Thus, in the present paper, we reported about several processes of nonlinear absorption and transmission in dye-doped NLC: nonlinear absorption due to light induced reorientation of NLC doped by highly dichroic dyes; nonlinear transmission due to nonlinear birefringence, and self-extinguishing of the beam due to”critical opalescence,” critically enhanced light scattering at the pre-transition region of nematic-isotropic phase transition. All these processes have large application potential for laser beam control and characterization techniques.

Carbonaceous mesophases are discotic nematic liquid crystals that are spun into high performance carbon fibers using the melt spinning process. The spinning process produces a wide range of different fiber textures and cross-sectional shapes. Circular planar polar (PP), circular planar radial (PR) textures, ribbon planar radial (RPR), and ribbon planar line (RPL) textures are ubiquitous ones. This paper presents, solves, and validates a model of mesophase fiber texture formation based on the classical Landau-de Gennes theory of liquid crystals, adapted here to carbonaceous mesophases. The effects of fiber cross-sectional shape and elongational flow on texture formation are characterized. Emphasis is on qualitative model validation using existing experimental data [1, 2]. The results provide additional knowledge on how to optimize and control mesophase fiber textures.

Linear stability analysis of capillary instabilities in a thin nematic liquid crystalline cylindrical fiber embedded in an immiscible viscous matrix is performed by formulating and solving the governing nemato-capillary equations, that include the effect of interfacial viscous shear forces due to flow in the viscous matrix. A representative axial nematic orientation texture is studied. The surface disturbance is expressed in normal modes, which include the azimuthal wavenumber m to take into account non-axisymmetric modes. Capillary instabilities in nematic fibers reflect the anisotropic nature of liquid crystals, including the orientation contribution to the surface elasticity and surface bending stresses. Surface gradients of bending stresses provide additional anisotropic contributions to the capillary pressure that may renormalize the classical displacement and curvature forces that exist in any fluid fiber. The exact nature (stabilizing and destabilizing) and magnitude of the renormalization of the displacement and curvature forces depend on the nematic orientation and the anisotropic contribution to the surface energy, and accordingly capillary instabilities may be axisymmetric or non-axisymmetric, with finite or unbounded wavelengths. Thus, the classical fiber-to-droplet transformation is one of several possible instability pathways while others include surface fibrillation. The contribution of the viscosity ratio to the capillary instabilities of a thin nematic fiber in a viscous matrix is analyzed by two parameters, the fiber and matrix Ohnesorge numbers, which represent the ratio between viscous and surface forces in each phase. The capillary instabilities of a thin nematic fiber in a viscous matrix are suppressed by increasing either the fiber or matrix Ohnesorge number, but estimated droplet sizes after fiber breakup in axisymmetric instabilities decrease with increasing matrix Ohnesorge number.

Flow modelling of mesophase pitches is performed using a previously formulated mesoscopic viscoelastic rheological theory [1] that takes into account flow-induced texture transformations. A complete extra stress tensor equation is developed from first principles for liquid crystal materials under non-homogeneous arbitrary flow. Predictions for a given simple shear flow, under non-homogeneous conditions, for the apparent shear viscosity and first normal stress differences are presented. The rheological functions are explained using macroscopic orientation effects, which predominate at low shear rates. The predicted normal stress differences and apparent shear viscosity are in agreement with experimental measurements.

Liquid crystalline thermosets (LCTs) have been under intensive study because of their outstanding mechanical performance and low viscosity during processing. We have successfully synthesized four kinds of nematic bismaleimide thermosets that differ in the nature of a pendant group substitution and that feature a thermally stable nematic phase. Importantly, the pendant groups were found to be efficient in depressing the melting point to a level that allows for flow and cure at reasonably low temperatures. By blending two particular monomers with monofunctional maleimide that we synthesized, N-(4-hydroxyphenyl) maleimide (HPMI), the melting point is depressed and the curing process is postponed to higher temperatures. Such a blend has a processing window amenable to much-needed thermal and rheological characterization, particularly the evolution of disclination density during shear flow as cure progresses. Our previous work on disclination density measurements during shear flow has revealed particular scaling of dimensionless disclination density with dimensionless shear rate. The influence of shearing on the disclination density, flow patterns, and molecular orientation though gelation of thermosetting liquid crystals remains unexplored. In this presentation, we will follow a description of material synthesis with a report on the results of optical rheology experiments applied to the ternary blends containing HPMI and two bismaleimide monomers, detailing the influence of shear on disclination density though gelation.

