A. G. Vanakaras, A. F. Terzis and D. J. Photinos, Molec.Cryst. Liq. Cryst., 362, 67-78 (2001).
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.
A. G. Vanakaras and D.J. Photinos,
Chem. Phys. Lett.,
341(1-2), 129-134 (2001).
Abstract: We have used Monte Carlo (MC) NPT computer simulations to study a system of spherical-fan-shaped particles made of three hard discs fused along a common diameter. The calculated equation of state indicates a strong, entropy driven. first-order transition from the isotropic phase to a layered phase that has the basic characteristics of the usual smectic-A mesophase but with strongly correlated rotations about the symmetry axes of neighbouring molecules.
A. F. Terzis, A. G. Vanakaras and D.J. Photinos,
Molec.Cryst. Liq. Cryst.,
352, 699-708 (2000).
Abstract: We investigate theoretically the liquid crystalline phase behaviour of globular dendrimers consisting of a soft inner scaffold and peripheral mesogenic units attached to the scaffold by means of flexible chains. We use a generalised Onsager molecular theory to analyse model systems exhibiting conformational rod-sphere and rod-disc interconversions. Phase transitions normally not encountered in low molar mass mesogens are reported. For a system with spherical lowest energy conformation a thermotropic re-entrance to the isotropic phase is observed.
A. F. Terzis, A. G. Vanakaras and D.J. Photinos, Molec.Cryst. Liq. Cryst., 330, 1761-1768 (1999).
Abstract: We present a simple statistical theory of anisotropic fluid phases consisting of deformable molecules and we use it to describe the self-organization of liquid crystalline dendrimers in the limit of very densely attached mesogenic units. We perform calculations for model-systems of spherical molecules that can deform into rod-like or disc-like states and we determine their mesomorphic range as a function of molecular shape parameters and intrinsic conformational probability.
A. G. Vanakaras, D.J. Photinos and E. T. Samulski, Phys. Rev. E, 57(5), R4875-R4878 (1998).
Abstract: We show through explicit molecular modeling how tilt may be induced in layered mesophases and propagate across the-layers, using a concrete representation of flexible tail-core-tail calamitic mesogens in conjunction with-the variational cluster expansion. The results demonstrate that spontaneous symmetry breaking observed in smectic-liquid crystals-the tilt of the director relative to the layers-can be induced by excluded. volume interactions, both in the synclinic and in the anticlinic configurations.
A. G. Vanakaras, S. C. McGrother, G. Jackson and D. J. Photinos, Molec.Cryst. Liq. Cryst., 323, 199 (1998).
Abstract: The possibility of using H-bonding interactions to promote the stabilization of phase biaxiality in nematic binary mixts. of oblate and prolate thermotropic mesogens is discussed. Onsager’s theory of the isotropic-nematic transition was extended to allow for such selective assocns. among unlike species and it is used to calc. the phase diagram of binary mixts. consisting of hard spherocylinders and cut spheres. Directional, short-ranged attractions between rods and disks strongly stabilize the biaxial nematic mixt. against demixing and suggest that interactions of H-bonding type may provide an efficient mechanism for sustaining phase biaxiality in binary mixts. of real thermotropic nematogens. Preliminary Monte Carlo simulations designed to test such predictions are discussed.
A. G. Vanakaras and D.J. Photinos, Molec.Cryst. Liq. Cryst., 299, 65-71 (1997).
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.
A. G. Vanakaras and D.J. Photinos, Mol. Phys., 85(6), 1089-1104 (1995).
Abstract: 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 interaction centres. 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.
A. G. Vanakaras and D.J. Photinos, Molec.Cryst. Liq. Cryst., 262, 463-471 (1995).
Abstract: We formulate the probability distribution of orientations and conformations of flexible chains by explicitly taking into account short range repulsions and el ectrostatic interactions with the solvent molecules. The theory is used calculate the segmental order parameters associated with proton NMR measurements on n-alkanes and to study the apparent shift of the gauche-trans energy in the nematic phase. The foundation of the modular description of chain ordering is investigated in the light of the above developments. Excluded volume and electrostatic effects on the conformation energy of dimethoxyethane are evaluated.
