoxybenzylidene)-4-butylaniline (MBBA). Fumed Al2O3/MBBA suspensions exhibit flowability with nanoparticle volume fraction (ϕ) = 0.001 and 0.007, and become solid-like gels at a nanoparticle ϕ = 0.014 and beyond. The dynamic rheology of gel-like suspensions follows the soft glass rheology (SGR) model. The effective noise temperature remains close to 1 for these Al2O3/MBBA suspensions, which serves as an indication of the presence of glassy dynamics. Further, the optical microscopy and the differential scanning calorimetry (DSC) reveal that the incorporation of fumed Al2O3 nanoparticles causes a significant depression in the nematic-isotropic phase transition temperature (TNI). The gel-like suspensions are less sensitive to pre-shear and show a large structural recovery after shear when compared to Al2O3/silicone oil suspensions. At and above a critical nanoparticle ϕ = 0.005, the suspensions exhibit a reversible nematic-isotropic phase transition under the application of E.

Quantum and Thermodynamic Estimation of Mesostate Behaviour of Alkyl Benzoic Acids in Dielectric Medium: Comparative Study

Quantum and thermodynamic estimation of mesostate behaviour of p-n-alkyl benzoic acid with pentyl (PBA), hexyl (HBA) groups has been studied in dielectric medium (benzene). The complete neglect differential overlap (CNDO/2) method has been implemented for the charges, dipole moment and its components analysis. The long-range intermolecular interactions have been examined using the modified Rayleigh–Schrodinger perturbation method coupled with multicentred multipole expansion method. For understanding the short-range interactions, a ‘6-exp’ potential function has been used. The interaction energy values have been used to estimate the probability of each configuration in a dielectric medium (benzene) using the Maxwell–Boltzmann formula at room temperature (300 K) and at nematic–isotropic transition temperatures. Thermodynamic view has been presented based on Helmholtz free energy and entropy. A correlation has been made between quantum statistical and thermodynamic data to estimate the mesostate behaviour of the title molecules.

Nanoparticle-based hollow microstructures formed by two-stage nematic nucleation and phase separation

Rapid bulk assembly of nanoparticles into microstructures is challenging, but highly desirable for applications in controlled release, catalysis, and sensing. We report a method to form hollow microstructures via a two-stage nematic nucleation process, generating size-tunable closed-cell foams, spherical shells, and tubular networks composed of closely packed nanoparticles. Mesogen-modified nanoparticles are dispersed in liquid crystal above the nematic-isotropic transition temperature (TNI). On cooling through TNI, nanoparticles first segregate into shrinking isotropic domains where they locally depress the transition temperature. On further cooling, nematic domains nucleate inside the nanoparticle-rich isotropic domains, driving formation of hollow nanoparticle assemblies. Structural differentiation is controlled by nanoparticle density and cooling rate. Cahn-Hilliard simulations of phase separation in liquid crystal demonstrate qualitatively that partitioning of nanoparticles into isolated domains is strongly affected by cooling rate, supporting experimental observations that cooling rate controls aggregate size. Microscopy suggests the number and size of internal voids is controlled by second-stage nucleation.

Effect of the anchoring strength on the phase behaviour of discotic liquid crystals under face-on confinement

In this study we have performed molecular dynamics simulations to study a Gay–Berne discotic fluid confined in a slab geometry for a fixed confinement length. Four different anchoring strengths with a homeotropic (face-on) configuration were studied. We found that changing the anchoring strength changes the normal component of the stress tensor, which in turn changes the density of the system’s bulk. This phenomenon leads to a shift in the isotropic-nematic transition temperature. We observe that the temperature regions where the nematic phase is present diminishes as the anchoring strength increases. The anchoring strength also affects the nematic-columnar coexistence temperature-region: it spans over more temperatures at higher anchoring strengths.

Local Chain Alignment via Nematic Ordering Reduces Chain Entanglement in Conjugated Polymers

Chain entanglements govern the dynamics of polymers and will therefore affect the processability and kinetics of ordering; it follows that through these parameters chain dynamics can also affect charge transport in conjugated polymers. The effect of nematic coupling on chain entanglements is probed by linear viscoelastic measurements on poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and poly((9,9-dioctylfluorene-2,7-diyl)-alt-(4,7-di(thiophene-2-yl)-2,1,3-benzothiadiazole)-5′,5″-diyl) (PFTBT) with varying molecular weights. We first verify the existence of nematic phases in both PFTBT and PCDTBT and identify nematic–isotropic transition temperatures, TIN, between 260 and 300 °C through a combination of differential scanning calorimetry, polarized optical microscopy, temperature-dependent X-ray scattering, and rheology. In addition, both PCDTBT and PFTBT show a glass transition temperature (Tg) and TIN, whereas only PFTBT has a melting temperature Tm o...

