John L. West

Organic thin-film transistor-driven polymer-dispersed liquid crystal displays on flexible polymeric substrates

We have fabricated organic thin-film transistor (OTFT)-driven active matrix liquid crystal displays on flexible polymeric substrates. These small displays have 16×16 pixel polymer-dispersed liquid crystal arrays addressed by pentacene active layer OTFTs. The displays were fabricated using a low-temperature process (<110 °C) on flexible polyethylene naphthalate film and are operated as reflective active matrix displays.

Polymer Dispersed Liquid Crystals for Display Application

Abstract An overview of polymer dispersed liquid crystal (PDLC) materials, their physical properties, and potential applications in the optic and electrooptic industry is presented. These optoelectronic materials have unique properties which are expected to expand liquid crystal technology into new display and light shutter applications. Recent research by small and large industrial and university laboratories on device physics and chemistry has provided substantial progress towards the commercialization of these materials. Work to date on such features as response times, switching voltage, and contrast as well as material preparation procedures and unique optical characteristics will be reviewed. These materials are also of interest to basic physics because of new kinds of physical phenomena brought on by the confinement of a nematic liquid crystal to small submicron-size droplets. Enhanced surface-to-volume ratio, large nematic deformations and problems associated with molecular anchoring at a polymer w...

Control of Reflectivity and Bistability in Displays Using Cholesteric Liquid-Crystals

It is demonstrated that the reflective properties and bistability of cholesteric liquid crystals can be controlled by proper surface treatment and dispersed polymers. Dispersing a polymer in the liquid crystal or using a cell with an inhomogeneous surface anchoring creates permanent defects which result in long‐term bistability, high contrast at large viewing angles, and gray scale. The wide‐angle, reflective feature makes cholesteric materials suitable for displays without backlights and bistability provides flicker‐free operation.

Ferroelectric nematic suspension

We report on the development of a dilute suspension of ferroelectric particles in a nematic liquid-crystal (LC) host. We found that the submicron particles do not disturb the LC alignment and the suspension macroscopically appears similar to a pure LC with no readily apparent evidence of dissolved particles. The suspension possesses enhanced dielectric anisotropy, and is sensitive to the sign of an applied electric field.

Phase Separation of Liquid Crystals in Polymers

Abstract New optoelectronic materials based on polymer dispersed liquid crystals (PDLC) show great potential for application in displays, temperature sensors, optical computing and for solar energy control. We report liquid crystal, termoset or thermoplastic materials. PDLC materials may be formed by several different processes. The liquid crystal may be dissolved in low molecular weight polymer precursors, in a thermoplastic melt or with a thermoplastic in a common solvent. Subsequent polymerization, cooling of the polymer melt or solvent evaporation lead to liquid crystal immiscibility, droplet formation and growth, and polymer gelation. The optoelectronic properties of these materials are affected by the droplet morphology. Specific examples are presented for each of these processes and it is demonstrated how the droplet morphology and density, and thus device performance, can be controlled by each method. The thermoplestics are suitable for forming films by a variety of techniques. A range of polymers...

Orientational Coupling Amplification in Ferroelectric Nematic Colloids

We investigated the physical properties of low concentration ferroelectric nematic colloids, using calorimetry, optical methods, infrared spectroscopy, and capacitance studies. The resulting homogeneous colloids possess a significantly amplified nematic orientational coupling. We find that the nematic orientation coupling increases by approximately 10% for particle concentrations of 0.2%. A manifestation of the increased orientational order is that the clearing temperature of a nematic colloid increases by up to 40 degrees C compared to the pure liquid crystal host. A theoretical model is proposed in which the ferroelectric particles induce local dipoles whose effective interaction is proportional to the square of the orientational order parameter.

Angular discrimination of light transmission through polymer‐dispersed liquid‐crystal films

The refractive index of the polymer np is adjusted relative to the ordinary refractive index of dispersed droplets of nematic liquid crystal no to regulate the angle of maximum transmission through a film in the presence of an applied field. Maximum transmission occurs at normal incidence when np≤no with the breadth of the angle of view being largest at np=no. When np>no, maximum transmission is peaked at an angle away from normal incidence, the value of the angle depending upon the ratio np/no. Angular discriminating filters are therefore possible with these films. The optical response of these materials as a function of applied field is also shown to be dependent on the ratio np/no as is the contrast exhibited by information displays. Methods for measuring the scattering cross section of a nematic droplet are also described.

A method for the formation of polymer walls in liquid crystal/polymer mixtures

We have investigated the formation of polymer walls for high polymer content liquid crystal (LC) formulations, using a patterned electric field to induce phase separation. The effect of this field on the phase separation temperature of a LC/monomer mixture is studied as a function of the photopolymerizable monomer concentration. The phase separation temperature increases with the patterned field strength. The application of a patterned field results in segregation of the LC molecules in the high electric field regions, i.e., pixels, whereas the monomers segregate in the low-field regions, i.e., interpixels. Subsequent photopolymerization results in the formation of polymer walls around the pixels. The structure of the polymer walls was investigated using scanning electron microscopy.

Fast birefringent mode stressed liquid crystal

We report a stressed liquid crystal (SLC) that produce a large shift in phase retardation at submillisecond speeds. The SLC consists of uniformly aligned micro-domains of a liquid crystal dispersed in a polymer structure. Mechanical stress produces uniform alignment, essentially eliminates light scattering, and substantially improves the electro-optic performance. A 22-μm-thick SLC film switches more than 2μm of phase retardation in less than 1ms. The system has a linear voltage response with essentially no hysteresis.

Light scattering from polymer‐dispersed liquid crystal films: Droplet size effects

The effects of droplet size on light scattering by polymer‐dispersed liquid crystal (PDLC) films have been studied theoretically and experimentally. Rayleigh–Gans calculations predict that, provided the droplets scatter independently, backscattering will be maximum at wavelength λ when the droplet radius is about λ/7. However, correlation effects become noticeable for droplet volume fractions η above about 0.2 and shift the radius for maximum backscattering to about λ/5 as η increases to 0.5. These calculations are consistent with hemispherical transmittance and reflectance measurements on PDLC films in which multiple scattering is known to be important. These measurements indicate that solar attenuation by PDLC films will probably be maximized by choosing droplet size to maximize backscattering of visible radiation. This choice also increases solar attenuation by absorption of radiation trapped inside a PDLC film by total internal reflection.