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

Abstract

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.surface dynamics | self-organization | switchable jagged surfaces | polydomain liquid crystal polymer | light activation I n nature, living creatures have developed a series of motion and surviving strategies based on unique topographic patterns on their surfaces. The surface topographies range from static patterns as, for instance, found on the leaves of the lotus flower, repelling water and dirt particles, to the dynamic structures that appear under certain conditions as found in many mammals. An example is the pilomotor reflex at the skin of mammals which creates insulation in cold conditions and provides protection by scaring away predators when the body appears larger. To date, many studies have been devoted to the use of static surface topographies fabricated by wrinkling (1, 2), embossing (3), or lithography. However, there is an increasing demand to make these surface structures switchable in modern applications, such as adjustable gas and water flow behavior at corrugated surfaces (4), autonomously adjusting lenses (5), controlled wettability (6), and modulated optics such as scattering, diffraction, or reflection (7-9). Also, the mechanical properties can be altered by the appearance or disappearance of protrusions at the surface (10, 11), such as friction, stick, and adhesion which manifest themselves in, e.g., haptic applications (12).Our focus here is to develop dynamic topographic patterns at su...

High‐Power Actuation from Molecular Photoswitches in Enantiomerically Paired Soft Springs

“…Under illumination, the trans ‐azobenzene isomerizes and induces an anisotropic deformation of the liquid‐crystal network (Figure 1 c) 35. This photomechanical property has been used to switch surface roughness36, 37 and to transform twisted shapes before 13, 38, 39. In the artificial valves that we designed, the two sets of bars undergo different shape and size modifications upon activation of 1 .…”

Photoswitchable Nanomaterials Based on Hierarchically Organized Siloxane Oligomers

“…[13] Different modes of motion arise with more complex molecular arrangements. For example, expansion of chiral nematic LCs along the helical axis has been utilized to create polymer coatings with photoswitchable topologies [14][15][16][17] and polymer films with macroscopic helical motion. [18] Additionally, polymer films with splay-bend configuration, in which LC-mesogens gradually change orientation throughout the film cross-section, have shown to exhibit inherently anisotropic bending as well as achieve significantly faster and larger bending when compared to uniaxially aligned films.…”

Enhanced Deformation of Azobenzene-Modified Liquid Crystal Polymers under Dual Wavelength Exposure: A Photophysical Model

“…This actuation can be reversed by exposing the material to visible (VIS) light or heat, which accelerate the cis azobenzenes to fall back to the trans state. The current consensus in experimental [5,[7][8][9] and theoretical studies [10][11][12][13] is that the amplitude of the photoinduced deformation is related to the statistical building up of molecules in the cis state. However, a recent experimental study [14] has revealed that this is only one side of the story.…”

“…Photophysics model.-A two-light nonlinear penetration model is developed based on previous theoretical studies [12,15]. In Ref.…”

“…[15], a two-light model was proposed in which one light only activates the trans-to-cis forward reaction and the other only the cis-to-trans backreaction, thus accounting for two reactions during isomerization. Here, we extend our single-light illumination model [12] to double-wavelength exposure by accounting for the transto-cis forward reaction, the cis-to-trans backreaction, and the thermally activated backreaction for both lights, thus accounting for six reactions in total. We consider a LC-Azo coating with thickness w, subject to 365 and 455 nm light with intensities I 0;1 and I 0;2 , respectively.…”