Uniaxial Smectic Phases Research Articles

Molecular simulation study of polar order in orthogonal bent-core smectic liquid crystals.

We explore the phase behavior and structure of orthogonal smectic liquid crystals consisting of bent-core molecules (BCMs) by means of Monte Carlo molecular simulations. A simple athermal molecular model is introduced that describes the basic features of the BCMs. Phase transitions between uniaxial and biaxial (antiferroelectric) orthogonal smectics are obtained. The results indicate the presence of local in-plane polar correlations in the uniaxial smectic phase. The macroscopic uniaxial-biaxial transformation is rationalized in terms of local polar correlations giving rise to polar domains. The size of these polar domains grows larger under the action of an external vector field and their internal ordering is enhanced, leading to field-induced biaxial order-disorder transitions.

Development of polar order and tilt in lamellar liquid crystalline phases of a bent-core mesogen

A new bent-core mesogen combining a 4-cyanoresorcinol unit with two terephthalate based rod-like wings and terminated by two long alkyl chains, was synthesized and investigated by DSC, XRD, optical, electrooptical and dielectric methods. A series of liquid crystalline phases in the unique sequence SmA-SmA(P)-SmCPR-(M1/SmCPα)-SmCsPA-SmCPA-SmCaPA, mainly distinguished by the degree and mode of correlation of tilt and polar order, was observed. The development of polar order is associated with the emergence of a small tilt (<10°). With decreasing temperature the tilt changes from random (SmA) via synclinic to anticlinic, while the coherence length of the polar domains grows. This small tilt gives rise to an only weak layer coupling which is in competition with the polar coupling and this leads to new modes of self assembly in lamellar phases of bent-core mesogens, among them the SmCPR and the SmCPα phases. The SmCPR phase is an only slightly tilted biaxial smectic phase with randomized polar order and the SmCPα phase is a slightly tilted and antiferroelectric switching, but uniaxial smectic phase. For this phase a regular change of the in-plane polarization vector between the layers by an angle between >0° and <90° is proposed.

Chiral Liquid Crystal Trimer Exhibiting an Optically Uniaxial Smectic Phase with a Double-Peak Polarization

We prepared a chiral liquid crystal trimer consisting of three different mesogenic units connected via flexible spacers with different parity. We then investigated its liquid crystalline properties using polarized optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). A POM study revealed that the trimer has a phase sequence of isotropic liquid–optically uniaxial smectic A (SmA*)–helical smectic C (SmC*) on cooling. However, XRD measurements revealed a discontinuous decrease in layer spacing occurring at T – TIA* = −4 K in the SmA* phase. Double-peak polarization in the current response and tristable switching were observed in the low-temperature SmA* phase below T = TIA* – 4 °C. In a binary phase diagram of the trimer and its enantiomer, the transition temperature from the low-temperature SmA* phase to the SmC* phase increased continuously with decreasing optical purity. The results indicate that the low-temperature SmA* phase forms an optically uniaxial structure in which the molecules tilt to the same direction in each layer, as they do in the SmC phase. We discuss the organization of the supermolecules in the unusual smectic phase.

Sequence of Four Orthogonal Smectic Phases in an Achiral Bent-Core Liquid Crystal: Evidence for theSmAPαPhase

The mesomorphic properties of an achiral bent-core liquid crystal derived from 4-cyanoresorcinol are studied by polarizing optical microscopy, x-ray diffraction, and second harmonic electro-optic response. It shows a novel sequence of four nontilted or orthogonal smectic phases on cooling: SmA-SmAP(R)-SmAP(X)-SmAP(A). Here SmAP(X) is the new orthogonal polar uniaxial smectic phase. The electric-field-induced transformations in the SmAP(X) phase give rise to two biaxial states separated by a uniaxial one. The second harmonic electro-optic response in this phase is interpreted in terms of the polar interaction with the electric field. A comparison of the experimental results with the next-nearest-neighbor model for the structure of the SmAP(X) phase shows it to be an SmAP(α) phase.

