Chiral Liquid Crystalline Elastomer for Twisting Motion without Preset Alignment of Mesogens

Design, Synthesis, and Performance of Photo-Responsive Liquid Crystal Polymers with Stepwise Deformation Capability

Photo-responsive liquid crystal polymers (LCPs) have potential application value in flexible robots, artificial muscles, and microfluidic control. In recent years, significant progress has been made in the development of LCPs. However, the preparation of LCPs with continuous and controllable stepwise deformation capabilities remains a challenge. In this study, visible photo-responsive cyanostilbene monomer, UV photo-responsive azobenzene monomer, and multiple hydrogen bond crosslinker are used to prepare photo-responsive LCPs capable of achieving continuously and controllable stepwise deformation. The comprehensive investigation of the multiple light response ability and photo-induced deformation properties of these copolymers is conducted. The results reveal that in the first stage of photo-induced deformation under 470 nm blue light irradiation, the deformation angle decreases with a reduction in cyanostilbene content in the copolymer component, ranging from 40° in AZ0-CS4 to 0° in AZ4-CS0. In the second stage of photo-induced deformation under 365 nm UV irradiation, the deformation angle increases with the increase of azobenzene content, ranging from 0° of AZ0-CS4 to 89.4° of AZ4-CS0. Importantly, the deformation between these two stages occurs as a continuous process, allowing for a direct transition from the first-stage to the second-stage deformation by switching the light source from 470 to 365 nm.

Liquid Crystal Elastomer Artificial Tendrils with Asymmetric Core-Sheath Structure Showing Evolutionary Biomimetic Locomotion

The sophisticated and complex haptonastic movements in response to environmental-stimuli of living organisms have always fascinated scientists. However, how to fundamentally mimic the sophisticated hierarchical architectures of living organisms to provide the artificial counterparts with similar or even beyond-natural functions based on the underlying mechanism remains a major scientific challenge. Here,  liquid crystal elastomer (LCE) artificial tendrils showing evolutionary biomimetic locomotion are developed following the structure-function principle that is used in nature to grow climbing plants. These elaborately designed tendril-like LCE actuators possess an asymmetric core-sheath architecture which shows a higher-to-lower transition in the degree of LC orientation from the sheath-to-core layer across the semi-ellipse cross-section. Upon heating and cooling, the LCE artificial tendril can undergo reversible tendril-like shape-morphing behaviors, such as helical coiling/winding, and perversion. The fundamental mechanism of the helical shape-morphing of the artificial tendril is revealed by using theoretical models and finite element simulations. Besides, the incorporation of metal-ligand coordination into the LCE network provides the artificial tendril with reconfigurable shape-morphing performances such as helical transitions and rotational deformations. Finally, the abilities of helical and rotational deformations are integrated into a new reprogrammed flagellum-like architecture to perform evolutionary locomotion mimicking the haptonastic movements of the natural flagellum.

Defected twisted ring topology for autonomous periodic flip-spin-orbit soft robot

Periodic spin-orbit motion is ubiquitous in nature, observed from electrons orbiting nuclei to spinning planets orbiting the Sun. Achieving autonomous periodic orbiting motions, along circular and noncircular paths, in soft mobile robotics is crucial for adaptive and intelligent exploration of unknown environments-a grand challenge yet to be accomplished. Here, we report leveraging a closed-loop twisted ring topology with a defect for an autonomous soft robot capable of achieving periodic spin-orbiting motions with programmed circular and re-programmed irregular-shaped trajectories. Constructed by bonding a twisted liquid crystal elastomer ribbon into a closed-loop ring topology, the robot exhibits three coupled periodic self-motions in response to constant temperature or constant light sources: inside-out flipping, self-spinning around the ring center, and self-orbiting around a point outside the ring. The coupled spinning and orbiting motions share the same direction and period. The spinning or orbiting direction depends on the twisting chirality, while the orbital radius and period are determined by the twisted ring geometry and thermal actuation. The flip-spin and orbiting motions arise from the twisted ring topology and a bonding site defect that breaks the force symmetry, respectively. By utilizing the twisting-encoded autonomous flip-spin-orbit motions, we showcase the robot's potential for intelligently mapping the geometric boundaries of unknown confined spaces, including convex shapes like circles, squares, triangles, and pentagons and concaves shapes with multi-robots, as well as health monitoring of unknown confined spaces with boundary damages.

