Liquid Crystal Elastomers Research Articles

Experimental study on the dynamic mechanical response of a blue light-stimulated acrylate side chain liquid crystal elastomer.

This study demonstrates a breakdown analysis of the dynamics of a liquid crystal elastomer (LCE) including quality check, geometric measurement, thermal characterization, and comparison of heat- and light-induced contractions. A blue light-responsive acrylate side chain LCE with 1% azobenzene dye was characterized. From a classical viewpoint, photo-thermal contraction is considered a dominating effect, while direct photo-mechanical deformation can be neglected due to a low dye percentage. However, the findings of this research suggest that a low percentage of azobenzene dye does not necessarily lead to heat-dominating dynamics of LCE. This phenomenon has not yet been quantitatively studied before. The approach reported in this Letter can potentially be used to extract the data to improve the dynamics models of light-driven LCEs.

Liquid crystalline elastomers/glycerol-based solid polymer electrolytes with shape memory properties for Zn-ion battery applications

A new type of solid polymer electrolytes (SPEs) for zinc-ion batteries was fabricated by combining liquid crystalline elastomer (LCE) with glycerol. LCEs were selected for their flexibility and low transition temperatures. However, these materials exhibit a degree of crystallinity at ambient temperatures, limiting high ionic conductivity. Glycerol was introduced as both an antinucleating agent and plasticiser to reduce crystallinity and increase flexibility of this system. As a result, adding 15 wt% glycerol enhanced the ionic conductivity to 1.87 × 10−5 S cm−1 while maintaining stable charge-discharge cycles for 200 hrs. Besides, this modification reduced the nematic-isotropic transition temperature and storage modulus from 78 °C to 66 °C and 4.7 MPa to 0.6 MPa, respectively. Furthermore, these materials indicated excellent shape fixity and shape recovery of 98.3 % and 99.6 %. The successful fabrication of this LCE/glycerol system highlights its potential for developing shape memory SPE materials tailored for Zn-ion battery applications.

Creasing instability of polydomain nematic elastomers in compression

Polydomain liquid crystalline (nematic) elastomers exhibit unique mechanical properties such as soft elasticity, where the material largely deforms at nearly constant stress, due to microstructural evolution. In this paper, we numerically study the effect of such remarkable soft behavior on the surface instability of a half-space polydomain nematic elastomer, which is uniformly compressed parallel to the interface under a plane-strain condition. We compare the creasing instability of nematic elastomers with that of neo-Hookean elastomers by presenting bifurcation diagrams, stress and strain development in the elastomers, energy relaxation, and surface morphology at the creased state. Our results reveal that soft elasticity stabilizes nematic elastomers in plane-strain compression. Remarkably, the critical strain and stress at which the crease nucleates depend nonlinearly on the degree of anisotropy in nematic elastomers. Moreover, we find that the morphology of the creased surface in nematic elastomers exhibits the universal cusp shape previously observed in neo-Hookean elastomers.

Enhanced Fatigue Resistance of Liquid Crystal Elastomers Enabled by Dynamic Sacrificial Bonds

Enhanced Fatigue Resistance of Liquid Crystal Elastomers Enabled by Dynamic Sacrificial Bonds

Mechanochromic Displays Based on Photoswitchable Cholesteric Liquid Crystal Elastomers

Stimuli responsive optical materials are attractive for many areas, from healthcare to art design. However, creating intricate color‐changing patterns for visual information is still a challenge. This work describes the preparation of mechanochromic structural colored intricate pictures imprinted in cholesteric liquid crystal elastomers by using a chiral isosorbide molecular photo‐switch. The photo‐switch contains a photoisomerizable cinnamate moiety and was incorporated in a main chain liquid crystal oligomer with photopolymerizable acrylate end groups. After coating, the structural colored film was irradiated with ultraviolet (UV) light in air causing E/Z isomerization of the cinnamate units leading to a redshift of the structural color of the film. A grayscale photomask was used to spatially control the photoisomerization reaction and imprint colorful pictures such as portraits and landscapes, in the cholesteric liquid crystal films with high resolution. Photopolymerization in a nitrogen atmosphere led to a mechanochromic cholesteric liquid crystal elastomer with striking structural colors that blueshift upon strain independently. The sharp details of the patterns were preserved even under deformation and the system returned to the initial state upon strain removal. Our work offers a simple photo‐switch approach to prepare stimuli responsive optical polymers imprinted with color‐changing pictures of unprecedented complexity.

