Progress in the development of polymer cholesteric liquid crystal flakes for display applications

Polymer cholesteric liquid crystal (PCLC) flake technology is being developed as an alternative display technology for flexible, reflective particle displays. The motion of PCLC flakes suspended in a host fluid can be controlled with an electric field, creating a way to electrically control the flakes' ability to brightly reflect light that is circularly polarized. The PCLC flake/host fluid dispersion has been successfully microencapsulated both in a polymer matrix and in gelatin micro-capsules. Microencapsulation will not only expand the applications scope of the technology, but also may aid in addressing some potential problem areas that are inherent to many forms of particle display technology. A second important development in PCLC flake technology involves the manufacture of shaped flakes based on soft lithography techniques. The size and shape of a flake impact its reorientation, and uniformly shaped flakes respond in a similar manner. The unique reflective properties of PCLC flakes also provide possible applications in areas such as optics and photonics, switchable ‘smart windows’ or conformal coatings, and information displays such as ‘electronic paper.’     

Enhanced Electro‐Optic Behavior for Shaped Polymer Cholesteric Liquid‐Crystal Flakes Made Using Soft Lithography

Polymer cholesteric liquid‐crystal (PCLC) flakes were investigated for their electro‐optical behavior under an applied alternating‐current field. Shaped flakes, fabricated using soft lithography and suspended in dielectric‐fluid‐filled cells, reoriented more uniformly than randomly shaped flakes made by fracturing of PCLC films. Extensive characterization found shaped flakes to be smooth and uniform in size, shape, and thickness. Reorientation in applied fields as low as tens of mV_rms μm^–1 was fastest for flakes with lateral aspect ratios greater than 1:1, confirming theoretical predictions based on Maxwell–Wagner polarization. Brilliant reflective colors and inherent polarization make shaped PCLC flakes of interest for particle displays.     

Cover Picture: Enhanced Electro‐Optic Behavior for Shaped Polymer Cholesteric Liquid‐Crystal Flakes Made Using Soft Lithography (Adv. Funct. Mater. 2/2005)

The cover shows a variety of shaped flakes fabricated from polymer cholesteric liquid‐crystal material using soft lithography. In work reported by Jacobs and co‐workers on p. 217, the micrometer‐sized flakes exhibit brilliant circularly polarized selective reflection colors without polarizers or color filters when placed in a fluid‐filled electro‐optic cell. With the application of a low‐magnitude alternating current field, the flakes reorient in hundreds of milliseconds and the colors disappear. Polymer cholesteric liquid‐crystal (PCLC) flakes were investigated for their electro‐optical behavior under an applied alternating‐current field. Shaped flakes, fabricated using soft lithography and suspended in dielectric‐fluid‐filled cells, reoriented more uniformly than randomly shaped flakes made by fracturing of PCLC films. Extensive characterization found shaped flakes to be smooth and uniform in size, shape, and thickness. Reorientation in applied fields as low as tens of mV_rms μm^–1 was fastest for flakes with lateral aspect ratios greater than 1:1, confirming theoretical predictions based on Maxwell–Wagner polarization. Brilliant reflective colors and inherent polarization make shaped PCLC flakes of interest for particle displays.     

Electric-field-induced motion of polymer cholesteric liquid-crystal flakes in a moderately conductive fluid

Polymer cholesteric liquid-crystal flakes suspended in a fluid with nonnegligible conductivity can exhibit motion in the presence of an ac electric field. The plateletlike particles with a Grandjean texture initially lie parallel to the cell substrates and exhibit a strong selective reflection that is diminished or extinguished as the flakes move. Flake motion was seen within a specific frequency bandwidth in an electric field as low as 5 mV(rms)/microm. Flakes reoriented about their longest axis to align parallel to theapplied field, and a rise time of 560 ms was seen in a 40-mV(rms)/microm field at 100 Hz.