Switching between 3D Surface Topographies in Liquid Crystal Elastomer Coatings Using Two-Step Imprint Lithography.

While dynamic surface topographies are fabricated using liquid crystal (LC) polymers, switching between two distinct 3D topographies remains challenging. In this work, two switchable 3D surface topographies are created in LC elastomer (LCE) coatings using a two-step imprint lithography process. A first imprinting creates a surface microstructure on the LCE coating which is polymerized by a base catalyzed partial thiol-acrylate crosslinking step. The structured coating is then imprinted with a second mold to program the second topography, which is subsequently fully polymerized by light. The resulting LCE coatings display reversible surface switching between the two programmed 3D states. By varying the molds used during the two imprinting steps, diverse dynamic topographies can be achieved. For example, by using grating and rough molds sequentially, switchable surface topographies between a random scatterer and an ordered diffractor are achieved. Additionally, by using negative and positive triangular prism molds consecutively, dynamic surface topographies switching between two 3D structural states are achieved, driven by differential order/disorder transitions in the different areas of the film. It is anticipated that this platform of dynamic 3D topological switching can be used for many applications, including antifouling and biomedical surfaces, switchable friction elements, tunable optics, and beyond.

Facile Stratification-Enabled Emergent Hyper-Reflectivity in Cholesteric Liquid Crystals.

Cholesteric liquid crystals (CLCs) are chiral photonic materials with selective reflection in terms of wavelength and polarization. Helix engineering is often required in order to produce desired properties for CLC materials to be employed for beam steering, light diffraction, scattering, and adaptive or broadband reflection. Here, we demonstrate a novel photopolymerization-enforced stratification (PES)-based strategy to realize helix engineering in a chiral CLC system with initially one handedness of molecular rotation throughout the layer. PES plays a crucial role in driving the chiral dopant bundle consisting of two chiral dopants of opposite handedness to spontaneously phase separate and create a CLC bilayer structure that reflects left- and right-handed circularly polarized light (CPL). The initially hidden chiral information therefore becomes explicit, and hyper-reflectivity, i.e., reflecting both left- and right-handed CPL, successfully emerges from the designed CLC mixture. The PES mechanism can be applied to structure a wide range of liquid crystal (LC) and polymer materials. Moreover, the engineering strategy enables facile programming of the center wavelength of hyper-reflection, patterning, and incorporating stimuli-responsiveness in the optical device. Hence, the engineered hyper-reflective CLCs offer great promise for future applications, such as digital displays, lasing, optical storage, and smart windows.

Liquid crystal-based structural color actuators.

Animals can modify their body shape and/or color for protection, camouflage and communication. This adaptability has inspired fabrication of actuators with structural color changes to endow soft robots with additional functionalities. Using liquid crystal-based materials for actuators with structural color changes is a promising approach. In this review, we discuss the current state of liquid crystal-based actuators with structural color changes and the potential applications of these structural color actuators in soft robotic devices.

Pigmented Structural Color Actuators Fueled by Near-Infrared Light.

Cuttlefish can modify their body shape and both their pigmentary and structural colors for protection. This adaptability has inspired the development of appearance-changing polymers such as structural color actuators, although in most cases, the original shape has been confined to being flat, and pigmented structural color actuators have not yet been reported. Here, we have successfully created a pigmented structural color actuator using a cholesteric liquid crystal elastomer with a lower actuation temperature where both actuation and coloration (structural and pigmental) are tunable with temperature and NIR light. The shape, structural color, and absorption of the NIR-absorbing dye pigment of the actuator all change with temperature. Light can be used to trigger local in-plane bending actuation in flat films and local shape changes in a variety of 3D-shaped objects. A cuttlefish mimic that can sense light and respond by locally changing its appearance was also made to demonstrate the potential of pigmented structural color actuators for signaling and camouflage in soft robotics.

Controlling the Phase Behavior and Reflection of Main-Chain Cholesteric Oligomers Using a Smectic Monomer.

Cholesteric liquid crystals (CLCs) are a significant class of temperature-responsive photonic materials that have the ability to selectively reflect light of a specific wavelength. However, the fabrication of main-chain CLC oligomers with dramatic reflection band variation upon varying the temperatures remains a challenge. Here, a feasible method for improving and controlling the responsiveness of main-chain cholesteric liquid crystal oligomers by the incorporation of a smectic monomer is reported. The smectic monomer strengthens the smectic character of the oligomers and enhances the magnitude of the change of the pitch as a function of temperature upon approaching the cholesteric-smectic phase transition temperature. The central wavelength of the reflection band can be easily modified by mixing in an additional chiral dopant. This promising method will open the door to the preparation of temperature-responsive photonic devices with excellent responsiveness.

