Structures and Applications of Photo-Responsive Shape-Changing Liquid Crystal Polymers

Mingxin Zheng; Min Zeng; Xi Chen; Jinying Yuan

doi:10.7536/PC200737

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (6) : 914-925 DOI: 10.7536/PC200737

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Structures and Applications of Photo-Responsive Shape-Changing Liquid Crystal Polymers

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Abstract

Liquid crystal polymers (LCPs) are soft intelligent materials, which can change shape under external stimuli. The change of arrangement of liquid crystal molecules in microstructure induces a macroscopic shape-changing of LCP materials. Without changing environment, light irradiation allows remote and localized actuation of photo responsive LCPs, which makes photo-responsive LCPs especially attractive as soft actuators. Photo-responsive groups are introduced into liquid crystal polymers to obtain photo-deformable materials with important applications. This review focuses on recent research progress on photo-responsive shape-changing liquid crystal polymers. Different mechanisms of light-driven deformation are first discussed to understand the essence of photo-responsive behaviors of liquid crystal polymers, including photo-isomerization, photo-thermal effect and multi-stimuli-response. Their design principles are introduced in detail, especially the newly designed molecular structures. Furthermore, applications of photo-responsive liquid crystal polymers are highlighted, including bio-inspired actuator materials, energy-converted materials and soft robots. Finally, a brief outlook on the future development of this field is presented.

Contents

1 Introduction

2 Photo-responsive mechanisms of liquid crystal polymer devices

2.1 Photo-isomerization of liquid crystal polymer

2.2 Shape changing driven by photo-thermal effect

2.3 Multi-stimuli-responsive liquid crystal polymer

3 Applications of photo-responsive liquid crystal polymer

3.1 Application of bionic function

3.2 Application of energy conversion

3.3 Application of soft robots

4 Conclusions

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Keywords

liquid crystal polymer / photo-responsive / soft actuator / intelligent material / azobenzene

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Mingxin Zheng, Min Zeng, Xi Chen, Jinying Yuan. Structures and Applications of Photo-Responsive Shape-Changing Liquid Crystal Polymers. Progress in Chemistry, 2021, 33(6): 914-925 DOI:10.7536/PC200737

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1 Introduction

Shape-changeable polymer is a kind of soft matter material that can be used to make flexible actuators, which has the advantages of high degree of freedom, strong adaptability to the environment, low raw material price, diverse driving forms and multi-functional integration^[1,2]. Polymers respond to external stimuli (such as heat, light, humidity, chemical environment, electric current and magnetic field) and exhibit reversible shape changes to achieve specific functions or perform mechanical work^[3,4]. In recent years, it has become an important research field of great concern, which has greatly promoted the development of bionic devices, smart surfaces, sensors and flexible robots^[5⇓⇓~8]. Among these smart materials, liquid crystal polymer materials have ordered orientation of liquid crystals on the nanometer scale and are fixed by polymerization. The alignment direction of the ordered liquid crystal is controllable and easy to prepare, for example, rubbing orientation or mechanical stretching can make the liquid crystal units align along the rubbing or strain direction. The most prominent advantage of this material is the cooperative movement at the molecular level, which can change the arrangement of liquid crystal molecules and produce macroscopic deformation in a predetermined direction by applying a small amount of energy. According to the differences in chemical structure, crosslinking degree and thermo-mechanical properties of polymers, polymers with liquid crystal ordering are usually called liquid crystal polymer (LCP), liquid crystal elastomer (LCE) and liquid crystal network (LCN)^[9,10].

Photoresponsive polymers can achieve remote control under light stimulation, and complete a variety of high-resolution operations through precise regulation of irradiation time and area. Parameters such as wavelength, intensity and polarization direction of light are easy to control, and light energy is a clean and safe energy source^[11]. Light-responsive shape-changing flexible actuators can be obtained by introducing photosensitive groups or reagents into liquid crystal polymer systems. In 2001, Finkelmann et al. Published the first study on the combination of liquid crystal elastomers with photoresponsive azobenzene molecular switches, revealing that they could contract reversibly under ultraviolet irradiation^[12]. Since this pioneering work, light-responsive deformable liquid crystal polymers have been explored for many years, not only to develop three-dimensional bending motion, but also to develop complex motion and intelligent applications such as twisting, rolling and oscillating^[13,14].