Unraveling Dynamic Helicity Inversion and Chirality Transfer through the Synthesis of Discrete Azobenzene Oligomers by an Iterative Exponential Growth Strategy

Unraveling the chirality transfer mechanism of polymer assemblies and controlling their handedness is beneficial for exploring the origin of hierarchical chirality and developing smart materials with desired chiroptical activities. However, polydisperse polymers often lead to an ambiguous or statistical evaluation of the structure-property relationship, and it remains unclear how the iterative number of repeating units function in the helicity inversion of polymer assemblies. Herein, we report the macroscopic helicity and dynamic manipulation of the chiroptical activity of supramolecular assemblies from discrete azobenzene-containing oligomers (azooligomers), together with the helicity inversion and morphological transition achieved solely by changing the iterative chain lengths. The corresponding assemblies also differ from their polydisperse counterparts in terms of thermodynamic properties, chiroptical activities, and morphological control.

Steerable and Agile Light-Fueled Rolling Locomotors by Curvature-Engineered Torsional Torque

On-demand photo-steerable amphibious rolling motions are generated by the structural engineering of monolithic soft locomotors. Photo-morphogenesis of azobenzene-functionalized liquid crystal polymer networks (azo-LCNs) is designed from spiral ribbon to helicoid helices, employing a 270° super-twisted nematic molecular geometry with aspect ratio variations of azo-LCN strips. Unlike the intermittent and biased rolling of spiral ribbon azo-LCNs with center-of-mass shifting, the axial torsional torque of helicoid azo-LCNs enables continuous and straight rolling at high rotation rates (≈720 rpm). Furthermore, center-tapered helicoid structures with wide edges are introduced for effectively accelerating photo-motilities while maintaining directional controllability. Irrespective of surface conditions, the photo-induced rotational torque of center-tapered helicoid azo-LCNs can be transferred to interacting surfaces, as manifested by steep slope climbing and paddle-like swimming multimodal motilities. Finally, the authors demonstrate continuous curvilinear guidance of soft locomotors, bypassing obstacles and reaching desired destinations through real-time on-demand photo-steering.

Multiple Shape Manipulation of Azobenzene-Containing Polyimide by Combining Shape Memory Effect, Photofixity, and Photodeformation

The integration of different shape manipulation could greatly expand the versatility and functionality of smart materials, for which the achievement of synergism of different shape control is crucial. Here, we seek to create one kind of polyimide with integrated multiple shape manipulations by constructing the chemical network bearing azobenzene as a side chain. Trifunctional cross-linkers serving as net points of the chemical network render polyimide thermal-induced shape memory effects, which enables shape transformation. Azobenzene as a photoresponsive group is employed to achieve the photofixity and reversible photodeformability. Such photosensitive behaviors are independent of molecular prealignment and remain available after thermally shaping and fixing. As a result, these noninterfering performances induced by heat and light allow us to arbitrarily combine them to meet different needs. By integrating different shape manipulations, various shape changes and functional execution are conveniently achieved. The combination of the shape memory effect with photofixity enables the setting of diverse shapes, while the merging of it with reversible deformation facilitates the construction of actuators capable of executing functions. This study provides a new approach for the preparation of multifunctional actuators and has potential applications in the field of intelligent drivers.

Recent Advances in 4D Printing of Liquid Crystal Elastomers

Liquid crystal elastomers (LCEs) are renowned for their large, reversible, and anisotropic shape change in response to various external stimuli due to their lightly cross-linked polymer networks with an oriented mesogen direction, thus showing great potential for applications in robotics, bio-medics, electronics, optics, and energy. To fully take advantage of the anisotropic stimuli-responsive behaviors of LCEs, it is preferable to achieve a locally controlled mesogen alignment into monodomain orientations. In recent years, the application of 4D printing to LCEs opens new doors for simultaneously programming the mesogen alignment and the 3D geometry, offering more opportunities and higher feasibility for the fabrication of 4D-printed LCE objects with desirable stimuli-responsive properties. Here, the state-of-the-art advances in 4D printing of LCEs are reviewed, with emphasis on both the mechanisms and potential applications. First, the fundamental properties of LCEs and the working principles of the representative 4D printing techniques are briefly introduced. Then, the fabrication of LCEs by 4D printing techniques and the advantages over conventional manufacturing methods are demonstrated. Finally, perspectives on the current challenges and potential development trends toward the 4D printing of LCEs are discussed, which may shed light on future research directions in this new field.

