Biaxial smectic A liquid crystal in a pure compound

Simple molecular model for ferroelectric nematic liquid crystals exhibited by small rodlike mesogens

Nematic liquid crystals (NLCs) are the prime example of a liquid medium with an apolar orientational order. In the past couple of years, the ferroelectric nematic (FN) phase has been discovered in some compounds with small rodlike molecules with large longitudinal dipole moments and very restricted chemical structures, as the temperature is lowered from the NLC. We propose a simple model in which the molecules are idealized as cylindrical rods with longitudinal surface charge density waves. The usually strong electrostatic inter-rod interactions favoring antiparallel structures are shown to be subdued in magnitude, and those of parallel structures enhanced, by reducing the amplitudes of the half-waves at both ends of the rods. By introducing an additional increased amplitude of one interior wave, the energy per rod of a cluster of molecules with a pseudohexagonal order is shown to favor the ferroelectric order compared to the antiparallel order, below some value of the inter-rod separation. The model broadly accounts for the restriction in molecular structures for a compound to exhibit the FN phase. It is suggested that the weakly first-order nature of the NLC to FN transition arises from a coupling of the polar order and the density of the medium.

Microscopic shear flow simulations of a biaxial smectic A liquid crystal based on the soft ellipsoid string-fluid

We have studied the behaviour of a biaxial smectic A liquid crystal based on the soft ellipsoid string-fluid in shear flow by molecular dynamics simulation using the SLLOD equation of motion. This is facilitated by the fact that the biaxial symmetry allows linear relations between the pressure and the velocity gradient. This means that linear irreversible thermodynamics can be applied independently of the simulations to obtain the torques determining the orientations of the system and that the predictions of this theory can be cross-checked by the simulations. It turns out that there is a torque turning the smectic layers to the orientation parallel to the vorticity plane if the simulation is started in another orientation. In the orientation parallel to the vorticity plane where the director formed by the long axes of the molecules, nw, is perpendicular to the vorticity plane there is another torque keeping the director formed by the normals of the broadsides of the molecules, nu, parallel to this plane at a constant alignment angle, ψ relative to the streamlines independently of the strain rate. Moreover, this alignment angle seems to be the one where the irreversible energy dissipation rate, , is minimal. This is in agreement with a recently proven theorem according to which is minimal in the linear regime of a nonequilibrium steady state. Finally, we studied the orientation of nu when the smectic layers are parallel to the shear plane. In a simulation this orientation is stabilised by the periodic boundary conditions. Then we found that there was a nonlinear torque turning nu to the orientation perpendicular to the streamlines thus minimising the value of even though this value is larger than the value of in the orientation parallel to the vorticity plane. This means that is minimized given the external boundary conditions.

Simple model for the stripe phase in compounds with bent-core molecules which exhibit a lower-temperature ferroelectric smectic-A phase

Bent-core (BC) molecules usually exhibit polar packing in smectic layers in which the long or bow axes tilt with respect to the layer normal. In many compounds, the tilt angle goes to zero, and typically the polarization (P) of neighboring layers has an antiferroelectric order (SmAP_{AF}). A careful molecular engineering has led to the discovery of the ferroelectric SmAP_{F} phase in a few BC compounds. Detailed experimental studies [Zhu et al., J. Am. Chem. Soc. 134, 9681 (2012)10.1021/ja3009314] have shown that one of the compounds undergoes a weak first order transition to a striped phase (SmAP_{Fmod}), in which the repolarization switching occurs at a threshold electric field, the latter increasing with temperature. The main optical birefringence of the SmAP_{Fmod} phase increases rapidly with the field at lower temperatures of its range. A small (<10%) fraction of the BC molecules can be expected to have less bent conformations in excited states (ESs), and we argue that the ES conformers rotate freely about the bow axes, and aggregate to form regions without any polar order to gain rotational entropy. In turn, a favorable divP term leads to the formation of stripes in the polarized regions made of ground state conformers. Based on this physical picture, we develop a simple phenomenological model to reproduce all the experimental trends qualitatively. In sample cells with insulating layers neighboring stripes prefer to have an antiparallel orientation of the mean P direction. The dielectric constant of the stripe phase increases with a dc bias electric field, a trend which is opposite to that in other ferroelectric liquid crystals.

