Typical applications of polymer-dispersed of liquid crystal (PDLC) films include large and flexible information &splays and windows with electronically controlled transmission. Whenever these applications are intended for outdoor usage as, for example, when the films are to be applied to automobile sunroofs, selection criteria for the materials and choice of preparation method become important in order to satisfy the environmental temperature and humidity extremes. This paper reviews and compares different technologies for preparing PDLC films, giving special emphasis to their microstructure and ability to exhibit good electro-optic performance over an extended temperature range.

Films consisting of micron-sized nematic liquid crystal droplets dispersed in a polymer matrix (NCAP) represent an important new class of electro-optical devices. These films strongly scatter light in the tm powered state, but achieve a high degree of clarity when an electric field is applied. In this report we describe the aspects of liquid crystal and polymer composition that control the magnitude of the electric field required to orient the nematic droplets. The droplet shape is found to be an important factor in the electro-optical response of these films. In films deposited from aqueous solutions the nematic cavities in the film are usually oblate in nature, with the short axis perpendicular to the film plane. The nematic, which adopts a bipolar configuration within the cavity, is preferentially aligned so that each droplet's symmetry axis is aligned parallel to the film plane in the rest state, but rotates to lie parallel with the field in the powered state. Capacitance data is presented which supports this picture. It is shown that the nematic droplet shape can be a major factor in determining the thermodynamics of droplet orientation.

Basic studies of a liquid crystal confined to a small droplet have been motivated by the use of polymer dispersed liquid crystals' for a new generation of light shutters and displays.2 The structure of such a droplet is characterized by a specific director configuration and the spatial dependence of the orientational order parameter. Depending on temperature, droplet size, surface interaction, type of liquid crystal and external field, the phase of the material enclosed in the droplet can be either the isotropic (paranematic), nematic or boundary layer nematic phase.3 Director configurations in droplets have been studied extensi% ely,4-7 but the spatial dependence of the order parameter has been treated only for the case of a liquid crystal in contact with a solid planar surface.3.8-10 Lsing the Landaude Gennes approach," the order parameter profile was evaluated, showing how the orientational order varies with distance from the planar surface. In addition, for some values of the surface-liquid crystal coupling, the existence of the boundary layer nematic phase was predicted in a narrow tem-perature range between the isotropic (paranematic) and nematic phases.3

Polymer-dispersed liquid crystal (PDLC) films are derived from the polymerization of solutions of liquid crystals (LCs) in monomers. Upon polymerization:a phase separation occurs causing the liquid crystal to separate into discrete droplets. The liquid crystal material within these droplets still retains the electro-optic properties of bulk material. We describe the preparation of PDLC films in which the polymer is derived from a number of photopolymerizable acrylate-based monomers and the liquid crystal is a commercially available mixture with a positive dielectric anisotropy. It was found that the morphology of the resulting PDLC film can be controlled by the composition and concentration of the monomer/liquid crystal solution, the type of photoinitiator, and the rate of polymerization. Two distinct morphologies were observed: (1) A "polymer ball" morphology, in which the liquid crystal droplets were large and irregularly shaped, and (2) a "Swiss cheese" morphology, in which the liquid crystal droplets were small and nearly spherical. Modification of the polymerization mechanism from a chain polymerization to a stepwise polymerization had a dramatic effect on the resulting morphology.

The response times and operating voltages for light shutters made from polymer dispersed liquid crystals' are studied. Alterations in the resistivity of the polymer binder are made by varying the degree of cross-linking to demonstrate and quantitize the strong effect of resistivity on the switching voltage. The voltage response from transitions in the nematic director configuration is studied in radially configured droplets and the molecular anchoring energy at the droplet wall is measured. Measurements of the relaxation time, response time, and voltage response are also reported on uniformly elongated and aligned droplets to study the effect of droplet shape. The results of these measurements are compared with calculations based on simplified models for ellipsoidal shaped droplets with major and minor axes a and b respectively. Response equations in terms of the aspect ratio ℓ = a/b are experimentally examined.

A new class of high-brightness, high color contrast reflective-mode displays can be constructed from nematic droplet/polymer (NCAP) films. In these films, a high order parameter pleochroic dye can be dissolved in the nematic, leading to a film with both controllable absorbance and scattering. The physics behind the operation of these films is discussed. The intrinsic optical order parameter of a guest-host mixture is related to the performance of the NCAP film. It is shown that the scattering effects inherent in these films can be used to amplify the effects of the controllable dye absorbance, leading to excellent optical performance for a reflective-mode display. A typical construction of a display cell is given, and examples of applications are discussed. Touch switches may easily be fabricated within the display, so that an integrated control/display module can be constructed.

