Photo-Induced Bending Behavior of Post-Crosslinked Liquid Crystalline Polymer/Polyurethane Blend Films

Photoinduced deformation of amorphous polyimide enabled by an improved azobenzene isomerization efficiency

A newly designed azo-PI, without pre-stretching or polarized-light irradiation, exhibits reversible bending behaviors under alternate UV and visible light irradiation, providing a facile route to deformable 2D/3D structure actuators.

A versatile and low-toxicity material for photothermal therapy in deeper tissues

The limited depth of the near infrared (NIR) response is one of the major flaws of the present photothermal therapy (PTT). In this article, thermosensitive polyurethane urea (TPUU) was synthesized by polymerization. Subsequent experiments showed that, compared with classical photosensitizers, TPUU has higher photothermal effects and lower cytotoxicity. These valuable properties could make the present PTT research provide more therapeutic options among different tissues and organs. As a practical example, TPUU was applied to regulate the intestinal flora through external NIR irradiation, which implied its promising expanded applications in deeper tissues.

Active Ester Functionalized Azobenzenes as Versatile Building Blocks

Azobenzenes are important molecular switches that can still be difficult to functionalize selectively. A high yielding Pd-catalyzed cross-coupling method under mild conditions for the introduction of NHS esters to azobenzenes and diazocines has been established. Yields were consistently high with very few exceptions. The NHS functionalized azobenzenes react with primary amines quantitatively. These amines are ubiquitous in biological systems and in material science.

Reversible Patterning Cross-Linked, Humidity-Responsive Polymer Films with Programmatically and Accurately Controlled Deformation

A series of novel humidity-responsive and photosensitive polymer films (PCA-PAA-PEG) are prepared. These films can be patterning cross-linked by the photodimerization of coumarin pendant groups. The humidity-induced deformation can be well controlled by the pattern because of the different modulus and hydrophilicity between cross-linked and un-cross-linked segments. In addition, the pattern can be erased and the deformation direction can be changed programmatically by the de-cross-linking-re-cross-linking approach due to the reversible photodimerization of coumarin groups. The cross-linking degree also affects the humidity responsiveness of the film. The deformation of the gradient patterning cross-linked film can be more accurately controlled. Moreover, the length and width ratio (L/W_s/W_h) of the un-cross-linked segment to the cross-linked segment affects the deformation of the films as well. When L/W_s/W_h is 5/2/1 or 5/3/1, the deformation is controllable, and when L/W_s/W_h is 5/1/1 or 5/4/1, the deformation is random at the initial stage, but the whole film will bend along the short axis in the end.

Structural relationships for the design of responsive azobenzene-based lyotropic liquid crystals

Light-responsive binary (azobenzene + solvent) lyotropic liquid crystals (LCs) were investigated by structural modification of simple azobenzene molecules. Three benzoic acid-containing azobenzene molecules 4-(4-(hydroxyphenyl)diazenyl)benzoic acid (AZO1), 3-(4-(hydroxyphenyl)diazenyl)benzoic acid (AZO2) and 5-(4-(hydroxyphenyl)diazenyl)isophthalic acid (AZO3) were produced with various amide substitutions to produce tectons with a variety of hydrophobicity, size and branching. The LC mesophases formed by binary (azobenzene + solvent) systems with low volatility solvents dimethylsulfoxide (DMSO) and N,N-dimethylformamide (DMF) as well as the protic ionic liquids ethylammonium formate (EAF) and propylammonium formate (PAF), were investigated using a combination of small-angle X-ray and neutron scattering (SAXS and SANS) as well as polarising light microscopy (PLM). Increasing alkyl group length was found to have a strong influence on LC phase spacing, and changes in the position of substitution on the benzene ring influenced the preferred curvature of phases. UV-induced trans to cis isomerization of the samples was shown to influence ordering and optical birefringence, indicating potential applications in optical devices.

