Hydrogen-Bonded Supramolecular Liquid Crystal Polymers: Smart Materials with Stimuli-Responsive, Self-Healing, and Recyclable Properties.

Hydrogen-bonded liquid crystalline polymers have emerged as promising "smart" supramolecular functional materials with stimuli-responsive, self-healing, and recyclable properties. The hydrogen bonds can either be used as chemically responsive (i.e., pH-responsive) or as dynamic structural (i.e., temperature-responsive) moieties. Responsiveness can be manifested as changes in shape, color, or porosity and as selective binding. The liquid crystalline self-organization gives the materials their unique responsive nanostructures. Typically, the materials used for actuators or optical materials are constructed using linear calamitic (rod-shaped) hydrogen-bonded complexes, while nanoporous materials are constructed from either calamitic or discotic (disk-shaped) complexes. The dynamic structural character of the hydrogen bond moieties can be used to construct self-healing and recyclable supramolecular materials. In this review, recent findings are summarized, and potential future applications are discussed.

Fully (Re)configurable Interactive Material through a Switchable Photothermal Charge Transfer Complex Gated by a Supramolecular Liquid Crystal Elastomer Actuator.

Charge transfer complexes (CTCs) based on self-assembled donor and acceptor molecules allow light absorption of significantly redshifted wavelengths to either the donor or acceptor. In this work, we demonstrate a CTC embedded in a hydrogen-bonded liquid crystal elastomer (LCE), which in itself is fully reformable and reprocessable. The LCE host acts as a gate, directing the self-assembly of the CTC. When hydrogen bonding is present, the CTC behaves as a near-infrared (NIR) dye allowing photothermal actuation of the LCE. The CTC can be disassembled in specific regions of the LCE film by disrupting the hydrogen bond interactions, allowing selective NIR heating and localized actuation of the films. The metastable non-CTC state may persist for weeks or can be recovered on demand by heat treatment. Besides the CTC variability, the capability of completely reforming the shape, color, and actuation mode of the LCE provides an interactive material with unprecedented application versatility.

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.

The Interplay between Different Stimuli in a 4D Printed Photo-, Thermal-, and Water-Responsive Liquid Crystal Elastomer Actuator.

Multi-stimuli responsivity in 3D-printed objects is receiving much attention. However, the simultaneous interplay between different environmental stimuli is largely unexplored. In this work, we demonstrate direct ink writing of an oligomeric ink containing an azobenzene photo-switch with an accessible hydrogen bond allowing triple responsivity to light, heat, and water. The resulting printed liquid crystal elastomer performs multiple actuations, the specific response depending on the environmental conditions. Bilayer films formed by printing on a static substrate can rapidly change shape, bending almost 80 degrees if irradiated in air or undergoing a shrinkage of about 50 % of its length when heated. The bilayer film assumes dramatically different shapes in water depending on combined environmental temperature and lighting conditions.

The development of luminescent solar concentrator-based photomicroreactors: a cheap reactor enabling efficient solar-powered photochemistry.

Sunlight strikes our planet every day with more energy than we consume in an entire year. Therefore, many researchers have explored ways to efficiently harvest and use sunlight energy for the activation of organic molecules. However, implementation of this energy source in the large-scale production of fine chemicals has been mostly neglected. The use of solar energy for chemical transformations suffers from potential drawbacks including scattering, reflections, cloud shading and poor matches between the solar emission and absorption characteristics of the photochemical reaction. In this account, we provide an overview of our efforts to overcome these issues through the development of Luminescent Solar Concentrator-based PhotoMicroreactors (LSC-PM). Such reactors can efficiently convert solar energy with a broad spectral distribution to concentrated and wavelength-shifted irradiation which matches the absorption maximum of the photocatalyst. Hence, the use of these conceptually new photomicroreactors provides an increased solar light harvesting capacity, enabling efficient solar-powered photochemistry.

Liquid-Crystalline Polymer Particles Prepared by Classical Polymerization Techniques.

