Showing papers on "Liquid crystal published in 1990"

Liquid to solidlike transitions of molecularly thin films under shear

Abstract: We have measured the shear forces between two molecularly smooth solid surfaces separated by thin films of various organic liquids. The aim was to investigate the nature of the transitions from continuum to molecular behavior in very thin films. For films whose thickness exceeds ten molecular diameters both their static and dynamic behavior can usually be described in terms of their bulk properties, but for thinner films their behavior becomes progressively more solidlike and can no longer be described, even qualitatively, in terms of bulk/continuum properties such as viscosity. The solidlike state is characterized by the ordering of the liquid molecules into discrete layers. The molecular ordering is further modified by shear, which imposes a preferred orientation. All solidlike films exhibit a yield point or critical shear stress, beyond which they behave like liquid crystals or ductile solids undergoing plastic deformation. Our results on five liquids of different molecular geometry reveal some very co...

Side Chain Liquid Crystal Polymers

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Cubic phases in surfactant and surfactant-like lipid systems

Abstract: Cubic liquid crystalline phases are common in surfactant and surfactant-like lipid systems at temperatures above the Krafft point. They are optically isotropic and very stiff. Therefore, they are often not recognized as independent phases and separated in pure state. The liquid crystalline nature is evidenced by a low-angle diffraction pattern with sharp reflections having Bragg-values above 20 A coupled with a diffuse wide-angle reflection at 4.5 A, proving that the hydrocarbon moiety is in a liquid state. The cubic phases occur in a variety of lipid/water systems (also with liquid organic solvents), such as simple soaps, amphiphilic lipids of biological origin, and extracts from membrane lipids. The location of the cubic phases in a phase diagram varies. The original concept of a cubic structure composed of closed globular aggregates, either of “oil-in-water” or “water-in-oil” type in face-centered array seems to be obsolete. The present structure concepts include closed anisotropic aggregates, short rod-like aggregates forming continuous networks or lamellar aggregates with “zero” curvature forming networks of Infinite Periodic Minimal Surfaces (IPMS). The structure is mostly primitive or body-centered cubic.

Arrested Tumbling in Shearing Flows of Liquid Crystal Polymers

Abstract: The doi equation for the three-dimensional time-dependent orientation-distribution function for rodlike polymers in the nematic liquid crystalline state in the presence of shearing flow is solved by an expansion in spherical harmonic functions. The results confirm calculations by Marrucci and Maffettone, who showed that a regime of steady-state negative first normal stress difference N 1 exists at shear rates high enough to frustrate the tumbling of the average molecular orientation that occurs at lower shear rates

Polymer Blends Containing Liquid Crystals: A Review

Abstract: This paper reviews the literature of polymer blends containing low and high molar mass liquid crystals. Low molar mass liquid crystals have been used as plasticizers for thermoplastic polymers and in applications such as electro-optics, optical recording media, and membranes. High molar mass liquid crystalline polymers have been primarily used in polymer blends as processing aids and as an incipient reinforcing phase for “self-reinforced” materials. This review discusses the phase behavior, rheology, and mechanical properties of these blends.

Electrically induced light scattering from anisotropic gels

Abstract: Anisotropic gels were produced by photopolymerization of an oriented liquid‐crystalline (LC) mixture containing LC diacrylates and conventional LC molecules. Gels produced in this way were clear and did not give rise to excess scattering of light. Upon application of an electric field across the gel, the system turned translucent. Light scattered by the gel was found to be highly dependent on the direction of polarization and wavelength of the incident light.

Discotic liquid crystals

Abstract: Recent studies have established that many compounds composed of disc-shaped molecules exhibit stable thermotropic liquid crystalline phases. They are now referred to as discotic liquid crystals. Structurally, most of them fall into two distinct categories, the columnar and the nematic. The columnar phase, in its simplest form, has long-range translational periodicity in two dimensions and liquid-like disorder in the third, whereas the nematic phase is an orientationally ordered arrangement of discs without any long-range translational order. This review article describes the detailed structures of the various discotic phases and some of their important physical properties.

