Demetri J. Photinos

Biaxial nematic phases

An overview over the current state of research in the field of biaxial nematic liquid crystalline materials is given. After a short introduction, providing some general aspects and summarizing the classical approaches, the main part focuses on recent developments of new concepts for designing biaxial nematics. First, the major theoretical concepts are outlined, including the classification to different symmetries, the importance of cooperativity and cluster formation for the development of biaxial order and the conditions for the establishment of field induced and spontaneous biaxiality in nematic phases. These new concepts also require the re-evaluation of the tools used for the identification of phase biaxiality, which are discussed briefly. In the second part, recent progress in the design of potential biaxial nematic materials, especially focussing on bent-core molecules with nematic phases, is reported and, finally, comparisons with phase biaxiality as observed in smectic liquid crystals are made.

Thermotropic biaxial nematic liquid crystals: Spontaneous or field stabilized?

An intermediate nematic phase is proposed for the interpretation of recent experimental results on phase biaxiality in bent-core nematic liquid crystals. The phase is macroscopically uniaxial but has microscopic biaxial, and possibly polar, domains. Under the action of an electric field, the phase acquires macroscopic biaxial ordering resulting from the collective alignment of the domains. A phenomenological theory is developed for the molecular order in this phase and for its transitions to purely uniaxial and to spontaneously biaxial nematic phases.

Biaxial nematics: symmetries, order domains and field-induced phase transitions †

We studied the symmetry and spatial uniformity of the orientational order of the biaxial nematic phase in the light of recent experimental observations of phase biaxiality in thermotropic bent-core and calamitic-tetramer nematics. Evidence is presented supporting monoclinic symmetry, instead of the usually assumed orthorhombic symmetry. The use of deuterium nuclear magnetic resonance to differentiate between the possible symmetries is described. The spatial aspects of biaxial order are presented in the context of the cluster model, wherein macroscopic biaxiality can result from the field-induced alignment of biaxial and possibly polar domains. The implications of different symmetries on the alignment of biaxial nematics and on the measurements of biaxial order are discussed in conjunction with the microdomain structure of the biaxial phase.

Symmetries and alignment of biaxial nematic liquid crystals

The possible symmetries of the biaxial nematic phase are examined against the implications of the presently available experimental results. Contrary to the widespread notion that biaxial nematics have orthorhombic symmetry, our study shows that a monoclinic (C(2h)) symmetry is more likely to be the case for the recently observed phase biaxiality in thermotropic bent-core and calamitic-tetrapode nematic systems. The methodology for differentiating between the possible symmetries of the biaxial nematic phase by NMR and by IR spectroscopy measurements is presented in detail. The manifestations of the different symmetries on the alignment of the biaxial phase are identified and their implications on the measurement and quantification of biaxiality as well as on the potential use of biaxial nematic liquid crystals in electro-optic applications are discussed.

The phase behavior of a binary mixture of rodlike and disclike mesogens: Monte Carlo simulation, theory, and experiment

The phase behavior of a binary mixture of rodlike and disclike hard molecules is studied using Monte Carlo NVT (constant number of particles N, volume V, and temperature T) computer simulation. The rods are modeled as hard spherocylinders of aspect ratio LHSC/DHSC=5, and the discs as hard cut spheres of aspect ratio LCS/DCS=0.12. The diameter ratio DCS/DHSC=3.62 is chosen such that the molecular volumes of the two particles are equal. The starting configuration in the simulations is a mixed isotropic state. The phase diagram is mapped by changing the overall density of the system. At low densities stabilization of the isotropic phase relative to the ordered states is seen on mixing, and at high densities nematic–columnar and smectic A–columnar phase coexistence is observed. Biaxiality in the nematic phase is not seen. The phase diagram of the mixture is also calculated using the second virial theory of Onsager for nematic ordering, together with the scaling of Parsons and Lee to take into account the high...

On the Origins of Spontaneous Polarization in Tilted Smectic Liquid Crystals

The rapid electrooptic response of ferroelectric liquid crystals-stratified (smectic) melts exhibiting spontaneous electric polarization-makes them candidates for the next generation of electrooptic shutters and liquid crystal displays. The model advanced here suggests that polarity is indigenous to tilted smectics and that spontaneous polarization derives from the packing constraints mesogens experience, a tilt-dependent correlation of molecular conformation with orientation. First-rank orientational order parameters (CxX) are nonzero with a tilt- (temperature-) dependent magnitude, and under certain packing conditions, the sign of CxX varies with tilt. These results have a direct bearing on experimental observations because most ferroelectric liquid crystals exhibit a tilt-dependent spontaneous polarization and some show an inversion of the sign of the polarization.

Hydrogen-bonding and Phase Biaxiality in Nematic Rod-Plate Mixtures

Abstract We study the possibility of using hydrogen-bonding interactions to promote the stabilisation of phase biaxiality in nematic binary mixtures of oblate and prolate thermotropic mesogens. We extend Onsagers theory of the isotropic-nematic transition to allow for such selective associations among unlike species and we use it to calculate the phase diagram of binary mixtures consisting of hard spherocylinders and cut spheres. The results show that directional, short-ranged attractions between rods and discs strongly stabilise the biaxial nematic mixture against demixing and suggest that interactions of hydrogen-bonding type may provide an efficient mechanism for sustaining phase biaxiality in binary mixtures of real thermotropic nematogens. Preliminary Monte Carlo simulations designed to test such predictions are discussed.

Synthesis and mesogenic properties of novel board-like liquid crystals

The syntheses of novel octa- and tetra-substituted systems based on tetrabenz[a,c,h,j]anthracene are reported. The disc-shaped octa-substituted material exhibits a columnar mesophase, whereas the tetra-substituted material exhibits a lamellar phase. Binary mixtures of the two materials were examined in order to explore the change over from discotic to calamitic behaviour as a function of concentration. At the crossover point, the possibility of the formation of a biaxial phase was investigated.

Dielectric studies of a laterally-linked siloxane ester dimer

This paper reports measurements of the dielectric response over the frequency range 102 to 109 Hz of a liquid crystal dimer consisting of two ester mesogens laterally linked by an alkoxy chain containing a siloxane group. The synthesis and phase behaviour of the siloxane dimer are also reported. Results show that there are two relaxations in the isotropic phase and four in the nematic phase of the material. The possible molecular origins for these modes are given. It is found that there is a coupling between internal and external modes which gives rise to a cooperative mode as the temperature in the nematic phase is lowered towards a glass transition.

Electrostatic interactions in liquid crystals: ordering of rigid solutes in nematic solvents

The orientational distribution of a probe molecule in a nematic solvent is derived in terms of the intermolecular potential describing steric, dispersional, and electrostatic interactions with the solvent molecules. Orientational correlations among the latter molecules are neglected in the derivation, whereas probesolvent correlations are treated explicitly. The theory is used to analyse NMR measurements of order parameters of small solute molecules in nematic solvents. Tensor components of the solute potential of mean torque up to fourth rank are evaluated. The various electrostatic contributions (dipole-dipole, dipole-quadrupole, quadrupole-quadrupole, etc.) are assessed quantitatively. It is found that the residual overall electrostatic interaction is sensitive to the configuration of the permanent electrostatic moments within the molecular frame and to the shapes of both the solute and the solvent molecules. Contributions from the induced moments are rather small. An accurate and consistent descriptio...