Novel type of combined photopatternable and electro-switchable polymer-stabilized cholesteric materials

Abstract

A novel type of photopatternable polymer network-stabilized cholesteric materials for creation of electrooptical switching was elaborated. For this purpose cholesteric composites based on a commercial low-molar-mass nematic mixture, a chiral dopant, different amount of polymerizable mesogenic diacrylate (1–6 wt%) and a chiral-photochromic substance consisting cinnamoyl and isosorbide fragments (∼4 wt%) were prepared. A small amount of photoinitiator sensitive mostly to light with wavelength of 365 nm was also added. UV irradiation of the complex mixture with this wavelength light results in the polymerization of the diacrylate and formation of the polymer network stabilizing the initial planar texture in the electrooptical cell. Electrical field application induces a transition from the transparent planar cholesteric texture to a scattered focal conic one; at higher voltage the selective light reflection peak disappears. Irradiation of the cell with shorter wavelength light (313 nm) leads predominantly to the E–Z isomerization of the chiral-photochromic dopant followed by irreversible helix untwisting and to the shift of the selective light reflection from blue to green or red spectral regions. In this case, the competing photopolymerization process almost does not take place. Subsequent irradiation of the cell by longer-wavelength UV light (365 nm) allows one to obtain a polymer network and stabilize the new texture. Electrooptical properties of cells with different values of helix pitch were studied in detail. These data demonstrate the new opportunities for creation of photopatternable electrooptically switchable devices, which can be used in photonics and electrooptics.