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Photodeformable Composite Materials Based on Liquid Crystalline Polymers

Shuai Huang , Yu Tao , Yinliang Huang

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (9) : 2012 -2023.

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Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (9) : 2012-2023 DOI: 10.7536/PC211103
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Photodeformable Composite Materials Based on Liquid Crystalline Polymers
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Abstract

Deformable liquid crystalline polymers have attracted a lot of research interests in recent years, because of the great potentials in the development of intelligent soft systems such as artificial muscles, soft robots and smart optical devices. However, the response of conventional liquid crystalline polymers is based on thermal control methods, which limits the applications due to the low thermal conductivity of the polymer matrix and the high dependence on external heating devices. In contrast, the light control method has shown many advantages, including non⁃contact, remote in⁃situ, and precise manipulation capabilities, which could contribute to developing diverse unrestricted and remotely operable smart soft devices. Recently, there have been many important advances in the development of photodeformable liquid crystalline polymers fabricated by introducing organic or/and inorganic light⁃responsive components as functional additives. Among them, the multiple functions of the introduced components can be easily combined with the orientational deformation behavior of the liquid crystalline polymer through the interaction among the functional module, the liquid crystal phase and the polymer matrix. Here, this review focuses on the design strategy, manufacturing method and working principle of the photo⁃manipulatable liquid crystalline polymer composite systems incorporated with light⁃sensitive components. Moreover, the possible applications and future development are briefly summarized at the end of the review.

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photodeformation / liquid crystalline polymer / smart soft materials

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Shuai Huang, Yu Tao, Yinliang Huang. Photodeformable Composite Materials Based on Liquid Crystalline Polymers. Progress in Chemistry, 2022, 34(9): 2012-2023 DOI:10.7536/PC211103

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Contents

1 Introduction
2 Organic dyes/photoresponsive liquid crystalline polymer composites
2.1 Composites based on photothermal dyes
2.2 Composites based on photochemical molecules
3 Inorganic nanomaterials/photoresponsive liquid crystalline polymer composites
3.1 Composites doped with nanoparticles
3.2 Composites doped with nanorods/nanotubes
3.3 Composites doped with nanosheets
4 Conclusion and outlook