Design of Collective Motions from Synthetic Molecular Switches, Rotors, and Motors

Precise control over molecular movement is of fundamental and practical importance in physics, biology, and chemistry. At nanoscale, the peculiar functioning principles and the synthesis of individual molecular actuators and machines has been the subject of intense investigations and debates over the past 60 years. In this review, we focus on the design of collective motions that are achieved by integrating, in space and time, several or many of these individual mechanical units together. In particular, we provide an in-depth look at the intermolecular couplings used to physically connect a number of artificial mechanically active molecular units such as photochromic molecular switches, nanomachines based on mechanical bonds, molecular rotors, and light-powered rotary motors. We highlight the various functioning principles that can lead to their collective motion at various length scales. We also emphasize how their synchronized, or desynchronized, mechanical behavior can lead to emerging functional properties and to their implementation into new active devices and materials.

Photodeformable Azobenzene-Containing Liquid Crystal Polymers and Soft Actuators

Photodeformable liquid crystal polymers (LCPs) that adapt their shapes in response to light have aroused a dramatic growth of interest in the past decades, since light as a stimulus enables the remote control and diverse deformations of materials. This review focuses on the growing research on photodeformable LCPs, including their basic actuation mechanisms, the various deformation modes, the newly designed molecular structures, and the improvement of processing techniques. Special attention is devoted to the novel molecular structures of LCPs, which allow for easy processing and alignment. The soft actuators with various deformation modes such as bending, twisting, and rolling in response to light are also covered with the emphasis on their photo-induced bionic functions. Potential applications in energy harvesting, self-cleaning surfaces, sensors, and photo-controlled microfluidics are further illustrated. The existing challenges and future directions are discussed at the end of this review.

Photo Actuation Performance of Nanotube Sheet Incorporated Azobenzene Crosslinked Liquid Crystalline Polymer Nanocomposite

Crosslinked liquid crystalline polymers (CLCPs) containing azobenzene (AZO-CLCPs) are a type of promising material due to their significance in the design of light-driven smart actuators. Developing AZO-CLCP composites by incorporating AZO-CLCPs with other materials is an effective way of enhancing their practicability. Herein, we report an AZO-CLCP/CNT nanocomposite prepared by the in situ polymerization of diacrylates containing azobenzene chromophores on carbon nanotube (CNT) sheets. The liquid crystal phase structure of CLCP matrix was evidenced by the two-dimensional X-ray scattering. The prepared pure AZO-CLCP films and AZO-CLCP/CNT nanocomposite films demonstrated strong reversible photo-triggered deformation under the irradiation of UV light at 366 nm of wavelength, as a result of photo-induced isomerization of azobenzene moieties in the polymer network. But compared to pure AZO-CLCP films, the AZO-CLCP/CNT nanocomposite films could much more rapidly return to their initial shapes after the UV light irradiation was removed due to the elasticity effect of CNT sheets. The deformation behavior of AZO-CLCP/CNT nanocomposite films under the light irradiation was also different from that of the pure AZO-CLCP films due to the interfacial interaction between a polymer network and CNT sheet. Furthermore, incorporation of a CNT sheet remarkably increased the mechanical strength and robustness of the material. We also used this AZO-CLCP/CNT nanocomposite as a microvalve membrane actuator, which can be controlled by light, for a conceptual device of a microfluidic system. The results showed that this AZO-CLCP/CNT nanocomposite may have great potential in smart actuator applications for biological engineering, medical treatment, environment detection and microelectromechanical systems (MEMS), etc.

A Cut-and-Weld Process to 3D Architectures from Multiresponsive Crosslinked Liquid Crystalline Polymers

Crosslinked liquid crystalline polymers (CLCPs) have garnered extensive attention in recent years for their significant values in the design of light-driven soft actuators. However, poor processabilities due to the insoluble and infusible crosslinked networks prevent their practical applications severely. In this study, a weldable azobenzene-containing CLCP is designed with photo- and humidity-responsive actuations, which enables a cut-and-weld process to 3D CLCP architectures. The tensile properties and stability are almost unchanged after welding, much better than those of the films pasted by common adhesive tapes. Meanwhile, the mechanisms of the welding process are clarified on the base of surface hydrogen bonding and further crosslinking. By taking advantage of the cut-and-weld process, a 3D "claw" integrated into a robotic arm is realized for grabbing millimeter-scale objects by remote control. This work enhances significantly not only the processability of CLCP films but also the utilization of leftover pieces, which provides an efficient approach to create functional 3D structures from film precursors for the potential application in the smart materials.