In recent years, researchers have designed a series of novel chemical structures and developed more advanced technical methods to improve the mechanical performance, optical control performance and processability of light-responsive liquid crystal polymer actuators. In this paper, we will summarize the construction of photoresponsive deformable liquid crystal polymer flexible actuators from the perspective of molecular structure design, mainly discuss the scientific problems solved by specific molecular design, and briefly introduce the application of photoresponsive liquid crystal polymers as functional materials in recent years.

2 Molecular Design of Photoresponsive Liquid Crystalline Polymer

2.1 Photoisomerization response type

Azobenzene molecules undergo reversible trans-cis isomerization upon irradiation with light of appropriate wavelengths (Fig. 1A), and are not susceptible to photochemical degradation, so they have been most widely used and studied in the design of light-responsive liquid crystal polymers^[15,16]. The molecular structure of azobenzene in the trans configuration is similar to that of a rod, and azobenzene can be used as a liquid crystal unit or mixed with other liquid crystal molecules to form a stable liquid crystal phase. The corresponding cis-isomer structure has a certain angle, which will make the liquid crystal alignment unstable. Therefore, the isomerization of azobenzene derivatives from trans to cis in crosslinked liquid crystalline polymers causes an isotropic phase transition from liquid crystalline order to disorder in an isothermal manner, and accumulates and amplifies to a macroscopic shape change. The research of Ikeda et al shows that the order parameter of the system decreases with the formation of cis-isomer under light irradiation, and the material shrinks under light irradiation when the alignment of the liquid crystal units is parallel to the irradiated surface^[17,18]. When the liquid crystal is aligned perpendicular to the surface, the material expands under light (fig. 1b). Recent studies have also shown that when the azobenzene group is located in the crosslinking unit, the decrease of the liquid crystal order parameter and the phase transition are the main factors causing the above deformation. However, if the azobenzene group is located only in the side chain, the increase in free volume caused by its conversion to the cis isomer will be the main factor, causing deformation in the opposite direction (Fig. 1C)^[19]. Current studies have explored the preparation of photoresponsive materials with different parameters such as concentration, relative position and network topology of azobenzene molecules.For example, it is found that the concentration of azobenzene group mainly affects the degree of deformation and macroscopic deformation mode, while the position mainly affects the stress and contraction length of deformation, and the length of flexible spacing also affects the photoinduced deformation behavior^[20⇓~22]^[23]^[24]. When azobenzene molecules are non-covalently linked to the liquid crystal polymer network by supramolecular interactions, the shape change can also be driven by light^[25].

In the previous reported work on azobenzene-containing liquid crystal polymer networks, the materials usually return to their original shape in a short time after the light stops, because the cis-azobenzene molecules are prone to thermal relaxation and transition to the trans-state^[26]. Fletcher and Katsonis et al. Designed azobenzene materials with ortho-fluorine substitution, which can improve the thermal stability of the photoresponsive molecules, and they also adjusted the crosslinking density of the polymer network appropriately^[27]. After significant deformation induced by light, the shape can be maintained for several days after the light is stopped. Overcoming thermal relaxation is the key to convert light energy into mechanical work when designing flexible actuators using photosensitive molecules. In addition to azobenzene and its derivatives, other photoisomerizing groups have also recently been applied to liquid crystal polymer systems. Aprahamian and Katsonis et al. Developed a new photoisomerized perylene-based molecular switch in liquid crystal polymer networks, which can reversibly convert between rod-like and "L" -shaped molecular structures under ultraviolet and visible light, so the shape of the polymer changes under illumination^[28]. This hydrazone molecule is not easily thermally isomerized at room temperature, so the shape change of the material can be maintained for a long time in the absence of light (Fig. 2). The response mechanism is also different from the traditional azobenzene photoresponse system, the ordered structure of the liquid crystal is only slightly destroyed during the photoisomerization process, and the main contribution to the macroscopic deformation comes from the mechanical stress generated by the light-induced molecular shape change. This requires that the photoresponsive molecule must act as a cross-linking group in the polymer network in order to be more effective in causing the shape transition. Zhang Hailiang et al. Reported that α-cyanostilbene derivatives were bound to the backbone of poly (4-vinylpyridine) through hydrogen bonds, and the molecule had the characteristics of UV-induced trans-cis structure conversion similar to azobenzene^[29]. The polymer exhibits good photo-induced deformability both in terms of bending speed and bending angle. These works expand the molecular design options of photochemical effect deformation systems, and confirm that the design of molecular switches has a decisive impact on the mechanism of photoresponse. In the future, further exploration in this area will help to improve the photoresponse performance of liquid crystal polymer materials.

