Phase Diagrams For The Blue Phases Of Highly Chiral Liquid Crystals

Polarizing microscopy and optical-activity measurements are used to determine the phase diagram for the blue phases of chiral-racemic mixtures of terephthaloyloxy-bis-4-(2\u27-methylbutyl) benzoate. Contrary to an earlier report, it is the second blue phase (BP II) rather than the first blue phase (BP 1) that is not stable relative to the other blue phases at high chirality. With this development, all phase diagrams for the blue phases reported to date have the same topology. Using similar data for two other highly chiral systems, it is found that a simple scaling of the temperature and chiral-fraction axes produces phase diagrams in quantitative agreement with the present results. Thus, in spite of differences in molecular structure, the number of chiral centers, and phase-transition temperatures, these three systems possess remarkably similar phase diagrams and lend evidence for a universal phase diagram for the blue phases

Nonstandard electroconvection in a bent-core oxadiazole material

Electroconvection (EC) phenomena have been investigated in the nematic phase of a bent-core oxadiazole material with negative dielectric anisotropy and a frequency dependent conductivity anisotropy. The formation of longitudinal roll (LR) patterns is one of the predominant features observed in the complete frequency and voltage range studied. At voltages much above the LR threshold, various complex patterns such as the "crisscrossed" pattern, bimodal varicose, and turbulence are observed. Unusually, the nonstandard EC (ns-EC) instability in this material, is observed in a regime in which we measure the dielectric and conductivity anisotropies to be negative and positive respectively. A further significant observation is that the EC displays distinct features in the high and low temperature regimes of the nematic phase, supporting an earlier report that EC patterns could distinguish between regions that have been reported as uniaxial and biaxial nematic phases

An experimental and computational study of calamitic and bimesogenic liquid crystals incorporating an optically active [2,2]-paracyclophane

Two liquid-crystalline materials containing an optically active (R)-4-hydroxy-[2,2]-paracyclophane group were prepared, one in which the chiral group is a bulky terminal unit and one in which it forms part of a terphenyl-like mesogenic unit. Both materials exhibit monotropic chiral nematic phases. Partial phase diagrams were constructed for mixtures of both materials with 5CB, allowing us to extrapolate pitch lengths and helical twisting power values (HTP) for each material. The HTP value of the material with a ‘locked’ paracyclophane is 70% higher than that of a ‘free’ paracyclophane and this is rationalised as being due to the reduction in conformational freedom of the former material relative to the later

Developments in liquid-crystalline dimers and oligomers

Liquid-crystalline dimers and bimesogens have attracted much attention due to their propensity to exhibit the spontaneously chiral twist-bend mesophase (NTB), most often by dimers with methylene spacers. Despite their relative ease of synthesis, the number of ether-linked twist-bend materials significantly lags behind those of methylene-linked compounds. In this work, we have prepared and studied a range of ether-linked bimesogens homologous in structure to the FFO9OCB; as with methylene-linked systems, it appears that it is molecular topology and the gross molecular shape that are the primary drivers for the formation of this phase of matter. Dimers and bimesogens are well studied within the context of the twist-bend phase; however, present understanding of this mesophase in oligomeric systems lags far behind. We report our recent efforts to prepare further examples of oligomeric twist-bend nematogens, including further examples of our ‘n+1’ methodology, which may allow the synthesis of high-purity, monodisperse materials of any given length to be prepared. We have observed that there is a tendency for these materials to exhibit highly ordered soft-crystalline mesophases as opposed to the twist-bend phase

Pressure induced twist grain boundary phase

We report high pressure studies on a binary system of 4-(2'-methyl butyl)phenyl 4'-n-octyl biphenyl-4-carboxylate (CE8 from BDH) and 4-n- dodecyloxy biphenyl-4'-(2'-methyl butyl)benzoate (C12). For the concentration range 0.32<X<0.62, where X is the weight fraction of C12, the system shows the following sequence of transitions at room pressure: Isotropic- Cholesteric (Ch)-Twist Grain Boundary (TGB)-Smectic A (A)Smectic C− (C∗ ). For X=0.25, the Ch phase transforms directly to the A phase at 1 bar, but there is an induced TGB phase between the Ch and A phases at elevated pressures. Analysis of the topology of the pressure-temperature diagram in the vicinity of the Ch-TGB-A meeting point indicates that is a critical end point rather than a bicritical point as expected from the mean field theory. At X=0.64 there is a direct TGB-C∗ transition at 1 bar, but there appears a pressure induced A phase between the TGB and C∗ phases. The topology of the phase diagram suggests that the TGB-A-C∗ meeting point is a bicritical point

Observation of the Smectic C -- Smectic I Critical Point

We report the first observation of the smectic C--smectic I (C--I) critical point by Xray diffraction studies on a binary system. This is in confirmity with the theoretical idea of Nelson and Halperin that coupling to the molecular tilt should induce hexatic order even in the C phase and as such both C and I (a tilted hexatic phase) should have the same symmetry. The results provide evidence in support of the recent theory of Defontaines and Prost proposing a new universality class for critical points in layered systems.Comment: 9 pages Latex and 5 postscript figures available from [email protected] on request, Phys.Rev.Lett. (in press

Critical Point For The Blue-Phase-III-Isotropic Phase Transition In Chiral Liquid Crystals

The highly chiral compound S,S-4 \u27\u27-(methylbutyl)phenyl-4\u27-(methylbutyl) biphenyl carboxylate (S,S-MBBPC) undergoes a continuous supercritical evolution from the isotropic (I) phase to the third blue phase (BPI III). Mixtures of S,S-MBBPC and its racemate have been studied with high-resolution calorimetry capable of quantitative latent heat determinations and with optical activity measurements. Both experiments indicate that the first-order BP III-I transition line ends at a critical point when the chiral mole fraction X(c) similar or equal to 0.45. Analysis of C-p and optical activity data for the near-critical mixture with X = 0.45 indicates mean-field behavior instead of the theoretically predicted Ising fluctuation behavior, which would be analogous to that at the liquid-gas critical point of a simple fluid. It is speculated that the Ginzburg criterion can explain this mean-field behavior since the critical regime may be too small for experimental observation, as is the case for almost all Smectic-A -Smectic-C transitions

Discontinuous Change in the Smectic Layer Thickness in Ferrielectric Liquid Crystals

The temperature dependence of the thickness of thick free-standing films is studied using a high-resolution film thickness measurement technique. A small discontinuity in the temperature dependence of the smectic layer thickness at every phase transition between ferro-, ferri-, and antiferroelectric phases is observed. We show that the major contribution to it arises from a change in the smectic tilt angle

Phase Transition between the Cholesteric and Twist Grain Boundary C Phases

The upper critical temperature Tc2 for the phase transition between the Cholesteric phase (N*) and the Twist Grain Boundary C phase with the layer inclination tilted to the pitch axis (TGBct) in thermotropic liquid crystals is determined by the mean field Chen-Lubensky approach. We show that the N*-TGBct phase transition is split in two with the appearance of either the TGBA or the TGB2q phase in a narrow temperature interval below Tc2. The latter phase is novel in being superposed from two degenerate TGBct phases with different (left and right) layers inclinations to the pitch axis.Comment: Phys. Rev. E, to be publ; 24 pages, RevTeX + 3 ps figure