Polymer-stabilized ferroelectric liquid crystals (PSFLCs) are made by photopolymerizing 3% reactive mesogenic monomer on a quasi bookshelf texture of a ferroelectric liquid crystal (FLC). We observe the formation of nano-scaled polymer fibrils templated by the two dimensionally ordered host. The polymer fibrils capture the orientation of the host with thin polymer fibrils interweaving the smectic layers. The SEM study highlights the difference in morphology of the polymer fibrils depending on polymerization conditions; that is, polymerizing with or without the presence of an electric field. The polymer networks suppress the cone of rotation of FLC and thus, facilitate the switching and shorten the response time of the PSFLCs. We observed the threshold-less switching behavior in PSFLCs from samples polymerized with and without the field.

Flexible plastic liquid crystal displays (LCDs) offer a number of advantages over traditional glass displays. Roll-to-roll manufacturing of such displays makes possible large size, inexpensive displays. While this type of manufacturing is still in the future, here we report solutions of some of the pressing problems needed to achieve this goal. We utilize liquid crystal effects that do not require polarizers and can therefore utilize commercially available, birefringent substrates such as polyesters and polyethylene terephthalate. We achieve excellent optical and mechanical performance of these flexible displays, even when bent, by creating polymer networks and polymer walls that adhere the front and back substrates and maintain uniform spacing between the substrates. Roll-to-roll processing of flexible plastic displays also requires that the photolithographic techniques used to pattern the electrodes on the current glass LCD’s be replaced by printing techniques and the development of techniques to rapidly etch unwanted electrode material. We must also develop and optimize techniques for rapidly forming polymer walls. Here we report methods of patterning ITO electrodes on flexible plastic substrates using conventional wax transfer printing techniques and rapid etching and cleaning techniques. We also report experiments that elucidate the basic science of rapidly forming polymer walls. We combine these new achievements and understanding and demonstrate their utility by fabricating a flexible plastic LCD using techniques compatible with roll-to-roll manufacturing. Finally, we discuss the critical issues that must be addressed if flexible plastic LCD’s are to reach their full potential.

Beamsteering using liquid crystals can be achieved with refractive or diffractive implementations. The common thread in these various structures is that the liquid crystal is employed as an optical phase modulator. Either nematic or smectic liquid crystal phases can be used to shift the phase of light and steer an optical beam. Various liquid crystal optical phase modulating schemes will be described. Examples include polarization independent and quasi-achromatic modulators. Model predictions and experimental results demonstrating the optical phase modulation and beamsteering made possible using different liquid crystal based designs will be presented.

We have investigated the dynamics of formation of a reflection hologram in a photosensitive formulation containing pre-polymer and liquid crystal. Kogelnik’s two beam coupling theory of an isotropic material has previously been expanded to account for variations of refractive index Δn in the x, y, and z directions. This theory predicts a non-zero p-polarized coupling coefficient, κ p at 45o internal angle, only when a macroscopic anisotropy in the grating is present. A broadband source was used as a probe to monitor the diffraction efficiencies (DE) during exposure for both s- and p-polarized light. The onset of a macroscopic ordering of the liquid crystal is observed at the same time as the onset of scattering. We report here the effects of laser writing power on the temporal evolution of s- and p-polarized diffraction efficiency and ppolarized scattered intensity.