Multiresponsive polymeric microstructures with encoded predetermined and self-regulated deformability

Dynamic functions of biological organisms often rely on arrays of actively deformable microstructures undergoing a nearly unlimited repertoire of predetermined and self-regulated reconfigurations and motions, most of which are difficult or not yet possible to achieve in synthetic systems. Here, we introduce stimuli-responsive microstructures based on liquid-crystalline elastomers (LCEs) that display a broad range of hierarchical, even mechanically unfavored deformation behaviors. By polymerizing molded prepolymer in patterned magnetic fields, we encode any desired uniform mesogen orientation into the resulting LCE microstructures, which is then read out upon heating above the nematic-isotropic transition temperature (TN-I) as a specific prescribed deformation, such as twisting, in- and out-of-plane tilting, stretching, or contraction. By further introducing light-responsive moieties, we demonstrate unique multifunctionality of the LCEs capable of three actuation modes: self-regulated bending toward the light source at T < TN-I, magnetic-field-encoded predetermined deformation at T > TN-I, and direction-dependent self-regulated motion toward the light at T > TN-I We develop approaches to create patterned arrays of microstructures with encoded multiple area-specific deformation modes and show their functions in responsive release of cargo, image concealment, and light-controlled reflectivity. We foresee that this platform can be widely applied in switchable adhesion, information encryption, autonomous antennae, energy harvesting, soft robotics, and smart buildings.

Influence of zinc oxide nanorods on an orientationally ordered fluid comprising soft-bent dimers

Bent-core liquid crystals (LCs) introduced a whole new dimension to the science of LCs. Besides re-emphasizing the importance of the shape of the molecule, they brought in phases with symmetry, not known earlier. Another sub-class of systems that is emerging is that of soft-bent molecules. In contrast with the bent-core molecules, here the bend is achieved through the parity of the aliphatic linker that connects two monomers. They hold the unique advantage that a simple variation of temperature can favour different conformer states and thus govern the self-assembled structure. A highlight of the power of this route is seen in terms of the discovery of a new type of nematic, viz., twist–bend nematic. Investigations on not only this phase but also the regular nematic that often precedes it have received significant attention in the last few years. Here we present results on the regular nematic phase of a binary mixture comprising such a soft-bent dimer, known in the literature as CB7CB, by incorporating zinc oxide nanorods (NRs) into the system. The NRs with an aspect ratio of \({\sim }\)7.2, not very different from that of LCs, further accentuate the importance of shape and shape anisotropy of the entities. Specifically, we observe that the nematic–isotropic transition temperature increases by \({\sim }\)1.9 K even for a low concentration of 4% NRs. While the dielectric anisotropy decreases, birefringence shows a substantial increase, adding to the complexity of the influence. Upon addition of minute amount of NRs (1%), while the splay elastic constant gets enhanced, its bend counterpart not only gets reduced but retains the convex-shaped thermal profile seen for the parent mixture.

A soft-bent dimer composite exhibiting twist-bend nematic phase: Photo-driven effects and an optical memory device

We report the influence of photoisomerization on a guest-host composite comprising a soft-bent dimer exhibiting the presently sought-after twist-bend nematic (NTB) phase, doped with small amounts of a calamitic photoactive azobenzene-based dimer that assumes a bent shape when photo-driven. Trans-cis photoisomerization of the latter compound leads to a substantial decrease in the nematic-isotropic and nematic-NTB transition temperatures. The results bring out the puzzling feature that the thermal stability of the NTB phase, having a helical structure, despite the molecules being achiral, is influenced more than the regular nematic. Differential influence is also seen in the Frank elastic constants of the regular nematic: while the splay elastic constant is significantly diminished its bend counterpart is hardly affected. Molecular conformational aspects of the entities are proposed to explain these features. Through a judicious combination of this photo-driven effect and an AC electric field, an attractive type of optical memory device is demonstrated.

Dielectric properties of a strongly polar nematic liquid crystal compound doped with gold nanoparticles

ABSTRACTThis study focuses on the electrical characteristics of a strongly polar nematic liquid crystal, Hexyloxy-cyanobiphenyl (6OCB), doped with a low concentration (2% by weight) of citrate buffer stabilised gold nanoparticles (GNPs) at low frequencies between 20 Hz and 35 MHz. The doped samples have lower values of nematic–isotropic transition temperature, permittivity (both parallel and perpendicular to the field direction) and dielectric anisotropy; however, relaxation time and activation energy were increased. The observed results could be explained on the basis of weakly anisotropic nature of GNPs and a local rearrangement of liquid crystal molecules surrounding the nanoparticles. Moreover, a complimentary suggestion on a possible change in the dipole–dipole correlation is made to explain the difference in changes (qualitative and quantitative) observed for permittivity of the host nematic liquid crystal doped with GNP. Temperature dependent dielectric relaxation studies indicate an increase in viscosity and potential barrier; and hence a change in strength of inter-molecular and intra-molecular interactions is suggested.