Colored polymer dispersed liquid crystal, PDLC, shutters are formed by the inclusion of isotropic dyes in the epoxy resin and liquid crystal used to make the shutter. Upon curing, the dye is dissolved in both the epoxy binder and the liquid crystal droplets. The shutters are highly colored and scattering in the absence of an aligning electric field, OFF state. Application of an electric field across the film aligns the director of the liquid crystal in the droplets, switching the shutter to a lightly colored, transparent, ON state. In the ON state, light passes through the film unscattered and its path length is essentially equal to the thickness of the cell. In the scattering, OFF, state the path length of light is increased due to Rayleigh-Gans and multiple scattering. The optical density, OD, of the shutter is directly proportional to the path length of the light. The shutter OD is measured in both the ON and OFF states using a LTV-visible spectrophotometer equipped with an integrating sphere and an undyed PDLC shutter used as a reference. The ratio of the OD in the OFF to the ON state is a direct measure of the scattering efficiency of the shutter. Using this ratio, it is possible to calculate the dye concentration that will maximize the change in percent transmission through the film between the ON and OFF states. High contrast, colored displays have been made using isotropic dye containing PDLC films and complementary colored backgrounds.

We have established correlations between the liquid crystal (LC) droplet size and selected characteristics of polymer dispersed liquid crystal (PDLC) films, formed by photopolymerization of LC dissolved in a monomer. The LC droplet size was controlled by means of the ultraviolet (UV) exposure intenstiy, LC concentration, and concentration of polymerization initiator. Correlations were noted between the LC droplet size and the resulting PDLC threshold and operating voltages for optical transmission, contrast ratio, and electro-optical response times. Good contrast PDLC films were obtained which were operable with less than 20 V, rms of applied signal. Special types of films, including in situ gratings were obtained by varying the conditions of polymerization across the film to produce a periodic spatial variation in the LC droplet size.

The first large area, commercially available, electrically-controllable glazing products sold under the tradename VARILITETM are based on a new liquid crystal film technology called NCAP. The glazing products can be switched in milliseconds between a highly translucent state (for privacy and glare control) to a transparent state (for high visibility) with the application of an AC voltage. The optical and environmental properties are demonstrated to meet the general requirements for architectural glazing use. The first qualified indoor product is described in detail.

In this paper we discuss the application of liquid crystals to binary optical computing. In particular we are interested in using liquid crystals to form optical logic gates and flip-flops, the basis of binary computing. A fair amount of work has been done using liquid crystals incorporated in optical spatial light modulators. These will be reviewed and discussed and simple optical circuits using them will be described.

We present results of our recent research in the implementation of optical modulation devices utilizing polymer dispersed liquid crystal materials. Optical bistability, power limiting, self-pulsations, and optical deflection (diffraction) have been observed in these devices. Experimental results and potential applications are discussed.

The largest contribution to the nonlinear susceptibility of liquid crystalline materials arises from the effects of optical fields on orientational order. One mechanism of fundamental importance is the collective reorientation of the molecules by the radiation field which couples to the orientation via the polarizability anisotropy. The reorienting torque of the optical field is in general opposed by surface and elastic torques, and hence the response of the system depends on the sample geometry and surface anchoring effects. We consider the details of the interaction between the radiation field and the liquid crystalline medium and examine the nature of optical field induced elastic deformations. These reorientation effects can give rise to technologically important phenomena such as optical bistability and optical field induced scattering. We compare the predictions of theory with the results of recent experiments on homogeneous liquid crystals and on composite polymer dispersed liquid crystal (PDLC) films.

Tight-binding (molecular orbital) models of conjugated organic molecules were used to predict structure-activity correlations for molecules with large electronic hyperpolarizabilities [U. The dependence of the nonresonant second hyperpolarizability on chain length and molecular substituents was investigated. The calculations point out the influence of bond alternation and the presence of substitutional (and conjugation) "defects" on the size, sign, and chain length dependence of the molecular (and the bulk) hyperpolarizability. It is hoped that the calculations will provide guidance for future chemical synthesis by presenting strategies for systematically tuning the size and sign of the hyperpolarizability.

We discuss the conditions under which the electric fields produced by polarization space charge need to be included in calculating the response of ferroelectric liquid crystals to applied electric fields. We present a simple example to illustrate the basic physical effect of polarization space charge and apply the result to geometries of electro-optic interest.