Thermochromic Cholesteric Liquid Crystal Microcapsules with Cellulose Nanocrystals and a Melamine Resin Hybrid Shell.

Thermochromic coatings that can change their color in response to variations in ambient temperature have various potential applications. Cholesteric liquid crystals (CLCs) are promising thermochromic materials due to their selective light reflection and wide regulation range. However, it remains a challenge to fabricate thermochromic coatings that combine good responsivity, mechanical strength, fabrication feasibility, and flexibility. In this study, CLC microcapsules containing cellulose nanocrystals (CNCs) and a melamine-formaldehyde (MF) resin hybrid shell were fabricated via in situ polymerization using CNC-stabilized Pickering emulsions as templates. The CNCs were employed as both Pickering emulsifiers and alignment agents of CLCs to prepare CLC Pickering emulsions. The CLC microcapsules were mixed with curable binders to obtain coating slurries, and thermochromic coatings were prepared by painting the slurries on substrates and drying. The thermochromic coatings could adjust their color in the visible wavelength range in a temperature range of 12 to 42 °C. Moreover, the obtained thermochromic coatings displayed a relatively high reflectance of up to 30-40% and can even be applied to flexible substrates. The CLC microcapsules with CNCs and an MF hybrid shell are promising in the field of smart decorative paints, anti-counterfeit labels, and artificial skins.

Reversible Thermochromic Photonic Coatings with a Protective Topcoat.

The fabrication of reversible and robust thermochromic coatings remains challenging. In this work, a temperature-responsive photonic coating with a protective topcoat is fabricated. A cholesteric oligosiloxane liquid crystal possessing a smectic-to-cholesteric phase-transition temperature response is synthesized. A planar alignment of its cholesteric phase is possible with blade coating. By stabilizing with 3 wt % of a crosslinked liquid crystal network, the photonic coating shows a color change ranging from red to blue upon heating. High transparency is retained, and the structural color changes are fully reversible. A transparent polysiloxane layer can be directly applied on top of the cholesteric layer to protect it against damage without affecting its optical properties. This approach satisfies the basic requirements of thermochromic polymer coatings, as it combines easy processability, coating robustness, and a reversible temperature response.

Light-deformable dynamic surface fabricated by ink-jet printing.

Dynamic surfaces which can change their topography with external stimuli have wide application prospects. Liquid crystal network (LCN) is an ideal material for making dynamic surfaces, but traditional methods for LCN dynamic surface manufacturing are difficult to scale up, which limits its applications. This research proposes a new method to fabricate a responsive surface using ink-jet printing technology. Using a liquid crystal monomer mixture as the ink, we printed arrays of droplets onto a glass substrate with a homeotropic alignment layer and polymerized the droplets into deformable LCN hemispheres. An azobenzene diacrylate was copolymerized into the hemispheres to make them photo-responsive to UV light. Because the ink-jet printing method can potentially be used to print countless hemispheres on a large area substrate, large area dynamic surfaces consisting of a multitude of separate dynamic structures can be manufactured. Since the deformation of the entire surface is a periodic repetition of the deformation of a single hemisphere, we characterized the deformation of individual hemispheres, and found that the optical image of hemispheres between crossed polarizers shows a "maltese cross" texture, and 3D surface profiling shows the top surface depresses into a valley after UV-irradiation. This is caused by an order parameter decrease of the homeotropically aligned LC molecules, which leads to a contraction in the alignment direction. The deformation amplitude can be modulated by UV intensity and temperature. This kind of dynamic surface fabricated by ink-jet printing technology can easily be scaled up and is promising for applications such as adjustable micro-lenses or surface wettability.

Polymer Stabilized Cholesteric Liquid Crystal Siloxane for Temperature-Responsive Photonic Coatings.