1 Introduction

In nature, organisms have evolved the ability to adapt to their environment by adjusting their appearance, form, and movement. For example, the color change of chameleons, the predation of pitcher plants, and the circadian rhythm of Albizzia leaves are often achieved by manipulable microstructures[1⇓~3]. These natural phenomena have inspired scientists to develop smart soft materials through multi-level structural design. Flexible devices based on these materials can recognize external stimuli (including light, heat, electric field, magnetic field, mechanical force, etc.) And convert input signals into various chemical or physical outputs to perform a given task[4⇓~6]. Among various smart materials, liquid crystalline polymers with tunable anisotropic microstructure are one of the most attractive systems[7⇓⇓~10]. In general, liquid crystalline polymers contain mesogens in the main chain or side chain, which can combine the anisotropy, self-assembly, and stimuli-responsive properties of liquid crystals with the processability, corrosion resistance, and designable mechanical properties of high-performance polymers[11⇓⇓⇓~15]. Compared with other smart soft materials such as hydrogels, liquid crystal polymers have many advantages in developing multifunctional smart devices: (1) their stimuli-responsive behavior does not depend on environmental conditions; (2) The response speed is fast, and it is easy to operate accurately in situ; (3) Anisotropic response can be achieved by controlling the orientation of liquid crystals for the development of programmable complex devices[16⇓~18].
At present, researchers classify liquid crystal polymers into linear liquid crystal polymers, glassy liquid crystal polymer networks, and liquid crystal elastomers. Linear liquid crystal polymers have excellent processability because of the physical interaction between molecular chains, such as van der Waals force and chain entanglement. However, the stimulus-response behavior of such systems is usually irreversible, and interchain sliding prevents the alignment of the chain segments and the liquid crystal from returning to the initial state[19]. In contrast, both glassy liquid crystalline polymer networks and liquid crystalline elastomers are cross-linked systems and thus exhibit reversible response behavior, but these two types of materials have different mechanical properties. In general, glassy liquid crystal polymer networks are tightly crosslinked networks with glass transition temperatures above room temperature, which represent the mechanical properties of plastics. However, liquid crystal elastomers are slightly crosslinked systems, often containing flexible main chains, and their glass transition temperatures are lower than room temperature, showing the mechanical properties of rubber[20,21]. At present, researchers have developed a variety of intelligent soft devices by using the advantages of liquid crystal polymers, such as artificial muscles, soft robots, microfluidic controllers, reversible adhesives, tunable surfaces, new optical devices and 3D printed smart devices[22,23][24⇓~26][27,28][29,30][31,32][33,34][35,36]. Among them, the liquid crystal actuator, which can transform various external stimuli into macroscopic deformation, is a very potential direction. This response is generally caused by the phase change or orientation change of the liquid crystal. In addition, these deformations are usually directional and steerable. In thermotropic liquid crystals, the deformation of the liquid crystal actuator is triggered simply by heating it to trigger the order-disorder transition of the liquid crystal phase (Figure 1)[16,37].
In particular, the deformation mechanisms of glassy liquid crystalline polymer networks and liquid crystalline elastomers are different. In a glassy liquid crystalline polymer network, the deformation mechanism is that an increase in internal temperature reduces the order of the liquid crystal in the compact network and causes a rapid conformational change, resulting in a local modulus change. However, in liquid crystal elastomers, the conformational change is greater but slower when driven due to the relatively loose network, which has less restriction on molecular motion, and the synergistic effect of phase transition and segmental motion in the polymer backbone[35]. In other words, different polymer network densities have a great influence on the mechanical properties and deformation behavior of materials. However, the low thermal conductivity inside the liquid crystal polymer limits the heat transfer, making it difficult to achieve uniform heating of the material, further affecting the response speed of the actuator. To solve this problem, researchers have achieved efficient and rapid deformation by implanting a heat source into the liquid crystal polymer[38]. However, due to the difficulty of regionalized control of the heat source and the existence of thermal diffusion, it is difficult to generate local driving force without affecting the state of molecules around the heat source to achieve accurate regionalized deformation control.
In contrast, optically manipulated liquid crystal actuator systems have many advantages, such as non-contact, remote in-situ and localized precise manipulation capabilities, which can be used in optically controlled motors, wave transmitters, optically manipulated artificial irises and microfluidic optical controllers[39,40][41][24][42][27]. However, in order to endow liquid crystal polymers without photoresponsive groups with the ability to respond to light, there are two commonly used strategies: one is to achieve fast and efficient photoresponse by doping components with photothermal conversion ability; The other is to use the photochemical response mechanism of some functional groups to trigger the order-disorder transition of liquid crystal phase through the change of molecular conformation or configuration. Among them, the phase transition of liquid crystal is the medium for the conversion of light energy to mechanical energy. Therefore, the interaction between mesogens and dopants becomes an important factor in determining the properties of materials. For the system based on photothermal effect, the liquid crystal temperature range of the mesogen should be able to match the photothermal conversion ability of the dopant, especially the clearing point of the liquid crystal should not be too high or too low (generally 50-100 ℃). Too high can result in high dopant content or high excitation light intensity to drive the system, and too low can result in too slow shape recovery process[20]. For the system based on photochemical effect, the dopant should generally contain a rigid structure and the molecular configuration should change greatly before and after the photochemical reaction, so as to induce the liquid crystal phase transition through the supramolecular synergistic effect with the liquid crystal units to drive the material to deform[9,17]. In addition, the incorporation of dopants may disturb the order of the liquid crystal phase and the internal structure of the material, and adversely affect the directional deformation and even the mechanical properties of the material. Therefore, it is necessary to carefully design the internal structure (including molecular structure and aggregation structure) of these composites in order to obtain optically controlled intelligent devices with excellent performance[35].
In this paper, we will focus on the design ideas and application directions of photodeformable liquid crystal polymer composites and devices. This kind of material is mainly based on photothermal effect or photochemical reaction to achieve photodeformation by introducing photosensitive organic dyes or inorganic nano-components. Its function and characteristics can be designed by adjusting the ordered structure of liquid crystal and the interaction between doping components, thus realizing various remote, in situ and precise control. Through this strategy, materials can be endowed with intelligent characteristics. In this paper, we focus on the design strategies, preparation methods, working principles, and potential applications of liquid crystalline polymer composites using organic dyes or inorganic nanocomponents as functional dopants. More importantly, the addition of photosensitive dyes or nano-components can combine the various functions of additives with the directional deformation behavior of liquid crystal polymers, and realize the cooperative work of functional modules, liquid crystal phases and polymer matrices, which provides an effective strategy for the manufacture of smart soft devices capable of precise manipulation in a broad spectrum.

2 Organic dye/photoresponsive liquid crystal polymer composite material

Organic materials often have tailorable and functionalizable properties from the molecular to the macroscopic scale. Therefore, functions derived from different organic functional groups can be integrated by chemical design. In addition, incompatibilities between components in composites often adversely affect mechanical properties and fatigue resistance[43,44]. However, for organic systems, the compatibility between components can be easily improved by their ease of chemical modification. Generally speaking, there are two main design strategies for organic dye/photoresponsive liquid crystal polymer/composite materials: the introduction of organic photothermal dyes and the introduction of molecules containing photoresponsive functional groups. The mechanism of both strategies is to trigger a phase transition to produce a local volume change by changing the orientational order of the liquid crystal molecules. The sensitive wavelength range of liquid crystal polymer composite system based on photothermal effect is usually more designable. At present, light-controlled systems from the ultraviolet spectral region to the near-infrared spectral region have been reported. Most of the systems based on photochemical reactions are controlled by ultraviolet and visible light. In addition, these two strategies have an impact on the response speed of the device and the light intensity threshold of the trigger drive[45,46].