The speed of electro-optic switching in the surface stabilized ferroelectric liquid crystal (SSFLC) cell is dependent upon the polarization density and orientational viscosity of the FLC material. These properties are, of course, related to the chemical structure of the FLC components. In an effort to design new, high performance FLC materials in a directed way, we have developed a stereochemical model for the molecular origins of the polarization based upon the concept that the polarization is a manifestation of a novel form of molecular recognition occurring in the FLC phase.1 Recent results of work aimed at testing this model will be described. Specifically, a novel series of chiral fluorinated FLC components have been prepared and characterized with respect to polarization, viscosity and electro-optic response. Comparison of the properties of diastereomeric materials provides experimental evidence for the details of orientation of organic functional groups occurring in the FLC phase.

Ferroelectric liquid-crystal (FLC) light-shutter arrays for half-tone recording are studied. The shutter arrays differ from traditional LC arrays in that they have three terminals in every shutter cell. The shutters perform a stopping action with a curtain-like diaphragm when appropriate signals are applied to the three terminals. The diaphragm area is varied by controlling one of the three signals. This function is applied to a printing head recording half-tone images by dot area modulation. A 105-step gray scale is recorded with this shutter array. This system enables half-tone recording without a drop in resolution (unlike the dither method) or recording speed. Only one signal line every unit shutter is proposed for this shutter array, for recording resolution as good as that of a binary printing head.

The increasing density and complexity of integrated circuits illustrates the quick evolution of their technology. As a consequence, new methods of internal testing are now necessary for failure analysis that allow for the visualization of the internal functioning of these circuits. In this way such methods as electronic microscopy working in the voltage contrast process have been developed a longtime ago. An alternate promising method uses liquid crystals for the visualization of electric fields present on the surface of the chip. In this article we investigate the various potentialities of the nematic and smectic mesophases for such a visualization. We will especially underline the use of new ferroelectric liquid crystals which could allow for the dynamical analysis of integrated circuits.

Polycrystalline silicon thin film transistors (poly-Si TFTs) have been applied to active-matrix liquid-crystal displays (LCDs) because of high mobility and stability which enable the integration of driver circuits on the glass substrates. For small size LCDs up to 5 inches in diagonal, high temperature processes using a thermally grown gate oxide film have been used. Poly-Si TFT driver circuits with sufficient electrical characteristics can be integrated on the substrates in such displays. For large area displays of up to 10 inches in diagonal, a low temperature process which allows fabrication of poly-Si TFTs on normal glass substrates at about 600°C has been developed. The electrical properties of low temperature processed poly-Si TFT circuits have approached the level necessary for integration. To increase the carrier mobility and decrease the threshold voltage, plasma hydrogenation, solid-phase grain growth, and laser annealing have been used.

Characteristics of an active-matrix LCD (Liquid Crystal Display) module utilizing two-terminal nonlinear element have been analyzed. The two-terminal nonlinear element, consisting of Metal/SiNxATO (Indium Tin Oxide) shows symmetric and steep diode characteristics. This nonlinear element is applied to 640 x 400 dots LCD which realizes contrast ratio of more than 20 in a reflective mode. Since the two-terminal element, composed of three thin film layers, has a simple structure, it can be produced with only two photolithographic fabrication processes. The LCD with those elements has the advantage of a large area display device capability. In this paper, the relations between the electro-optical characteristics of the LCD panel and the I-V (current voltage) characteristics of SiNx diode are discussed. The I-V diode characteristics can be adjusted to drive the liquid crystal by controlling the Si and N atomic ratio of the SiNx layer. Several parameters of the liquid crystal cell required for the diode matrix LCD are also discussed.

New advances in the development and commercialization of Liquid Crystal Active Matrix Display products have increased the density of the rows and columns that must be electrically interfaced to the driver board. In the past the "Planar" type of conductive adhesive interconnect system (CAIS) having up to 80 connections per inch as depicted in Figure #1 were developed for the pocket TV sets and other Liquid Crystal Displays. The active matrix color displays now being developed require up to 164 connections per inch. To meet this challenge, a high density CAIS has been developed that also satisfies MIL-STD 810C requirements. This system differs from the "Planar" type in that it uses an Anisotropic Conductive Thermoset Film (AECT) at the bond area over conductive traces etched out of 9 micron copper foil laminated to Polyester base film.

Recently, transient phase response associated with surface mode molecular relaxation from highly deformed nematic liquid crystal (LC) directors has been employed to achieve high speed liquid crystal modulators.1-3 This transient nematic effect was analyzed experimentally and numerically. The transient decay time was found to be fast, insensitive to cell thickness, but cell proportional to (X/An0)2; being the optical wavelength and Ano the corresponding birefringence of LC. Based on both computer simulations and experimental results, decay time of the fast LC amplitude modulator was derived. Together with multiple pass technique and optimal temperature effect, the response speed of transient nematic effect can be further improved. Nematic liquid crystal modulators with frame time <40 pis have been achieved in the visible spectral region.