Covalently crosslinked polymers usually do not have the properties of melt or solution processing, which leads to high processing requirements in the preparation process and makes it difficult to achieve thermal remodeling and recycling of materials, which also becomes an obstacle to large-scale application of materials^[30,31]. The liquid crystalline glass polymer (vitrimer) developed in recent years can solve this problem, and its representative molecular design is to replace the traditional permanent covalent bonds with exchangeable ester bonds. At present, a variety of exchangeable dynamic covalent bonds have been developed, which can be used in the preparation of photoresponsive liquid crystalline polymers to endow crosslinked liquid crystalline polymers with good processability.

Ji Yan and Wei Yan et al. Studied a new type of liquid crystal polymer elastomer containing allyl sulfide groups in the backbone, in which the allyl sulfide groups in the main chain of the crosslinked liquid crystal network are sensitive to free radicals, and photoinitiators produce free radicals under UV irradiation, resulting in chain transfer between the groups^[32]. The surface is coated with polydopamine with photothermal effect, the local liquid crystal arrangement is restored to isotropy and contracted under the irradiation of infrared light, and the material is bent towards the light source. This approach allows a single liquid crystal polymer actuator to be designed for different drive modes and to be reconfigurable.

Chen Aihua et al. designed a solderable photoresponsive crosslinked liquid crystal polymer, which was amidated by N-hydroxysuccinimide modified azobenzene four-arm block copolymer and amino groups of high molecular weight branched polyethyleneimine at room temperature^[33]. Because of the formation of hydrogen bonding interaction and further crosslinking, it is possible to weld the system. Under appropriate pressure and heating, the cross-linked liquid crystal polymer can be welded into the desired shape, and it is proved that self-welding has better compatibility, stability and mechanical properties compared with ordinary adhesive bonding. Zhao Yue et al. Designed a liquid crystal polymer network, which was formed by crosslinking azobenzene liquid crystal polyester in the main chain by dynamic Diels-Alder reaction^[34]. Because the Diels-Alder reaction is thermoreversible, it can be heated to high temperature to initiate the reverse reaction to form a melt state for reprocessing, and then form bonds to restore the network structure when cooled. When the liquid crystal order-disorder phase transition is induced by UV light or heat, the polymer is capable of light-driven motion (fig. 3). Compared with the exchangeable covalent bond vitrimer, this new dynamic network can not only be directly melt processed, but also be dissolved in hot solvents to achieve reshaping.

Linear liquid crystalline polymer are designed to have sufficient mechanical property and readily available liquid crystal orientation without that need for chemical crosslinking, and can also be processed to fabricate complex structural material^[35,36]. Yu Yanlei et al. reported for the first time a linear liquid crystal polymer with a flexible alkyl backbone containing double bonds and a side chain of azobenzene groups. The flexible spacer provides enough free volume for the azobenzene groups to form a highly ordered liquid crystal alignment structure and to undergo rapid photoresponse (Fig. 4A)^[37]. The polyolefin synthesized by living ring-opening metathesis polymerization has narrow distribution and high molecular weight (the molecular weight reaches the 3.6×10⁵g/mol), and the high molecular weight enables the polyolefin to have better toughness and strength. Tubular microactuators with different shapes were designed based on the material. Under local illumination, the azobenzene groups were rearranged due to the Weigert effect, resulting in asymmetric deformation to promote the movement of the liquid in the tube. Therefore, the polymer can be used for manufacturing flexible microfluidic devices, and has potential applications in microreactors, lab-on-a-chip and micro-opto-mechanical systems. On this basis, linear random copolymers containing side chain azobenzene groups and biphenyl groups were further designed to increase the light penetration depth and photodeformability (Fig. 4B)^[38]. When the copolymer is coated on the surface of an ethylene-vinyl acetate copolymer microtube, a light-controlled double-layer flexible microtube device can also be constructed and applied to microfluidics (fig. 4 C). Yu Yanlei et al. Also synthesized high molecular weight linear waist-grafted liquid crystalline polymers with azobenzene side chains by ring-opening metathesis polymerization, and prepared films by melt shear orientation method^[39]. The film has a rapid and significant photoinduced bending behavior under ultraviolet light, and can be recycled by melt or solution processing due to its linear structure.

