Switchable mirrors of chiral liquid crystal gels

Abstract

Chiral nematic liquid crystal compositions containing mono- and di-acrylates in mixtures with non-reactive liquid crystal molecules were produced. Defect-free planar macroscopic alignment of the molecules was induced between uniaxially rubbed substrates. Polymerization of the acrylate groups was initiated photochemically leading to the formation of a new class of chiral gels consisting of a lightly cross-linked network containing non-reactive molecules. In this way, the chiral pitch and, hence, the reflection colour became permanently fixed. Under the influence of an electric field the gels could be switched to the homeotropic state and reverted to the planar defect-free state upon removal of the electric field. Depending on the composition of the gel, either the position of the reflection band remained the same while its magnitude and/or width decreased, or its position shifted to smaller wavelengths with increasing electric field. Some of the gels showed sharp switching characteristics with a hysteresis making them suitable for passive matrix addressing with a high degree of multiplexability. The gels also showed fast switching times as they reverted to the defect-free optical state upon removal of the electric field. The phase structure of the gels was studied using dielectric spectroscopy which showed that the gels have an inhomogeneous structure.