Molecular Orientational Structures in Ferroelectric, Ferrielectric and Antiferroelectric Smectic Liquid Crystal Phases as Studied by Conoscope Observation

Abstract: Molecular orientational structures in MHPOBC were studied by means of conoscope observation. Contrary to a ferroelectric response of the conoscope to an electric field in the smectic C* phase, the conoscopic figure in the antiferroelectric smectic CA* phase does not shift its center and is biaxial with its optic plane perpendicular to the field direction. This conoscope change is only due to a dielectric contribution, indicating the existence of the inherent threshold in the electric field induced transition to the ferroelectric phase. In between the ferroelectric and antiferroelectric phases, a kind of ferrielectric phase was clearly distinguished in the field response of the conoscopic figure; the shift of the conoscope center perpendicular to the applied field and the biaxial optic plane parallel to the field suggest a novel molecular orientational structure.

Computer simulation studies of anisotropic systems. XIX. Mesophases formed by the Gay-Berne model mesogen

Abstract: We report the results of a molecular dynamics computer simulation of particles interacting via the Gay-Berne potential with parameters selected to approximate those of mesogenic molecules. The system was found to form a variety of mesophases as the temperature was lowered. We have characterized these phases with the aid of computer graphics techniques to visualize the molecular organization within configurations taken from the production stage of the simulations. The phases have been identified, on the basis of such images, as isotropic, nematic, smectic A, smectic B and crystal.

Liquid crystal display having essentially single crystal transistors pixels and driving circuits

Abstract: A display panel is formed using essentially single crystal thin-film material that is transferred to substrates for display fabrication Pixel arrays form light valves or switches that can be fabricated with control electronics in the thin-film material prior to transfer The resulting circuit panel is than incorporated into a display panel with a light emitting or liquid crystal material to provide the desired display

Phase diagram of a system of hard spherocylinders by computer simulation

Abstract: We report a computer-simulation study of the phase diagram of a system of hard spherocylinders with aspect ratio L/D between 0 and 5. For L/D≤3 only isotropic liquid and crystalline solid phases can occur. For L/D=3 a mechanically stable smectic phase occurs, but it is thermodynamically unstable. For larger L/D values, stable smectic and nematic phases are present.

Liquid crystal apparatus

Abstract: A liquid crystal apparatus comprises a) a liquid crystal device comprising an electrode matrix composed of scanning electrodes and data electrodes, and a ferroelectric liquid crystal; and b) a driving means. The driving means includes a first drive means for applying a scanning selection signal two or more scanning electrodes apart in one vertical scanning so at to effect in one picture scanning in plural times of vertical scanning, and a second drive means for applying data signals in synchronism with the scanning selection signal.

Laser-induced gratings and spectroscopy in monodomains of liquid-crystalline polymers

Abstract: We present a detailed experimental characterization of an optical storage technique that uses monodomain films of liquid-crystalline polymers (LCP’s). This technique employs the trans–cis isomerization of cyanoazobenzene mesogenic groups of the LCP’s, which is photoinduced in the glassy state. Changes in the molecular geometry leading to nonmesogen moieties and to a reorientation of the optical axis were found to be responsible for the observed strong variations of the anisotropic optical properties. These variations were characterized by means of UV–visible spectroscopy with polarized light. The optically induced trans–cis isomerizations as well as the complete thermal relaxation back to the trans state were found to be similar to those for amorphous polymers containing azobenzene-type groups. However, permanent changes were observed in the irradiated LCP monodomains. An altered orientational distribution in the illuminated area is responsible for the observed long-time stability. The original uniform orientational order of the LCP monodomains could be fully restored by annealing the samples well above the glass-transition temperatures of the polymers. Laser-induced grating experiments were performed to characterize the formation of a periodic modulation of the anisotropic refractive index as a function of time and intensity. A kinetic model that describes the grating formation due to the isomerization reaction is introduced and experimentally confirmed. A very high resolving power (3000 lines/mm) combined with large diffraction efficiencies (50%) could be achieved. Finally, the first hologram reversibly stored in a LCP is presented as practical example. Such a hologram has already lasted for more than two years at ambient.

