Morphology and electro-optical properties of reverse mode polymer dispersed liquid crystals

We have investigated the morphology and electro-optical properties of reverse mode polymer dispersed liquid crystals as a function of liquid crystal loading. Reverse mode shutters have been obtained by a polymerization-induced phase separation of mixtures, consisting of a liquid crystalline monomer and a non-reactive nematic liquid crystal, placed between rough conductive surfaces. Such surfaces are able to keep the photopolymerizable mixtures homeotropically aligned without the use of any aligning polymer substrate. OFF state transmittances are always larger than 80% and the switching fields decrease if the non-reactive liquid crystal percentage is increased. Both rise and decay times are always lower than 10 ms. The electro-optical properties have been related to the sample morphology and a simple mode is proposed.     

Morphology and electro-optical properties of reverse mode polymer dispersed liquid crystals

We have investigated the morphology and electro-optical properties of reverse mode polymer dispersed liquid crystals as a function of liquid crystal loading. Reverse mode shutters have been obtained by a polymerization-induced phase separation of mixtures, consisting of a liquid crystalline monomer and a non-reactive nematic liquid crystal, placed between rough conductive surfaces. Such surfaces are able to keep the photopolymerizable mixtures homeotropically aligned without the use of any aligning polymer substrate. OFF state transmittances are always larger than 80% and the switching fields decrease if the non-reactive liquid crystal percentage is increased. Both rise and decay times are always lower than 10 ms. The electro-optical properties have been related to the sample morphology and a simple mode is proposed.     

UV tuning of the electro-optical and morphology properties in polymer-dispersed liquid crystals

Polymer-dispersed liquid crystal (PDLC) films operating in reverse mode are transparent electro-optical devices, which can be turned into an opaque state by application of a suitable electric field. The effect was investigated of different UV powers, used during the polymerization process, on the electro-optical and morphology properties of PDLCs, working in reverse mode operation. Films were obtained by UV polymerization of mixtures of a low molecular weight nematic liquid crystal and a photopolymerizable liquid crystal monomer, homeotropically aligned by rough conductive surfaces. The electro-optical and morphology properties of samples were related to the polymerization conditions. Samples polymerized by lower UV powers exhibited “polymer ball” morphology and an electro-optical response due to the liquid crystal director reorientation, whereas samples obtained at higher UV powers showed a “Swiss cheese” morphology and an electro-optical response due to dynamic scattering. In addition, we observed by conductivity and IR measurements that UV exposure induces a degradation of the nematic liquid crystal.     

The electro-optical response of nematic emulsions

Switchable nematic emulsions are micron-sized droplets of nematic liquid crystal, floating in isotropic fluid matrices. Such droplets can be switched from an opaque (off) to a transparent (on) state by application of very low electric fields. It is known that the electro-optical properties of liquid crystal dispersions are affected by several parameters, including the liquid crystal loading. The electro-optical response of nematic emulsions has been investigated as a function of liquid crystal weight percentage. Almost transparent films with a reduced contrast ratio are obtained with lower liquid crystal contents. A macroscopic phase separation is observed when liquid crystal content exceeds 45 wt %. On the contrary, large contrast ratios and very low switching fields can be obtained if liquid crystal ranges from 25 to 35 wt %. Consequently, nematic emulsions prepared in this liquid crystal range can be used as promising systems for electro-optical applications. In addition to technological developments, these results can help computational and basic studies of phase separation in novel multiphase liquid crystalline materials.     

Physical properties of chlorinated liquid crystals

Dielectric, optical and electro-optical properties of four chlorinated nematic liquid crystal compounds and a eutectic mixture were characterized. Some chlorinated liquid crystals are found to exhibit a wide nematic range, modest dielectric and optical anisotropies, low viscosity and small UV absorption. Potential application of mixtures containing chlorinated liquid crystals for information displays in the visible spectral region is foreseeable.     

Adequate measuring techniques for ions in liquid crystal layers

The ion contamination in twisted nematic liquid crystals should be limited to assure good electro-optical performance of AM-LCDs. Today, pure liquid crystal mixtures are produced, though little information is available on long term stability, and on the kind of ions that contaminate the liquid crystal. The first step in the determination of the ion source is the development of an appropriate measuring technique that characterizes the ions in the liquid crystal. In this article such a new method is proposed. A comparison with former techniques is made.     