A low-molecular-weight liquid crystalline amphiphilic compound (Az6Bu) and a liquid crystalline polymethacrylate [P(Az6Bu)] with amphiphilic-mesogenic side-chains are reported in this present paper. A polyethyleneimine chain with 2-hydroxyethyl side-groups was used as the water-soluble spacer. Az6Bu exhibited a lyotropic columnar phase and no thermotropic mesophase. P(Az6Bu) showed thermotropic and lyotropic liquid crystalline properties. P(Az6Bu) formed thermotropic smectic A and C mesophases on the heating and cooling processes. The lyotropic systems consisting of P(Az6Bu) and water exhibited a smectic C lyomesophase with oilystreak and schlieren textures. The layered structures formed in the thermotropic and lyotropic mesophases are composed of a sublayer produced by the anisotropic alignment of aromatic-mesogenic groups and a sublayer of hydrophilic groups.

We investigated the third-order nonlinear optical response of a Gay-Berne system at the isotropic, nematic, and smectic phases by molecular dynamics simulation. The components of the optical response were calculated for the three different axes with respect to the director of the system, separately. In the nematic phase, in particular, we observed that the response function does not vanish at long times. This means that the orientation of the director of the system is permanently changed by an instant irradiation of polarized light, as a result of third-order nonlinear optical response. In the smectic phase, however, all the components of the response function decay quickly. Our results give a theoretical background at molecular level on the interpretation of the reported experimental observations of peculiar dynamics of liquid crystalline systems at irradiation of laser lights.

In recent years, a considerable amount of research has been dedicated to synthesizing highly ordered nanostructured materials. One way to achieve this is through templated polymerizations. A novel route to fabricating such materials is through the use of lyotropic liquid crystals (LLCs) that possess highly ordered nanostructures. However, LLC phases lack the necessary physical robustness. So, templating LLC phase morphology onto other materials such as organic polymers would give a nanostructure retained as part of a robust polymeric matrix. This study focuses on the photopolymerization behavior and structure retention of hydroxyethyl acrylate (HEA) / dodecyltrimethyl ammonium bromide (DTAB) / water system in a select LC phase. The results suggest that the polymerization behavior is heavily dependent on the type of LLC structure. Specifically, lamellar aggregates polymerize faster than either cubic or isotropic morphologies due to diffusional limitations on the growing polymer chain. Monomer segregation also plays a role in determining the polymerization rates. Results also indicate that the original LLC order of these systems is largely retained upon photopolymerization although some LLC phases do change upon cure. This order would be useful in applications such as ultrafiltration membranes, separation media, and drug delivery systems.

Optical responses and orientational photorefractivities of nematic liquid crystal cells doped with fullerene were investigated with a sandwiched structure. The photorefractive effect was attributed to the reorientation and switching of liquid crystals using a photoalignment layer for vertical alignment. When nematic liquid crystals were injected at a nematic phase, a better cell texture could be obtained due to the capillary flow-induced orientation effect. Samples with this method showed the higher photorefractive effects than those injected at an isotropic phase. Therefore, the state of molecular ordering is important for the orientational photorefractive effects.

Several new benzoic acid derivatives having perfluorinated substituents were synthesized and their hydrogen-bonded complexes with 4, 4’-dipyridyl were prepared. In these acid/base complexes the incompatibility between perfluoroalkyl moieties and hydrocarbon parts is large, which can lead to organize nano-segregation structures. We have found in one of such complexes, that has a long flexible fluorinated moiety, a thermotropic cubic phase with Ia3d symmetry formed by double gyroid of two interpenetrating jointed rod networks with an estimated cell parameter of 10.9 nm. Furthermore another complex with a branched long perfluoroalkyl terminal chain exhibits a first order smectic A to smectic C phase transition, which is confirmed with thermal analysis detecting a large enthalpy change of 5.3 kJxmol-1, X-ray scattering experiments revealing a tilt angle jump, and polarized optical microscopy observing a remarkable texture change at the phase transition temperature.

This paper presents a comparison of moisture permeation in liquid crystalline and conventional epoxy systems. The permeability is obtained using a dynamic method. It is found that both epoxy systems exhibit Fickian behavior. The liquid crystalline epoxy network exhibits higher barrier properties for moisture transport than the conventional epoxy network. The efficient chain packing within the smectic mesophase of the liquid crystalline thermoset (LCT) is suggested as the main factor for this difference. The stoichiometry has a large effect on the moisture permeation. The diffusion coefficient decreases monotonically with increasing amine/epoxide functional ratio. The permeability (P) and solubility coefficient (S) reach a minimum at a functional ratio of one. The mechanism of the permeation is described in terms of the two-phase morphology present and hydrogen bonding between absorbed water and the network.

