Until recently, it was an empirical fact that creation of a chiral liquid crystal phase required enantiomerically enriched molecules. In addition, to date known ferroelectric and antiferroelectric smectics have also been composed of enantiomerically enriched molecules. Herein are described the first examples of the formation of chiral and antiferroelectric supermolecular liquid crystalline structures from achiral molecules. In one case (apparently metastable) the liquid crystal structure is macroscopically chiral, with samples composed of heterochiral macroscopic domains: a liquid conglomerate.

A brief review of non-display devises based on ferroelectric liquid crystals (FLC‘s) is presented. The development of ferroelectric liquid crystal materials and corresponding electrooptical elements for non-display applications have been considered.

Dynamic light scattering was used to examine ferrielectric liquid crystalline phases in porous media. Whereas in larger pores (200 Å) ferrielectric phases were observed, they were not found in the smallest pores (200 Å). Additionally, the temperatures of SmC - SmA phase transition were found to be suppressed in the pores relative to bulk, while SmCA - SmCγ phase transition is not affected by the confinement. These observations have been explained by the structural aspects of antiferroelectric liquid crystalline materials in a confined geometry and show the importance of long range electrostatic interaction for existence of ferrielectric phases.

The field dependent birefringence and the tilt angle are measured for a series of four electroclinic liquid crystals. The zero-field birefringence decreased significantly and monotonically with the length of the siloxane side chain. For all four samples, the birefringence and the electroclinic tilt angle increased substantially with the application of a field. At 10 V/µm, the electroclinic tilt angle in DSiKN65 and TSiKN105 exceeded 22.5 degrees and the phase retardation due to the change in the birefringence exceeded a quarter wave. In all four liquid crystals, the electroclinic tilt angle and the field-dependent birefringence were shown to be strongly correlated, suggesting a common origin. Moreover, this correlation persists even at high fields where both processes undergo saturation. A simple orientational model, based on a distribution in the molecular optical axis, was developed to explain the large change in birefringence in the substituted compounds as well as the dependence of δn on electric field.

The smectic structure obtained by cooling the material from the cholesteric phase often produces different structures in the twisted ferroelectric configuration, which depends not only on the nature of the material, but also on the rate of cooling. In this paper, we have investigated the origin of this behavior and have concluded that the twist in the cholesteric phase is responsible for the apparent unpredictability. Furthermore, we show that for a given chirality of material and the same rubbing condition at the surface, two possible smectic domains are produced. While the electro-optic properties of both these domains show gray-scale capabilities, these domains have different optical properties. This comparison was made possible by using a specially prepared sample in which we rubbed two glass plates, each with two areas in which the rubbing direction was both horizontal and vertical. Using this sample, we have been able to show that the presence of pitch inversion close to the cholesteric to smectic phase transition is the apparent reason the results can not be reproduced. Furthermore, our experiments clarify the importance of sample cooling rate. If rapidly cooled during the fabrication from the cholesteric to the smectic phase, the sample retains a memory of the non-equilibrium pitch, providing us with a means to capture the desired pitch. This important discovery will aid us in modifying the material as well as carefully tailoring its pitch in order to insure that cooling of the cholesteric material leads naturally to the twisted smectic structure.

We present here a rheo-optical study of the flow behavior of two cholesteric liquid crystals, one with a large pitch and one with a small pitch. The large pitch compound has been investigated as a possible fixed wavelength polarizer in the mid-wavelength infra-red region (3-5 micron). The investigation of these compounds is driven by their low melt viscosity and ability to vitrify order, and thus functionality, into films with a wide range of thickness. In our attempts to obtain consistent thin films with reproducible contrast ratios, we explored the defect textures of both compounds under a polarizing optical microscope. These materials were sheared at various strain rates and at various temperatures in an attempt to determine the best processing window for defect free films. The pitch lengths of the two materials investigated were 160 and 1330 nm. The flow behavior of the large pitch material resembles a pure nematic with defect refinement taking place under flow. The short pitch material exhibited the typical Grandjean oily streaks upon shearing followed by coarsening. Observations made during this rheological study were used to identify a processing technique for the large pitch materials. Upon application of a conventional buffed alignment layer, films with consistent quality were routinely made. The measured polarization contrast of >70:1 exceeds the values obtained from state-of-the art commercial polarizers in this wavelength regime.