Wu Si et al. Also synthesized a linear azobenzene polymer with high molecular weight, which has processability and good mechanical properties by entanglement of polymer chains^[40]. Based on the photoresponsive trans-cis isomerization of azobenzene, the liquid crystalline polymer exhibits a photoinduced bending behavior (fig. 5). The study shows that the light-induced solid-liquid transition properties of azobenzene liquid crystalline polymers are conducive to light-responsive shape change and light-induced surface repair, which provides a new strategy for the design of advanced photoactuators.

Xie Helou and Zhang Hailiang reported an acrylate copolymer containing azobenzene group and ureidopyrimidinone group in the side chain^[41]. Wherein the ureidopyrimidinone group can form intermolecular multiple hydrogen bonds, so that the linear polymer chains are crosslinked to form a network structure. This non-covalent cross-linking allows the material to remain highly processable at elevated temperatures, as well as self-healing at low temperatures. The cis and trans isomerization of the azobenzene group under ultraviolet irradiation enables the prepared film or fiber material to be bent due to light responsiveness.

2.2 Photoinduced thermal response type

The polymer chains in the polymer with liquid crystal orientation are generally in an anisotropic and stretched conformation. Once heated above the liquid crystal-isotropic phase transition temperature, the liquid crystal units will lose their previous orientation direction and arrange randomly, so that the polymer chains will return to a random coil conformation. As that phase transition proceed, the polymer will contract in a direction parallel to the initial orientation of the liquid crystal while expanding in a direction perpendicular to the orientation. When it is cooled below the transition temperature, it will spontaneously restore the direction of alignment and return to the original shape. Based on this, complex deformation can be achieved by properly adjusting the alignment direction of the liquid crystal, for example, a gradually inclined alignment direction (the bottom layer is parallel to the surface and the top layer is perpendicular to the surface) can produce a rapid and large change in the curved shape^[42]. The use of light to control this process is more advantageous than direct heating to achieve localized heating, driving region-selective shape change, allowing remote control and rapid response (Figure 6)^[43]. A variety of photothermal reagents that can form composites with liquid crystal polymers have been reported, including carbon nanotubes, graphene, metal nanoparticles, organic molecular dyes, conjugated polymers and polydopamine.They can absorb light in a specific wavelength range, convert light energy into heat energy, and release heat into the polymer matrix to drive shape change^{[44⇓⇓⇓~48]}^[49⇓~51]^{[52⇓⇓⇓~56]}^{[57⇓⇓⇓⇓⇓⇓~64]}^[65]^[66⇓~68]. Azobenzene and its derivatives as dye molecules can also produce photothermal effect, using the heat released by the photoisomerization of liquid crystal polymers doped with a small amount of azobenzene to induce the order-disorder transition of liquid crystal and material deformation^[69⇓~71]. In the use of various developed photothermal reagents and the design of new photothermal reagents, the following factors generally need to be considered, including the required absorption wavelength, the quantum yield of photothermal energy conversion, and the dispersibility or surface adhesion in the liquid crystal polymer matrix. In addition, the liquid crystal-isotropic phase transition temperature of the polymer itself should not be too high or too low, otherwise it will lead to the difficulty of photo-responsive thermal deformation or the slow recovery of shape.

