Rheology and flow-induced liquid crystal phase transitions in thermotropic polyethers

This work investigates the interdependence of the phase behaviour, viscosity, temperature, molecular weight and shear rates of thermotropic liquid crystalline polyethers. The viscosity of the isotropic and nematic phases are quantitatively compared; a positive variation in viscosity with respect to temperature is found, with the isotropic viscosity being about an order of magnitude higher than the liquid crystalline viscosity. The dependence of viscosity upon molecular weight of well defined fractions is investigated in both the liquid-crystal and isotropic phases. In the liquid crystalline state the viscosity scales with M ^3.5–5. Variations in the viscosity due to temperature changes affect the isotropic phase more than the liquid crystal phase. No evidence for a negative first normal stress difference is seen. Finally, it is shown how the phase diagram of the material can be altered by shearing the material in the isotropic phase. This is evident by the onset of a shear thinning region at temperatures slightly above T _i, which can be attributed to the formation of shear induced liquid crystallinity.     

Gelation versus liquid crystal phase transitions in suspensions of plate-like particles

Gelation is a common effect in aqueous suspensions of charged colloidal clay platelets at concentrations as low as 1 wt%. However, in systems of charged gibbsite [Al(OH)3] platelets, gelation can be delayed to concentrations as high as 50 wt% depending on the ionic strength. We investigated the phase behaviour of this system approaching the state of gelation in the delicate region between attractive and repulsive states that originate from competition between Coulomb repulsion and van der Waals attraction. As a function of the ionic strength, isotropic-nematic, nematic-columnar and isotropic-columnar phase separations were observed. Moreover, compression by gravitational forces allowed us to observe phase separation that is arrested by gelation in the homogeneous suspensions.     

High pressure studies of liquid crystalline transitions: Recent results

For sometime now we have been conducting high pressure studies on various types of liquid crystalline phase transitions. Two topics of current interest in liquid crystalline systems are (i) the phenomenon of reentrance, and (ii) multicritical points. In this paper we present some of the recent results of our pressure studies dealing with these two aspects.     

Study of flow in a smectic liquid crystal in the X-ray Surface Forces Apparatus

We describe the newly invented X-Ray Surface Forces Apparatus (X-SFA) which allows the simultaneous measurement of forces and collective structures of confined complex fluids under static and flow conditions. The structure of the smectic liquid crystal 8CB (4-cyano-4-octylbiphenyl) f confined between two mica surfaces with separation ranging from 4000 to 20,000 A was measured. At small gaps and no shear, the smectic layers take on distinct stable orientations, including the bulk forbidden h orientation. which persist under low shear ( 30 s^–1). However, at higher shear rates 360 s^–1) the shear acts to dramatically order and align the smectic layers into a singlea orientation.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Quartz crystal microbalance studies of polymer gels and solutions in liquid environments

Spreading of liquids and soft solids on a rigid surface in a surrounding liquid medium is investigated by utilizing the lateral sensitivity of the quartz crystal microbalance (QCM). While the QCM has been used extensively to study systems with spatial variations in the direction normal to the crystal's electrodes, few studies have exploited the QCM's ability to sense changes in loading in the plane of the electrodes. We propose equations to describe the predicted response of the QCM to a generalized viscoelastic material spreading at the QCM surface at the expense of the surrounding liquid medium. Several experimental examples are given in order to support the validity of the proposed equations, including situations where the spreading material is a Newtonian liquid, a viscoelastic liquid, or one of two viscoelastic solids. The first viscoelastic solid is a physically cross-linked gel based on a styrene/ethylene-butene/styrene triblock copolymer in mineral oil, and the second is a cross-linked poly(ethylene glycol) hydrogel.     