Mechanochromic Displays Based on Photoswitchable Cholesteric Liquid Crystal Elastomers.

Stimuli responsive optical materials are attractive for many areas, from healthcare to art design. However, creating intricate color-changing patterns for visual information is still a challenge. This work describes the preparation of mechanochromic structural colored intricate pictures imprinted in cholesteric liquid crystal elastomers by using a chiral isosorbide molecular photo-switch. The photo-switch contains a photoisomerizable cinnamate moiety and was incorporated in a main chain liquid crystal oligomer with photopolymerizable acrylate end groups. After coating, the structural colored film was irradiated with ultraviolet (UV) light in air causing E/Z isomerization of the cinnamate units leading to a redshift of the structural color of the film. A grayscale photomask was used to spatially control the photoisomerization reaction and imprint colorful pictures such as portraits and landscapes, in the cholesteric liquid crystal films with high resolution. Photopolymerization in a nitrogen atmosphere led to a mechanochromic cholesteric liquid crystal elastomer with striking structural colors that blueshift upon strain independently. The sharp details of the patterns were preserved even under deformation and the system returned to the initial state upon strain removal. Our work offers a simple photo-switch approach to prepare stimuli responsive optical polymers imprinted with color-changing pictures of unprecedented complexity.

Regulating Actuations and Shapes of Liquid Crystal Elastomers through Combining Dynamic Covalent Bonds with Cooling-Rate-Mediated Control

Regulating Actuations and Shapes of Liquid Crystal Elastomers through Combining Dynamic Covalent Bonds with Cooling-Rate-Mediated Control

Rejuvenating liquid crystal elastomers for self-growth

To date, only one polymer can self-grow to an extended length beyond its original size at room temperature without external stimuli or energy input. This breakthrough paves the way for significant advancements in untethered autonomous soft robotics, eliminating the need for the energy input or external stimuli required by all existing soft robotics systems. However, only freshly prepared samples in an initial state can self-grow, while non-fresh ones cannot. The necessity of synthesizing from monomers for each use imposes significant limitations on practical applications. Here, we propose a strategy to rejuvenate non-fresh samples to their initial state for on-demand self-growth through the synergistic effects of solvents and dynamic covalent bonds during swelling. The solvent used for swelling physically transforms the non-fresh LCEs from the liquid crystal phase to the isotropic phase. Simultaneously, the introduction of the transesterification catalyst through swelling facilitates topological rearrangements through exchange reactions of dynamic covalent bonds. The rejuvenation process can also erase growth history, be repeated several times, and be regulated by selective swelling. This strategy provides a post-modulation method for the rejuvenation and reuse of self-growing LCEs, promising to offer high-performance materials for cutting-edge soft growing robotics.

The interfacial behavior of a liquid crystal elastomer film bonded to an orthotropic substrate under light illumination

Bilayer soft actuators composed of a liquid crystal elastomer (LCE) film and a soft substrate have been widely used in soft robotics, bioengineering, and other fields. The existence of stress concentration at the interface edge easily leads to delamination damage of layered structures. Therefore, this article focuses on the distribution of interfacial stresses of the light-driven LCE film/orthotropic substrate system. The governing equations of the LCE film/substrate system are derived. The distributions of peeling and shear stresses on the interface are given and validated by using the finite element method (FEM). The effects of light illumination intensity, light attenuation distance and the director direction on the distribution of interfacial stresses are comprehensively analyzed. It is found that the signs of the shear stress and peeling stress depend on the director direction. This indicates that aligning the director in a certain direction may enhance the interface strength. The results may be useful for designing light-responsive LCE bilayer drivers.

13C CSA Tensors and Orientational Order of Model and Dimer Mesogens Comprising of Phenyl Benzoate.