In addition to light absorption properties, some photothermal reagents can also improve the mechanical properties, electrical conductivity and thermal conductivity of materials. Xie Helou and Chen Erqiang grafted polymers on the surface of multi-walled carbon nanotubes to improve their solubility and dispersion in liquid crystal polymer materials^[72]. A small amount of doping in the film can lead to a rapid increase in local temperature under near-infrared light irradiation, and then the bending deformation of the material occurs. Ware et al. Reported that carbon nanotubes can be used as an alignment layer of liquid crystal elastomers on the basis of enhancing the mechanical properties and thermal conductivity of materials (Fig. 7)^[73]. By controlling the arrangement direction, the position and the quantity of the carbon nanotubes in the liquid crystal elastomer/carbon nanotube composite material, an executive device with dual response to visible light and current can be constructed. Under appropriate illumination, rapid shape change and complex local deformation can occur based on photothermal response drive. When a DC voltage is applied, the polymer can also undergo a similar response deformation under the electrothermal drive. Zhang Aimin et al. Doped silver nanowires into azobenzene liquid crystal network, proving that silver nanowires can also improve the thermal conductivity of the composite, improve the photothermal conversion efficiency, and increase the photoresponse deformation amplitude of the polymer^[74].

2.3 Multiple effect binding response

2.3.1 Multi-wavelength light response type

A plurality of light response systems which do not interfere with each other are designed in the liquid crystal polymer material to respond to light of different wavelength bands. In this way, even under the conditions of the same scanning position, direction and light intensity, the polymer can be deformed in different areas or in different ways due to the difference of photon energy absorption efficiency. Such actuators can simultaneously exhibit reversible and complex responses to several common light sources.

Depending on the chemical structure of the modification, the trans-absorption peak of the azobenzene group can range from ultraviolet to visible light. Van Oosten et al. Used two kinds of azobenzene dye monomers with trans absorption peaks of 358 and 490 nm to polymerize and crosslink with acrylate containing liquid crystal groups, respectively, to construct a two-part microactuator with independent response to ultraviolet and visible light (green light)^[75]. The actuator mimics the movement of natural cilia through a light-controlled bending motion. Schenning and Broer introduced a pH-sensitive azo-merocyanine dye into liquid crystal polymer network film, which can be reversibly converted to hydroxyazopyridine form under the action of acid^[69]. Thus, selective patterning can be achieved by a localized acid treatment of the polymer film and the pattern can be restored by a base treatment. The two forms of the dye have different absorption wavelengths (405 and 530 nm), and different colors of light can be used to control the polymer to fold in different modes. Based on the design, the double-photoresponse liquid crystal polymer actuator which is easy to pattern and can design the response mode for multiple times can be prepared.

By combining the visible/infrared light-induced photothermal effect and the UV-induced trans-cis isomerization of azobenzene, the prepared multi-component liquid crystal polymer can also perform different transformations by modulating the wavelength of light. Zhang Aimin et al. Prepared a double-layer flexible actuator based on graphene oxide and azobenzene doped liquid crystal polymer network. Different driving layers responded to near-infrared light and ultraviolet light respectively and caused bending in opposite directions^[76]. And the graphene oxide layer can be subjected to fine patterning treatment according to the microchannels of the liquid crystal network layer so as to generate a bionic actuator capable of customizing deformation. Yang Hong et al. Prepared a two-layer polysiloxane-based liquid crystal elastomer flexible actuator, which contains azobenzene and organic near-infrared dyes respectively^[61]. By adjusting the wavelength of light stimulation from ultraviolet to near-infrared, the material can perform different three-dimensional reversible transformations (bending and chiral twisting), simulating the two basic movement modes of natural plant tendrils. In their recent work, they used a similar strategy to achieve different chiral helical coiling of actuators under different wavelength control^[77]. Zhao Yue et al. Doped grafted gold nanorods into azobenzene liquid crystal dynamic network and synthesized polymer nanocomposites with near-infrared and ultraviolet response^[53]. A "crane" actuator based on liquid crystal polymer (LCP) is designed to realize complex macroscopic motion by using near-infrared light controlled stretching and ultraviolet light induced bending deformation.

In addition to pre-defined shape changes that are controlled in different ways or in different regions according to wavelength selectivity, the combination of photothermal and photochemical effects can also enable reversible, all-optical shape programming. Priimagi and Zeng et al. reported that trans-cis isomerization of azobenzene groups can be used to control the local mechanical properties of actuators (without shape change) by synergistic use of photochemical and photothermal effects in photosensitive liquid crystal polymer networks.The photothermal effect releases the stress generated by the non-uniform distribution of cis-azobenzene, which triggers the shape change under red light irradiation (Fig. 8)^[78]. Cis-azobenzene can be isomerized to trans-azobenzene induced by blue-green light radiation, which restores the system to its initial state, and then it can be reconfigured by ultraviolet light, so the actuator has photoreconfigurability.