The chemical crosslinking of the linear "LC-side chain polymers" yields elastomers ("rubbers") which exhibit the liquid crystalline state. The mechanical deformation of these elastomers by elongation or compression causes a macroscopic orientation of the liquid crystalline molecules linked to the polymer network similar to the electric field orientation of low molar mass liquid crystals. In this way elastomer films can be prepared, whose macroscopic optical properties are similar to the optical properties of an anisotropic single crystal having the same dimension. There is, however, an additional feature to these elastomers: a local deformation of the macroscopically aligned film causes a local chance of the optical properties. If the plane surface of the film is brought in contact with a "stamp", which has a defined pattern in the micrometer range, the pattern deforms the elastomer surface and consequently the local direction of the optical axis. It is obvious that this procedure enables the simple realisation of elements for the integrated optics. The light conductors can be mechanically pressed into the liquid crystalline elastomer film (similar to the production of a record) and durably stored in the glassy state.

Nematic curvilinear aligned phase (NCAP) liquid crystals have been found useful for high information content video displays. NCAP materials are liquid crystals which have been encapsulated in a polymer matrix and which have a light transmission which is variable with applied electric fields. Because NCAP materials do not require polarizers, their on-state transmission is substantially better than twisted nematic cells. All dimensional tolerances are locked in during the encapsulation process and hence there are no critical sealing or spacing issues. By controlling the polymer/liquid crystal morphology, switching speeds of NCAP materials have been significantly improved over twisted nematic systems. Recent work has combined active matrix addressing with NCAP materials. Active matrices, such as thin film transistors, have given displays of high resolution. The paper will discuss the advantages of NCAP materials specifically designed for operation at video rates on transistor arrays; applications for both backlit and projection displays will be discussed.

This paper presents the recent developments in our Active Matrix Liquid Crystal Displays for military, avionic and automotive applications. Data is presented for PIN diode driven active matrix liquid crystal display, with full color and high resolution (164 lines per inch). Optimization of color coordinates versus transmission has led us to better overall display performance. The contrast ratios were measured for various viewing angles (+/-60H and +45,-15 V) at different temperatures (-54 C to +85 C). The display shows good high ambient contrast ratio both by theoretical model and experimentally. The discrimination index model is used together with measurements to get the index for different high ambient light levels.

Progress in the development of GaAs photoaddressed liquid crystal spatial light modulators is reported. The imaging responses of nematic and ferroelectric liquid crystals are compared, and an edge-enhancement application of the differentiating nematic configuration is presented. Experiments demonstrating charge-trapping effects in GaAs in relation to photoaddressing are reported.

An actively surface stabilized ferroelectric liquid crystal (ASSFLC) cell is proposed to increase the time interval for which a ferroelectric liquid crystal (FLC) keeps its switched homogeneous structure. The principle of an ASSFLC cell is based on the use of a solid ferro-electrics as a surface producing FLC anchoring energy dependent on the external electric field. An external electric field orientates both the polarization in solid ferroelectrics and spontaneous polarization of FLC in one direction. After switching off the electric field the polarization in solid ferroelectrics remains nonzero and stabilizes the uniform structure of FLC.

Individual organic molecules in a liquid crystal array on a graphite surface have been imaged with the scanning tunneling microscope (STM) with near-atomic resolution 1,2. Two-dimensional order has been identified in 4-n-octy1-4'-cyanobiphenyl (8CR) and 5-nony1-2-n-nonoxylphenylpyrimidine (PYT) 909). The high resolution of the STM allows direct observation of intermolecular distances, angles and orientations. From the data, we find in the case of 803 a variety of surface phases can exist whereas for PYP 909 we find only one simple configuration. The graphite substrate appears to influence the molecules in two ways: 1) the molecular axes lie parallel to the surface so that the STM images a cross-section of the classical smectic planes; and 2) molecules of one plane are registered with those of adjacent planes, a degree of order which is not normally observed in the bulk.

Ultraviolet (UV) absorption of some nematic liquid crystals (LCs) with different conjugation length was investigated under polar and nonpolar diluted solutions, vapor phase, nematic phase, and isotropic phase. Results of the vapor phase and nonpolar solution are very similar. For nonpolar LC molecules, little red shift is observed. For polar molecules, the icelectron resonant wavelength exhibits a significant red shift in the nematic phase when compared with the results measured in solvents or vapor phase. Short range intermolecular interaction is thought to be responsible for the observed red shift.

