Nematic-isotropic phase transition: An extended mean field theory

Screening out the non-Arrhenius behaviour of nematic-isotropic transition by room temperature ionic liquid

Differential Scanning Calorimetry (DSC) and optical polarization microscopy of a mixture of the liquid crystalline material (N-(4-methoxybenzylidene)-4-butylaniline, MBBA) and a Fe-based room temperature ionic liquid 1-ethyl-3-methylimidazolium tetrachloroferrate ([Emim](+) [FeCl4](-), EMIF) indicate a decrease in the nematic-isotropic (N-I) phase transition temperature (T(NI)) with an increase in EMIF concentration, explained by a proposed model of Coulomb "screening" of MBBA quadrupoles by the EMIF ions along with ionic "self screening." DSC studies of EMIF-MBBA and pure EMIF and comparison with pure MBBA results show that the major transitions in pure EMIF have Arrhenius behaviour, but more importantly the previously found convex Arrhenius behaviour of the pristine MBBA [K. Dan et al., Europhys. Lett. 108, 36007 (2014)] becomes Arrhenius in the mixture, indicating a conversion of the entropic N-I activation barrier to an enthalpic one. In presence of EMIF, a drastic decrease in the intensity of out-of-plane distortions of benzene rings in MBBA is found from Fourier transform infrared spectroscopy, consistent with significant reduction in the conformational states of MBBA. This suppression of large amplitude motion is again consistent with a Coulomb screening and gives a molecular basis for the entropic-to-enthalpic conversion of the N-I activation barrier.

Generalized Onsager theory of liquid crystals

The Onsager theory is known to be inaccurate in its prediction of the critical transition density for small aspect ratio hard rods. In this paper we generalize the Onsager theory in two dimensions by taking into account the short-range order as well as the higher-order virial coefficients, up to the fourth order. By carrying out molecular dynamics (MD) simulations on "molecules" comprising linked hard disks with an aspect ratio ℓ ranging from 5 to 13, we show that the generalized theory is much improved as compared to the traditional theory, with its predictions of the transition density agreeing well with the simulation results. This indicates the importance of short-range order considerations (in conjunction with steric repulsion) for molecules with ℓ≤10, a group which includes the most commonly encountered thermotropic liquid crystals. MD simulations further yield evidence for hexagonal order for molecules with ℓ≤8, indicating an intermediate hexagonal phase before solidifying at higher densities.

Relationship between thermodynamic parameter and thermodynamic scaling parameter for orientational relaxation time for flip-flop motion of nematic liquid crystals

Thermodynamic parameter Γ and thermodynamic scaling parameter γ for low-frequency relaxation time, which characterize flip-flop motion in a nematic phase, were verified by molecular dynamics simulation with a simple potential based on the Maier-Saupe theory. The parameter Γ, which is the slope of the logarithm for temperature and volume, was evaluated under various conditions at a wide range of temperatures, pressures, and volumes. To simulate thermodynamic scaling so that experimental data at isobaric, isothermal, and isochoric conditions can be rescaled onto a master curve with the parameters for some liquid crystal (LC) compounds, the relaxation time was evaluated from the first-rank orientational correlation function in the simulations, and thermodynamic scaling was verified with the simple potential representing small clusters. A possibility of an equivalence relationship between Γ and γ determined from the relaxation time in the simulation was assessed with available data from the experiments and simulations. In addition, an argument was proposed for the discrepancy between Γ and γ for some LCs in experiments: the discrepancy arises from disagreement of the value of the order parameter P2 rather than the constancy of relaxation time τ1(*) on pressure.

Thermodynamic analysis of the low frequency relaxation time in the smectic A and C phases of a liquid crystal

Pressure-temperature-volume (pVT) measurements were carried out on 2-(4-hexyloxyphenyl)-5-octyl-pyrimidine, a substance exhibiting nematic and smectic A and C polymorphism. Analysis of the longitudinal relaxation times obtained recently for elevated pressures [Czub et al., Z. Naturforsch. A: Phys. Sci. 58, 333 (2003)] was performed for isobaric, isothermal, and isochoric conditions within the two smectic phases. Several relationships linking the dynamical and thermodynamical quantities, derived recently for isotropic glass formers [Roland et al. Rep. Prog. Phys. 68, 1405 (2005)], were found to hold for the liquid crystal, revealing a striking similarity of behaviors for these two types of materials. The parameter gamma characterizing the steepness of the interaction potential was derived in different ways. It is interesting that the liquid crystal gives relaxation time versus TV(-gamma) plots that are linear, unlike results for glass formers, implying that the dynamics of the former is thermally activated.

Critical behavior at the isotropic-to-nematic phase transition in a bent-core liquid crystal

Magnetic birefringence and dynamic light scattering measurements of orientational order parameter fluctuations at the isotropic-nematic phase transition of a bent-core liquid crystal reveal a pretransitional temperature dependence consistent with the standard Landau-deGennes mean field theory. However, as follows: the transition in the bent-core compound is more weakly first order (TNI-T* approximately 0.4 degrees C), the leading Landau coefficient is approximately 30 times lower, the viscosity associated with nematic order fluctuations is approximately 10 times higher, and the density change is approximately 10 times lower, than typically observed in calamitic (rod-shaped) liquid crystals. One consistent explanation for these anomalies is an optically isotropic phase composed of microscopic complexes or "clusters" of bent-core molecules.

Monte Carlo simulation of the nematic-isotropic transition in an isothermal-isobaric ensemble

Monte Carlo simulation of a system consisting of 512 cylindrically symmetric particles interacting with each other via a potential which has an isotropic, density dependent part as well as an anisotropic part has been used to simulate the nematic state. The usual Metropolis algorithm is used and the particles are allowed to have translational degrees of freedom along with the orientational one. The simulation has been carried out in an isothermal-isobaric (NPT) ensemble and the multiple histogram technique of Ferrenberg-Swendsen with appropriate modification for the NPT ensemble has been used. The results reveal realistic values of the pseudo-spinoidal temperature and pressure as well as that for the pressure dependence of the nematic-isotropic transition temperature.

Phase transitions in liquid crystals under negative pressures

We report the first measurements of orientational order parameters and phase transition temperatures in nematic and smectic A liquid crystals under negative pressures generated by an isochoric cooling of small droplets embedded in a glass former. Comparison of isobaric and isochoric measurements allows us to estimate the coefficients coupling the order parameter and density of an extended Landau--de Gennes model of the nematic phase.