Due to the very limited penetration of ultraviolet light, flexible actuator systems driven by ultraviolet light generally have strict restrictions on the direction of illumination, and can only effectively drive thin polymer films. Polymer actuators with a thickness of more than a few hundred microns tend to respond slowly to ultraviolet light, limiting their application. Yang Hong et al. Selected three organic dyes (520, 808 and 980 nm) with sharp absorption peaks and almost non-interfering visible-near-infrared absorption to prepare liquid crystal elastomer materials with layered structure (Figure 9)^[79]. Three wavelengths of light can selectively stimulate specific regions of the layered actuator to freely change the direction of motion. In addition, infrared light has high tissue penetration ability, and these light-responsive flexible actuators have broad application prospects in biomedical technology. Using similar strategies, multilayer liquid crystal polymer actuators with four or more levels of photoresponse can be further constructed in the future, and only photothermal or photochemical effect reagents with non-overlapping response wavelength intervals need to be found in the ultraviolet, visible or infrared regions. The multi-wavelength modulation flexible actuator system prepared by the method can controllably realize a plurality of motion modes under the condition of only changing the wavelength of the irradiation light, prepare a multi-level response flexible actuator which is easy to optically control and operate, and improve the practical application potential.

2.3.2 Light and other stimulus response combination type

Similar to light response, ambient temperature, humidity, and magnetic fields can all be controlled contactlessly on liquid crystal polymer actuators without connecting the polymer to conventional electronics, increasing its versatility. Therefore, combining the light response with these stimulus responses into a single system can trigger different movement patterns independently of each other, expanding the versatility of the device.

Schenning and Debije et al. Proposed a bilayer structure composed of a photothermal liquid crystal polymer network and a magnetic polydimethylsiloxane, in which magnetic and optical stimuli trigger different mechanical motion responses, respectively^[80]. Illumination causes the deformation of the material itself from bending to stretching, while the magnetic field makes the whole material rotate freely and controllably. Induction heating under a sufficiently strong magnetic field can also directly activate the shape change of the liquid crystal network layer. They further used a similar polydimethylsiloxane material containing magnetic iron powder, coated on the central area of a light-responsive liquid crystal polymer film containing azobenzene dye, to prepare a flexible mechanical claw with both magnetic and optical response, which can load, transport, rotate and release objects (Fig. 10)^[81]. The grasping and lifting motions are controlled from bending to unfolding of the actuator under the guidance of light, while the translation and rotation of the actuator are guided by magnetic response. The combination of optical and magnetic response can realize the remote control of unconstrained flexible robots to perform tasks, which is significantly different from the traditional devices that rely on contact or electronic circuit connection to control the execution of actions.

The development of humidity responsive actuators is important and attractive due to the widespread presence of water vapor in the environment and the dynamic changes in its content. Some plants, such as morning glory, not only have phototactic movements, but also show morphological changes in response to humidity. Yu Yanlei et al. First reported a cross-linked liquid crystal polymer film driven by both humidity and ultraviolet response^[82]. When the surface humidity of the film increases, the hydration of the surface oxygen-containing groups causes anisotropic expansion perpendicular to the alignment direction of the liquid crystal. Base on that change of molecular orientation caused by the photoisomerization of the azobenzene group, the molecule is bent unde the irradiation of ultraviolet light. Based on this, a flexible electronic device driven by humidity and light is realized for the first time, which can output different signals in response to humidity change and ultraviolet light respectively. Chen Aihua et al. Also reported crosslinked liquid crystal polymer films with light-responsive and humidity-responsive bending behavior^[33]. The photoresponsiveness comes from the azobenzene liquid crystal monomer, and the added high molecular weight branched polyethyleneimine contains abundant hydrophilic amino groups, which can form hydrogen bonds with water to sense humidity and cause bending. A three-dimensional flexible mechanical claw is designed by using the liquid crystal polymer material, and the three-dimensional flexible mechanical claw can be remotely controlled to grasp a millimeter-scale object under the combined control of multiple drives. Priimagi and Schenning et al. Have proposed a new type of multiresponsive liquid crystalline polymer whose response is controlled by a subtle interaction between light and humidity^[83]. An artificial flower is designed using the polymer, which is closed when the humidity is low or the light intensity is high, and opens in the dark and when the humidity is high. The liquid crystal polymer network contains azobenzene dye molecules as photothermal response groups, and a large number of carboxyl functional groups are introduced, so that the surface of the film after alkali treatment has obvious hygroscopicity, and the film is stretched due to water absorption deformation in a high humidity environment. Under illumination, the photothermal effect causes the evaporation of surface water and drives the material to restore the bending state, which is more sensitive than the traditional photothermal drive and can respond effectively under lower light intensity. Combining light response with humidity response promises more intelligent and versatile applications.