2.1 Photothermal dye based composite system

Organic dyes can absorb a certain wavelength of light, and then convert the light energy into heat energy, which can trigger the phase transition of liquid crystals when the heat accumulates to a certain extent. However, the incorporated dye must have a large molar extinction coefficient and photothermal energy conversion efficiency if the driving of the material through the mechanism of this phase transition is to be further achieved[47]. Although the photothermal efficiency of organic dyes is generally difficult to compare with inorganic materials, they have the advantage of good compatibility with polymer matrices. So far, researchers have reported a variety of organic dyes including small molecule dyes, organometallic complexes, and conjugated polymers. The molecular structure of these dyes contains π conjugated system, which is beneficial to the absorption of photons from ultraviolet to near-infrared and the efficient photothermal conversion.

2.1.1 Doped small molecular organic dye as photosensitizer

Broer et al. Found that the light stabilizers commonly used in the market have good photothermal effects, so they systematically studied the performance of these small molecule light stabilizers with absorption wavelength range of 365 ~ 600 nm as photothermal dopants[48]. It is found that these dyes can cause a significant increase in the internal temperature of the material under the irradiation of light with a specific wavelength, thus driving the movement of the liquid crystal polymer film. On this basis, they made the temperature of the material oscillate and realized the self-oscillating motion through a design strategy of "self-shadowing". Among them, organic small molecular dopants are considered to rapidly dissipate the absorbed light energy in the photothermal conversion process through molecular vibration or rotation.
Another typical organic small molecule dye is an azobenzene derivative containing a π-conjugated structure. One of the main advantages of these materials is that their molecular tautomerization can be combined with the orientation control of liquid crystal elements to prepare multi-mode programmable liquid crystal actuators. A typical example is the azo dye, Disperse Red II, which can be converted in situ to a hydroxyazopyridinium salt by acid treatment. The dye in these two States showed quite different response behaviors under the stimulation of 405 and 530 nm visible light, which realized the reversible acidification programming of the actuator[49].
The control light source of the above system is mainly ultraviolet light and visible light, in contrast, near-infrared light has better penetration and lower biological hazards[4]. However, the design of near-infrared responsive organic small molecule dyes has always been a difficult problem. Inspired by croconine dyes, Yang et al. Obtained near-infrared photoresponsive liquid crystalline organic dyes with "rigid-flexible" structure by modifying the rigid core of thiophene croconine chromophore with two symmetrical flexible alkyl chains[50]. The dye shows a sharp and intense absorption band around 796 nm wavelength. At the same time, because the dye itself also has liquid crystal properties, it is more compatible with the liquid crystal elastomer matrix, and it is easier to obtain good dispersion in the matrix, thus realizing rapid and reversible photodeformation behavior[51⇓~53]. This near-infrared responsive organic dye has also been further used to design sophisticated smart soft devices. Inspired by the camouflage behavior of organisms in the natural environment, Li et al. Reported a bionic soft actuator with synergistic light-controlled fluorescence color change and programmable deformation ability[16]. They chemically bonded a terminally functionalized tetraphenylethylene derivative and a photochromic spiropyran group into a liquid crystal polymer network. The luminescence behavior of the smart device can be controlled by switching ultraviolet/visible illumination, and the incorporation of the above organic dye allows the motion of the soft device to be manipulated by near-infrared light. Using this mechanism, researchers have developed soft robots that can simultaneously crawl and camouflage according to the environment to perform tasks in complex environments, as well as biomimetic devices that can imitate complex biological functions, such as the color change and deformation of the neck of ruffled lizards with color change behavior (Fig. 2).

2.1.2 Doped organometallic complexes as photosensitizer

As mentioned above, it is difficult to design and synthesize organic small molecule dyes with long wavelength response, which can be easily overcome by using organometallic complexes. Zhao et al. Introduced a nickel-phosphonium dithiolene complex with high photothermal activity at 980 nm into liquid crystal polymers to achieve near-infrared photocontrol. Through the programming of liquid crystal orientation and the design of photothermal motion and thermal feedback loop, they obtained a composite actuator that can perform a variety of autonomous motions (such as crawling, climbing, bending and twisting) under continuous 980 nm laser irradiation (Figure 3)[54].