A Model mesogen and its symmetrical Dimer made up of phenyl benzoate core unit are investigated by 13C NMR spectroscopy. The existence of layer order in smectic A and smectic C phases of Dimer mesogen is established by powder X-ray diffraction. The chemical shift anisotropy (CSA) tensors of Model mesogen are determined by 2D separation of undistorted powder patterns by effortless recoupling (SUPER) experiment and are utilized for calculating the order parameters employing the alignment-induced chemical shifts (AIS). Additionally, 2D separated local field (SLF) NMR is availed for extracting 13C-1H dipolar couplings for both mesogens and used for computing the order parameters. A good agreement in the order parameters calculated from 13C-1H dipolar couplings and AIS is observed. Accordingly, the main order parameter (Szz) for the phenyl rings of the Model mesogen is found to be in the range 0.54 - 0.82, and for the Dimer mesogen, the values span 0.64 - 0.82 across mesophases. Since the phenyl benzoate core unit is frequently employed structural moiety for constructing the main chain as well as side chain liquid crystalline polymers and liquid crystalline elastomers, the CSA tensors reported here will be of immense utility for the structural characterization of these materials.

Reconfigurable light-responsive liquid crystal elastomer /carbon nanotube nanocomposite actuators operated by photothermal effects and dynamic exchange reaction

Monodomain liquid crystal elastomers (MLCEs), exhibiting a light-induced dynamic exchange reaction (DER) and light-triggered actuation abilities, were prepared using dispersed multiwalled carbon nanotubes (MWCNTs) and allyl sulfide linkages incorporated in the MLCE matrix. MWCNTs were dispersed in the MLCE matrix using a newly synthesized dispersant, poly(4-cyanobiphenyl-4′-oxyundecylacrylate-co-pyrene methyl acrylate); this dispersant exhibited amphiphilic properties of π–π interactions with the MWCNT fillers (attributed to the pyrene moieties) and miscibility with the MLCE matrix (attributed to the cyanobiphenyl mesogenic moieties). A low amount of well-dispersed MWCNTs (≤0.05 wt%) in the MLCE matrix induced excellent photothermal effects under near-infrared (NIR) light irradiation, leading to considerable reversible contraction/extension actuation (∼40 % change in length). The bilayer film comprising the MLCE and PLCE (PLCE: polydomain LCE) composite films bonded via DER without glue exhibited excellent bending actuation under NIR light irradiation and was configured into several actuators such as a flower-shaped actuator, a weightlifter, a circular rolling band, an oscillator mimicking a hummingbird, and a casting catapult. Therefore, this novel MLCE/CNT composite film enhances the design ability for the arbitrary shaping and functionalization of NIR light-triggerable LCE actuators, which can be applied to several interesting soft robots.

Self-spinning of liquid crystal elastomer tubes under constant light intensity

Self-oscillating motion have the capacity to autonomously converting ambient power into repetitive motion without requiring an additional control unit, and designing more self-oscillating can broaden their utilization in energy extraction, robotic systems, and sensors. However, cyclic self-oscillating motions often cause structural instability and increase friction. To address these challenges, we creatively developed a zero-energy-mode self-spinning liquid crystal elastomer (LCE) tube-mass system under constant light intensity. By proposing a nonlinear dynamic model and using fourth-order Runge-Kutta method, the computational findings suggest that the LCE tube stays stationary when exposed to vertical light while develops into a zero-energy-mode self-spinning state under non-vertical light. The self-spinning state is self-sustained through harvesting ambient light energy, helping counteract the damping loss. In addition, the self-spinning frequency is controllable by tuning the light angle, contraction coefficient, light intensity, elastic modulus, radius, and damping coefficient. The translational damping has no impact on the self-spinning frequency, and the elastic modulus does not affect the X-axis displacement of the free end. The proposed self-spinning LCE tube system, differing from numerous existing self-oscillating systems, offers advantages like zero-energy-mode motion, structural simplicity, and controllability across multiple parameters, promising expanded design opportunities for applications such as motors, soft robotics, energy collectors, micro-machines, and beyond.

Self-rotation of a liquid crystal elastomer rod under constant illumination

Self-sustaining systems can generate and maintain periodic or chaotic motion under constant external stimulation, and has potential applications in fields such as soft robotics, energy harvesting, and active machinery. However, self-sustaining systems often come with excessive oscillations and increased friction, which limit their applications. Unlike oscillatory self-sustaining systems, we have developed a novel steadily self-sustaining system, which is composed of a liquid crystal elastomer rod and a support sleeve. Experiments demonstrate that the liquid crystal elastomer rod on a support sleeve can self-rotate steadily and continuously under constant small area illumination. During the self-rotation, the shape of the liquid crystal elastomer rod remains unchanged, which avoids excessive oscillations and decreases friction. Based on a photothermal-responsive liquid crystal elastomer model, we derived the lateral curvature and the actuating rotation moment. Numerical simulations reveal that the liquid crystal elastomer system balances damping dissipation during motion by absorbing heat converted from constant illumination. The angular velocity of the self-rotation is influenced by parameters such as heat flux, heat transfer coefficient, length of the rod, and damping rotation moment. The theoretical predictions match the experimental results. The novel steadily self-rotating system not only has the advantages of maintaining shape and reducing friction, but also offers benefits such as structural simplicity, small illumination area, and higher energy efficiency compared to other steadily self-sustaining systems driven by large illumination areas or hot surfaces. This research is anticipated to offer valuable insights for applications in soft robotics, energy harvesting, and active machinery.

