Liquid crystals have had a large presence in the display industry for several decades, and they continue to remain at the
forefront of development as the industry delves into flexible displays and electronic paper. Among the emerging
technologies trying to answer this call are polymer cholesteric liquid crystal (PCLC) flakes. The motion of PCLC flakes
suspended in a host fluid is controlled with an electric field, whereby the flakes reorient to align parallel with the applied
field. A PCLC device easily switches from a bright state, where light of a given wavelength and polarizationis selectively
reflected, to a dark, non-reflective state. The device returns to a bright state when the flakes relax to their original
orientation after removal of the applied field. Progress has been made in addressing several key device issues: the need to
switch flakes back to a reflective state quickly, the development of bistability, the ability to produce flexible devices, and
the necessity to produce both high brightness and a large contrast ratio. Improvements in the technology have been made
by addressing the optical, mechanical, chemical, and electrical features and characteristics of the PCLC flake/fluid host
system. The manufacture of "custom" flakes by the process of formation of specific flake shapes, the addition of dopants,
or the formation of layered flake composites results in particles with improved reflectivity and response times along with
the ability to respond to both AC and DC fields. Specially designed driving waveforms provide a new means for
controlling flake motion. PCLC flake micro-encapsulation allows for the possibility of flexible and potentially bistable
devices. Here we report on the wide variety of approaches toward improving PCLC flake devices and their results.
Polymer cholesteric-liquid-crystal (PCLC) flakes suspended in a fluid are used as the active medium in a novel particle-based, electro-optic technology. The motion of PCLC flakes is controlled with an electric field so that PCLC flake devices are brightly reflective in their "off" state and appear dark when an electric field is applied, causing the flakes to reorient 90°. Basic devices using a mildly conductive host fluid such as propylene carbonate are not bistable, and flakes relax to their original position within tens of seconds to minutes after the electric field is removed. We seek to control flake orientation by designing waveforms that follow the initial drive voltage. Shaped pulses were investigated to accelerate flake relaxation. The optimal pulse for motion reversal was found to be a 1.5-s sawtooth pulse with a 3-V amplitude. We also examined the use of holding voltages, which follow the driving voltage, but have amplitudes a fraction of the driving-voltage magnitude. The holding voltage prevents flakes from relaxing, while saving on power consumption. Cells driven at several volts were found to retain their brightness with the application of a holding voltage between 0.4 to 0.5 V.
When flakes of polymer cholesteric liquid crystals (PCLC's) are dispersed in a fluid host and subjected to an applied electric field, their bright, polarization-selective reflection color is extinguished as they undergo field-induced rotation. Maxwell-Wagner (interfacial) polarization is the underlying physical mechanism for flake motion and results from the large difference in dielectric properties of the flake and fluid hosts. Flake reorientation times can be as short as 300 ms to 400 ms at exceedingly low driving fields (10 to 100 mV<sub>rms</sub>/μm) and are dependent on flake size and shape, fluid host dielectric constant and viscosity, and drive-filed frequency and magnitude. These attributes make this new materials system of special interest in electro-optical and photonics applications, where reflective-mode operation, polarization selectivity, and low power consumption are of critical importance (e.g., reflective displays).
Until very recently, the electro-optical reorientation of PCLC flakes has been studied only in sandwich-type cells using glass substrates. In this work, we report on the dc field-induced reorientation behavior of PCLC flakes contained in confined spherical or near-spherical fluid-filled cavities formed by microencapsulation of the flake/fluid host dispersion in a water-borne flexible binder. This PCLC flake-fluid host/binder emulsion is coated onto either rigid or flexible condutive-coated substrates and then overcaoted (uniformly or patterned) using a conductive emulsion or paint that is either absorbing (black) or reflecting (silver). In addition to providing a unique environment to study flake motion, this device geometry also extends the application scope of the technology to conformal, electrically switchable coatings for large planar areas and flexible media for information display applications (e.g., electronic paper).