Interpenetrating polymer networks of liquid-crystalline azobenzene polymers and poly(dimethylsiloxane) as photomobile materials

Formation of topological defects at liquid/liquid crystal interfaces in micro-wells controlled by surfactants and light

Topological defects, the fundamental entities arising from symmetry-breaking, have captivated the attention of physicists, mathematicians, and materials scientists for decades. Here we propose and demonstrate a novel method for robust control of topological defects in a liquid crystal (LC), an ideal testbed for the investigation of topological defects. A liquid layer is introduced on the LC in microwells in a microfluidic device. The liquid/LC interface facilitates the control of the LC alignment thereby introducing different molecules in the liquid/LC phase. A topological defect is robustly formed in a microwell when the liquid/LC interface and the microwell surface impose planar and homeotropic alignment, respectively. We also demonstrate the formation/disappearance of topological defects by light illumination, realized by dissolving photo-responsive molecules in the LC. Our platform that facilitates the control of LC topological defects by the introduction of different molecules and external stimuli could have potential for sensor applications.

Photochromic Azobenzene Inverse Opal Film toward Dynamic Anti-Fake Pattern

Azobenzene mesogens have garnered considerable research attention in the realm of photo-responsive materials due to their reversible trans-cis isomerization. In this paper, we demonstrate an azobenzene inverse opal film synthesized via photo-polymerization from a SiO2 opal template. The proposed design exhibits intriguing optical properties, including dynamic fluorescent features, distinct fluorescent enhancement, and an anti-fake micropattern with a switchable structure color. This work holds significant importance for advancing the development of novel optical devices.

Bioinspired humidity-responsive liquid crystalline materials: from adaptive soft actuators to visualized sensors and detectors

Inspired by nature, humidity-responsive materials and devices have attracted significant interest from scientists in multiple disciplines, ranging from chemistry, physics and materials science to biomimetics. Owing to their superiorities, including harmless stimulus and untethered control, humidity-driven materials have been widely investigated for application in soft robots, smart sensors and detectors, biomimetic devices and anticounterfeiting labels. Especially, humidity-responsive liquid crystalline materials are particularly appealing due to the combination of programmable and adaptive liquid crystal matrix and humidity-controllability, enabling the fabrication of advanced self-adaptive robots and visualized sensors. In this review, we summarize the recent progress in humidity-driven liquid crystalline materials. First, a brief introduction of liquid crystal materials, including liquid crystalline polymers, cholesteric liquid crystals, blue-phase liquid crystals and cholesteric cellulose nanocrystals is provided. Subsequently, the mechanisms of humidity-responsiveness are presented, followed by the diverse strategies for the fabrication of humidity-responsive liquid crystalline materials. The applications of humidity-driven devices will be presented ranging from soft actuators to visualized sensors and detectors. Finally, we provide an outlook on the development of humidity-driven liquid crystalline materials.

Programmable Shape Change in Semicrystalline Liquid Crystal Elastomers

Liquid crystal elastomers (LCEs) are stimuli-responsive materials capable of reversible and programmable shape change in response to an environmental stimulus. Despite the highly responsive nature of these materials, the modest elastic modulus and blocking stress exhibited by these actuating materials can be limiting in some engineering applications. Here, we engineer a semicrystalline LCE, where the incorporation of semicrystallinity in a lightly cross-linked liquid crystalline network yields tough and highly responsive materials. Directed self-assembly can be employed to program director profiles through the thickness of the semicrystalline LCE. In short, we use the alignment of a liquid crystal monomer phase to pattern the anisotropy of a semicrystalline polymer network. Both the semicrystalline-liquid crystalline and liquid crystalline-isotropic phase transition temperatures provide controllable shape transformations. A planarly aligned sample's normalized dimension parallel to the nematic director decreases from 1 at room temperature to 0.42 at 250 °C. The introduction of the semicrystalline nature also enhances the mechanical properties exhibited by the semicrystalline LCE. Semicrystalline LCEs have a storage modulus of 390 MPa at room temperature, and monodomain samples are capable of generating a contractile stress of 2.7 MPa on heating from 25 to 50 °C, far below the nematic to isotropic transition temperature. The robust mechanical properties of this material combined with the high actuation strain can be leveraged for applications such as soft robotics and actuators capable of doing significant work.

