Periodic Surface Undulation in Cholesteric Liquid Crystal Elastomers

Cholesteric liquid crystal elastomer (CLCE) films, whose helical axis is parallel to a direction in the film plane, exhibit a pefiodic surface undulation with a period corresponding to a half-pitch (about 100 mu m) of the helical director configuration. Temperature variation drives not only a finite macroscopic uniaxial deformation along the helical axis but also a marked variation in the surface undulation. AFM results demonstrate that the surface undulation varies thermoreversibly like a stationary wave with an amplitude of about 600 nm. The top and bottom positions at 40 degrees C become the bottom and top ones, respectively, at 130 degrees C, while the undulation disappears near 100 degrees C. Microspectroscopy reveals the periodic spatial variation in optical birefringence, which reflects the quasi-helical superstructure of local directors in the CLCE films. The combined analysis of the results of AFM and microspectroscopy indicates that the thermally induced variation in the surface undulation becomes largest at positions with the most vertical and planar alignment in the quasi-helical director configuration.