A Light-Powered Self-Circling Slider on an Elliptical Track with a Liquid Crystal Elastomer Fiber.

In this paper, we propose an innovative light-powered LCE-slider system that enables continuous self-circling on an elliptical track and is comprised of a light-powered LCE string, slider, and rigid elliptical track. By formulating and solving dimensionless dynamic equations, we explain static and self-circling states, emphasizing self-circling dynamics and energy balance. Quantitative analysis reveals that the self-circling frequency of LCE-slider systems is independent of the initial tangential velocity but sensitive to light intensity, contraction coefficients, elastic coefficients, the elliptical axis ratio, and damping coefficients. Notably, elliptical motion outperforms circular motion in angular velocity and frequency, indicating greater efficiency. Reliable self-circling under constant light suggests applications in periodic motion fields, especially celestial mechanics. Additionally, the system's remarkable adaptability to a wide range of curved trajectories exemplifies its flexibility and versatility, while its energy absorption and conversion capabilities position it as a highly potential candidate for applications in robotics, construction, and transportation.

Cholesteric Liquid Crystal Elastomer Coatings with Brilliant Structural Colors and Mechanochromic Response Fabricated by Spray Deposition

Cholesteric Liquid Crystal Elastomer Coatings with Brilliant Structural Colors and Mechanochromic Response Fabricated by Spray Deposition

Mesogen Organizations in Nematic Liquid Crystal Elastomers Under Different Deformation Conditions.

Liquid crystal elastomers (LCEs) exhibit unique mechanical properties of soft elasticity and reversible shape-changing behaviors, and so serve as potentially transformative materials for various protective and actuation applications. This study contributes to filling a critical knowledge gap in the field by investigating the microscale mesogen organization of nematic LCEs with diverse macroscopic deformation. A polarized Fourier transform infrared light spectroscopy (FTIR) tester is utilized to examine the mesogen organizations, including both the nematic director and mesogen order parameter. Three types of material deformation are analyzed: uniaxial tension, simple shear, and bi-axial tension, which are all commonly encountered in practical designs of LCEs. By integrating customized loading fixtures into the FTIR tester, mesogen organizations are examined across varying magnitudes of strain levels for each deformation mode. Their relationships with macroscopic stress responses are revealed and compared with predictions from existing theories. Furthermore, this study reveals unique features of mesogen organizations that have not been previously reported, such as simultaneous evolutions of the mesogen order parameter and nematic director in simple shear and bi-axial loading conditions. Overall, the findings presented in this study offer significant new insights for future rational designs, modeling, and applications of LCE materials.

Easy Processable Photomechanical Thin Film Involving a Photochromic Diarylethene and a Thermoplastic Elastomer in Supramolecular Interaction.

A novel supramolecular photoactuator in the form of a thin film of centimetric size has been developed as an alternative to traditional liquid crystal elastomers (LCE) involving azobenzene (AZO) units or photochromic microcrystals. This thin film is produced through spin coating without the need for alignment or crosslinking. The photoactuator combines a photochromic dithienylethene (DTE) functionalized with ureidopyrimidinone (UPy) units, and a telechelic thermoplastic elastomer, also functionalized with UPy, allowing quadruple hydrogen bonding between the two components. Upon alternating ultraviolet (UV) and visible light exposure, the film exhibits reversible bending and color changes, studied using displacement and absorption tracking setups. For the first time, the photomechanical effect (PME) is quantitatively correlated with photochromism, showing that DTE units drive the movement under both UV (photocyclization) and visible (photoreversion) light. In situ illumination techniques reveal that the PME arises from photoinduced strain within 160 nm UPy-bonded DTE domains, which expand and contract by approximately 50% under UV and visible light, respectively. The semicrystalline nature of the elastomer and a robust supramolecular network connecting both components are critical in converting microscopic photostrain into macroscopic actuation.

