Reverse switching of surface roughness in a self-organized polydomain liquid crystal coating

Significance Switching surface topographic patterns in a dynamic way controls properties such as friction, sticking–release, light reflection, aero- and hydrodynamics, self-cleaning, and touch experience. We present a method to make coatings which are initially flat but form a spiked surface structure under light exposure. The surface turns flat again immediately as the light switches off. The polydomain liquid crystal provides a convenient method that makes fabrication of responsive coatings significantly easier than previous methods. The polydomain texture is simulated by finite elements and the topographic deformation can be understood in terms of director-dependent absorption and related changes in local order. Moreover, the dynamic creation of free volume further enhances the surface modulation to the record value of 24% of the film thickness. In this work we propose randomly ordered polydomain nematic liquid crystal polymer networks to reversibly generate notable jagged relief patterns at a polymer coating surface by light illumination. The domain size is controlled by the addition of traces of partly insoluble fluorinated acrylate. The photoresponse of the coating is induced by a small amount of copolymerized azobenzene monomers. Upon exposure to UV light, azobenzene undergoes trans to cis isomerization, resulting in a change in molecular order and packing within each domain. The extent of this effect and its directionality depends on the domain orientation. Localized to domain level, this morphological change forms large 3D spikes at the surface with a modulation amplitude of more than 20% of the initial thickness. The process is reversible; the surface topographical patterns erase within 10 s by stopping the light exposure. A finite element model is applied to simulate the surface topography changes of the polydomain coating. The simulations describe the formation of the topographic features in terms of light absorption and isomerization process as a function of the director orientation. The random director distribution leads to surface structures which were found to be in close agreement with the ones measured by interference microscopy. The effect of domain size on surface roughness and depth modulation was explored and related to the internal mechanical constraints. The use of nematic liquid crystal polydomains confined in a polymer network largely simplifies the fabrication of smart coatings with a prominent triggered topographic response.