Liquid-crystalline polymer particles prepared by classical polymerization techniques are receiving increased attention as promising candidates for use in a variety of applications including micro-actuators, structurally colored objects, and absorbents. These particles have anisotropic molecular order and liquid-crystalline phases that distinguish them from conventional polymer particles. In this minireview, the preparation of liquid-crystalline polymer particles from classical suspension, (mini-)emulsion, dispersion, and precipitation polymerization reactions are discussed. The particle sizes, molecular orientations, and liquid-crystalline phases produced by each technique are summarized and compared. We conclude with a discussion of the challenges and prospects of the preparation of liquid-crystalline polymer particles by classical polymerization techniques.

NIR-vis-UV Light-Responsive High Stress-Generating Polymer Actuators with a Reduced Creep Rate.

Untethered, light-responsive, high-stress-generating actuators based on widely-used commercial polymers are appealing for applications in soft robotics. However, the construction of actuators that are stable and reversibly responsive to low-intensity ultraviolet, visible, and infrared lights remains challenging. Here, transparent, stress-generating actuators are reported based on ultradrawn, ultrahigh molecular weight polyethylene films. The composite films have different draw ratios (30, 70, and 100) and contain a small amount of graphene in combination with ultraviolet and near-infrared-absorbing dyes. The composite actuators respond rapidly (t0.9 < 0.8 s) to different wavelengths of light (i.e., 780, 455, and 365 nm). A maximum photoinduced stress of 35 MPa is achieved at a draw ratio of 70 under near-infrared light irradiation. The photoinduced stress increases linearly with the light intensity, indicating the transfer of light into thermally induced mechanical contraction. Moreover, the addition of additives lead to a reduction in the plastic creep rate of the drawn films compared to their nonmodified counterparts.

Wavelength-Selective Photopolymerization of Hybrid Acrylate-Oxetane Liquid Crystals.

We report on the wavelength-selective photopolymerization of a hybrid acrylate-oxetane cholesteric liquid crystal monomer mixture. By controlling the sequence and rate of the orthogonal free-radical and cationic photopolymerization reactions, it is possible to control the degree of phase separation in the resulting liquid crystal interpenetrating networks. We show that this can be used to tune the reflective color of the structurally colored coatings produced. Conversely, the structural color can be used to monitor the degree of phase separation. Our new photopolymerization procedure allows for structuring liquid crystal networks in three dimensions, which has great potential for fabricating liquid crystal polymer materials with programmable functional properties.

Flower-Like Colloidal Particles through Precipitation Polymerization of Redox-Responsive Liquid Crystals.

We report on the synthesis of monodisperse, flower-like, liquid crystalline (LC) polymer particles by precipitation polymerization of a LC mixture consisting of benzoic acid-functionalized acrylates and disulfide-functionalized diacrylates. Introduction of a minor amount of redox-responsive disulfide-functionalized diacrylates (≤10 wt %) induced the formation of flower-like shapes. The shape of the particles can be tuned from flower- to disk-like to spherical by elevating the polymerization temperature. The solvent environment also has a pronounced effect on the particle size. Time-resolved TEM reveals that the final particle morphology was formed in the early stages of the polymerization and that subsequent polymerization resulted in continued particle growth without affecting the morphology. Finally, the degradation of the particles under reducing conditions was much faster for flower-like particles than for spherical particles, likely a result of their higher surface-to-volume ratio.

Epoxide and oxetane based liquid crystals for advanced functional materials.

Liquid crystalline elastomers (LCEs) and liquid crystalline networks (LCNs) are classes of polymers very suitable for fabricating advanced functional materials. Two main pathways to obtain LCEs and LCNs have gained the most attention in the literature, namely the two-step crosslinking of LC side-chain polymers and the photoinitiated free-radical polymerisation of acrylate LC monomers. These liquid crystal polymers have demonstrated remarkable properties resulting from their anisotropic shapes, being used in soft robotics, responsive surfaces and as photonic materials. In this review, we will show that LCs with cyclic ethers as polymerisable groups can be an attractive alternative to the aforementioned reactive acrylate mesogens. These epoxide and oxetane based reactive mesogens could offer a number of advantages over their acrylate-based counterparts, including oxygen insensitivity, reduced polymerisation shrinkage, improved alignment, lower processing viscosity and potentially extended resistivity. In this review, we summarise the research on these materials from the past 30 years and offer a glimpse into the potential of these cyclic ether mesogens.

