Alexandros G. Vanakaras

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...

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.

Theory of Biaxial Nematic Ordering in Rod-Disc Mixtures Revisited

Abstract We use the variational cluster approximation to study the relative thermodynamic stability of the spatially uniform phases of binary mixtures of hard rods and discs. The factors promoting the stability of the biaxial nematic phase are identified and discussed. The results suggest that a stable thermotropic nematic biaxial mixture cannot be obtained from molecules of the sizes and electric dipole interaction strengths commonly encountered in real calamitic and discotic thermotropic phases.

ELECTRIC DIPOLES AND PHASE STABILITY IN NEMATIC LIQUID CRYSTALS

A theory for the nematic-isotropic (N-I) phase transition of prolate uniaxial molecules with longitudinal dipole moments is presented. The theory is based on the variational cluster expansion, truncated after the two-molecule term, and is implemented for polar hard spherocylinders with and without attractions, and for polar linear arrays of Lennard-Jones interactions centre. We find that the dipole interactions substantially shift the N-I transition temperature and strongly promote antiparallel molecular association, but have a weak effect on the order parameters, the pressure, and the N-I coexistence densities. The effect of dipoles on phase stability is very sensitive to their position within the molecular frame. Off-centre dipoles are shown to give rise to phase re-entrance according to the sequence N-I-N on heating at constant density. The theory does not predict a stable ferroelectric nematic phase.

Molecular simulation of hierarchical structures in bent-core nematic liquid crystals

The structure of nematic liquid crystals formed by bent-core mesogens (BCMs) is studied in the context of Monte Carlo simulations of a simple molecular model that captures the symmetry, shape, and flexibility of achiral BCMs. The results indicate the formation of (i) clusters exhibiting local smectic order, orthogonal or tilted, with strong in-layer polar correlations and antiferroelectric juxtaposition of successive layers and (ii) large homochiral domains through the helical arrangement of the tilted smectic clusters, while the orthogonal clusters produce achiral (untwisted) nematic states.

On the Molecular Requirements for the Stabilisation of Thermotropic Biaxial Ordering in Rod-Plate Nematics

Abstract A nematic consisting of rod-like and plate-like molecular species is expected to exhibit phase biaxiality over a range of concentrations. Phase separation, however, prevents the achievement of such concentrations in mixtures of conventional, low molar mass, thermotropic calamitic and discotic mesogens. We present a theoretical study of the molecular requirements for phase biaxiality in three types of rod-plate nematics, namely in (i) binary mixtures of hard rods and discs, (ii) mixtures of rods and discs exhibiting molecular association, either by direct rod-disc interaction or through selective interactions with a third molecular species and (iii) single component systems with interconverting rod-like and plate-like conformations.

Extending the Maier–Saupe theory to cybotactic nematics

A theory of thermotropic nematic liquid crystals in which molecules form internally ordered clusters is presented. The formulation is based on the same molecular-field approximation and form of the anisotropic potential used in the Maier–Saupe theory. One uniaxial nematic and two macroscopically isotropic phases are predicted. The lower-temperature isotropic phase consists of thermodynamically stable clusters with internal orientational order. The transition from this phase to the nematic phase is characterised by the divergence of cluster size whilst the entropy and the order parameter change continuously.