Thermal dependence of the lateral shift of a light beam reflected from a liquid crystal cell deposited on a magnetic film

We study the influence of the thermo-optic effect and of thermal expansion on the lateral shift experienced by a Gaussian near-infrared beam upon reflection from a voltage-controlled nematic liquid crystal cell deposited on a magnetic yttrium-iron garnet film. Variations of temperature are considered in the range between room temperature and the nematic-isotropic phase transition temperature of the liquid crystal and induce changes in both the layer thicknesses and the permittivity tensor components of the constituents. We show that for all polarization configurations of the incoming and reflected beams, these changes modify the amplitude of the extrema of the lateral beam shift and their position with respect to the incidence angle of the beam and, except in the s–s polarization configuration, to the voltage applied to the liquid crystal cell. In the p–p and p–s polarization configurations, this drift can thus be controlled at some incidence angles by tuning the applied voltage. Moreover, in the p–s configuration, the lateral shift can be also controlled by a magnetization reversal in the magnetic layer. Finally, we discuss the possibility of temperature monitoring using the temperature dependence of the lateral shift in this system.

Photopyroelectric Calorimetry Investigations of 8CB Liquid Crystal–Microemulsion System

In this work, the photopyroelectric technique has been used to investigate the phase transitions in a liquid crystal microemulsion by combining the simultaneous high temperature resolution thermal diffusivity measurements and optical polarization microscopy observations. It has been found that, during the conversion from the isotropic phase into the nematic one, the micelles are expelled from the nematic domains and remain confined in islands of isotropic material which survive down to the smectic temperature range. A hysteresis in the thermal diffusivity profiles between heating and cooling run over the isotropic–nematic transition temperature range has been observed which has been ascribed to the different micelles distribution into the sample volume during cooling and heating runs. Finally, the almost bulk-like behavior of the thermal diffusivity over the nematic–smectic phase transition confirms that a significant fraction of the micelles are expelled during the nucleation of the nematic phase.

Reducing the actuation threshold by incorporating a nonliquid crystal chain into a liquid crystal elastomer.

Liquid crystal elastomers (LCEs) are important smart materials that can undergo reversible deformation in response to liquid crystal (LC) phase transitions. A low threshold temperature for LC phase transition is advantageous because the LCE material can be more conveniently actuated by the applied stimulus. In this work, we investigated the effect of a nonliquid crystal chain on the reduction of threshold temperature of the LC phase transition by linking a nonliquid crystal side chain, 4-methoxyphenyl-1-hexenyloxy (MOCH3), to the network backbone of a classical polysiloxane-based side-chain nematic LCE. The nematic–isotropic transition temperature (Tni) of the MOCH3 incorporated nematic LCE was lower than that of the normal nematic LCE without the incorporation of a nonliquid crystal chain by about 27 °C. Compared to the normal nematic LCE or its nanocomposite, the MOCH3 incorporated nematic LCE or its nanocomposite demonstrated more rapid thermo-actuated deformation or photo-actuated deformation, and can be actuated to attain full axial contraction at an obviously lowered temperature or by light with obviously lowered intensity, while the maximum contraction ratio basically did not vary. These research results indicate that some nonliquid crystal chains show potential for improving the characteristics and enhancing the application significance of LCE materials.

Nanoparticles-enhanced photonic liquid crystal fibers

Optical properties of photonic crystal fibers (PCFs) as well as micro-capillaries infiltrated with a 6CHBT liquid crystal (LC) doped with spherical silver (Ag) nanoparticles (NPs) have been studied. Three different concentrations of Ag NPs dopants were taken into consideration and their presence in the 6CHBT LC was found to modify the spectral properties of the infiltrated PCF. A significant improvement of switching times was observed for the NPs-doped LC-infiltrated PCF. Moreover, the Ag NPs-doped 6CHBT LC inside both micro-capillaries and PCFs was found to reduce the nematic-isotropic phase transition temperature.

Phase behaviour and electro-optical response of systems composed of nematic liquid crystals and poly (2-ethylhexylacrylate)

ABSTRACTExperimental studies of phase diagram and morphology of mixtures composed of linear poly(2-ethylhexylacrylate) (poly-2EHA) of molecular weight Mw = 92000g/mol and E7, an eutectic blend of nematic liquid crystals (LCs), were performed. The latter consists of four cyanoparaphenylenes, i.e. 4-cyano-4ʹ-n-pentyl-biphenyl (5CB), 4-cyano-4ʹ-n-heptyl-biphenyl (7CB), 4-cyano-4ʹ-n-oxyoctyl-biphenyl (8OCB) and 4-cyano-4ʹ’-n-pentyl-p-biphenyl (5CT). Thermal properties of the poly-2EHA/E7 system were studied by differential scanning calorimetry and the sample morphologies were investigated by optical microscopy. Phase diagram analysis revealed an increase of the nematic–isotropic transition temperature when E7 was added to poly-2EHA, compared to pure E7. This effect, related to a preferential miscibility phenomenon of E7 components towards poly-2EHA, was evidenced by gas chromatography analysis coupled to mass spectrometry, allowing to qualify and quantify the content of phase separated LC domains. Thermograms of model LC mixtures including 5CB, 7CB, 8OCB and 5CT, which correspond to their composition in segregated LC domains of poly-2EHA/E7, present strong deviations from eutectic behaviour. In order to access the impact of such variations for practical applications, such a model LC mixture was introduced in photochemically crosslinked 2-EHA (polycross-2EHA). Surprisingly, the electro-optical response of the obtained polycross-2EHA/LC films was considerably improved compared to the original polycross-2EHA/E7 system.