James T. Gleeson

The polarization and mass density modulation associated with antiferroelectric ordering in two ferroelectric nematic liquid crystals, a prototypical single compound (referred to as DIO) and a commercial mixture (FNLC919), are studied experimentally as a function of temperature. The wave number of the polarization modulation, q A , is determined by small-angle x-ray diffraction from the weak, accompanying density wave (wave number q S = 2 q A ) using a conventional laboratory source. Combined results for q S and the saturated value of the polarization are analyzed in terms of Landau theory previously developed to describe the paraelectric/antiferroelectric/ferroelectric sequence of phase transitions in solid ferroelectrics. The analysis of q S indicates that the polarization modulation can be reasonably well approximated by a simple sinusoid in DIO, whereas in FNLC919 a sharp decrease in q S close to the antiferroelectric to ferroelectric transition indicates development of a solitonlike profile.

Among the recently developed ferroelectric nematic liquid crystals, FNLC-919, synthesized by Merck Electronics KGaA, stands out for its stable, room-temperature, ferroelectric nematic (N_F) phase. This renders it a promising candidate for both fundamental research and device-level applications. In this study, we present a comprehensive experimental investigation of FNLC-919, focusing on its structural, optical, dielectric, and elastic properties in the paraelectric nematic (N) and the intermediate antiferroelectric phase (dubbed N_X) that occur in a temperature range between the N and N_F phases. Key material parameters such as ferroelectric polarization, viscosity, and nanostructure are characterized as functions of temperature in all mesophases, while the orientational elastic constants are determined only in the N and N_X phases. Our findings are compared with prior results concerning the benchmark compound DIO that also exhibits the phase sequence N-N_X-N_F and reveals a smectic-like mass density wave coinciding with antiferroelectric ordering in the N_X phase.

A dynamic light scattering study of director fluctuations in the antiferroelectric (AF) phase of the ferroelectric nematic liquid crystal DIO is reported. The AF phase occurs in a temperature range between nonpolar (paraelectric) and ferroelectric nematic states. The behavior of the observed fluctuation modes is consistent with the smectic-Z_A model of the AF phase -- namely, a smectic layer structure defined by fluid layers of alternating electric polarization, with the polarization field and nematic director both oriented parallel to the layer planes. This model has been advanced to explain a large body of previous experimental results on DIO. We discuss the wave vector dependence of the modes in terms of a dynamical theory based on the elastic free energy density of a smectic-C phase in the limit of 90^ο director tilt and an approximation of the viscous stresses by their form for an incompressible uniaxial fluid. We also present the pretransitional temperature dependencies of the splay, twist, and bend elastic constants and the corresponding viscosities in the paraelectric nematic phase.

The piezoelectric phenomena observed in a wide range of materials from crystalline solids to 3D liquids are reviewed. Special emphasis is given to recent measurements of the piezoelectric responses of fluid ferroelectric nematic liquid crystals.

Nematic liquid crystals are anisotropic fluids which have long-range one-dimensional orientational order and short-range spatial correlations corresponding to the molecular length L. In X-ray studies this is manifested as diffuse peaks along the average direction of the molecular long axis at Q = 2π/L and weaker harmonics at 2Q and 3Q wave numbers. This is the case for the recently discovered ferroelectric nematic (N_F) liquid crystals as well. Here we synthesized highly polar three ring rod-shaped compounds with a terminal thiophene ring which on cooling from the isotropic fluid directly transition to the N_F phase that shows the strongest spatial correlations corresponding to 1/3 of the molecular length (L/3). The set of thiophene compounds reported here have ferroelectric polarizations about 20% larger than that of usual ferroelectric nematic liquid crystal materials. This is the result of the tighter molecular packing and larger mass density, due to the lack of flexible terminal chains of these thiophene compounds compared to most of the N_F materials. Below the N_F phase, compounds with a single nitro or two cyano polar groups on the terminal benzene ring exhibit a so far never observed smectic phase with periodicity ∼1/3 the molecular length. Based on our experimental results, we propose a model of this phase featuring antipolar packing of the molecules within the layers.

Freedericksz transitions in the nematic and smectic ZA phases of DIO reveal important information about the elastic properties of this material.

We report dynamic light scattering measurements of the orientational (Frank) elastic constants and associated viscosities among a homologous series of a liquid crystalline dimer, trimer, and tetramer exhibiting a uniaxial nematic (N) to twist-bend nematic (N_TB) phase transition.

We report results on the high-field magneto-optical response of four aqueous, lyotropic, chromonic liquid crystal formulations in the isotropic phase. Measurements of the field-induced birefringence at temperatures above the isotropic-nematic coexistence region at high magnetic fields reveal qualitative differences in different materials; these differences can be attributed to the nature of aggregation and are discussed within the context of competing aggregation models. Extending these measurements to very high fields and large optical phase differences reveals the presence of an unexpected optical phenomenon: magnetic field-induced circular birefringence, measured in the Voigt geometry, in a system containing no molecularly chiral species. Possible origins of this effect are discussed.

The existence of the elusive biaxial phase has been the subject of much discussion since it was predicted by Freiser in 1970. More recently, there have been numerous attempts to find a thermotropic liquid crystal that exhibits a biaxial phase and with this, conflicting reports about whether such a phase has been positively identified in bent-core liquid crystals. One reason for the discrepancy is that there is currently no way to rule out surface effects or anchoring transitions, both of which may give a false positive identification of a uniaxial-biaxial nematic transition. We have developed a technique that uses a magnetic field to align the uniaxial director, thus widening its application to any bent-core nematic material.

We report small angle x-ray scattering (SAXS) studies of isotropic, nematic, and smectic mesophases formed by binary mixtures of bent-core (BC) and rod-shaped (RS) molecules. While optical studies indicate that the components are fully miscible, SAXS reveals fascinating structures that are consistent with segregation on a nanoscopic scale. We find that tilted smectic clusters, which have been previously reported in both the nematic and isotropic states of the pure BC materials, are also present in mixtures with up to 50 wt% of the RS compound; this is consistent with previous dielectric and flexoelectric studies on such mixtures. Unexpectedly in this concentration range the clusters are present in the isotropic and in the induced smectic phase range, as well as throughout the nematic phase. The results in the smectic phase also reveal complex layering phenomena, providing important insight into the interaction between bent and rod-shaped molecules. These studies will be crucial in the design of promising new functional nanomaterials.