Thermal behaviour of switchable nematic emulsions

We have investigated the electro-optical performance of switchable nematic emulsions as a function of temperature. The electro-optical properties of nematic emulsions are highly dependent on temperature because several parameters such as droplet size, number density, viscosity, surface free energy, elastic constant, refractive indices, dielectric anisotropy, and liquid crystal/monomer solubility are affected by temperature. In particular, both ON state transmittances and decay times show a decreasing behaviour with increasing temperature. On the contrary, the OFF state transmittances increase, and the rise times do not change in an appreciable way.     

Thermal behaviour of switchable nematic emulsions

We have investigated the electro-optical performance of switchable nematic emulsions as a function of temperature. The electro-optical properties of nematic emulsions are highly dependent on temperature because several parameters such as droplet size, number density, viscosity, surface free energy, elastic constant, refractive indices, dielectric anisotropy, and liquid crystal/monomer solubility are affected by temperature. In particular, both ON state transmittances and decay times show a decreasing behaviour with increasing temperature. On the contrary, the OFF state transmittances increase, and the rise times do not change in an appreciable way.     

Calorimetric studies of poly (2-phenoxyethylacrylate)/5CB mixtures

The thermophysical properties of mixtures of poly (2-phenoxyethylacrylate) and 4-cyano-4'-pentyl-biphenyl, 5CB, are investigated using polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). The polymer has a molar mass M w = 181 000 g mol -1 ; the low molecular mass liquid crystal exhibits a nematic to isotropic transition at 35.3°C and crystallizes below 23°C. The phase diagram exhibits miscibility gaps in certain regions of temperature and composition where coexisting nematic and isotropic phases are found. From a practical point of view when considering the electro-optical applications of these systems, it proves to be useful to know precisely the amount of small liquid crystal molecules dissolved in the polymer matrix and the concentration of polymer in the nematic phase. The former quantity has a mechanical impact due to a plasticizing effect, an optical impact since it changes the polymer refractive index, while the polymer in the nematic phase shifts the transition temperatures influencing the electro-optical response of the liquid crystal. The present work addresses these important aspects using POM and DSC.     

Electro-optic behaviour of a non-polar nematic liquid crystal and its mixtures

In this work we report preliminary results on the properties of a non-polar bicyclohexane nematic liquid crystal. Moreover, its binary mixtures both with a low viscosity phenylcyclohexane and with a normal polar nematic liquid have been investigated. The elastic, viscous and electro-optical properties of these compounds are presented. The non-polar compound, and its mixture with a low percentage of the well known liquid crystal MBBA, exhibit an electrohydrodynamic behaviour in which the conducting regime is absent, while the dielectric regime spreads to low frequencies. Additionally, at higher frequencies of the applied electric field, a regime whose thresholds are linear in frequency is observed. On the other hand, the mixture formed by 50 wt % of the non-polar compound with MBBA exhibits at low frequencies the usual behaviour, followed at higher frequencies by the linear regime.     

Electro-optic behaviour of a non-polar nematic liquid crystal and its mixtures

In this work we report preliminary results on the properties of a non-polar bicyclohexane nematic liquid crystal. Moreover, its binary mixtures both with a low viscosity phenylcyclohexane and with a normal polar nematic liquid have been investigated. The elastic, viscous and electro-optical properties of these compounds are presented. The non-polar compound, and its mixture with a low percentage of the well known liquid crystal MBBA, exhibit an electrohydrodynamic behaviour in which the conducting regime is absent, while the dielectric regime spreads to low frequencies. Additionally, at higher frequencies of the applied electric field, a regime whose thresholds are linear in frequency is observed. On the other hand, the mixture formed by 50 wt % of the non-polar compound with MBBA exhibits at low frequencies the usual behaviour, followed at higher frequencies by the linear regime.     

Calorimetric studies of poly (2-phenoxyethylacrylate)/5CB mixtures

The thermophysical properties of mixtures of poly (2-phenoxyethylacrylate) and 4-cyano-4′-pentyl-biphenyl, 5CB, are investigated using polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). The polymer has a molar mass M _w = 181 000 g mol ^-1 the low molecular mass liquid crystal exhibits a nematic to isotropic transition at 35.3°C and crystallizes below 23°C. The phase diagram exhibits miscibility gaps in certain regions of temperature and composition where coexisting nematic and isotropic phases are found. From a practical point of view when considering the electro-optical applications of these systems, it proves to be useful to know precisely the amount of small liquid crystal molecules dissolved in the polymer matrix and the concentration of polymer in the nematic phase. The former quantity has a mechanical impact due to a plasticizing effect, an optical impact since it changes the polymer refractive index, while the polymer in the nematic phase shifts the transition temperatures influencing the electro-optical response of the liquid crystal. The present work addresses these important aspects using POM and DSC.     