2.1.3 Doped polymeric dye as photosensitizer

Although the light absorption range of small molecular dyes is very narrow and their light selectivity is excellent, their photothermal efficiency is difficult to further improve, which is not conducive to the preparation of fast response devices[20]. In addition, the small molecule dye, while acting as an energy conversion site, also acts as a plasticizer in the polymer matrix, adversely affecting the mechanical strength of the resulting liquid crystalline polymer system. Therefore, replacing small molecule dyes with polymeric dyes is an effective strategy to obtain efficient photothermal conversion capability without adversely affecting the mechanical properties of the liquid crystal polymer matrix. The structural basis of high photothermal conversion efficiency is the π conjugated structure in the molecule. Yang et al. Demonstrated that a classical conjugated polymer, polyaniline, can be implanted into liquid crystal elastomers to prepare photoresponsive actuators[55]. In addition, the emeraldine salt form of polyaniline exhibits better photothermal properties than emeraldine base. The polyaniline/liquid crystal elastomer composite has excellent mechanical properties, and can lift an object 200 times of its own weight under near-infrared light irradiation.
Another example of a polymeric dye is the low-cost and eco-friendly mussel-inspired polydopamine material. Although polydopamine does not have the large conjugated structure of polymers such as polyaniline, it has been reported that the molar extinction coefficient and photothermal conversion efficiency of polydopamine nanoparticles are extremely high under light irradiation at 808 nm wavelength[56]. In addition, polydopamine can be easily prepared by self-polymerization of dopamine, a component of adhesion proteins in mussels, and is easily made into organic nanoparticles or surface coatings. Ji et al. Found that polydopamine dispersed in or coated on the surface of a liquid crystal elastomer matrix caused a significant temperature rise under near-infrared illumination, reaching 160 ° C in a few seconds[57]. Cai et al. Demonstrated that polydopamine-coated liquid crystal elastomers can work in air and water and have the advantages of fast response and large deformation[58]. These excellent performances enable polydopamine-based programmable soft actuators to be used as polymer oscillators for application scenarios such as solar energy conversion[59].

2.2 Composite system based on photochemical molecules

Compared with the photoresponsive liquid crystal polymer system based on photothermal effect, the system based on photochemical reaction has a mature design strategy, so it is more common in organic composite systems[9]. In general, photoresponsive groups capable of photoisomerization or photodimerization, such as azobenzene, stilbene, dithienylethene, spiropyran, cinnamate, and chalcone structures, have been shown to be useful for designing smart devices such as molecular motors or molecular switches[60⇓⇓~63]. When they are introduced into liquid crystal polymers, the molecular shape change caused by the tautomerization process can be amplified by the effect of supramolecular cooperative motion, which disturbs the ordered structure of liquid crystals and drives the macroscopic motion of materials[17,61]. In most studies, these photoresponsive groups are chemically bonded into the liquid crystal polymer backbone, so the molecular structure and synthetic route must be carefully designed[64⇓⇓~67]. In order to simplify the preparation of such systems, researchers have directly doped molecules that can undergo photochemical reactions into liquid crystal polymers, and found that these functional components can also drive the movement of the matrix[68⇓⇓~71].

2.2.1 Dye with photochemical reaction doped in liquid crystal polymer

Azobenzene is a typical photoactive molecule that is often incorporated into liquid crystal polymer systems. Azobenzene can undergo reversible cis-trans isomerization under the irradiation of light with specific wavelength. Palffy Muhoray et al. Found that the liquid crystal elastomer matrix doped with azo dye disperse orange I bent at an angle of more than 60 ° (60 ms at 1.3 W) when irradiated by 514 nm green light, and the bending speed was more than two orders of magnitude faster than the previously reported covalently bonded system[68]. In addition, the elastomer can float on water and swim under the illumination of green light.
The response wavelength of azobenzene dyes can be modulated by adjusting the substituents on the benzene ring, and even can be made to respond to ambient light to prepare adaptive actuators regulated by the environment. For example, a liquid crystal polymer network film doped with a fluorinated azobenzene dye can undergo disordered oscillation under sunlight, and further studies have found that it can be driven to undergo continuous oscillation by simultaneous irradiation with blue light (405 nm) and green light (530 nm)[69]. This material has great potential for applications such as autonomous motion, soft robots with continuous self-propulsion, and self-cleaning surfaces driven by sunlight. Since then, doping strategies have been increasingly used to design photodeformable liquid crystalline polymer systems due to the simplicity of the preparation process. However, the molecular motion of the dopant needs to be able to affect the packing state of the chain segment to produce a better driving effect, so the choice of the liquid crystal polymer matrix is also very important.

