Liquid crystal polymer networks: switchable surface topographies

Abstract

Coating of liquid crystal (LC) networks modified with azobenzene moieties as crosslinks was reviewed to create surface topographies in the micrometer range by exposure with UV light. Various configurations were worked out. Homogeneous cholesteric networks and homeotropic nematic networks could form well-defined protrusions by local exposure. The protrusions can be dynamic, i.e. disappear as soon as light is switched off, or be permanent keeping stable topological structures even after the exposure stops. The formation of the protrusions was proven to be induced mainly by a decrease in density when the order in the LC network is reduced under the trans to–cis isomerization of the azobenzene units. Next to the photochemical effect also heating by absorption played a role. When the polymer network coating was build up from alternating cholesteric and homeotropic areas a uniform exposure led to the surface topography. When exposed, the cholesteric areas are protruding and the homeotropic areas are indenting. In this the deformation is enhanced by the orchestrated linear geometric shear deformations in the neighboring areas where the homeotropic areas tend to expand in the lateral direction and the cholesteric areas in the direction perpendicular to the coating.