Programmable liquid crystal elastomer microactuators prepared via thiol-ene dispersion polymerization.

Narrowly dispersed, 10 micron-sized, liquid crystalline elastomer polymer actuators were first prepared via thiol-ene dispersion polymerization and then embedded and stretched in a polyvinyl alcohol film, followed by photopolymerization of the residual acrylate groups. Prolate micro spheroids in which the mesogens are aligned parallel to the long axis were obtained and showed reversible thermally driven actuation owing to nematic to isotropic transition of the liquid crystal molecules. The particles were also compressed to form disk-shaped oblate microactuators in which the mesogens are aligned perpendicular to the short axis, demonstrating that the reported method is a versatile method to fabricate liquid crystal elastomer microactuators with programmable properties.

Unravelling humidity-gated, temperature responsive bilayer actuators.

By spraying liquid crystal mixtures onto stretched polyamide 6 (PA6) substrates, dual-responsive heat/humidity bilayer actuators are generated. The oriented PA6 guides the self-organization of the liquid crystal monomers into well-aligned, anisotropic liquid crystal networks. The bilayer responds to changes in the environmental relative humidity, resulting in bending of the actuator with the liquid crystal network inside the curvature. In contrast, in conditions of constant high humidity (80%RH), increasing the temperature triggers the liquid crystal network coating to bend the bilayer in the opposing direction. The dual-responsivity to changes in environmental humidity and temperature is examined in detail and discussed theoretically to elucidate the humidity-gated, temperature responsive properties revealing guidelines for fabricating anisotropic bilayer actuators.

Liquid Crystal Networks on Thermoplastics: Reprogrammable Photo-Responsive Actuators.

Arbitrary shape (re)programming is appealing for fabricating untethered shape-morphing photo-actuators with intricate configurations and features. We present re-programmable light-responsive thermoplastic actuators with arbitrary initial shapes through spray-coating of polyethylene terephthalate (PET) with an azobenzene-doped light-responsive liquid crystal network (LCN). The initial geometry of the actuator is controlled by thermally shaping and fixing the thermoplastic PET, allowing arbitrary shapes, including origami-like folds and left- and right-handed helicity within a single sample. The thermally fixed geometries can be reversibly actuated through light exposure, with fast, reversible area-specific actuation such as winding, unwinding and unfolding. By shape re-programming, the same sample can be re-designed and light-actuated again. The strategy presented here demonstrates easy fabrication of mechanically robust, recyclable, photo-responsive actuators with highly tuneable geometries and actuation modes.

Dual-responsive "smart" window and visually attractive coating based on a diarylethene photochromic dye.

Controlling the intensity and manipulating the spectral composition of sunlight are critical for many devices including "smart" windows, greenhouses, and photomicroreactors, but these are also important in more decorative applications. Here, we use a diarylethene dye incorporated in a liquid crystal host to create a dual-responsive "smart" window regulated both by an electrical trigger and by specific wavelengths of light. By incorporating the same diarylethene dye in a polymerizable host and using inkjet printing, coatings can be made with complete freedom in the applied pattern design, although the electrical response is lost. The color change of the diarylethene dye can be controlled in simulated sunlight by concurrent light exposure from an LED source, allowing a manual override for outdoor use. Photoluminescence of the closed isomer of the diarylethene from the light guide edges could be used for lighting or electricity generation in a luminescent solar concentrator architecture.

Implementing a Practical, Bachelor's-Level Design-Based Learning Course To Improve Chemistry Students' Scientific Dissemination Skills.