2.2.2 Composite liquid crystal polymer into polymer matrix

A photoresponsive liquid crystal polymer dopant can impart photoresponsive characteristic to a photoinert substrate. Yu et al. Blended azobenzene liquid crystalline polymers containing reactive groups into a polyurethane matrix[70]. After post-crosslinking treatment, the liquid crystalline polymer in the blend film was well dispersed, and the blend film showed very rapid unidirectional photoinduced deformation behavior. Mechanical stretching treatment can induce the rearrangement of liquid crystal units and obtain reversible photoinduced deformation characteristics.
Direct doping of liquid crystalline polymer assemblies into the polymer matrix can also impart photoinduced deformation behavior to the composite film. As shown in fig. 4, Yu et al. Prepared liquid crystal polymer particles containing azobenzene groups, and doped these particles into a photoinert polymer to obtain a "polymer dispersed liquid crystal" film, and obtained ordered liquid crystal orientation by mechanical stretching[71]. Wherein the rough morphology of the hydrophobic liquid crystal polymer particles enhances the interaction between the dopant and the hydrophilic matrix, so that the dopant can be uniformly dispersed, and the material has excellent photoinduced deformation characteristics. In theory, based on this strategy of preparing liquid crystal polymer composites, there are many ways to achieve the combination of light and mechanical behavior of materials.
Another method of preparing composite films is to fill stretched porous polyethylene with a polymerizable azobenzene group-containing liquid crystalline monomer mixture[66]. After polymerization, a liquid crystalline network with good orientation was formed in the pores of the matrix. Under the irradiation of ultraviolet light, the composite film can realize controllable deformation. Another more convenient strategy is to directly coat the photoresponsive liquid crystal polymer on the polyimide film. The orientation of liquid crystal can be effectively controlled by pre-rubbing the surface of polyimide film. Based on the reversible photobending properties of the composite film, the researchers have proved that this kind of material has broad application prospects in the preparation of biomimetic devices (such as imitating the circadian rhythm of Albizzia julibrissin)[1].
In addition, the double-layer structure composite film obtained by post-crosslinking the liquid crystal polymer coated on the surface of the polyimide film shows a rapid bending to the polyimide side under UV irradiation, and recovers immediately after the light source is removed. This property was used to continuously propel the membrane over the water surface to simulate the forward motion of a dolphin by hitting the liquid/air interface (Figure 5)[72]. Further studies have found that if the polyimide film is replaced by a thermoplastic polymer matrix (such as polyethylene terephthalate), the liquid crystal polymer network/polymer composite film can have programmable characteristics. Based on the thermoplasticity of the photoinert polymer substrate, the shape can be arbitrarily programmed in the bilayer film to form various shapes including origami-like folding, left-handed and right-handed[73]. Among them, the crosslinked liquid crystal polymer network helps to fix the preset shape, and reversible driving of the material can be achieved with light.
These pioneering works demonstrate that the functionality and characteristics of light-responsive actuators can be tuned by changing the polymer matrix, and thus liquid crystal polymer composites have great potential for applications. Moreover, photothermal and photochemical mechanisms can work synergistically in organic composite systems. For example, Yang et al. Reported a soft actuator that mimics plant curling with both light-tunable bending and chiral twisting motion modes[74]. In this soft actuator, the researchers used a two-layer, two-component strategy to achieve two different reversible motion modes that can be tuned by adjusting the wavelength of the incident light (ultraviolet vs near-infrared) (Figure 6).

3 Inorganic nanomaterials/photoresponsive liquid crystal polymer composite

The above organic dyes can be well dispersed in the polymer matrix, but because the absorbed light energy is easily dissipated by molecular motion, the final device usually does not have a particularly outstanding conversion efficiency of light energy and mechanical energy. The sensitive light source of most organic dyes is short wave (mostly ultraviolet and visible light), which has low penetration and lacks bio-friendliness, while the synthesis of near-infrared responsive organic dyes is very difficult. Therefore, currently available organic dyes are still very limited. In contrast, inorganic photothermal nanomaterials have very high molar extinction coefficient and photothermal conversion efficiency, which can induce the conversion of light energy to mechanical energy in liquid crystal polymers even at very low contents[75]. For example, Zhao et al. Reported that the temperature of a liquid crystal network system doped with gold nanorods (0.05 wt%) could rise to 175 ° C within 40 s under near-infrared light (6.7 W/cm2) irradiation at 785 nm[26]. Moreover, the sensitive wavelength of the composite material itself can be changed by adding inorganic nanomaterials that absorb long wavelength light and emit short wavelength light, that is, upconversion materials. Therefore, inorganic nanomaterials are widely used in the fabrication of photodeformable liquid crystalline polymer composites and devices. At present, the most common inorganic nanomaterials, such as gold nanorods and graphene, show high photothermal conversion efficiency, which can raise the internal temperature of the material to above the liquid crystal clearing point (mostly about 100 ℃ or even higher) in tens of seconds under light[76,77]. In addition, the shape and arrangement of these components often play an important role in regulating material function.