3 Functional Materials Based on Light-Responsive Liquid Crystal Polymer

Based on the in-depth research and innovative development of the molecular design of photoresponsive deformable liquid crystal polymers, some recent research work has been devoted to the design of reasonable structures for these materials to achieve applications, and many functional smart material systems have been constructed.

3.1 Bionic function application

Nature breeds sophisticated, complex and intelligent biological systems, which also become the source of inspiration for material design. Flexible liquid crystal polymer materials have the advantage of adaptability in simulating soft biological structures, and smart Microsystems imitating biological behavior can be designed by combining light response actuation. Priimagi and Zeng et al. First reported a light-driven actuator mimicking a Venus flytrap, which can sense the physical properties of tiny objects for recognition and perform appropriate autonomous closure (Fig. 11A)^[84]. Through the opening in the middle of the liquid crystal elastomer and the connection of optical fibers, when a small object appears at a suitable distance and position, the light will be reflected or scattered to the polymer surface. The polymer is bent and deformed inwards under the action of the photothermal effect and clamps the target. The size, shape and surface absorbance of micro-objects will affect this process, and then realize the autonomous recognition and distinction of different objects. After that, the light source is turned off, and the object can be easily released. They also designed liquid crystal elastomers similar to the iris tissue in the eye, which responded to different degrees of shape change depending on the power density of the incident light (Fig. 11B)^[85]. When the light intensity increases, the polymer bends inward and blocks the light. When the maximum blocking amplitude is reached, it can reduce the transmitted light by 7 times, and when the light intensity decreases, it will return to the open state. This is similar to the principle that the natural iris regulates the size of the pupil under different lighting conditions, and then regulates the light entering the eye. At present, a large number of light-responsive liquid crystal polymers have been used to construct bionic functional applications, which can imitate the phototaxis of plants, the curling of vines, the blooming and color change of flowers, the wriggling of insects, the swinging of fish and the bending of cilia^[86,87]^[61]^[88]^[58,89,90]^[91]^[92]. Or simulate natural structures such as arteries to design micro-tubular devices, and use paper-cut art to create complex three-dimensional structures^[37]^[93]. These results provide creative inspiration for the functional application of photoresponsive liquid crystalline polymers.

3.2 Energy Conversion Application

Liquid crystal polymers can oscillate significantly and rapidly in response to light, making them valuable for collecting solar energy and generating electricity. Broer, Debije, and Schenning et al. prepared a liquid crystalline polymer film doped with a visible light-responsive azobenzene fluoride, which was capable of continuous irregular oscillatory motion when exposed to sunlight^[71]. The principle is that the simultaneous irradiation of blue and green light in sunlight makes the azobenzene group switch repeatedly between cis and trans. This work is the first time to realize a photoresponsive actuator with continuous self-motion driven by sunlight. By coating polydopamine on the local surface of liquid crystal polymer network, Yang Huai and others ingeniously designed and prepared a new type of photoresponsive polymer oscillator with fast and stable oscillating motion (Fig. 12)^[67]. The polydopamine coating has good bonding ability with a plurality of different types of materials, and has strong absorption capacity and good light stability in the near-infrared region, and the near-infrared wavelength in sunlight is an important component. When the coated area is irradiated by near-infrared light from a specific direction, the film will bend and deform due to the photothermal effect of the polydopamine coating, resulting in the uncoated area blocking the light. The photothermal effect is then interrupted and the bent polymer will return to its planar shape and be re-exposed to near-infrared light to cycle the process. Compared with other methods, because the polydopamine coating can be removed by chemical methods, the oscillation of the film can be adjusted not only by light intensity and film thickness, but also by removing and re-coating to change the position or pattern of the surface polydopamine. They have also demonstrated the possibility of creating solar generators through rapid oscillatory behavior, which can successfully convert solar energy into mechanical and electrical work.