Layer-by-layer transitions in liquid crystals

Employing our free-standing film calorimetric system. we have identified three remarkable layer-by-layer transitions in several liquid crystal compounds. Each of these transitions can be well described by the simple power-law form:L = L ₀ t. The layering transitions liMind near the smectic-A- hexatic-B and smectic-A crystal-B transitions are well characterized by the exponent v 1 3. This value is consistent with models based on a van der Wads-like dominant intermolecular interaction. Another novel layer-by-layer thinning transition has been discovered above the hulk smectic-A isotropic transition of a perfluorinated liquid crystal compound. The value of the exponent obtained, v 3 4. cannot he easily explained using familiar models.Paper presented at the Twelfth Symposium on Thermophysical Properties. June 19–24, 1994 Boulder Colorado U.S.A.     

Characterization and studies on grafting of ferroelectric liquid crystal onto polybutylacrylate

Ferroelectric liquid crystalline (FLC) elastomer as a class of electroactive polymers, exhibiting high strain at very low voltage compared to other materials. This giant eletrostriction exploring new applications for ferroelectric materials and these materials are suitable candidates for designing nanoscale artificial muscles, sensors, actuators, pumps and motors. Due to these properties, considerable attention has been paid to these materials in recent years. In this study, ferroelectric liquid crystalline (FLC) monomer was synthesized in a mini reactor in a four‐step procedure. The synthesized materials were characterized with HNMR and FTIR in each step. Then the monomer in the presence of benzoylperoxide as an initiator and n‐butyl acetate as a solvent was grafted onto polybutylacrylate. Grafting was carried out in different processing conditions and products were characterized with HNMR and DSC. The level of initiator, method of initiator's injection to reactor, reaction time, and reactants concentration were part of parameters that were investigated. The analysis of results shows that the reaction time is an important parameter and appropriate grafting needs a long time. Components concentration illustrated important effect on the level of grafting and final structure of grafted chains.     

Phase transitions on quantum crystal interfaces

The quantum solid-on-solid (SOS) model for the free surface of a crystal is studied at zero and finite temperatures. In addition to the usual roughening transition, this model also exhibits a superfluid transition. The phase diagram is constructed. We also formulate and study a quantum SOS model for the solid-liquid interface, which takes into account the motion and the compressibility of the liquid and solid. The existence of a phase transition at zero temperature is shown. The phases differ in the form of the excitation spectrum. In both interface models the interface at zero temperature is smooth. The quantum effects only slightly reduce the temperature of the roughening transition in comparison to the classical value.     

Phase transitions in an ionic conductor CuTeBr crystal

The dilatometric and pressure studies were carried out for CuTeBr crystal. The P–T phase diagram was constructed within the range of 3kbar. A triple point was found at 70 °C and 0.3 kbar, beyond which a new phase appears.     

X-ray diffraction studies of NbTe<Subscript>2</Subscript> single crystal

NbTe₂ is a member of transition metal dichalcogenide (TMDC) group. Single crystals of niobium ditelluride (NbTe₂) have been grown by a chemical vapour transport technique using iodine as transporting agent. The composition of the grown crystals was confirmed on the basis of energy dispersive analysis by X-ray (EDAX) and remaining structural characterization was also accomplished by X-ray diffraction (XRD) studies. Lattice parameters, volume and X-ray density have been carried out for the grown crystals. The particle size for a number of reflections has been calculated using Scherrer’s formula.     

Rotatable liquid crystal waveplate

A photoresist well of size 60 × 60 × 15 μm³ has been used to confine a droplet of nematic liquid crystal to create a rotatable waveplate. The optical texture of the droplet between crossed polarisers is consistent with the nematic n-director running substantially along a diameter of the droplet and connecting two nematic defects on the curved edges of the droplet. Electric field induced azimuthal rotation of the axis of the nematic liquid crystal droplet has been demonstrated. At higher temperatures, 30 °C and above, the droplet is more circular in shape and can be switched to arbitrary rotation angles. At lower temperatures, 25 °C and below, the sides of the droplet are straightened by the interaction with the well walls and the switching tends to favour discrete orientations of the optic axis. The shape of the time–voltage switching response curve for rotation by an angle of 40° also depends on the temperature of the droplet. A switching time that is inversely proportional to the voltage squared results when the droplet is nearest to circular in shape.     

