Dispersed Liquid Crystal Research Articles

Captured Orientational Order in Polymer Network Assemblies

Abstract The “marriage” between low molecular weight liquid crystals and polymers burgeoned in the 1980s with the idea of dispersing liquid crystal droplets in a rigid polymer matrix to create an electrically controllable light scattering medium. The orientation of the liquid crystal droplets, and hence the refractive index match and scattering, can be systematically controlled with an electric field. Today, dispersions of liquid crystals and polymers are found in many forms depending on the concentration of polymer, which can be as large as 70% or as small as 1%. The systems most understood are those of larger concentration where the liquid crystal is segregated out in the form of droplets randomly distributed throughout the surrounding polymer (see page 2). Dispersions of liquid crystals and polymers differ from macroscopic bulk liquid crystals because of the large surface-to-volume ratio and symmetry breaking non-planar geometry imposed by the polymer. Their composite nature profoundly affects the ordering of the liquid crystal, and their susceptibility to external fields makes them suitable for many new electro-optic applications, as well as intellectually challenging to study from the basic science perspective.

Light scattering spectral behavior of liquid crystal dispersions in silica glasses

Dispersions of nematic liquid crystal microdroplets in silica glasses have been obtained by sol-gel processes, forming gel-glass dispersed liquid crystals (GDLCs). GDLC microdroplet size may be adjusted precisely by controlling the sol-gel reaction. This allows the preparation of scattering media with different spectral response, which can be further modified by reorienting the liquid crystal microdroplets with external electric fields. Opaque/transparent and dichroic GDLC thin films have been prepared. An extension of the general scattering theory is used for explaining their optical behavior.

Gel dispersed liquid crystals

Microemulsion based gels (MBG) can be used as carrier materials for dispersed thermotropic liquid crystals (LC). The viscosity of the carrier material can be specifically changed by varying the gelatin content. The LC droplet sizes and their distribution in the MBG system are influenced by both the ratio between AOT surfactant and alcohol co-surfactant and the length of the carbon chain of the co-surfactant. LC droplets without or with only a small amount of alcohol co-surfactant have the same droplet size and show a radial structure.

Birefringence dispersion and inversion in lyotropic liquid crystals

The birefringence of a nematic cylindrical lyomesophase has been measured by polarized interferometry and optical microscopy in the wavelength interval 300-800 nm. An inversion of the optical signal is observed at 387.5 nm; Δn>0 for smaller λ values and Δn<0 for larger λ values. The experimental results are well fitted to a theoretical curve with two dispersion terms, due to ultraviolet and infrared absorption bands, of respectively positive and negative signals

Proton NMR Relaxation Study of Molecular Motions in a Liquid Crystal with a Strong Polar Terminal Group

Abstract Liquid crystalline compounds containing a cyano terminal group often exhibit peculiar molecular organizations of their mesophases. In this work we present proton NMR relaxation studies, performed by means of standard NMR and fast field-cycling NMR techniques, in the nematic (N) and bilayered smectic-A phase (SA2) of 4-pentyl-phenyl 4'-cyanobenzoyloxy-benzoate. The field-cycling measurements were used to clarify the relaxation behaviour in the low Larmor frequency range, where conventional techniques are not applicable. Self-diffusion and rotational reorientations are found to be the essential relaxation mechanisms at MHz frequencies in the smectic mesophase, while the contribution of collective modes appears only at lower frequencies in the kHz range. In the nematic mesophase the order director fluctuations mechanism dominates the relaxation dispersion up to 10 MHz, where the rotational reorientations become important, with minor corrections from the self-diffusion process. The agreement between the experimental findings and model fits could be improved by an additional relaxation mechanism in the kHz regime, ascribed to the interaction between protons and fast relaxing quadrupolar nitrogen 14N nuclei. Though all four processes are present in the nematic and smectic-A2 phases, the overall T1 frequency dependence is quite different in the two cases. This behaviour is discussed in terms of available theoretical calculations of the proton relaxation dispersion in liquid crystals, and it is also compared with data known from other cyano compounds.

Electro-optics of polymer dispersed ferroelectric liquid crystals

Abstract A new electrooptical material, planar-oriented films of polymer dispersed ferroelectric liquid crystals, has been investigated. Its polarizable, volt-amplitude and dynamic characteristics have been measured here.

Reverse Vesicles: Counter Structure of Biological Membranes

It was found that stable reverse vesicles of sucrose monoal-kanoate form in decane + hexanol in the presence of a small amount of water. They are the dispersion of lamellar liquid crystal which swell a considerable amount of oil. The interlayer spacing of the bilayers is increased upon addition of hexanol. As a result, hexanol is effective in making the bilayers flexible and in dispersing the liquid crystal as reverse vesicles. The structure of the reverse vesicles was confirmed by video-enhanced microscopy, fluorescence microscopy, and freeze-fracture transmission electron microscopy(Cryo-TEM). The orientation of amphiphilic molecules is completely opposite to that in normal vesicles, i.e., with the hydrophilic part located inward. The thickness of the bilayers (about 10 nm) observed from Cryo-TEM photography is in good agreement with SAXS (small-angle X-ray scattering) data. The smaller vesicles with sizes of submicrometer order are very stable and are not precipitated for several months.