Light-driven soft crawling robots capable of multidirectional locomotion and cargo transport

This work presents a liquid crystal elastomer (LCE)-based light-driven crawling robot capable of rapid multidirectional locomotion and cargo transport. The proposed LCE device consists of two small front legs, two large rear legs, and a concave cargo platform connecting the legs. The device’s locomotion, inspired by the principle of snake-like rectilinear movement, is generated by activating the rear legs with periodic near-infrared (NIR) illumination. The device’s structure was formed monolithically using a lithography-based molding process followed by an LCE stretching process for shape programming. To enhance NIR absorption efficiency, the rear end of the device was coated with a photothermal film consisting of acrylic-based black paint and liquid metal nanoparticles. The device’s speed, which exceeds two body lengths per minute, is among the fastest recorded in published LCE-based, light-driven crawling devices. Furthermore, the device can change the locomotion direction by controlling the actuation sequences of NIR illumination on the rear legs. This research also demonstrated and characterized the device’s load-carrying capability, as it was able to move at about half its maximum speed while carrying a load heavier than itself.

Hyperinelasticity. Part II: A stretch-based formulation

A generalisation of the hyperinelasticity modelling framework devised in Part I of this sequel is formulated here, by presenting a (principal) stretches-based hyperinelastic deformation energy function WF. This generalisation is based on the premise that the (principal) stretches λj may assume any arbitrary real-valued exponents, rather than being restricted to the prescriptive powers 2 and −2, as in principal invariants-based models. The motivation behind this extension is to reduce the overall number of model parameters and thereby increase the versatility of the application of the hyperinelasticity framework, as well as to provide a more universal model. The ensuing hyperinelastic model is then applied to a wide range of extant experimental datasets encompassing foams, glassy and semi-crystalline polymers, hydrogels and liquid crystal elastomers, over both elastic and inelastic deformation ranges including yield, softening and plateau, and hardening behaviours, under tensile and compressive deformations. Upon demonstrating the favourable simulation of the foregoing behaviours by the model, its application is then extended to account for other nuanced aspects of inelasticity such as the effects of rate of deformation, crystallinity volume and angle of printing in 3D printed lattice structures. This augmentation is done via devising a generalised modelling framework which allows for the incorporation of a generic tensorial (including rank zero scalar) field of inelasticity-inducing factors into the core model, resulting in the model parameters to evolve with an appropriate measure of the factor of interest; e.g., deformation rate, crystallinity volume ratio etc. The proposed modelling framework will be shown to capture these effects proficiently. Given the simplicity of this modelling approach, as essentially an extension in the application of hyperelasticity, its versatility of implementation, and the favourable capturing of both elastic and inelastic behaviours, the devised hyperinelasticity framework is presented for application to the large elastic and inelastic deformation of polymers and elastomers.

Fundamental Aspects of Stretchable Mechanochromic Materials: Fabrication and Characterization.

Mechanochromic materials provide optical changes in response to mechanical stress and are of interest in a wide range of potential applications such as strain sensing, structural health monitoring, and encryption. Advanced manufacturing such as 3D printing enables the fabrication of complex patterns and geometries. In this work, classes of stretchable mechanochromic materials that provide visual color changes when tension is applied, namely, dyes, polymer dispersed liquid crystals, liquid crystal elastomers, cellulose nanocrystals, photonic nanostructures, hydrogels, and hybrid systems (combinations of other classes) are reviewed. For each class, synthesis and processing, as well as the mechanism of color change are discussed. To enable materials selection across the classes, the mechanochromic sensitivity of the different classes of materials are compared. Photonic systems demonstrate high mechanochromic sensitivity (Δnm/% strain), large dynamic color range, and rapid reversibility. Further, the mechanochromic behavior can be predicted using a simple mechanical model. Photonic systems with a wide range of mechanical properties (elastic modulus) have been achieved. The addition of dyes to photonic systems has broadened the dynamic range, i.e., the strain over which there is an optical change. For applications in which irreversible color change is desired, dye-based systems or liquid crystal elastomer systems can be formulated. While many promising applications have been demonstrated, manufacturing uniform color on a large scale remains a challenge. Standardized characterization methods are needed to translate materials to practical applications. The sustainability of mechanochromic materials is also an important consideration.