Janus Photochemical/Photothermal Azobenzene Inverse Opal Actuator with Shape Self-Recovery toward Sophisticated Motion

Azobenzene actuators have aroused enormous research interest due to their excellent performance and promising applications in the fields of soft robots, artificial muscles, etc. However, there are still challenges for the fabrication of azobenzene actuators with a sophisticated actuation mode owing to the unitary actuation direction and slow thermal relaxation of cis- to trans-azobenzene mesogens. To solve these problems, this paper presents a facile fabrication method of a Janus azobenzene inverse opal actuator with one side made of the monodomain azobenzene polymer and the other side made of the polydomain azobenzene inverse opal structure. Gradient-layer spacing structure of the film in its cross section is proven by synchrotron small-angle X-ray diffraction. The introduction of the inverse opal structure mainly provides a polydomain mesogen alignment, large specific surface area, low elastic modulus, and structure color. The synergetic actuation of the photochemical/photothermal mode produces multiple actuation directions, a larger actuation force, and an alteration of the structure color. Shape self-recovery of this Janus azobenzene actuator contributes to some promising applications, such as crawling on a smooth surface, driving an engine axis, and logic electric circuit for the coding technique. This work is of great significance for the design and fabrication of novel-type photoactuators.

Triple-Shape-Memory Soft Actuators from an Interpenetrating Network of Hybrid Liquid Crystals

In this work, the formation of triple-shape-memory liquid crystalline-interpenetrating polymer network (LC-IPN) actuators based on a hybrid acrylate-oxetane LC mixture is reported. Orthogonal polymerization of the oxetane and acrylate liquid crystals creates polymer films with two distinct glass-transition temperatures. The use of these two transitions for one-way triple-shape-memory actuation and two-way bending actuation with a broad temperature window for actuation is demonstrated. Our results combine shape memory polymers with liquid crystal-based soft actuators having advanced stimuli-responsive properties.

Dynamic Semi IPNs with Duple Dynamic Linkers: Self-Healing, Reprocessing, Welding, and Shape Memory Behaviors

Thermoset polymers show favorable material properties, while bringing about environmental pollution due to non-reprocessing and unrecyclable. Diels-Alder (DA) chemistry or reversible exchange boronic ester bonds have been employed to fabricate recycled polymers with covalent adaptable networks (CANs). Herein, a novel type of CANs with multiple dynamic linkers (DA chemistry and boronic ester bonds) was firstly constructed based on a linear copolymer of styrene and furfuryl methacrylate and boronic ester crosslinker. Thermoplastic polyurethane is introduced into the CANs to give a semi Interpenetrating Polymer Networks (semi IPNs) to enhance the properties of the CANs. We describe the synthesis and dynamic properties of semi IPNs. Because of the DA reaction and transesterification of boronic ester bonds, the topologies of semi IPNs can be altered, contributing to the reprocessing, self-healing, welding, and shape memory behaviors of the produced polymer. Through a microinjection technique, the cut samples of the semi IPNs can be reshaped and mechanical properties of the recycled samples can be well-restored after being remolded at 190 °C for 5 min.

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.

Inkless Rewritable Photonic Crystals Paper Enabled by a Light-Driven Azobenzene Mesogen Switch

Rewritable paper, as an environment-friendly approach of information transmission, has potential possibility to conserve energy and promote a sustainable development of our society. Recently, photonic crystals (PCs) have become a research hotspot in the development of rewritable paper. However, there are still many shortcomings that limit the further application of PC paper, such as slow response sensitivity, short-cycle lifetime, poor storage stability, and so on. Herein, we constructed an optically rewritable azobenzene inverse opals (AZOIOs) with a thin film (ca. 1 μm) plated on an inverse opal structure based on the UV/vis switchable structure color of the sample. The top thin film acts as a protective layer to avoid the large deformation of the pore structure and the bottom inverse opal structure with refractive index/pore structure change that provides reversible structure color. Large, reversible, and rapid bandgap shift (ca. 60 nm, 2 s) of AZOIOs can be repeated more than 100 times under alternating UV/vis irradiation based on isomerization of high content of the azobenzene group. On-demand long-time preservation pattern can be obtained by the appearance of azobenzene's intrinsic color. The proof of concept for rewritable PC paper is demonstrated herein. Such inkless rewritable colorful paper paves a way for developing novel display technology.

Amplified photo-responses in sequentially polymerized azobenzene-containing polymer networks: the role of isomer interconnection

Sequentially polymerized azobenzene-containing polymer networks with isomer-interconnected features can greatly enhance photo-actuation responses.

Topographical changes in photo-responsive liquid crystal films: a computational analysis

Switchable materials in response to external stimuli serve as building blocks to construct microscale functionalized actuators and sensors. Azobenzene-modified liquid crystal (LC) polymeric networks, that combine liquid crystalline orientational order and elasticity, reversibly undergo conformational changes powered by light. We present a computational framework to describe photo-induced topographical transformations of azobenzene-modified LC glassy polymer coatings. A nonlinear light penetration model is combined with an opto-mechanical constitutive relation to simulate various ordered and corrugated topographical textures resulting from aligned or randomly distributed LC molecule orientations. Our results shed light on the fundamental physical mechanisms of light-triggered surface undulations and can be used as guidelines to optimize surface modulation and roughness in emerging fields that involve haptics interfacing, friction control and wetting manipulation.