Chiroptical Elastomer Film Constructed by Chiral Helical Substituted Polyacetylene and Polydimethylsiloxane: Multiple Stimuli Responsivity and Chiral Amplification

Chiral and circularly polarized luminescence (CPL) materials with multiple stimuli responses have become a focus of attention. Meanwhile, elastomers have found substantial applications in a wide variety of fields. However, how to design and construct chiral elastomers, in particular CPL-active elastomers, still remains an academic challenge. In the present study, chiral helical substituted polyacetylene is chemically bonded with polydimethylsiloxane (PDMS) by hydrosilylation to form a chiroptically active elastomer. A CPL-active film was further fabricated by adding achiral fluorophores. Compared with the corresponding chiral helical polymer, the chiral films show much enhanced thermal stability in terms of chiroptical properties. The films also demonstrate reversible tunability in optical activity and CPL property when being subjected to a stretching-restoring process and exposed to a solvent like toluene. Further, noticeable chiral amplification is observed when the chiral PDMS film is superimposed with a pure PDMS film. This interesting finding is proposed to be due to the photoreflectivity of PDMS. This study provides an alternative strategy to exploit novel CPL-active elastomer materials with multiple stimuli responsivity and tunability, which may open up new opportunities for developing novel chiroptical devices.

Force-Induced Synergetic Pigmentary and Structural Color Change of Liquid Crystalline Elastomer with Nanoparticle-Enhanced Mechanosensitivity

The ability of some animals to rapidly change their colors can greatly improve their chances of escaping predators or hunting prey. A classic example is cephalopods, which can rapidly shift through a wide range of colors. This ability is based on the synergetic effect of the change of pigmentary and structural colors exhibited by their own two categories of color-changing cells: supernatant chromatophores offer various pigmentary colors and lower iridophores or leucophores reflect the different structural colors by adjusting their periodicities. Here, a mechanochromic liquid crystalline elastomer with force-induced synergetic pigmentary and structural color change, whose mechanosensitivity is enhanced by the stress-concentration induced by the doped nanoparticle, is presented. The materials have a large color-changing gamut and high mechanochromic sensitivity, which exhibit great potential in the field of mechanical detectors, sensors, and anti-counterfeiting materials.

Controlling the Multiple Chiroptical Inversion in Biphasic Liquid-Crystalline Polymers

While controlling the chirality and modulating the helicity is a challenging task, it attracts great research interest for gaining a better understanding of the origin of chirality in nature. Herein, structurally similar azobenzene (Azo) vinyl monomers were designed in which the alkyl chains comprised the chiral stereocenter with different achiral tail lengths. Combining the synchronous polymerization, supramolecular stacking and self-assembly, the multiple chiroptical inversion of the Azo-polymer supramolecular assemblies can be modulated by the tail length and DP of Azo blocks during in situ polymerization. The DP-, UV light-, temperature-, aging time-dependent chiroptical properties and liquid-crystalline (LC) characterization indicated that the amorphous-to-LC phase transition and biphasic LC interconversion allow the transcription of intra-chain π-π stacking, inter-chain H- and J-aggregation, thereby controlling the dynamic multiple reversal of supramolecular chirality.

Engineering Achiral Liquid Crystalline Polymers for Chiral Self-Recovery

Flexible construction of permanently stored supramolecular chirality with stimulus-responsiveness remains a big challenge. Herein, we describe an efficient method to realize the transfer and storage of chirality in intrinsically achiral films of a side-chain polymeric liquid crystal system by combining chiral doping and cross-linking strategy. Even the helical structure was destroyed by UV light irradiation, the memorized chiral information in the covalent network enabled complete self-recovery of the original chiral superstructure. These results allowed the building of a novel chiroptical switch without any additional chiral source in multiple types of liquid crystal polymers, which may be one of the competitive candidates for use in stimulus-responsive chiro-optical devices.

Helical Self-Assembly of Amphiphilic Chiral Azobenzene Alternating Copolymers

Imposing chirality to supramolecular architectures is an important step forward toward understanding and utilization of chiral nanomaterials. This article reports the self-assembly of amphiphilic chiral alternating copolymers of poly(binaphthyl azobenzene-alt-hexaethylene glycol) (P(BNPAzo-alt-EG₆)) into helical supramolecular rods. Unlike conventional chiral assembly of copolymers largely through intermolecular organization, the intrachain stacking of chiral units along the main chain into single molecular micelles with amplified axial chirality of binaphthyl is key to the formation of helical supramolecular rods, which takes advantage of the particular chiral unit and soft unit alternating topological structure of the backbones. Moreover, the supramolecular self-assembly is light reversible because the azobenzene rings in the backbone scarcely execute trans- to cis-isomerization upon UV irradiation, and therefore the supramolecular rods keep their sublevel chirality even though the helical appearance was destroyed. This work paves an effective route to construct and regulate chiral supramolecular architectures and reveals an insight into natural and artificial chiral self-assembly.

Induction, fixation and recovery of self-organized helical superstructures in achiral liquid crystalline polymer

A flexible chiral storage based on an achiral polymer system can be successfully achieved by chiral doping and covalent cross-linking.