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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