Crystallisation of nematic liquid crystal mixtures

In this paper, we propose a method to accelerate the crystallisation of nematic liquid crystal mixtures based on crystallisation theory. This method is to hold a nematic liquid crystal sample at a temperature suitable for crystal growth after aging it at a temperature suitable for nucleation. After we specified these temperatures of a nematic liquid crystal mixture using differential scanning calorimetry, we demonstrate that the two-temperature aging method is effective for the crystallisation of other nematic liquid crystal mixtures in which the crystal-liquid crystal transition temperature has so far been undetectable.     

Preliminary communication Electron beam cured liquid crystal-polymer composite materials: electro-optical enhancement effect

Polymerization induced phase separation initiated by electron beam radiation has been used as a powerful method to obtain well defined PDLC films. Mixtures of different amounts of an aromatic polyester acrylate, an oligoether triacrylate and nematic liquid crystal material E7 were exposed to the electron beam radiation. The electro-optical properties strongly depend on the liquid crystal concentration and on the amount of the oligoether triacrylate in the prepolymer mixture, and are highly reproducible. An unusual enhancement effect of the electro-optical sensitivity was found for samples containing 28 weight per cent and 34 weight per cent of the oligoether triacrylate.     

Generation of frustrated liquid crystal phases by mixing an achiral nematic-smectic-C mesogen with an antiferroelectric chiral smectic liquid crystal

By mixing the achiral liquid crystal HOAB, exhibiting a nematic (N)-smectic-C (SmC) mesophase sequence, with the chiral antiferroelectric liquid crystal (AFLC) (S,S)-M7BBM7, forming the antiferroelectric SmC(a)(*) phase, at least seven different mesophases have been induced which neither component forms on its own: a twist-grain-boundary (TGB(*)) phase, two or three blue phases, the untilted SmA(*) phase, as well as all three chiral smectic-C-type "subphases," SmC(alpha)(*), SmC(beta)(*), and SmC(gamma)(*). The nature of the induced phases and the transitions between them were determined by means of optical and electro-optical investigations, dielectric spectroscopy, and differential scanning calorimetry. The induced phases can to a large extent be understood as a result of frustration, TGB(*) at the border between nematic and smectic, the subphases between syn and anticlinic tilted smectic organization. X ray scattering experiments reveal that the smectic layer spacing as well as the degree of smectic order is relatively constant in the whole mixture composition range in which AFLC behavior prevails, whereas both these parameters rapidly decrease as the amount of HOAB is increased to such an extent that no other smectic-C-type phase than SmC/SmC(*) exists. By tailoring the composition we are able to produce liquid crystal mixtures exhibiting unusual phase sequences, e.g., with a direct isotropic-SmC(a)(*) transition or a temperature range of the SmC(beta)(*) subphase of about 50 K.     

Linear and non-linear liquid crystal materials, electro-optical effects and surface interactions. Their application in present and future devices

The structural, material and electro-optical properties of novel, halogenated nematic liquid crystals which contain quite different functional groups are correlated. Synergisms which lead to broad mesophases, low viscosities and large dielectric anisotropies further improve the performance of actively and passively addressed, high information content liquid crystal displays. Some recent developments, such as operation of supertwisted nematic displays with not only linear, but also circularly polarized light, are included. A recently presented, efficient liquid crystal colour projection concept, whose functional elements, i.e. polarizers, filters and modulators, consist entirely of liquid crystal devices, is reviewed. Its circular polarizers and filters are made up of novel, negative dielectric anisotropic cholesteric liquid crystals designed such that, dislocation-free, optically uniform, planar textures result from electric field alignment. Novel, non-linear optical ferroelectric liquid crystals which exhibit very large and stable second order harmonic coefficients d₂₂ = 5 pm V^-1 have the potential to be used in integrated optical devices, such as frequency converters and Pockels modulators. Photopolymerization of polymer-coated substrates with linearly polarized light is shown to induce anisotropic, uniaxial orientation of the polymer side chains without mechanical treatment. The resulting anisotropic dispersive surface interaction forces align adjacent liquid crystal molecules parallel. This new, photoinduced liquid crystal aligning technique renders the generation of azimuthal director patterns possible. It opens up interesting possibilities for realizing new optical and electro-optical devices, including hybrid and stereo liquid crystal displays.     