Liquid crystal displays (LCDs) will be classified into miniature display, reflective type, projection type, direct view type, and holography type. All of these LCDs will be widely utilized in the coming multimedia network era. Along with this trends, in the first part of this paper we will discuss the social background of this research. In this paper we will place an emphasis on a polymer stabilized (PS) FLCD that is featured by fast response speed (40 µsec), high contrast (230:1) with grayscale, wide viewing angle, and high resolution (4001p/mmn). The PS-FLCD, which is referred to as Liquid Crystal Optical Diode owing to its strong asymmetric electrooptical characteristics, will be a promising technology for displaying a moving video image, and optical computing.

Among the different methods used to prepare Polymer Dispersed Liquid Crystals (PDLC), the Polymerisation Induced Phase Separation technique (PIPS) can be exploited to obtain optical recording in these materials. The possibility of performing optical storage of binary images in PDLC during the curing process and post curing holographic recording, will be described and discussed.

Conventional (polymer/liquid crystal) composite films exhibit the normal mode electro-optical switching, that is, the light scattering-light transmission switching upon electric field OFF-ON states, respectively. The “reverse mode” electro-optical switching, that is, the light transmission-light scattering switching upon electric field OFF-ON states, respectively, has been investigated for a novel type of composite system being composed of a side chain type liquid crystalline polymer(LCP) and low molecular weight liquid crystals(LCs). The polarizing optical microscopic observation and X-ray diffraction study revealed that the two types of smectic-like short range ordering were present in the (LCP/LCs) composite system in a nematic state. It was confirmed from electric capacitance measurements of the homogeneous alignment cell that these phases with different smectic-like short range orderings each exhibited a different value of threshold voltage. Although the homogeneous alignment cell of the (LCP/LCs) composite system was in a light transmitting state in the absence of an electric voltage, the cell turned into a light scattering state upon the application of an electric voltage of 5-15 V for the cell of 14 µm thick (0.4−1.1 MV" m−1). The reversible change from the light scattering state to the transmission state was observed after removing the electric voltage. The light scattering state of the (LCP/LCs) composite system upon the application of electric voltage might be due to the appearance of optically heterogeneous structure induced by the different values of the threshold voltage between the two types of smectic-like phases with different short range orderings. These phases might be in a phase-separated state with an optical dimension (several hundred nm). The reversible “reverse mode” electro-optical switching was realized for the (LCP/LCs) composite system in a nematic state in which the two types of smectic-like short range orderings were separately formed in an optical size level.

We have analyzed the micro-phase separation during photo-polymerization in the liquid crystal/polymer composite films for a variety of light-control applications. The photo-polymerization induced phase separation process is remarkably affected by the reactivity in diacrylates and the fraction volume in liquid crystals. It is presumed that the nematic transition in a liquid crystal-rich phase and the reactivity play an important role to determine the morphology.

We discuss recent advances in reflective holographically-formed polymer dispersed liquid crystal materials in the context of their suitability for reflective display applications. A dual-domain phenomenon resulting in a broadened reflection peak is presented. A simple phenomenological model is developed to explain this unusual effect The effect of monomer functionality on the reflectance characteristics of these materials is also discussed. Monomers of effective functionality ∼4.5 yield the brightest holograms, however, the data suggests that these systems are currently far from optimal.

