Single-component Liquid Crystals Research Articles

The contribution of chirality and crosslinker concentration to reflection wavelength tuning in structurally chiral nematic gels

Tunable reflective material systems are potentially useful in a range of applications including displays, optical filters, photonics, and sensing. Presented here is a systematic study of thermally induced reflection wavelength tuning in structurally chiral nematic gels. The tuning is enabled by a swelling/de-swelling/re-swelling transition driven by order/disorder transitions in the liquid crystal and the liquid crystalline polymer network. By preparing liquid crystal polymer networks from single component liquid crystal monomers we isolate the contribution of chemical chirality and the crosslink density of the liquid crystal polymer network to thermally-induced reflection wavelength tuning. Further elucidating the fundamental relationships between the composition of the polymer network and the resulting optical properties is a key step towards enabling advancement and optimization necessary to realize the utility of this mechanism in devices such as mirrorless lasers.

Reentrant Nematic Phenomenon in a New Class of Low Molar Mass, Single-Component Liquid Crystals

We report the observation of reentrant nematic behavior in a relatively new class of compounds, namely, dimerlike mesogens that are made by covalently linking nematogenic cyanobiphenyl and nonmesogenic N-(n-alkyl)salicylaldimine segments through a central flexible spacer of varying length and parity. The existence of reentrant phenomenon in this class of mesogens, evidenced indubitably by means of several complementary studies, appears to be crucially dependent on the length of the terminal alkyl tail and parity of the spacer.

Triptycene-containing bis(phenylethynyl)benzene nematic liquid crystals

We report the synthesis of a new class of nematic liquid crystals with triptycenes built into a p-dialkoxy-bis(phenylethynyl)benzene mesomorphic core. Triptycenes are appended on the center or terminal ring of the mesogen, leading to symmetric liquid crystals and reduced symmetry liquid crystals, respectively. Both types displayed monotropic behavior, with the asymmetric compounds having unusual phase behavior, lacking distinct crystallization transitions, and forming a glassy mesophase. A chiral analogue was found to be non-mesomorphic, but induced chiral nematic phases when doped into achiral triptycene-containing analogues. Rotation is physically hindered normal to the director and, hence, this new liquid crystal architecture may allow for the synthesis of a single component liquid crystal that displays a biaxial nematic phase.

Fabrication of Ultrathin Films of Liquid Crystals Assisted by (Octaalkoxyphthalocyanato)cobalt at the Air/Water Interface

The generation of molecular films of hybrid liquid crystals (LCs) assisted by (octaeicosyloxyphthalocyanato)cobalt (C20PcCo) at the air/water interface is described. C20PcCo in a flat-laid orientation on a water surface has a space surrounded by the Pc ring plane and eight long-alkoxy chains as a wall so that hybrid molecular films are assembled by incorporating LC molecules due to hydrophobic interactions. Uniform molecular films are formed by spreading mixtures of C20PcCo and single-component LCs with hydrophobic terminal chains at subphase temperaures where LCs are in a nematic phase, while LCs bearing hydrophilic groups result in phase separation even in a nematic phase and give rise to no homogeneous hybrid molecular films. Homogeneous floating films of LCs were obtained even though the mixing ratio of C20PcCo:LC was 1:24, implying the multilayered structuring of LC molecules on a C20PcCo molecular sheet. Homeotropic orientation of guest LC molecules was suggested by using a liquid crystalline azobenzene.

SmA--SmC^* Phase Transitions in Liquid Crystals

For the SmA--SmC^* phase transition in liquid crystals, the free energy can be expanded in powers of two order parameters, namely, the tilt angle \theta and the polarization P, In this free energy, we have taken a nonlinear coupling between P and \theta. We have obtained the temperature dependence of P and \theta by fitting to the experimental data for a single component liquid crystal. We have also studied a binary mixture of liquid crystals under the influence of an applied electric field. Using the same free energy, we have obtained the electric field dependence of tilt angle and concentration dependence of polarization by fitting to the experimental data. For this mixture of liquid crystals, we have also deduced the critical exponents \beta for the polarization, \gamma for the susceptibility and the critical isotherm \delta.

Photomechanically induced phase transitions in ferroelectric liquid crystals

Abstract We report optically induced phase transtions occurring in two different host ferroelectric liquid crystals; SCE13 a multicomponentmixture optimised for room temperature performance, and CE8 a single component liquid crystal. These act as host liquid crystals for a novel guest azo dye, which can be made to photoisomerise using low power density U.V. illumination, resulting in dramatic changes in sample properties. We have shown that the magnitude of spontaneous polarisation of systems can be isothermally and reversibly induced or reduced, with the consequent appearance or disappearance of optical switching hysteresis. We discuss the parameters controlling the behaviour of the systems under U.V. illumination and suggest mechansims by which the transitions may occur.

The strong temperature dependent goldstone mode relaxation frequency in a broad range SmC* phase

Abstract Broad frequency range dielectric spectroscopy, X-ray diffraction, electro-optical methods and thermal analysis are used to characterize a single component liquid crystal with a monotropic broad range room temperature ferroelectric SmC* phase. The dielectric spectrum shows a Soft mode and a Goldstone mode. Unlike in other ferroelectric liquid crystals the Goldstone mode of this substance has a strongly temperature dependent characteristic frequency. It is proposed that glassy-state fluctuations are responsible for the appreciable increase of activation energy (from 37 to 91 kJ/mol) upon cooling from 318 K (TCA) to 278 K. Beside these modes a Domain mode is detected. Measurements in the high frequency range 1 MHz to 450 GHz show an additional relaxation process.

Chevron layer structures in surface stabilized ferroelectric liquid crystal (SSFLC) cells filled with a material which exhibits the chiral nematic to smectic C*phase transition

Abstract Single component liquid crystals which exhibit the chiral nematic (N*) to smectic C* (SC*) phase transition exhibit a large director tilt with respect to the local layer normal (typically 30 degrees or greater) in the SC* phase which appears immediately upon cooling through the N* - SC* transition. In the SSFLC geometry and in the presence of an applied AC field these materials have been observed to align with their molecular long axes oriented along the buffing direction (for a buffed polymer alignment treatment) and with two domains of the smectic layers symmetrically disposed at a large angle (approximately the director tilt angle) with respect to the buffing direction[1]. Furthermore, the layer domains are selectable with the application of a DC field applied normal to the bounding plates. We demonstrate, for the large tilt mixture SCE-10, that the layer domains in all cases adopt the chevron local layer structure[2] immediately upon cooling through the transition. With decreasing temperature...

High Pressure Studies of Liquid Crystalline Transitions

ABSTRACTSome of the important results emerging from our recent investigations on liquid crystals at high pressure are presented. In particular the following topics will be discussed: i) The relation between Pm, the maximum pressure of smectic stability, and the temperature range of the nematic phase in reentrant nematogenic compounds. ii) Piezothermal studies in the vicinity of the smectic A-nematic transition in 8 OCB. iii) The effect of molecular ordering on the pressure behaviour of the nematic-isotropic transition in reentrant nematic mixtures. iv) The first observation of a multicritical point in a single component liquid crystal.