Anomalous optical Freedericksz transition in an absorbing liquid crystal

Abstract: Laser-induced reorientation was studied in absorbing nematic films. The optical Freedericksz threshold occured at an intensity level of 50 W/cm2 in contrast to the normally observed few KW/cm2 value for transparent layers. Thermomechanical coupling is considered as a possible explanation of the observed anomaly.

Configuration transition in a nematic liquid crystal confined to a small spherical cavity.

Abstract: The director-field configuration of a nematic liquid crystal confined to a spherical cavity within a urethane polymer is observed to transform from a radial- to axial-type structure as the radius of the cavity, temperature, or strength of an applied electric field are varied. The phase diagram, anchoring strength at the droplet wall, and a value of the reduced field inside the droplet are determined for different polymer/liquid crystal interfaces.

Computer simulation of a biaxial liquid crystal

Abstract: We report the first results of Monte Carlo simulations using hard ellipsoids with three distinct semi-axes a, b, c chosen such that abc = 1, c/a = 10 and b/a varies between 1 and 10 A survey of the phase diagram provides evidence for the existence of isotropic, nematic, discotic, and biaxial liquid crystal phases; this is believed to be the first simulation of a biaxial phase of a bulk liquid with full rotational and translational freedom We find that the phase diagram is approximately symmetric under the transformation {a, b, c} → {a−1,b−1,c−1}, and that the biaxial phase is most stable at about the expected (self-conjugate) value b= √ac For this value, the isotropic phase transforms directly into the biaxial phase on compression, at a density at least 50 per cent higher than that at which-the nematic-isotropic transition occurs in the corresponding uniaxial systems These results are in semi-quantitative agreement with recent theories, but there are also some significant differences

Electrically controlled polarization-independent liquid crystal fresnel lens

Abstract: In successive zones of a layer of nematic liquid crystal material, the principal axes of at least some of the material in each zone are established orthogonal to the axes of at least some of the material in adjacent zones. By applying a uniform electric field to the zones of the entire layer, an electrically controlled polarization-independent Fresnel lens is achieved.

Electrically tunable optical filter for infrared wavelength using liquid crystals in a Fabry–Perot étalon

Abstract: Using the refractive index modulation in liquid crystals by an externally applied electric field, we show that common nematic liquid crystals can be used in a Fabry–Perot etalon to produce an electrically tunable optical filter for use in the infrared region. Using commercial liquid crystals we demonstrate that the wavelength can be tuned over 175 nm using less than 10 V. In our first device the free‐spectral range of the etalon is about 75 nm with a passband of about 1–2 nm which is determined by the quality of the mirrors that we have used.

Theory of the nonequilibrium phase transition for nematic liquid crystals under shear flow

Abstract: We consider the impact of shear flow on the isotropic-nematic transition in crystalline liquids by generalizing Leslie-Ericksen dynamics of nematic systems to include amplitude and biaxial degrees of freedom. Neglecting fluctuations, we find steady-state solutions to the equations of motion for the nematic order parameter and fluid velocity and interpret them in terms of nonequilibrium steady states. We predict a transition temperature increasing with shear rate up to a nonequilibrium critical point, and discuss the singular behavior of the order parameter and external stress near this point.

Light control systems with a circular polarizer and a twisted nematic liquid crystal having a minimum path difference of λ/2

Abstract: A new mode of operation is proposed for light control systems consisting of a combination of liquid crystal cells and polarizers. The direct operation of liquid crystal cells with circularly-polarized light is advantageous in many respects. The circularly-polarized light is produced either with conventional circular polarizers or with monomeric or polymeric cholesteric liquid crystals. The new mode of operation is possible under certain conditions for all liquid crystal cells which hitherto had to be operated with linearly-polarized light.