The synthesis, characterization and phase transition studies of higher homologues of N(p-nbutoxybenzylidene)p-n-alkylanilines, Viz., 4O.m series (m=14 and 16) are carried out by thermal microscopy, differential scanning calorimetry and density. The compounds exhibit a phase variant of nematic and smectic-A. Density studies reveal a first order nature of IN transitions and a weak first order NA transitions. An estimate of pressure dependence of the phase transition temperatures using the volume and enthalpy data are presented. A comparison of these results with other reported results in nO.m and other compounds are presented.

We describe a new polymer patterning technique that produces controllable morphological and optical anisotropy in a polymer network. This technique is based on the use of pattern-forming states of nematic and cholesteric liquid crystals as templates for forming ordered polymer networks. One and two-dimensional optical patterns are induced by applying an electric field across a narrow gap of an electro-optical cell. These field-induced optical patterns are then stabilized by UV-induced polymerization of a typically 5 wt% reactive monomer in liquid crystal host. Depending on specific conditions (e.g., thickness to pitch ratio of a cholesteric, applied electric field, and wavelength of UV illumination), the polymer captures various degrees of the orientational order and spatial periodicity of the pattern-forming states of liquid crystals. The fidelity of the templating effect is explored using polarizing optical microscopy and SEM. We also describe the effect of UV wavelength on the network morphology and the morphological control over the”third” dimension (normal to the cell substrates).

Holographically-formed Polymer Dispersed Liquid Crystal have been imaged using atomic force microscopy (AFM) tapping techniques to study the droplet cavity morphology. Reflection-mode Bragg gratings were created using a 532 nm beam expanded laser to holographically form a grating structure within the liquid crystal / polymer film. The films were removed from their glass substrates prior to imaging by an atomic force microscope, and the digital files were analyzed. The surface structure is not smooth, as anticipated, but contains cavities with a dimpled morphology. We report our investigations and analysis including AFM images, image analysis, and liquid crystal / polymer alignment considerations.

Holographic polymer dispersed liquid crystal (H-PDLC) films with partially fluorinated matrices were investigated. Electro-optical and morphological studies revealed that fluorinated composites were substantially different from non-fluorinated analogues. The addition of a fluorinated monofunctional acrylate monomer to a pentaacrylate-derived polymer matrix resulted in improved diffraction efficiency. These findings suggest that the partial fluorination of the host polymer decreases the compatibility between the matrix and liquid crystal phase. Morphological differences between fluorinated films and non-fluorinated control specimens were verified using low-voltage, high-resolution scanning electron microscopy (LVHRSEM).

New cholesteric polymeric material containing a liquid crystalline (LC) copolymer with side phenylbenzoate nematogenic and photochromic spiropyran groups and 5 mol % chiral photochromic dopant based on cinnamic acid and isosorbide was prepared. It was shown that UV irradiation and subsequent annealing of planarly-oriented mixture films lead to untwisting of the cholesteric helix and cause an irreversible shift of selective light reflection to the long-wavelength region. This process is explained by the E-Z isomerization of chiral photochromic groups of the dopant relative to a C=C bond accompanied by a decrease in their twisting ability. Upon UV irradiation of films at room temperature, a thermally reversible transformation of spiropyran groups into a merocyanine form takes place, which is followed by the appearance of an intense absorption maximum in the visible spectral region. It was shown, that the use of the material prepared in this work provides an opportunity to record two images at a time: one image is due to a change in the helical pitch and another image (above) is due to the photochromism of spiropyran groups. It is pertinent to note that using the approach developed in this work allows one to widely vary the range of materials exhibiting dual photochromism. It was demonstrated that the mixture under examination shows promise as new photosensitive material for optics, optoelectronics, and data recording.