3.1 Nanoparticle doped composite

Generally speaking, NPs are considered as zero-dimensional nanomaterials, which are spatially isotropic. Among them, the key factors determining the properties of the composite are the photothermal conversion efficiency of the nanoparticles and their compatibility with the polymer matrix. Metal nanoparticles are a kind of typical nanomaterials with surface plasmon resonance effect, which have high photothermal conversion efficiency[78]. However, the easy aggregation of nanoparticles and the poor compatibility between inorganic components and organic matrix limit the preparation of this kind of composites[76,78,79]. Therefore, it is a feasible strategy to modify the nanoparticles to act as crosslinkers to achieve good dispersion. For example, gold nanoparticles chemically bond with liquid crystal monomers to form crosslinkers, which are then covalently incorporated into the liquid crystal network. Not only is uniform distribution achieved, but the spherical appearance of the particles does not interfere with liquid crystal long-range order and does not adversely affect mechanical properties[79]. Wang et al. Proved that the conversion of light energy to mechanical energy of silver nanoparticle/liquid crystal elastomer composites can be realized based on the surface plasmon resonance effect. They found that visible light irradiation can drive the material to undergo rapid and reversible deformation even when the content of silver nanoparticles is low (0.04 to 0.20 wt%). More importantly, doping a small amount of nanoparticles (0.20 wt%) can even improve the tensile strength of the actuator by about 60% at temperatures close to the clearing point[80].
Upconversion particles are another class of materials commonly used in the preparation of liquid crystalline polymer composites, which can absorb continuous near-infrared light and transfer the available energy to other chromophore units. Yu et al. Incorporated NaYF4:Yb/Tm(~70 nm) upconversion nanophosphor into a photoresponsive liquid crystal network containing azo units, and the resulting composite film showed reversible rapid deformation ability under near-infrared light irradiation (Fig. 7A)[81]. The mechanism is that the energy transfer occurs when the upconversion luminescence triggers the photoisomerization of the azo unit and the rearrangement of the mesogen. However, this system needs to be excited by high-intensity laser with a power density above 15 W/cm2, which will cause overheating effect to adversely affect the material. Therefore, Yu et al. Further developed a two-dye organic upconversion system, using a fluorescent dye as an annihilator and a Pt (II) complex as a sensitizer, to achieve low-power infrared light-driven photodeformation based on triplet annihilation (Fig. 7B)[82]. Moreover, the photochemical component and the upconversion nanoparticle can work in concert. The researchers have successfully realized the shape memory with full wavelength response in the ultraviolet, visible, and near infrared with the polyurethane composite system doped with azobenzene dye and core-shell upconversion nanoparticles (~20nm,Yb 25 mol%/Er 2 mol%@NaYF4:Nd 30mol%) containing NaYF4[83].

