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  • New Liquid Crystal Promises Better Displays

Photonics Spectra
Jan 2001
Richard Gaughan

EINDHOVEN, Netherlands -- Researchers at the Dutch Polymer Institute of Eindhoven University of Technology have identified materials that may enable them to develop liquid crystal displays (LCDs) that generate contrast by controlling light-scattering behavior rather than polarization state. Displays based on the materials will not require linearly polarized light and also promise perfectly clear images.

Traditional LCDs employ electrical addressing to orient nematic compounds from a 90° twisted state into a homeo-tropically aligned state and vice versa. Mutually perpendicular polarizers at both sides of the LCD ensure that only one polarization state can be transmitted through the first polarizer and, depending upon the orientational state of the liquid crystal, so create dark and light regions. Unfortunately, using two polarizers reduces the brightness of the transparent state to 50 percent or less.

The new approach employs dendritic molecules, or dendrimers, which are large, highly branched molecules with a three-dimensional structure. The researchers modified the dendrimers with apolar alkyl chains at the periphery and mixed them with nematic liquid crystal to create a light-scattering polydomain structure.

The combination yielded a stable, pastelike material with many randomly oriented nematic domains. A 5-µm-thick electro-optical cell capillary that is filled with this two-phase material efficiently scatters visible light, but when it is exposed to an electric field, the differences in the internal refractive indices of the liquid crystal domains cancel each other out and the film becomes transparent.

Initial electro-optical tests of the cell have yielded transmission values of about 20 percent in the off state and 80 percent when voltage is applied. The transition between the light-scattering and transparent states begins with an applied voltage of about 1 to 1.5, significantly lower than competing approaches. In addition, the cell has minimal hysteresis and a switching time of 0.5 ms.

Road to commercialization

The initial results are encouraging, said Marysia van Boxtel, a researcher at the university, but improvements must be made before a viable display can be created. Transmittance in the off state is higher than that in other light-scattering approaches. The researchers have varied the cell thickness and dendrimer content and modified the dendrimer, and have found that they can reduce the transmittance.

Altering these parameters, however, influences transmittance in the on state and the viscosity of the mixture. Moreover, eventual manufacture depends on the rheological stability of the mixture, which is enhanced by the addition of the dendrimers to the liquid crystal.

In its current phase, the research requires expertise in several academic disciplines to meet the challenge of improving established technology. "The most intriguing aspect," said van Boxtel, "is that we work from the design of the molecules up to the large-scale application."

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