Spatial dispersion in chiral liquid crystals. Effects of higher orders

Abstract Spatial dispersion effects in chiral liquid crystals are reviewed. New spatial dispersion phenomena are observed in the vicinity of the Bragg reflection wavelength–an anomaly of the refractive index and an optical anisotropy of the cubic blue phases. These effects can be explained by taking into account the spatial dispersion correction of the dielectric tensor of the medium of higher orders in the ratio of the light wave vector to the structural wave vector of chiral phases.

Photoconductive Polymers in Liquid Crystal Spatial Light Modulators with High Resolution

Abstract Various types of organic polymer photoconductors have been successfully used in optically addressed liquid crystal (LC) light valves based on the electrically controlled birefringence, optical activity, cholesteric-nematic phase transition, dynamic scattering, and polymer dispersed LC. Record regime optimization in projection or holographic manner is necessary to obtain the best device characteristics. The highest parameters obtained in the pulse holographic regime were diffraction efficiency of 36%, limiting resolutions of 1500 mm−1, and spectral sensitivity of 10−8 J/cm2 %. Photoconductive polymers themselves may be used for LC alignment simplifying the light valve construction. High aperture devices were realized side by side with the small ones. Polymer dispersed LC's connected with the photoconductive polymers allow for the realization of entirely solid state valves. High resolution, mechanical and elecrical stability, and the possibility for sensitization make the use of the polymer photoco...

A semiempirical model for liquid-crystal refractive index dispersions

A single-band model and a three-band model which consists of one σ→σ* and two major π→π* electronic transitions were developed for understanding the refractive index dispersions of uniaxial liquid crystals. This information is particularly important for liquid-crystal-display applications. By comparing the three-band model with experimental results of 5CB and 7CB (two cyanobiphenyl homologs), the contribution of each band to both refractive indices and birefringence is evaluated quantitatively. It is found that the σ→σ* band makes the primary contribution to refractive indices, but its contribution to birefringence is not as significant. Conversely, the π→π* bands make the primary contribution to birefringence, but their contribution to refractive indices is not comparable to the σ→σ* band. These characteristics originate from the differences in transition wavelength, intensity, and absorption anisotropy of the σ→σ* and π→π* bands.

A three-band model for liquid-crystal birefringence dispersion

A three-band model which consists of one σ→σ* and two major π→π* electronic transitions was developed to account for the birefringence dispersion of a uniaxial liquid crystal. By comparing theory with experimental results, the contribution of each band to the overall birefringence is evaluated. The effect of each band on birefringence dispersion depends strongly on the conjugation length of the molecule. This model reveals new physical insights for designing a liquid-crystal molecule with desirable birefringence.

Relaxation dispersion and zero-field spectroscopy of thermotropic and lyotropic liquid crystals by fast field-cycling N.M.R.

Abstract Systematic field-cycling measurements of the T 1 relaxation dispersion in numerous nematic liquid crystals (azoxybenzenes, Schiff's bases, biphenyls, phenyl-cyclohexanes, cyclo-cyclo-hexanes) confirm our previous observations obtained for PAA and MBBA that order fluctuations of the nematic director are a significant relaxation contribution only at low Lamor frequencies v, i.e. far below the usual megahertz range. Their significance is demonstrated most convincingly by the characteristic square-root dispersion law, T 1 ∼ v 1/2, which occurs in the kilohertz range and which completely disappears above the nematic–isotropic phase transition. The strength of the collective relaxation mechanism varies by more than two orders of magnitude in the sequence (selection) PAA-d 8 PAA, PAA-d 6 PAB, OCB7, MBBA, CB7, PCH7, MBBA-d 6 MBBA-d 13 and CCH7. This finding can be understood almost quantitatively by the widely differing separations and orientations of proton pairs on the molecules, together with the diff...

Birefringence dispersions of liquid crystals.

The origins of liquid-crystal birefringence were investigated. Theory on the quantitative birefringence dispersions of liquid crystals was developed and found to have excellent agreement with experimental results throughout the entire visible and infrared spectral regions. New guidelines for selecting or synthesizing the liquid crystals with the desired birefringence are established. Novel applications of liquid crystals in the infrared region are foreseeable.

Solution Behavior and Coloration Phenomena of Diglycerol Alkyl Ether/Water Systems

The binary phase diagrams of certain diglycerol alkyl ethers (DGE) and water systems were determined and the solution behavior of these surfactants was studied. These surfactants form liquid crystalline phases with lamellar structures even at low concentrations at which ordinary nonionic surfactants with ethylene oxide chain give micellar solution. Aqueous solutions of 1% to 2% of DGE showed coloration which changed according to surfactant concentration. Furthermore, the color of these solutions observed by reflected light differed from that observed by transmitted light. On the basis of the phase diagram study and scanning electron micrographic determination, the structure of these colored solutions was concluded to be a dispersion of multi-lamellar liquid crystals containing large amounts of water within the interlayers. The coloration was thus considered to arise from the interference of light reflected at the interface of lamellar layers whose thickness was comparable to that of the wave length of visible light.