3.2 Nanorod or nanotube doped composite

Rod-like metallic nanomaterials can be uniaxially oriented and display unique shape-dependent localized surface plasmon resonance effects. Therefore, the alignment of the metal nanorods in the material will interact with the liquid crystal orientation in the liquid crystal polymer matrix. In addition, compatibility and easy aggregation are still common problems in the preparation of such composites[84]. Yang et al. Prepared near-infrared photoresponsive gold nanorod/liquid crystal elastomer fibers using a three-step thiolene click strategy[85]. It is found that the well-dispersed gold nanorods can be heated to 50 ℃ by irradiation with 808 nm near-infrared light for 10 min at a very low loading level (0. 09 wt%), which drives the liquid crystal elastomer to undergo reversible deformation. However, the chemical bonding of gold nanorods into liquid crystal polymers is not always required for the preparation of photodeformable liquid crystal actuators. Appropriate modification of nanorods is also an effective strategy. For example, poly (3-mercaptopropylmethylsiloxane) (PMMS) modified gold nanorods with thiol side groups have good stability and can be well dispersed in the liquid crystal elastomer matrix, and can be rapidly heated to more than 100 ℃ under 808 nm near-infrared light irradiation, which is nearly 80 ℃ higher than the blank control group[86]. These characteristics make the composite materials show extraordinary potential in the fabrication of multi-mode near-infrared light-driven devices.
The further addition of a dynamic crosslinking agent sensitive to ultraviolet light to the above system can make the material reprocessable, thus expanding its application in the precise control of multifunctional devices. By optimizing the alignment of liquid crystals and nanorods, the material will have fast response speed, recyclability, and the ability to transform from a monolithic film into a variety of shapes. With the help of patterned photomask, thermal gradients can be generated during illumination, and complex microstructures of materials can be edited at high spatial resolution, which will greatly broaden the deformation mode of wireless controlled adaptive soft robots[87].
Carbon nanotubes have a very large aspect ratio (generally more than one thousand), so they are considered to be a one-dimensional nanomaterial. Due to the characteristics of conjugated C — C bonds and the influence of shape and size, carbon nanotubes have excellent electrical conductivity, thermal conductivity, mechanical properties and high dielectric anisotropy. It is reported that the band gap transition of SWNTs makes them show strong characteristic absorption in the visible to near-infrared region, so they have excellent photothermal conversion efficiency[88]. Chen et al. Used rigid conjugated macromolecule poly (p-phenylene ethynylene) (PPE) to non-covalently modify multi-walled carbon nanotubes and dispersed them in a nematic liquid crystal elastomer matrix[89]. The as-prepared composites exhibited excellent infrared light-induced reversible strain (~ 30%) ability at very low carbon nanotube loading levels (0.1 wt% ~ 0.2 wt%).
In addition, this type of photoresponsive liquid crystal polymer can be driven by a broad band of light with an intensity of about 100 mW/cm2, improving the practicality of the material[90]. Further studies have shown that the content of carbon nanotubes can affect the mechanical and optical properties of materials, and significantly change the deformation behavior, driving speed and strain mode of materials[91,92].
Chen et al. Found that the incorporation of single-walled carbon nanotubes with near-infrared organic dyes into liquid crystal polymers can develop wavelength-selective, infrared-light-driven hinge devices that can power a variety of remote-controlled soft robots, including active origami structures, fly-trap-inspired grippers, and crawling inchworms (Figure 8 a)[93]. Carbon nanotube/liquid crystal polymer composites and devices have a variety of potential applications, such as smart actuation and microstructure control using patterning strategies[94]. In addition, a multi-domain carbon nanotube/liquid crystal elastomer composite film containing a patterned bubble-shaped single-domain region is made into a tactile actuator, which is used in a new Braille reading device to help the visually impaired[95].
In addition to SWNTs, MWNTs can also be used as optical functional additives for liquid crystal polymers. Compared with single-walled carbon nanotubes, multi-walled carbon nanotubes have better dispersion in polymer matrix, so they can significantly enhance the response of materials to infrared and visible light at lower content[96]. By controlling the orientation of carbon nanotubes, various types of light-controlled motion can be achieved in carbon nanotube/polymer composites, including two-dimensional phototropic/non-phototropic bending, three-dimensional spiral transformation, etc[97]. The regular array of carbon nanotubes also has a unique effect on the optomechanical behavior of materials. Peng and Yu et al. Found that in the carbon nanotube/liquid crystal network composite system containing azobenzene, the liquid crystal orientation can be perpendicular to the orientation direction of the nanotube, which makes the material bend reversibly under ultraviolet irradiation, and the regularly arranged carbon nanotubes also improve the mechanical strength and conductivity of the composite[98]. The carbon nanotube array is processed into a thin sheet and then compounded in a sandwich structure of a liquid crystal network containing azobenzene, which can induce the orientation of the liquid crystal, and the composite can be reversibly deformed under the alternating irradiation of ultraviolet and visible light (Fig. 8B)[99].
In addition, the incorporation of carbon nanotubes into liquid crystalline polymers containing photochemical functional groups can achieve wavelength-controlled selective motion. In order to combine photochemically induced phase transition and photothermally induced phase transition into one system, Yang et al. Doped carbon nanotubes into polysiloxane-based liquid crystal elastomers containing azophenyl groups on the side chain[100]. By reasonably designing the orientation of the liquid crystal element, the composite liquid crystal elastomer can be bent and deformed under light irradiation, and can be contracted and deformed under near-infrared light irradiation. These studies show that different optical and mechanical energy conversion mechanisms can be combined through the synergistic effect of organic and inorganic photoresponsive components, which further broadens the potential application of inorganic/organic liquid crystalline polymer composite systems in complex soft devices.