Sidechain liquid crystal polymers

Research has continued in the field sidechain liquid crystal polymers over recent years, but it is becoming clearer that this research is being directed away from the traditional technology areas of electro-optic devices and researchers are developing new and exciting applications for this novel state of matter.     

Graphene-based liquid crystal device

Graphene is only one atom thick, optically transparent, chemically inert, and an excellent conductor. These properties seem to make this material an excellent candidate for applications in various photonic devices that require conducting but transparent thin films. In this letter, we demonstrate liquid crystal devices with electrodes made of graphene that show excellent performance with a high contrast ratio. We also discuss the advantages of graphene compared to conventionally used metal oxides in terms of low resistivity, high transparency and chemical stability.     

Substrate-induced crystal plastic phase of a discotic liquid crystal

A new phase of a known discotic liquid crystal is observed at the interface with a rigid substrate. The structure of the substrate-induced phase has been characterized by atomic force microscopy, specular X-ray diffraction, and small-angle and wide-angle grazing incidence X-ray diffraction. The substrate-induced phase, which has a thickness of ～30 nm and a tetragonal symmetry, differs notably from the bulk phase. The occurrence of such phase casts a new light on alignment of discotic liquid crystals.     

New directions in liquid crystal science

While we are all familiar with liquid crystal displays, an industry currently worth more than US dollars 60 billion yr(-1) and growing rapidly, fewer people are aware of the breadth of the subject of liquid crystals--one that represents the study of the fourth state of matter. Liquid crystals are found as essential elements in biological systems, soaps and detergents, sensor technologies and in the manipulation of electromagnetic radiation of various wavelengths. This meeting was designed to highlight both the truly multidisciplinary nature of liquid crystal science and to feature those areas away from electro-optic displays; these issues are developed and summarized in more detail.     

Simulation studies of the hydrodynamics in high-temperature solutions for crystal growth. VI. Temperature fluctuations at the crystal/liquid interface

The temperature fluctuations at the crystal/liquid interface have been studied using model liquids. The character of the fluctuations varied depending on whether in the liquid there was only free convection or both free and forced convections were simultaneously present. It has been established that in the second case the amplitude of fluctuations is determined mainly by the rate of the resultant convection flow and the liquid viscosity. A maximum amplitude of fluctuations is observed at a resultant flow rate w_fl = O when the free and forced convections proceed in opposite directions, and at w_fl = min when both convection flows have the same direction. Larger amplitudes of temperature fluctuations are registered in low-viscosity liquids. The conditions under which a flat crystal/liquid interface showing no temperature fluctuations should be formed have been determined for crystal diameters of 10, 15 and 20 mm, respectively.     

Single-cell gap-transflective liquid crystal display using two optical modes of a bistable liquid crystal

We present a single-cell gap-transflective liquid crystal display (LCD) characterized by a π cell having bulk-type liquid crystal (LC) bistability and an optical film configuration. The π cell has two stable states, which are a twisted and a nontwisted LC state. We used the twisted LC state for the reflective part connected with nonpatterned electrodes, producing a vertical field in the proposed transflective LCD. Also the nontwisted LC state was for the transmissive part with interdigitated patterned electrodes driving an in-plane field in the device. The simulated and experimental results show good electro-optical characteristics in both the reflective and transmissive parts.     

Terahertz time-domain spectroscopy studies of the optical constants of the nematic liquid crystal 5CB

The optical constants of a nematic liquid crystal, 4'-n-pentyl-4-cyanobiphenyl (5CB), in the frequency range 0.3-1.4 THz were determined by terahertz (THz) time-domain spectroscopy. The real parts of the extraordinary refractive index n(e) and the ordinary refractive index n(o) of 5CB varied from 1.74 to 2.04 and from 1.59 to 1.83, respectively. Liquid-crystal 5CB exhibits a relatively small absorption loss in this frequency range. The birefringence of 5CB was found to be as large as 0.21. The experimental results indicate that liquid crystal 5CB is potentially useful for device applications in the THz frequency range.