Isotropic “Islands” in a Cholesteric “Sea”: Patterned Thermal Expansion for Responsive Surface Topologies

Abstract

Reactive mesogens are low-molecular-weight liquid-crystalline monomers that can be photopolymerized in order to form well-organized, high-molecular-weight structures. [1,2] The order and alignment of the molecules imposed by internal fields originating from cholesteric molecule interactions, [1] external forces originating from surfaces, [1] external fields, [3] shear flow, and solvents [4] can be covalently captured by a photopolymerization process.[5] Reactive mesogen films have been utilized for a plethora of applications including reflective color-filter arrays, [6] broadband reflective polarizers, [7] viewing-angle-enhancement technology for liquid-crystal displays, [8] responsive microstructures, [9][10][11] polymer microelectromechanical systems, [12] and on the fundamental side, the stabilization and subsequent investigations of low-molecular-weight liquidcrystal ordering templates.[13]A desired property in polymer systems is systematic and accurate control over molecular order in three dimensions. [5] The ability to tailor molecular order to such an extent would enable a wide variety of novel thin films for optical and electrical devices. Reactive mesogen materials come close to meeting this goal-molecular order and orientation can be controlled using a variety of methods that successfully manipulate low-molar-mass liquid crystals. These alignments can then be indefinitely captured through photopolymerization. [2,5] Reactive mesogens are often composed of a three-ring mesogenic unit with reactive side groups. The presence of reactive groups plays a minor role in the mesomorphic properties of the material, including the effect of lowering their phase transition temperatures by destabilizing the mesophases. The central ring of the mesogenic group can also be substituted with a methyl group in order to reduce the phase-transition temperatures.[2] In order to induce helicoidal order the side group can be substituted with a chiral methyl group [2] or the central ring can be exchanged by a chiral sorbitol moiety.[14]The reactive mesogen used to fabricate the thermally activated topological features described here was a nematic difunctional liquid-crystalline acrylate with a methyl group on the central ring of the mesogenic core doped with a chiral reactive mesogen additive. The use of these materials allows for accurate control of the molecular order in multiple dimensions, and each of the various liquid-crystalline states can be fixed by the polymerization process. [2,7,15] In reactive mesogen materials, the ordering of the molecules in the polymerized liquid-crystal network is closely reminiscent of the molecules in their monomeric state. The basic thermal-expansion behavior of planar aligned reactive mesogens has previously been reported; there is an anisotropy in their thermal-expansion behavior because of their liquid crystallinity. [12,16] The thermal expansion of the reactive mesogen is substantially lower along the long molecular axis than normal to it, owing to the fact that most of the covalent bonds...

Photo‐Switchable Surface Topologies in Chiral Nematic Coatings

“…For example, a coating with a transition form chiral nematic to isotropic areas where a single action occurs from the cholesteric region gives an expansion of around 5 %. [5] Expansion in the cholesteric region is accompanied with a change in the optical properties. The increase in pitch would give rise to the expectation that the reflection wavelength shifts to higher values based on the relation l r ¼ " np with l r is the reflection wavelength, "…”

“…Those two types of molecular order have opposite photo-mechanical responses, the combination of which results in a larger effect than that of the previous patterned systems. [5] In the chiral nematic area, where the molecules are oriented on average planar to the surface, reduction of the order parameter results in a positive expansion normal to the plane whereas the expansion in the plane is close to zero or even negative. In the homeotropic area the opposite occurs.…”

“…[12] By decreasing the degree of orientation, as quantified by the order parameter, the dimension tends to increase in the direction perpendicular to the direction of the average molecular orientation and to decrease in the direction parallel to it. [13] Based on this principle, morphing surfaces were reported for a thermally responsive system based on patterned chiral nematic networks, [5] where isotropic areas are integrated in media of helicoidally ordered polymer films (cholesteric films). We now advance this system into photo-switchable surface topologies through integration of azobenzene-containing crosslinkers into a structured chiral nematic network.…”

“…[4] This requires the direct vicinity of electrodes and wires for driving the process. Alternatively, thermal effects based on thermal expansion effects [5] or phase transitions can be used. [6] In buffer solutions surface switching is established in hydrogels by either pH or thermal effects and is often proposed to control protein or cell adhesion in biological systems.…”

Liquid Crystal Polymer Networks: Preparation, Properties, and Applications of Films with Patterned Molecular Alignment

“…The two types of molecular order have opposite photomechanical responses, the combination of which results in larger effects than those studied previously. 45 The surface deformation, expressed as the height difference between the activated regions and the nonactivated regions divided by the initial film thickness, can reach values of up to 24%. Figure 12 shows an example of a patterned film which upon exposure to UV light forms surface structures of around 0.4 μm.…”