3.3 Nanosheet doped composite

Two-dimensional nanomaterials can also be used as photosensitizers in photodeformable liquid crystalline polymers. Their lamellar or layered structure can have an impact on the mechanical properties and photoresponse behavior of the materials. Graphene is a typical two-dimensional nanomaterial, and its incorporation into polymer matrix can make it respond to near-infrared light[101]. However, the incompatibility of graphene with the polymer matrix and its tendency to self-aggregate often cause them to be too randomly distributed in the matrix. Weak interfacial adhesion often brings about uneven recombination, resulting in weak driving ability to the material, relatively small deformation and slow response[102].
The above problems can be solved by using liquid crystal monomer in-situ ultraviolet polymerization to prepare graphene/liquid crystal elastomer system and simultaneously carrying out hot stretching orientation[103]. Among them, the aromatic mesogens on the side chain of the polymer are beneficial to stabilize the graphene sheet and induce the uniform dispersion of graphene through the π π stacking between the aromatic rings and graphene. The stretching process can make the orientation of mesogens and graphene sheets have a certain cooperativity, so large shrinkage strain, high driving stress, high initial sensitivity and fast response are achieved in this composite system.
Graphene oxide is also an excellent photothermal conversion material, so it is often used in the preparation of reversible photoresponsive actuators. The hydrogen bonds between the hydroxyl and carboxyl groups in graphene oxide and the hydroxyl and ester groups in the polymer are beneficial to the dispersion of graphene oxide[103]. For example, PDLC films prepared by mixing graphene oxide and small molecular liquid crystals directly into polymer matrices have efficient optomechanical behavior controlled by near-infrared light, which has great application prospects in the development of micro-optomechanical systems[104]. Based on this strategy, the researchers have further fabricated an actuator with excellent mechanical strength and full-band response in the ultraviolet, visible, and near-infrared. This device can be rapidly deformed by sunlight and is suitable for smart devices driven by sunlight (Figure 9)[105].
It is found that the performance of graphene/liquid crystal polymer photoactuator can be adjusted by adjusting the hybridization degree of graphene oxide sp2, the type of liquid crystal unit, the length of spacer group, the matrix and the parameters of external light source, in addition to changing the orientation of dopant and liquid crystal by external force[103].

4 Conclusion and prospect

In this paper, the research progress of optically controlled actuators based on liquid crystal polymer composites is summarized. How to endow liquid crystal polymer materials with light manipulation properties by doping and the relationship between the microstructure of doped materials and the properties of the composite system are discussed. At present, the transition from liquid crystalline phase to isotropic phase in liquid crystalline polymers can be triggered by light energy with the help of special organic dyes and some inorganic nanocomponents. For composite systems functionalized with organic dyes, although their doping amount and compatibility can be adjusted on demand by chemical modification, the efficiency of optical and mechanical energy conversion has always been a key limiting factor for the further development and application of such materials. Therefore, efficient organic dyes need to be obtained by complex molecular structure design and massive chemical synthesis. For the composite system functionalized with inorganic components, the composite actuator based on the photothermal effect of metal or carbon nanomaterials shows stronger light absorption in a wide wavelength range, and can bring high optomechanical conversion efficiency at very low content. However, the aggregation of nanomaterials and the weak bonding with the polymer matrix are the main challenges in developing such systems. In addition, the shape and arrangement of nanomaterials also significantly affect the orientation of liquid crystals. In order to obtain more ideal optically controlled soft devices, combining the advantages of organic and inorganic functional components may be a reliable strategy for further development of optically controlled deformable liquid crystalline polymers with multiple functions.
As mentioned above, to date, a number of organic dyes and inorganic nanocomponents have proven to be effective photosensitizers for driving the deformation of liquid crystalline polymer materials. These studies have been heavily used to develop smart devices such as wireless soft robots (Figure 10)[20,52,106⇓⇓ ~109]. For example, Schenning et al. Reported a wireless soft transport robot prepared by assembling coiled liquid crystal polymer films containing different azobenzene dyes[108]. The robot can move in multiple directions under the manipulation of blue light (455 nm), pick up goods, and deliver the goods to the target location. In addition, integrating different photosensitizers into one device allows for discrete manipulation of different lights[106]. For example, Li et al. Combined different photochromophores and photothermal dyes to prepare a bionic soft robotic device, which can be camouflaged based on the synergistic modulation of deformation and luminescence[16]. Light manipulation can also work in concert with other external manipulation sources, enabling more complex functions in smart actuators. A recent study has shown that the integration of a flexible magnetically responsive PDMS column as a magnetically actuated rod and a light-responsive liquid crystal polymer as a light-controlled fixture can be used to design surface-anchored artificial aquatic coral polyps to mimic the complex movements of marine organisms in the water[107]. The device is driven by a magnetic field to attract a suspended target by stirring, and is controlled by light to capture or release the target. These integrated smart soft systems with synergistic function and motion can perform tasks that cannot be accomplished by single-component systems based on traditional liquid crystal polymers or other functional materials.
At present, the optically controlled deformable liquid crystal polymer system is mainly used to prepare smart soft devices with a size of centimeter or millimeter, and there are many directions for further research and development in the future. For example, how to develop smaller devices to perform special tasks in microscopic environments, such as micro- or nano-scale soft robots that can operate on designated cells or viruses; How to prepare large-size smart soft devices and wearable devices that can work in natural environment or even in outer space. In the development of programmable soft robots, how to realize the interaction between the movement of devices and the environment, and adapt to changes in the environment. In addition, the design, fabrication, manipulation, and application of smart soft devices based on optically controlled deformable liquid crystal polymers can be revolutionized by the controllable alignment of liquid crystals and high-precision spatial light modulation technology.

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Funding

National Natural Science Foundation of China(52003050)

Jiangsu Provincial Natural Science Foundation(BK20200343)

“Zhishan” Scholars Programs of Southeast University

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