This work presents an outline for a full-quartile design-based learning laboratory-based course suitable for final year Bachelor's students. The course has been run for 5 years in the department of Chemical Engineering and Chemistry. The course attempts to provide a complete laboratory experience for its students, including an authentic research project, experience in writing a research paper with realistic limitations of both space and time, and giving of a presentation appropriate for a scientific conference, finally culminating with a written exam, where the questions are based on the written reports and oral presentations of the other students, making the students also course "teachers". This article will discuss both the successful aspects of the course and point out the areas that still need improvement and provides enough information as to allow the transfer of the methodology to other educational curricula.

Energy-Efficient Solar Photochemistry with Luminescent Solar Concentrator Based Photomicroreactors.

The sun is the most sustainable light source available on our planet, therefore the direct use of sunlight for photochemistry is extremely appealing. Demonstrated here, for the first time, is that a diverse set of photon-driven transformations can be efficiently powered by solar irradiation with the use of solvent-resistant and cheap luminescent solar concentrator based photomicroreactors. Blue, green, and red reactors can accommodate both homogeneous and multiphase reaction conditions, including photochemical oxidations, photocatalytic trifluoromethylation chemistry, and metallaphotoredox transformations, thus spanning applications over the entire visible-light spectrum. To further illustrate the efficacy of these novel solar reactors, medicinally relevant molecules, such as ascaridole and an intermediate of artemisinin, were prepared as well.

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.

Temperature-Responsive Luminescent Solar Concentrators: Tuning Energy Transfer in a Liquid Crystalline Matrix.

Temperature-responsive luminescent solar concentrators (LSCs) have been fabricated in which the Förster resonance energy transfer (FRET) between a donor-acceptor pair in a liquid crystalline solvent can be tuned. At room temperatures, the perylene bisimide (PBI) acceptor is aggregated and FRET is inactive; while after heating to a temperature above the isotropic phase of the liquid crystal solvent, the acceptor PBI completely dissolves and FRET is activated. This unusual temperature control over FRET was used to design a color-tunable LSC. The device has been shown to be highly stable towards consecutive heating and cooling cycles, making it an appealing device for harvesting otherwise unused solar energy.

A Leaf-Inspired Luminescent Solar Concentrator for Energy-Efficient Continuous-Flow Photochemistry.

The use of solar light to promote chemical reactions holds significant potential with regard to sustainable energy solutions. While the number of visible light-induced transformations has increased significantly, the use of abundant solar light has been extremely limited. We report a leaf-inspired photomicroreactor that constitutes a merger between luminescent solar concentrators (LSCs) and flow photochemistry to enable green and efficient reactions powered by solar irradiation. This device based on fluorescent dye-doped polydimethylsiloxane collects sunlight, focuses the energy to a narrow wavelength region, and then transports that energy to embedded microchannels where the flowing reactants are converted.

Enhanced Amplified Spontaneous Emission in Perovskites Using a Flexible Cholesteric Liquid Crystal Reflector.

Organic-inorganic perovskites are highly promising solar cell materials with laboratory-based power conversion efficiencies already matching those of established thin film technologies. Their exceptional photovoltaic performance is in part attributed to the presence of efficient radiative recombination pathways, thereby opening up the possibility of efficient light-emitting devices. Here, we demonstrate optically pumped amplified spontaneous emission (ASE) at 780 nm from a 50 nm-thick film of CH3NH3PbI3 perovskite that is sandwiched within a cavity composed of a thin-film (∼7 µm) cholesteric liquid crystal (CLC) reflector and a metal back-reflector. The threshold fluence for ASE in the perovskite film is reduced by at least two orders of magnitude in the presence of the CLC reflector, which results in a factor of two reduction in threshold fluence compared to previous reports. We consider this to be due to improved coupling of the oblique and out-of-plane modes that are reflected into the bulk in addition to any contributions from cavity modes. Furthermore, we also demonstrate enhanced ASE on flexible reflectors and discuss how improvements in the quality factor and reflectivity of the CLC layers could lead to single-mode lasing using CLC reflectors. Our work opens up the possibility of fabricating widely wavelength-tunable "mirror-less" single-mode lasers on flexible substrates, which could find use in applications such as flexible displays and friend or foe identification.