Active matrix liquid crystal liquid crystal light valve including a dielectric mirror upon a leveling layer and having fringing fields

Abstract: An active matrix liquid crystal light valve is disclosed. The addressing matrix has an array of electrodes to create electric fields across the liquid crystal layer. The light valve has a continuous nonconductive reflector layer, separate and distinct from the electrode array of the active matrix. This light valve has a leveling layer disposed on the active matrix to form a planar surface. The reflector layer is formed as a continuous sheet on the planar surface of the leveling layer. An additional invention is the use of fringing fields to switch substantially all of the liquid crystal, including areas not directly over the electrodes.

Anchoring structure of smectic liquid-crystal layers on MoS 2 observed by scanning tunnelling microscopy

Abstract: THE molecular alignment of liquid crystals on solid substrates is important both to fundamental issues in physics and biology and in practical applications such as optoelectronic devices. The structure of adsorbed phases is determined to a large extent by the balance of molecule–molecule and substrate–molecule forces at the solid-adsorbate interface. Scanning tunnelling microscopy (STM) now allows the possibility of direct observation of the interfacial structure with molecular resolution, and has been used to study positional and orientational order in monolayers of organic molecules on graphite1–4. Here we report STM imaging of smectic liquid crystals (4'-n-octyl-4-cyanobiphenyl (8CB)) condensed on a molybdenum disulphide single crystal. The anchoring structure is different from that on graphite4, and is unusual in that the molecules are aligned so that intermolecular interactions are energetically unfavourable. The influence of interactions with the substrate is clearly important in this case, and this is further supported by an apparent degree of epitaxy in the adsorbed layer.

Tunable liquid crystal etalon filter

Abstract: A tunable liquid-crystal etalon filter comprising two dielectric stack mirrors defining an optical cavity into which is filled a liquid crystal Electrodes disposed on the outsides of the mirrors apply an electric field to the liquid crystal, changing its refractive index and thereby changing its optical length Thereby, the optical pass band of the filter can be electrically changed in a low-powered, compact, rugged, and economical structure

A simple model for pretilted nematic liquid crystal medium and its torsional surface coupling strength

Abstract: An explanation for the mechanism generating a pretilted nematic liquid crystal conformation aligned on rubbed alkylbranched-polyimide layers is given based on a steric interaction model. An analytical calculation using this model was shown to agree fairly well with the data of torsional coupling strength obtained in this work.

Nematic liquid crystal modulator with response time less than 100 μs at room temperature

Abstract: A high‐speed nematic liquid crystal modulator with response time less than 100 μs at room temperature was demonstrated. The effects that contribute to the observed high speed are the transient nematic effect, a new liquid crystal mixture, and a holding voltage technique.

A Phenomenological Theory of the Antiferroelectric Phase Transition in Smectic Liquid Crystals

Abstract: By applying the Landau theory, the appearance of the antiferroelectric phase in smectic liquid crystals and the double hysteresis loop are interpreted Several types of phase sequences including the antiferroelectric, ferroelectric and/or ferrielectric phase are obtained phenomenologically The D-E curves in the antiferroelectric phase are classified into three types depending on temperature and the coefficients of the fourth-order terms in the free energy

Transient and steady‐state rheology of a liquid crystalline hydroxypropylcellulose solution

Abstract: Cone and plate rheological measurements have been performed on aqueous solutions of hydroxypropylcellulose in the liquid crystalline phase. The time‐ dependent experiments (start‐up of shear and oscillatory shear flow) suggest the presence of a supramolecular structure which is modified by the flow. The characteristic time of structure evolution is estimated, and a tentative explanation of such a process is suggested in terms of polydomain rearrangement. Steady‐state measurements confirm some peculiar properties of liquid crystalline polymers, such as the presence of a shear thinning regime at the lowest shear rates and the occurrence of negative values of the first normal stress difference. Finally, an anomalous, yet reproducible, behavior of the dynamical properties is reported and discussed.