The orientational ordering of eight liquid crystals in their nematic phase at TNI - T = 4 K and 10K was studied by carbon-13 NMR. The technique applied is a combination of separated local field spectroscopy (SLF) and variable angle spinning (VAS). Using this method, a first order splitting pattern for each carbon signal was obtained and C-H dipolar coupling constants were determined. Order parameters for each aliphatic C-H segment and each phenyl and cyclohexyl ring were calculated for the compounds. Results are presented for four 1-(4'-cyanophenyl)-trans-4-alkenylcyclohexanes and two of their analogous 1-(4'-cyanophenyl)-trans-4-alkylcyclohexanes, as well as a 1-(4'-isothiocyanophenyl)-trans-4-alkylcyclohexane and a 4-alkyl-4'-cyanobiphenyl for comparison. The effects of substitution of a phenyl group for a cyclohexyl group, the presence and position of a double bond, the length of the aliphatic chain, the nature of the phenyl-ring substituent, and temperature are discussed.

Nitrile-containing polymers are used to prepare light-absorbing coatings for liquid crystal displays. The coatings, initially transparent to visible light, develop intense color when heated. Optical densities greater than 2 across the visible spectrum were obtained in 1 gm films of a copolymer of acrylonitrile and methyl acrylate baked at 400 C, and in 1 p.m coatings of the same polymer doped with a cyan dye and baked at 250 C. The films were etched in an oxygen plasma to form a black matrix for a three color filter array.

We give an introduction to the method of dielectric spectroscopy to study molecular motions on side chain liquid crystalline polymers. The principal relations between the dielectric permittivity and the microscopic dynamics for low molecular liquid crystals and their modifications due to polymer properties are presented. Refering to our previous works we summarize the models for the different molecular motions in the liquid crystalline side chain polymers. We discuss the effects of structural changes for some examples. At last we outline some aspects for potential applications, that result from our investigations.

Polymer-dispersed liquid crystal (PDLC) films, comprised of liquid crystal microdroplets dispersed in polymer matrices, are attractive for a variety of indoor and outdoor light control applications since they can be switched electrically from a light-scattering off-state to a transparent on-state. This paper reviews the electro-optic properties of PDLC films which govern their performance in such diverse applications as electronic information displays, signs, room dividers, and solar energy control in buildings and automobiles. Factors governing the operating temperature range of PDLC films will be identified and temperature-dependent transmittance and response-time characteristics of these films will be presented. Spectral transmittance characteristics will be discussed and used to determine contrast ratios of PDLC films. Dual frequency addressing of PDLC films will be demonstrated and shown to be a viable technique for increasing contrast ratio of PDLC displays. Solar attenuation properties of PDLC films will be reviewed.

The light transmission and scattering properties of a PDLC light shutter in the transparent (field applied) state are studied. It is found from transmission experiments of samples containing sub-micron size droplets that there is a broad distribution of about 10° about the field direction in droplet alignment, even in saturating fields. Evidence is presented that this distribution is principally caused by the non-spherical shape of the droplets. Measurements of the total scattering cross section imply that even in the transparent state there exists some multiple scattering or interdroplet scattering effects. Transmission and scattering measurements in samples with supra micron size droplets show effects of anomalous diffraction and are sensitive to distributions in droplet size. The theory of scattering from a nematic droplet in the anomalous diffraction regime agrees favorably with experiment.

Polymer dispersed liquid crystals are composite materials where nuclear magnetic relaxation is strongly affected by specific liquid crystal ordering and high surface to volume ratio. Frequency and temperature dependences of proton rotating frame relaxation and temperature dependences of deuteron spin-lattice relaxation are discussed. It is demonstrated how relaxation studies provide us with information on droplet ordering and details about molecular anchoring on the polymer surface.

Stability factors were studied for polymer dispersed liquid crystal (PDLC) films which had been formed by ultraviolet (UV) initiated photopolymerization and liquid crystal phase separation. These stability studies emphasized long term exposures to high intensity light as a function of UV protection filters, and to high humidity (95% relative humidity (R.H.)) at an elevated temperature of 70°C. Various liquid crystal compositions were evaluated in PDLC films formed between conductive glass electrodes. The UWVIS exposure lifetime depended strongly on the wavelength cutoff of the protection filters. With coatings which had a 1% T cutoff at 394 nm, lifetimes greater than 2,200 h were obtained in a 1.1 kW Heraeus Suntest exposure system. Samples also showed greater than 1,000 h of stability, without degradation of properties, in the 95% R.H./70°C lifetime test.