3.3 Flexible robot application

A variety of light-responsive liquid crystal polymers have been reported for remote light-controlled shape change and motion, but it is still a challenge to realize complex intelligent flexible robots. By integrating multiple liquid crystal polymer actuators, Toonder and Schenning first constructed a flexible transport robot driven entirely by blue light and capable of controllable multi-directional movement (Fig. 13)^[94]. The liquid crystal polymer network of the four "legs" of the flexible robot contains a light-responsive azobenzene group, while the "arms" contain another azobenzene group with different absorption wavelengths. The robot can move in any direction through the deformation of its "legs" under light, while another wavelength of light stimulates the control of picking up and releasing goods. This work shows us that through reasonable design strategies, combined with various light-responsive liquid crystal polymers developed so far, complex smart materials can be realized through constructive and cooperative utilization of different motions, which can be applied to various complex behaviors in practical situations.

4 Conclusion

In this paper, the recent research work on the molecular design methods and functional materials of light-responsive shape-change liquid crystalline polymers is summarized. In molecular design, the photoresponsive liquid crystal polymer mainly responds to light of a certain wavelength by introducing a photoisomerization molecular switch represented by azobenzene and adding various types of photothermal reagents. On this basis, a series of specific chemical designs with novel photoisomerized molecules, dynamic covalent bonds and linear structures without chemical crosslinking were developed. The combination of groups with different photoresponse wavelengths or other stimulus response modes can effectively expand the diversity of photoresponsive materials in shape change modes. Making full use of the rich deformation properties of light-responsive liquid crystal polymers, it is expected to achieve a variety of functions in practical applications, and promote innovative development in the fields of micro-mechanical manipulation, energy conversion materials and flexible robots. Drawing inspiration from nature through bionics is helpful for further design and development of new functional devices.

Light-responsive liquid crystalline polymer systems still face some important challenges. First, most of the current work is based on films or fibers. This makes light-driven polymers generally applicable only in scenes where mechanical forces are weak and the scale of motion is small. In recent years, a series of studies have been reported to demonstrate its potential applications at the micromechanical scale, and the preparation process has been continuously improved, for example, the topological rearrangement of the network is used to store larger strain energy during preparation, thus generating larger shrinkage stress during photoresponse^[95]. However, larger scale polymer actuators generate greater internal stress during fabrication and cause the material to be unstable and prone to fracture. However, the penetration of light is relatively limited, which also limits the macroscopic thickness of light-responsive liquid crystal polymer materials. In the future, the preparation process of the material can be further improved and the internal stress generated in the preparation of the material can be improved by using strategies such as dynamic covalent bonding. Or the mechanical strength and modulus of the material can be enhanced by introducing secondary interaction into the polymer material and constructing a multi-level crosslinking network and the like; At the same time, combined with engineering technologies such as optical fiber conduction and laser, the limitation of light penetration into large-scale materials can be improved by increasing light intensity or providing light energy from the inside of materials, which is expected to design larger-scale deformation materials under light response and generate stronger mechanical forces.

In addition, the stability of photosensitive molecules and the stability and reproducibility of shape changes in polymer photoresponse are still limited, which also limits the application in practical devices. Therefore, the exploration of new photoresponsive systems and the further development of new photosensitive molecules or groups with excellent chemical stability and photoresponsiveness for application in liquid crystal polymers remain crucial. For example, designing new monochromophores with visible light response and even multi-wavelength response can further overcome the dependence on ultraviolet light sources and endow materials with versatility. It is believed that there will be more systematic and in-depth research in the field of designing light-responsive shape-changing liquid crystal polymers, and further explore and broaden the space for their manufacture, manipulation and application.

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