Cholesteric liquid crystalline materials reflecting more than 50% of unpolarized incident light intensity

Abstract

Cholesteric liquid crystals (CLCs) selectively reflect light when the wavelength matches the helical pitch. The reflectance is limited to 50% of ambient, unpolarized light because only circularly polarized light of the same handedness as the helix is reflected. Here the elaboration procedure and the properties of a CLC gel whose optical characteristics go beyond the 50% reflectance limit are reported. Photopolymerizable monomers are introduced into the volume of a CLC exhibiting a thermally induced helicity inversion and the blend is then cured with UV light when the helix is right‐handed. The reflectance exceeds 50% when measured at the temperature assigned at a cholesteric helix with the same pitch but a left‐handed sense before reaction. The reflection properties are investigated in the infrared region. From scanning electron microscopy investigations, it is shown that the organization of the mesophase is transferred onto the structure of the network. The gel structure is discussed as consisting of a polymer network with a helical structure containing two populations of low molar mass LC molecules. Each of them is characterized by a band of circularly polarized light which is selectively reflected. The monitoring of the optical response with temperature offers the opportunity to discriminate the respective contributions of the bound and free fractions of LC molecules to the reflectance, and to give evidence of the progressive increase of the reflected flux when the temperature decreases from the curing temperature. Novel opportunities to modulate the reflection over the whole light flux range are offered. Potential applications are related to the light management for smart windows or reflective polarizer‐free displays with a larger scale of reflectivity levels.

**Now at Philips Research, Display Applications and Technologies, Postbox MS41, Building 34 (WB), High Tech Campus 34, S656 AE Eindhoven, The Netherlands.

Acknowledgements

We acknowledge Dr S. Rauch, Dr D. Loetzsch, Prof. G. Heppke and Ms M. Dzionara (Stranski‐Laboratorium für Physikalische und Theoretische Chemie in Technische Universität Berlin) who participated in the synthesis and supply of the helicity‐inversion chiral dopant DL6. C. Bourgerette (CEMES) participated in the material preparation for SEM‐FEG investigations and, with S. Leblond du Plouy (University Paul‐Sabatier, Toulouse), assisted in the observations.

Notes

**Now at Philips Research, Display Applications and Technologies, Postbox MS41, Building 34 (WB), High Tech Campus 34, S656 AE Eindhoven, The Netherlands.