Silylated bent-core molecules: the influence of the direction of the carboxyl connecting groups on the mesophase behaviour

Novel bent-core molecules based on phenyl benzoate rigid cores and heptamethyltrisiloxane units at one or at both ends of the aliphatic side chains were synthesized and investigated by polarized light microscopy, differential scanning calorimetry, X-ray diffraction and electrooptical methods and the effect of the direction of the carboxyl groups on the mesomorphic self assembly was investigated. A series of distinct liquid crystalline phases was found, among them ferroelectric and antiferroelectric switching and also non-switchable smectic, undulated smectic (USmC) and modulated smectic phases (columnar phases, Colob). All polar phases show field-induced inversion of chirality at least in a certain temperature range. Moreover, two non-polar and uniaxial smectic phases were observed, one with a hollow cone-like orientational director distribution (SmCR) and the other one with a maximum of the director distribution at zero tilt (SmA). These two phases were discussed in relation to the distinct models of de Vries type smectic phases.

Electric-Field-Induced Polar Biaxial Order in a Nontilted Smectic Phase of an Asymmetric Bent-Core Liquid Crystal

A polarization reorientation process has been studied by means of optical second-harmonic generation in an optically uniaxial smectic phase of an asymmetric bent-core liquid crystal. A nontilted polar smectic order with a biaxial order is induced by applying an electric field to the uniaxial nonpolar smectic phase. This phenomenon was well simulated by the two-dimensional Langevin process, i.e., electric-field-induced continuous molecular reorientation against thermal agitation. The simulation suggests that about 200s of molecules form a polar domain and cooperatively respond to the applied field. The existence of the polar domains and their reorientation are consistent to the dielectric measurement; a high dielectric constant of about 60 at the kHz range is markedly suppressed by applying a bias field.

Phase diagram of transitions from an isotropic phase to nematic and smectic (uniaxial, biaxial) phases in liquid crystals with achiral molecules

The phase diagrams of transitions from an isotropic phase to nematic and smectic phases are investigated within a simple phenomenological model of the Landau thermodynamic potential. The conditions of the isomorphic phase transition between two uniaxial smectic phases and the direct transition from the isotropic phase to the uniaxial and biaxial smectic phases are determined. The behavior of the order parameters is described along different thermodynamic paths. The theoretical results are discussed using the example of liquid-crystal phases in compounds with banana-shaped achiral molecules.

Liquid Crystalline Phase Transition in Hexakis(4-(N-(41-alkoxyphenyl)iminomethyl)phenoxy)cyclotriphosphazene

A new series of cyclotriphosphazenes bearing Schiff bases with different carbon number alkoxy end groups(HAICP) in the side chains were synthesized and their phase transitions and liquid crystallinity were studied using DSC measurements and polarizing microscope observations. We found a monotropic Sml phase for n = 3 and an enantiotropic Sml phase for n = 4 −5, enantiotropic Sml, SmC and SmA phases for n = 6 – 7, enantiotropic SmC and SmA phases for n = 8 – 10, and enantiotropic Sm2, SmC and SmA phases for n=11 and 12 in the HAICP series. The Sml phase is a low-order uniaxial smectic phase and Sm2 phase is a low-order biaxial smectic phase. The melting point decreased with an increase in the end alkoxy carbon number, n, of the cyclotriphosphazenes, but the clearing point decreased with an increase in n. As a result, the mesomophic region expands with an increase in n. The melting and clearing points do not show clear odd-even effects, which is caused by the peculiar shape of the cyclotriphosphazenes.

Synthesis of Some New Fluorene Esters with Mesomorphic Behavior

Abstract A number of 2-fluorenyl 4′-alkylbenzoates and 4′-alkoxybenzoates have been synthesized to study the effects of the central ester linkage on their mesomorphic behavior. These compounds typically show nematic (N) and the uniaxial smectic phases. Many isomeric fluorene esters with reversed ester linkage were also prepared, but no major differences were observed. The presence of a lateral substituent in the phenyl ring decreases the nematic-isotropic (N-I) temperature by about 28°C. Replacement of phenylene by trans-cyclohexylene has the unusual effect of increasing the N-I transition temperature by about 21°C. The presence of a nitro group at position (7) of the fluorene ring increases N-I temperature by about 66°C and stabilizes the SA.

Effect of structure on the mesomorphic properties of cholesteryl alkanoates. V. Electric field effects on a uniaxial smectic phase of mixtures of cholesteryl alkanoates

Effect of structure on the mesomorphic properties of cholesteryl alkanoates. V. Electric field effects on a uniaxial smectic phase of mixtures of cholesteryl alkanoates