Staining and drying-induced artifacts in electron microscopy of surfactant dispersions

Comparison of the results obtained from electron microscopy of air-dried and frozen hydrated specimens of stained dispersions of liquid crystals and vesicles of synthetic and natural amphiphiles indicates that rearrangement upon drying causes gross changes in the original microstructure of the examined systems. The addition of stain, usually a salt of a heavy atom, to the system may alter structure even before drying. Staining followed by drying, a technique widely used in research on vesicles and liquid crystals, leads to the formation in the specimen of dark and light fringes which have been accepted as indicative of lamellar structures, even in systems which originally are molecular or micellar solutions. The electron micrograph obtained is also dependent on focusing conditions. Defocus increases contrast by “phase-contrast” mechanisms, but may lead to errors in image interpretation if not controlled and well understood. Electron microscopy is a useful tool in the study of the microstructures of vesicles and liquid crystals, but specimens must be prepared with care, and possible changes in the original microstructure must be assessed. Whenever possible, at least two independent techniques should be used to obtain a reliable picture of the microstructure of the investigated system.

Characterization and aging of aqueous vesicular dispersions of sodium 4-(1′-heptylnonyl)benzenesulfonate

Vesicles formed by sonication of aqueous dispersions of liquid crystals of the double-tailed surfactant sodium 4-(1′-heptylnonyl)benzenesulfonate (SHBS) are examined with several techniques. The average diameter of the vesicles prepared in water is about 450 Å. The average size decreases when prepared in NaCl or at higher surfactant concentrations. The presence of a few large liquid crystallites in the dispersion, as detected by fast-freeze cold-stage transmission electron microscopy, is shown to severely bias the measurement of vesicle sizes by quasi-elastic light-scattering techniques. The commonly used techniques of gel-permeation chromatography and ultrafiltration are shown to be ineffective in separating liquid crystals from SHBS vesicle dispersions. Vesicle preparation in the presence of uranyl acetate is shown to dramatically reduce the vesicle size. The spontaneous, irreversible reversion of vesicles to liquid crystallites as the dispersions age is documented and proves that SHBS vesicles are not equilibrium structures in water or brine.

Vesicular Dispersion Delivery Systems and Surfactant Waterflooding

Abstract Surfactant waterflooding processes that rely on ultralow interfacial tensions suffer from surfactant retention by reservoir rock and from the need to avoid injectivity problems. New findings reported here open the possibility that by delivering the surfactant in vesicle form, more successful low-concentration, alcohol-free surfactant waterflooding processes can be designed. Basic studies of low concentration (⩽2 wt%) aqueous dispersions of lamellar liquid crystals of a model surfactant, Texas No. 1, have established the role of dispersed liquid crystallites in the achievement of ultralow tensions between oil and water. Recent work, including fast-freeze, cold-stage transmission electron microscopy (TEM), reveals that sonication both in the absence and the presence of sodium chloride converts particulate dispersions of Texas No. 1 into dispersions of vesicles, which are spheroidal bilayers or multilayers less than 0.1 µm in diameter filled with aqueous phase. Vesicles ordinarily revert only very slowly to the bulk liquid crystalline state. We find, however, that their stability depends on their preparation and salinity history, and that contact with oil can accelerate greatly the reversion of a vesiculated dispersion and enable it to produce low tensions between oil and water. Tests with Berea cores show that surfactant retention and attendant pressure buildup can be reduced greatly by sonicating Texas No. 1 dispersions to convert liquid crystallites to vesicles. In simple core-flooding experiments both the unsonicated liquid crystalline dispersions and the sonicated vesicle dispersions are able to produce substantial amounts of residual oil. We point out implications and directions for further investigation.

Staining and drying induced artifacts in liquid crystal and vesicle dispersions

Staining and drying induced artifacts in liquid crystal and vesicle dispersions

Some Optical Properties of dispersed Cholesteric Liquid Crystals

Abstract High iridescence has been achieved by the dispersion of cholesteric liquid crystals in polyurethane elastomers. A model based on Frank's disclination theory is proposed whereby the aggregate size and pitch determine a textural change from a spherulitic texture to an “aligned” plane texture as the size of the dispersed aggregate decreases. This is accomplished by the different free energy relationships of radial and surface defects in the dispersed state. Discussion of the model and experimental observations center on the morphological, chemical and physical parameters which determine high iridescence of dispersed cholesteric liquid crystals and hence the effect of these parameters on the topological role of defects in attaining high iridescence.

On the rotatory dispersion in cholesteric liquid crystals

It is shown that the dynamical theory of light propagation in a cholesteric liquid crystal does not predict a reversal in the sign of optical rotation on the long wavelength side of the reflection band. This conforms with the results of exact electromagnetic treatment.