Technology Papers
A brief abstract of the paper is provided under the title. Should you wish more, simply clicking on the title will open the full paper in a new window.
A brief abstract of the paper is provided under the title. Should you wish more, simply clicking on the title will open the full paper in a new window.
Trends in Liquid Crystal Research in the USA
In this paper we review the major research areas of the Liquid Crystal Institute, LCI, at Kent State University. The LCI is the largest academic program in the United States devoted to liquid crystal research. The LCI is also headquarters for the Center for Advanced Liquid Crystalline Optical Materials, ALCOM. ALCOM, a National Science Foundation Science and Technology Center, is a consortium of three northeastern Ohio Universities, Kent State University, Case Western Reserve University and the University of Akron. Over twenty North American Companies are members of the ALCOM Industrial Partnership Program. Member companies review ALCOM research and advise on research directions. A review of LCI and ALCOM research will therefore provide a clear indication of the trends in liquid crystal research throughout North America.
We present for the first time a 14 inch diagonal high resolution (200 dpi) Cholesteric liquid crystal display developed to eventually replace ordinary print on paper with an electronic equivalent. Bistability allows the full page size display, with a resolution of 2240 rows and 1728 columns, to be driven in a passive mode. Performance features in applications targeted to replace printed materials such as electronic document viewers and newspapers are reviewed.
Surface-Modified Reflective Cholesteric Displays
Bistable reflective cholesteric displays re constructed with different surface treatments and their electro-optical properties are studies. A phenomenological theory is developed to model the angular dependence of the reflectivity of the displays.
Reflective Cholesteric Displays
We report on the photometric and colorimetric properties of surface stabilized and polymer network stabilized reflective cholesteric displays. Experimentally we use both diffuse and partially diffuse illumination, the latter being an experimental approach to emulating room light conditions. Both types of display cells have good contrast, brightness, and color properties, proving them to be viable stabilized cells owe their brightness to the specular component of reflection while the surface stabilized cells have nearly indistinguishable specular and diffuse components of reflection. As such the former cells appear relatively dim when illuminated by a lighting scheme void of any specular component. Such partially diffuse illumination further illustrates that the hue properties of the surface stabilized cells are less sensitive to alternate lighting schemes.
Polymer Modified and Stabilized Cholesteric Liquid Crystal Materials for Flat Panel Application
Polymer dispersions, even in very low concentrations, can improve the performance of cholesteric liquid crystal materials necessary for their application in flat panel displays. Normal scattering mode, reverse scattering mode and reflective mode cholesteric liquid crystal materials are described. The roll of the polymer in each of these display modes is reviewed as are the unique electro-optic characteristics of the dispersions.
White Reflective Polymer Stabilized Cholesteric Displays
A white reflective display can be made with PSCT material and chiral monomers. The exact nature of the white reflection appears to be due to broadening of the reflected spectrum. The broadening of the reflected spectrum is possibly due to the overlap of reflected spectrum from a two pitch region formed by chiral incorporation into a polymer network. Moreover, two kinds of chiral networks have been observed in these displays: one forms a fiber like network and the other forms a spherical globule-like network. The latter is mainly composed by the chiral binder and exhibiting the highest chirality.
Transient dielectric measurement is used to study the transitions among the planar, focal conic, and homeotropic states of Cholesteric liquid crystals. If the initial state is the field-induced homeotropic state, at low bias fields, the liquid crystal transforms to the planar state in a sequence of homeotropic-transient planar-planar, at high bias fields, the liquid crystals transforms to the focal conic state. The hometropic-transient planar transition is on the order of 1 ms while the homeotropic-focal conic transition is on the order of 100 ms. Large hysteresis is observed in the transitions between the homeotropic and the focal conic state. Based on the rapid homeotropic-transient planar transition and the hysteresis effect in the focal conic-homeotropic transition, we have designed a drive scheme which can address bistable reflective cholesteric displays at the speed of one line per millisecond.
Multicolor Reflective Cholesteric Displays
Mutely-color reflective cholesteric displays (RCD’s) are developed, which show the three primary colors: red, green and blue. A tunable chiral material (TCM) whose chirality can be photo-chemically altered by exposing the cell to UV light is the key factor in the production of a mutely-color reflective cholesteric display. Pixellization of color pattern is achieved through photolithography.
New Chiral Monomers for Polymer Stabilized Cholesteric Textures
New mesomorphic chiral acrylates were synthesized. The additions to a nematic liquid crystalline mixture cause the director of nematogens molecules to adopt a helically twisted orientation. The pitch of the resulting helicoidally structure id determined by the concentration of the chiral dopant. Mesomorphic properties and actual pitches of several mixtures of these chiral dopants with nematic or cholesteric liquid crystalline solvents were studies. Photochemical polymerizations of these mixtures were performed in thin films which resulted in cross-linked polymer networks with a twisted superstructure. The use of such specific chiral additives in polymer stabilized cholesteric texture material resulted in white reflective display. The electro-optical properties of such devices were investigated.
Dynamic drive for bitable reflective cholesteric displays: A rapid addressing scheme
We have developed a novel scheme to drive bitable reflective Cholesteric displays, in which the optical state of a pixel is determined by a pulse shorter than 1.0 ms. This scheme takes advantage of the rapid transition of the cholesteric liquid crystal from the homeotropic state to the transient planar state. The voltage wave form is designed to implement this scheme in a high resolution passive matrix display.
A High Information Content Reflective Cholesteric Display
We present for the first time a 14 inch diagonal high resolution (100 dpi) Cholesteric liquid crystal display. Bistability allows the full page size display with a resolution of 1152 x 896 pixels to be passively addressed. Performance features in application as electronic books or newspapers will be reviewed.
Surface and Polymer Network Stabilized Reflective Cholesteric Liquid Crystal Displays
We report on the optical reflective properties of the planar texture of cholesteric liquid crystal displays. The Cholesteric liquid crystal is made bitable by either dispersing a low concentration of polymer or by treating the cell substrate surfaces. We determine the role that the polymer network and surface treatment has on the reflective properties as a function of viewing angle using both collimated and diffuse illumination. Both the polymer network and surface treatment have the effect of distributing the orientation of the cholesteric helix axes about the cell normal. Theoretically we characterize these cells by this distribution.
The bistable reflective cholesteric formulations exist in two stable states, a relatively transparent focal conic state and a reflecting planar state. The pitch and refractive index of the cholesteric phase determines the reflected wavelength and therefore the color of the display. Application of either a high or low voltage pulse switches the material to the planar or focal conic state respectively. Voltage pulses of intermediate intensity produce a mixture of the planar and focal conic states and an effective gray scale capability. Bistable reflective cholesteric displays are particularly well suited for electronic publishing and portable communication devices. They are much brighter than TN or STN displays and maintain high contrast even at wide viewing angles. The displays produce bright images without the need for back-lights, greatly reducing the power requirements. The bistable switching of these materials allows high resolution images to be produced using simple multiplex addressing schemes.