Technique Enables Flexible, Thin Displays
Laurel M. Sheppard
KENT, Ohio — Researchers have developed a method similar to that used to prepare polymer-dispersed liquid crystal devices, but resulting in very different internal geometry and performance. The technique provides a simpler and less expensive way to fabricate high-end displays because it involves fewer steps and can be easily adapted to existing equipment.
To prepare polymer/liquid crystal laminates, researchers start with a mixture of liquid crystals and prepolymer, which forms a polymer when exposed to UV light. Polymer sheets form closest to the light source, forcing the liquid crystal away from the glass plates, resulting in uniform layers of polymer and liquid crystal.
The phase-separated composite films method -- developed by researchers in Kent State University's department of physics and the Liquid Crystal Institute -- separates liquid crystal from a solution containing a prepolymer. This creates multilayer structures either parallel or perpendicular to a substrate, the simplest structure consisting of adjacent uniform layers of liquid crystal and polymer parallel to the substrate. To make perpendicular multilayer structures, which could be used for switchable gratings and other diffractive optics applications, the technique incorporates masks during phase separation.
To prepare the liquid crystal devices, developers first coat a pair of substrates with transparent electrodes of indium-tin oxide. They spin-coat one of the substrates with an alignment layer of commonly used polymers, such as polyimide or polyvinyl alcohol, and then rub it for the liquid crystal alignment. The substrates are separated by a glass rod or bead spacers typically used in liquid crystal displays (LCDs). The scientists introduce a solution of the photocurable prepolymer by capillary action at a temperature well above the isotropic-nematic phase transition, then initiate phase separation by exposing the cell to UV light.
The cell thickness can be easily controlled by varying the concentration, enabling devices with a film thickness comparable to optical wavelengths without needing such small spacers. Cells prepared with this method also tend to be very uniform. The Kent researchers have made devices with uniform and homogeneously aligned nematic films as small as 0.8 µm thick using 3-µm spacers, and thinner films should be possible.
Devices made using the new method are mechanically rugged, making them ideal for ferroelectric and antiferroelectric LCDs. These types of LCDs are up to 1000 times faster than nematic displays, but have not been commercialized because they tend to lose their alignment if mechanically deformed, according to Satyendra Kumar, physics professor and one of the principal investigators.
The mechanical strength also makes phase-separated composite film LCDs very flexible. Tests have shown that, at up to 40 percent deformation, devices still work with no degradation in performance, making them suitable for smart cards and head-mounted displays.
Several applications are under development. In a space shuttle experiment that is scheduled for 2002, researchers plan to use a ferroelectric liquid crystal cell that can operate at up to 50 kHz to modulate (at speeds of 20 kHz) a laser beam reflected to Earth for communications tests. Ferroelectric devices prepared with conventional surface stabilization methods could not withstand the rigors of the test and failed at 1 kHz.
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