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New Sol-Gel Derived Gel-Glass Dispersed Liquid Crystals (GDLC) Formulation Yields 10-25 Fold Reduction in Device Thickness

David Levy of the Instituto de Ciencia de Materiales de Madrid, CSIC, Spain and co-workers have, over the past decade, developed gel-glass dispersed liquid crystals (GDLC) as an alternative material system to polymer-dispersed liquid crystals PDLCs. Such materials are of great interest as electronic liquid crystal displays (LCD), optical switches, and light modulators. The GDLCs opened a new range of possibilities in the field of optical and electro-optical devices.

However, up to now, GDLCs have suffered from lack of stable precursor sol viscosity, rendering it very difficult to reproducibly deposit thin films. In his latest work, published in the June issue of Chemistry of Materials, Levy and co-worker Marcos Zayat from the Institute describe a new sol-gel formulation and approach to overcome this major drawback.

Using the sol-gel method, Levy and Zayat encapsulated LC microdroplets into gel-glass matrices. This yielded materials with better transparency and thermal stability than PDLC and with a larger refractive index differential between matrix and LC's leading to higher light scattering (opacity) in the OFF state. This is made possible by the sol-gel method allowing control of size and chemical environment of the voids the liquid crystal will be located in, enabling scientists to tailor the performance of the GDLC device.

The breakthrough came when Levy and Zayat dispersed micro droplets of 4'-pentyl-4-biphenylcarbonitrile LC in an ormosil gel-glass matrix prepared from TEOS and triethoxy derivatives. The blend of precursors, LC, and solvent resulted in a sol that is stable for at least 10 days and ensures that the critical phase separation step takes place during the gel-glass formation immediately after film deposition. This formulation allows, for the first time, multiple experiments with high repeatability and highly reproducible results.

The developed procedure forms 2 mm thin GDCL layers, a 10-25fold decrease compared to the state-of-the-art 20-50 mm reported in the literature while sustaining similar electro-optical responses.

Levy and Zayat found that larger organic substituents in the glass matrix drastically reduce the adhesion strength to the LC molecules causing a decrease in opacity in the OFF state. This situation lowers the driving voltage for operation and exhibits a slower relaxation time compared to silica matrix samples.

Levy and Zayat will use the new procedure to incorporate active organic groups into the pores to allow chemical tailoring of GDLC's performance parameters and to optimize the matrix composition.

Reported by Chemical & Engineering News

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