Temperature-responsive photonic coatings are appealing for a variety of applications, including smart windows. However, the fabrication of such reflective polymer coatings remains a challenge. In this work, we report the development of a temperature-responsive, infrared-reflective coating consisting of a polymer-stabilized cholesteric liquid crystal siloxane, applied by a simple bar coating method. First, a side-chain liquid crystal oligosiloxane containing acrylate, chiral and mesogenic moieties was successfully synthesized via multiple steps, including preparing precursors, hydrosilylation, deprotection, and esterification reactions. Products of all the steps were fully characterized revealing a chain extension during the deprotection step. Subsequently, the photonic coating was fabricated by bar-coating the cholesteric liquid crystal oligomer on glass, using a mediator liquid crystalline molecule. After the UV-curing and removal of the mediator, a transparent IR reflective polymer-stabilized cholesteric liquid crystal coating was obtained. Notably, this fully cured, partially crosslinked transparent polymer coating retained temperature responsiveness due to the presence of non-reactive liquid-crystal oligosiloxanes. Upon increasing the temperature from room temperature, the polymer-stabilized cholesteric liquid crystal coating showed a continuous blue-shift of the reflection band from 1400 nm to 800 nm, and the shift was fully reversible.

Preparation of an Interpenetrating Network of a Poly(ampholyte) and a Cholesteric Polymer and Investigation of Its Hydrochromic Properties.

A new water-responsive photonic coating based on a hygroscopic amphoteric poly(ampholyte) has been developed. The material consists of an interpenetrating network between the poly(ampholyte) and a cholesteric liquid crystalline polymer that reflects light. Swelling of this hybrid material upon contact with water causes a red-shift of the reflection band. As both cation and anion are incorporated in the ionic network, this coating possesses a high stability of its water responsiveness after prolonged and/or repeated exposure to water, even if the water contains dissolved ions. In addition, optimization of the water response of the coatings is demonstrated by changing the composition of the base cholesteric mixture, and color patterns were prepared through selective UV exposure.

Contactless Control of Local Surface Buckling in Photoaligned Gold/Liquid Crystal Polymer Bilayers.

Wrinkling is a powerful technique for the preparation of surface structures over large areas, but it is difficult to simultaneously control the direction, period, and amplitude of the wrinkles without resorting to complicated procedures. In this work, we demonstrate a wrinkling system consisting of a liquid crystal polymer network and a thin layer of gold, in which the direction of the wrinkles is controlled by the alignment of the liquid crystal molecules and the average amplitude and period are controlled by a high-intensity UV irradiation. The UV exposure represses the amplitude and period dictated by the total exposure. Using photoalignment and photomasks, we demonstrate an unprecedented control over the wrinkling parameters and were able to generate some striking optical patterns. The mechanism of the wrinkle suppression was investigated and appears to involve localized photodegradation at the polymer-gold interface, possibly due to the formation of mechanoradicals.

Easily Processable Temperature-Responsive Infrared-Reflective Polymer Coatings.

A temperature-responsive near-infrared reflective coating was fabricated based on a side-chain liquid crystal siloxane polymer using a simple wired-bar method. The cholesteric liquid crystalline polymer film showed a blue shift of the reflection band of ∼1000 nm in the IR region upon heating. The temperature-responsive change of the reflection band was reversible. Compared to that of the same mixture system in an alignment cell, the coating showed a significantly faster response. This research demonstrates an easy way to prepare a temperature-responsive IR-reflective coating that shifts its reflection to a shorter wavelength upon heating. As IR radiation of shorter wavelengths is more strongly represented in sunlight than longer wavelengths, this coating could be used to selectively reduce heating of an indoor space when the temperature is high. This is promising for the future application of smart climate control.

Humidity-responsive liquid crystalline polymer actuators with an asymmetry in the molecular trigger that bend, fold, and curl.

We show a versatile method for the preparation of a variety of humidity-responsive actuators based on a single sheet of a hydrogen-bonded, uniaxially aligned liquid crystal polymer network. In this approach, the asymmetry in the molecular trigger in the anisotropic polymer film plays a dominant role leading to programmed deformation events. The material is locally treated with a potassium hydroxide solution to create the asymmetry in the responsiveness toward humidity, which allows to prepare actuators that bend, fold, or curl.

Humidity-responsive bilayer actuators based on a liquid-crystalline polymer network.

A humidity-responsive bilayer actuator has been developed that consists of an oriented polyamide-6 substrate and a liquid-crystalline polymer coating. The oriented substrate acts as an alignment layer for the liquid crystal. The liquid-crystalline polymer consists of a supramolecular network having hydrogen-bonded entities that, after activation with an alkaline solution, exhibit deformation in response to a change in humidity. The bending behavior of the bilayer actuator was analyzed, showing a large response to a change in the humidity.

Engineering of complex order and the macroscopic deformation of liquid crystal polymer networks.

Rise or fall: Complex-structured freestanding polymer films with molecular order in three dimensions were prepared through photoalignment of polymerizable liquid crystals. The resulting films deform into cone and saddle shapes upon heating.