Orthogonal smectic and nematic ordering in three-ring polar bent-core molecules with anti-parallel arrangement

Three-ring based BLCs exhibiting orthogonal smectic and nematic phases with antiparallel arrangement as derived from SCXRD and X-ray scattering results.

A new family of four-ring bent-core nematic liquid crystals with highly polar transverse and end groups

Non-symmetrically substituted four-ring achiral bent-core compounds with polar substituents, i.e.., chloro in the bent or transverse direction in the central core and cyano in the lateral direction at one terminal end of the molecule, are designed and synthesized. These molecules possess an alkoxy chain attached at only one end of the bent-core molecule. The molecular structure characterization is consistent with data from elemental and spectroscopic analysis. The materials thermal behaviour and phase characterization have been investigated by differential scanning calorimetry and polarizing microscopy. All the compounds exhibit a wide-ranging monotropic nematic phase.

Improvement in device performance from a mixture of a liquid crystal and photosensitive acrylic prepolymer with the photoinduced vertical alignment method

In a multicomponent nematic liquid crystal (NLC) mixture of a liquid crystal (negative-type NLC) and a photosensitive acrylic prepolymer, photopolymerization upon UV irradiation induces the separation of the LC and photosensitive acrylic prepolymer layers, thereby leading to a vertical arrangement of LC molecules. In this study, we propose a simple vertical alignment method for LC molecules, by adding a chiral smectic A (SmA∗) liquid crystal having homeotropic texture characteristics to an NLC mixture solution. Measurements of electro-optical properties revealed that the addition of the SmA∗ LC not only strengthened the anchoring force of the copolymer alignment film surface, but also significantly enhanced the contrast ratio (∼73%), response time and grayscale switching performance of the device.

Ferroelectric behavior of orthogonal smectic phase made of bent-core molecules

Ferroelectric behavior in the recently reported orthogonal ferroelectric Sm-A(d)P(F) phase in an unsymmetric bent-core molecule with a carbosilane terminal group was studied. The ferroelectricity of the Sm-A(d)P(F) phase was unambiguously confirmed by optical second-harmonic generation activity in the absence of an electric field, ferroelectric response, and high dielectric strength. The long-range polar order is a consequence of weakened interlayer coupling due to the formation of carbosilane sublayers, which allows for the parallel order of dipole moments of bent-core molecules in the neighboring layers. It develops in the system gradually through the second-order phase transition from the orthogonal Sm-A(d) phase. In the Sm-A(d)P(F) phase the strong surface anchoring results in the splay of polarization across the sample thickness. The polar surface anchoring also brings about strongly thickness-dependent polar fluctuations, as proved by the dielectric measurements (Goldstone-like mode). The relaxation frequency and dielectric strength vary more than one order of magnitude with cell thickness; in particular the dielectric strength attains more than 2000 in a 25 μm-thick cell and continues to increase for thicker cells. Simple theory developed qualitatively explains the experimental results, supporting the polarization splay model proposed.

Polar switching in the smectic- A(d)P(A) phase composed of asymmetric bent-core molecules

We have studied mesogenic properties in the smectic- A(d)P(A) phase of an asymmetric bent-core liquid crystal by means of polarizing optical microscopy, second-harmonic generation (SHG), electro-optical (EO), and dielectric measurements. In homeotropically aligned cells, EO switching is clearly observed from a schlieren texture with both four- and two-brush defects to a uniform bright domain under the application of very low in-plane electric field below 1 V/microm. The phase is SHG active under an electric field, i.e., SHG gradually increases without a threshold and saturates with the increasing field. In homogeneous cells, by applying triangular wave field at saturated voltage, the splitting of the polarization switching current peak is observed, indicating that the mesophase is antiferroelectric. The dielectric studies indicate a Debye-type relaxation of the transverse dipoles associated with the rotation of the molecules about their long axis. The dc bias-dependent dielectric relaxation time and the dielectric strength suggest that a field-induced antiferroelectric-ferroelectric phase transition occurs continuously beyond 1.2 V/microm and is reversible. The field-dependent texture observation is consistent with the dielectric measurements. Two possible models are proposed to interpret the continuous phase transition.