Electrically switchable volume reflection holograms were written by inhomogeneous illumination of a prepolymer syrup containing a nematic liquid crystal and a multifunctional acrylate monomer. Switchable holograms are diffractive optics structures and the diffraction efficiency can be controlled by the application of an electric field. Reflection gratings with grating spacing varying between 0.16-0.27 µm were made during the phase separation of liquid crystals from the fast curing prepolymer syrup. The reflection efficiency of the holograms were electrically modulated with the applied field of ∼10-15V/µm. Real time study of the grating formation revealed that the maximum efficiency is reached in ∼15 seconds. The shrinkage of the host polymer during grating formation resulted in the blue shift of the reflection notch. The response time of the grating in an electric field is ∼50 µs. Low voltage scanning electron microscope studies showed the presence of discrete nematic droplet domains of sizes 30-60 nm in liquid crystal rich region.

The influence of elasticity on the phase behavior of a mixture of nematic liquid crystal (LC) and in-situ cross-linked polymer has been investigated. Unlike the phase diagram of the LC/pre-cured polymer, the experimental diagram of LC/cross-linked polymer phase supported by theoretical calculation showed no critical point. Instead, the binodal curve exhibits an upward asymptotic behavior as the LC volume fraction approaches unity due to domination arising from network elasticity, particularly at high LC volume fraction. An examination of the effect of cross-links segment length and network functionality on the phase diagram of LC/cross-linked polymer showed that the segment length between cross-links exerts a greater influence.

Ferroelectric liquid crystals (FLCs) have shown great potential for use in electro-optic and display technology due to their inherently fast switching speeds and bistability. Recently, considerable research has been devoted to FLCs mechanically stabilized by a polymer network. The network is formed typically by in situ polymerization of a monomer dissolved in the FLC. Because of the inherent order in the FLC, the polymerization behavior may be significantly different than what might be expected in solution polymerizations. These deviations result largely from the segregation properties of the monomer in the liquid crystal. One class of monomers, namely fluorinated acrylates, is a likely candidate for inducing novel segregation, polymerization and electro-optic behavior in polymer stabilized ferroelectric liquid crystals (PSFLCs). The use of fluorinated moieties has a significant impact on the phase and polymerization behavior of liquid crystal systems. This study focuses on the polymerization of a fluorinated diacrylate, octafluoro 1,6-hexanediol diacrylate (FHDDA), to form PSFLCs and the consequent impact of the polymerization on the ultimate performance. Interestingly, as the temperature is increased and the order of the system decreases, a dramatic increase in the polymerization rate is observed. This increase is especially prominent for polymerizations in the smectic C* phase for which the rate is more than five times that exhibited at much higher temperatures in the isotropic phase. As with other monomer/FLC systems, the segregation of the monomer plays a role in this polymerization behavior as the monomer segregates between the smectic layers of the liquid crystal. The segregation properties also significantly impact the ultimate electro-optic properties. Both ferroelectric polarization and response time of the PSFLC change markedly with different polymerization temperatures, and approach values very close to those of the neat FLC under appropriate polymerization conditions. This behavior not only provides a unique mechanism for rate acceleration in PSFLCs, but also paves the way for new methods to optimize performance in these materials.

In holographic Bragg gratings formed by anisotropic photopolymerization of free-radical monomers, an electric field is required to switch the grating between a diffracting and a transmitting state. The voltage necessary to turn the grating completely off is defined as the switching voltage. In this work, we report on the gradual increase of this switching voltage with time after initial fabrication. The switching field increases approximately 60% over a seven-day period for a pentaacrylate/E7 material system. Using dielectric measurements, it is observed that the resistance of the cell does not change over time while the capacitance decreases. FT-IR spectroscopy was also used to explore the continued post-polymerization after fabrication of the gratings. Increases in switching field were observed for samples fully illuminated with light as well as those kept in the dark. We speculate that the rise in the switching voltage is caused by post polymerization of residual reactive moieties located near the polymer/nematic droplet interface. An increase in the amount of interfacial area generated by post-polymerization yields more surface area for the LC molecules to bind; thus a larger field is needed to switch.