Morphology of spinodal decompositions in liquid crystal-colloid mixtures

We study the morphology of spinodal decompositions (SDs) in mixtures of a liquid crystal and a colloidal particle by solving time-dependent Landau-Ginzburg equations for a conserved order parameter (concentration) and two nonconserved order parameters (orientation and crystallization). We numerically examine the coupling between concentration, nematic ordering, and crystalline ordering in two dimensional fluid mixtures, coexisting a nematic and a crystalline phase. On increasing the concentration of colloidal particles, we have three different SDs: a nematic order-induced SD, a phase-separation-induced SD (PSD), and a crystalline-order-induced SD (CSD). In NSD, the phase ordering can lead to fibrillar and cellular networks of the minority colloidal-particle-rich phase in early stages. In the PSD, we find a bicontinuous network structure consisting of a nematic phase rich in liquid crystal and a crystalline phase rich in colloidal particles. In the CSD, nematic droplets can be formed in a crystalline matrix. Asymmetric mixtures of a liquid crystal and a colloidal particle lead to rich varieties of morphologies.     

Liquid crystal alignment on a ZrO2 thin film as a function of ion beam incident angle

Nematic liquid crystal alignment capabilities and electro-optical performance characteristics have first been embodied in a ZrO₂ layer using ion-beam irradiation. The study demonstrates that liquid crystal layers can be aligned homogeneously as a function of the incident angle of the ion beam device, which causes a uniform dense plasma. X-ray photoelectron spectroscopy indicated that full oxidation of the ZrO₂ thin-film surfaces was produced by ion irradiation, shifting the Zr 3d spectra to lower binding energies. In addition, the electro-optical performance characteristics of twisted nematic cells on a ZrO₂ thin-film layer after ion beam irradiation showed similar characteristics to those of a rubbed polyimide cell.     

Induced smectic phases in phase diagrams of binary nematic liquid crystal mixtures

To elucidate induced smectic A and smectic B phases in binary nematic liquid crystal mixtures, a generalized thermodynamic model has been developed in the framework of a combined Flory-Huggins free energy for isotropic mixing, Maier-Saupe free energy for orientational ordering, McMillan free energy for smectic ordering, Chandrasekhar-Clark free energy for hexagonal ordering, and phase field free energy for crystal solidification. Although nematic constituents have no smectic phase, the complexation between these constituent liquid crystal molecules in their mixture resulted in a more stable ordered phase such as smectic A or B phases. Various phase transitions of crystal-smectic, smectic-nematic, and nematic-isotropic phases have been determined by minimizing the above combined free energies with respect to each order parameter of these mesophases. By changing the strengths of anisotropic interaction and hexagonal interaction parameters, the present model captures the induced smectic A or smectic B phases of the binary nematic mixtures. Of particular importance is the fact that the calculated phase diagrams show remarkable agreement with the experimental phase diagrams of binary nematic liquid crystal mixtures involving induced smectic A or induced smectic B phase.     

Liquid crystal orientation in elliptic droplets in nematic emulsions

Elliptic droplets of nematic liquid crystal dispersed in a fluid organic monomer were obtained by phase separation from an isotropic mixture consisting of an organic monomer and a nematic liquid crystal contained in a poly(ethylene terephthalate) cell with inner surfaces treated with rubbed polyimide. The elliptic shape is a consequence of the constraint upon droplet growth along the direction perpendicular to the cell surfaces owing to the small thickness. Then, the resulting droplets will have a contact area with the inner surfaces of the cell treated with polyimide, which will impart a planar orientation on the liquid crystal in the droplet. By means of an optical microscope, using a simple pin hole of 5 μm, we have selected single droplets for a series of samples having different contact areas. By polarized infrared spectroscopy we have also studied the liquid crystal orientation in selected areas of the droplets. We then report the dependence of the order parameter of the liquid crystal on different contact areas with the alignment surface of the cell. The good degree of planar alignment of the liquid crystal in the elliptic droplets allows the use of such a technique for realizing electro-optical films operating in the reverse mode. We report the electro-optical transmission of reverse mode films with different sizes of elliptic droplet.