Polymer dispersed liquid crystal (PDLC) films with a partially fluorinated polymer matrix were investigated. The optical and morphological properties of fluorinated PDLC's were different from non-fluorinated films. The incremental addition of a fluorine-substituted monofunctional methacrylate monomer to a pentaacrylate-based standard PDLC formulation resulted in significant improvement in contrast ratio. In addition, results suggest that fluorine decreased compatibility between the polymer and liquid crystal phases. Scanning electron microscopy (SEM) was employed to elucidate morphological differences between fluorinated host matrices and non-fluorinated control films.

Thin films of polymers containing p-cyanoazobenzene side chains were studied to reveal its applicability for optical storage media, surface grating optics and aligning films for liquid crystal (LC) display devices. We followed the generation of optical anisotropy of LPLirradiated films of three polymethacrylates with azobenzene side chains exhibiting amorphous, crystalline and liquid-crystalline natures during heat treatment by mean of an ellipsometry and polarized UV-visible spectroscopy. Whereas photogenerated optical anisotropy of amorphous polymer films disappeared above glass transition temperature, the photo-induced anisotropy of films of liquid crystalline and crystalline polymer films was significantly amplified by heat treatment. In case of crystalline polymers amplified optical anisotropy was maintained even at 300°C whereas partial decomposition was observed.

Uniaxially oriented composites of high-density polyethylene and silver nanoparticles were prepared using solution-casting, melt-extrusion and solid-state drawing techniques. The absorption spectrum in the visible wavelength range of the drawn nanocomposites was observed to strongly depend on the polarization direction of the incident light. For instance, the nanocomposites appear bright yellow or red when the vibration direction of linearly polarized light is perpendicular or parallel, respectively, to the drawing axis. The optical anisotropy of the drawn nanocomposites originates from uniaxially oriented, pearl-necklace type of arrays of nanoparticles of high aspect ratios. The absorption spectrum of the nanocomposites can be shifted to higher wavelengths using appropriate annealing procedures. The annealing results in an increased size of the primary silver particles, due to Ostwald ripening, and consequently a range of polarization-dependent colors can be generated in the drawn nanocomposites. It is suggested that the drawn nanocomposite films can be used in liquid crystal displays (LCD's) were they serve a dual purpose in combining polarization filter and color filter. The new display configuration transmits colored light in both the on- and off-state of the device, this in contrast to a conventional color LCD which only transmits colored light in the off-state. As a consequence, an enhanced brightness and light (energy) efficiency of the new display set-up is envisioned.

We explored the formation of holographic gratings by means of a polymer azobenzene liquid crystal (PALC). When two writing beams (argon-ion laser, 488 nm, unpolarized) were interfered on the surface of the PALC film (thickness, 500 ∼ 600 nm), multiple diffraction beams were observed immediately, resulting from grating formation. Such generation of the diffraction beams was assumed to be due to a periodic induction of photochemical phase transition in the bright fringes of the interference pattern. The grating is namely made up of a periodic arrangement of nematic (N, dark fringes) and isotropic (I, bright fringes) phases. Observation of recorded interference patterns under a cross-polarized optical microscope supported our speculation for the structure of the grating. On the basis of these results, we succeeded in recording an image hologram of a photomask as an object.

Photo-induced reorientational behavior of films of a liquid crystalline polymer with azobenzene side chains was studied by means of UV-Vis absorption spectroscopy and Fourier transform infrared transmission and reflection-absorption spectroscopy. The incident directions of light for photoisomerization played a critical role in the photocontrol of out-of-plane (three-dimensional) orientation of the azobenzenes in films. Tilted directions of the azobenzene residues were evaluated by using polarized UV-Vis absorption spectra. Photoreoriented states of azobenzene chromophores including photodichroism and tilted alignment were enhanced by successive annealing of photoirradiated films at temperatures close to the glass transition temperature of the polymer.