Monodomain Liquid Crystal Elastomers Research Articles

Multiple shapes from a single nematic elastomer sheet activated via patterned illumination

Liquid crystal elastomers (LCEs) undergo a large uniaxial contraction upon thermal or optical stimulation. LCE sheets are often fabricated with a spatially patterned direction of contraction, which can sculpt the sheet into a Gauss-curved surface. Here, we instead consider LCE sheets subject to patterned stimulation intensity, leading to a control of contraction strength. We show such patterns may also sculpt a complex surface, but with the advantage that arbitrarily many surfaces may be achieved sequentially in the same sample, thus breaking the link between microstructure and shape. We first consider a monodomain LCE in which some regions are actuated and others are not. We discuss how to join actuated and unactuated regions compatibly, and use this design rule to generate patterns for cones, anti-cones, arrays of cones and a rolling bi-strip. We validate the patterns numerically via elastic shell simulations and demonstrate them experimentally via patterned photo-chemical actuation. Secondly, we consider an LCE disk with an azimuthal director profile actuated by a radially varying stimulus. We show, theoretically and numerically, how to design a stimulation profile to sculpt any surface of revolution. Such re-configurable actuation offers enticing possibilities for haptics, robotics and locomotion.

Multisensory Flexible Braille Interactive Device Based on Liquid Crystal Elastomers

Traditional electronic braille devices’ rigid and complex structure limits their portability while satisfying braille output. Main-chain monodomain liquid crystal elastomers (m-LCEs) can achieve high contraction deformation under external stimulation conditions; it has broad application prospects in the human–machine interaction with the biological tissue safety and flexibility of LCEs. A multisensory flexible braille interactive device based on LCEs is proposed in this article. It utilizes the good reversible shape-memory performance of m-LCEs to solve the problems of complex structure design, poor portability, high cost, and low control stability in traditional rigid mechanical braille readers. The fixed-point stretchable convex lattice and the nonactuation performance-based film are accurately distinguished and fabricated by compression molding. Combined with the ultrathin flexible electric heating layer, the reversible contraction of the braille point is realized, and the braille point is given the dual perception of height difference and temperature difference. Moreover, matching circuit design can realize braille input by press and braille output by convex–concave conversion. Furthermore, we try to introduce thermochromic powders, referring to the principle of light-emitting diode (LED) imaging, to transform the pattern concept of visually impaired people in minds into color information that ordinary people can observe.

A High-Fidelity Preparation Method for Liquid Crystal Elastomer Actuators.

Three-dimensional (3D) structural actuators based on monodomain liquid crystal elastomers (mLCEs) show a wide range of potential applications. A direct ink writing technique has been developed to print LCE structures. It is still a challenge to print high-precision 3D-mLCE actuators. Here, a method of wet 3D printing combined with freeze-drying is proposed. The coagulation bath is designed to restrain the nascent fiber disturbance of the capillary wave and weight by adjusting the ink viscosity and printing speed to control the LC molecular order, enabling uniform (B = 1.02) fibers with a high degree of orientational alignment (S = 0.45) of the mesogens. Furthermore, dynamic disulfide bond formation was used as the cross-linking point, which can allow the LCE network structure to be continuously cured to ensure adjacent layers are effectively bonded and, in combination with freeze-drying, produce the 3D-mLCE actuators of fidelity architecture (98.37 vol %) by printing. The actuators have excellent actuating strain (45.12%), and the dynamic disulfide bond makes them programmable. Finally, a printed bionic starfish and a printed bionic hand can easily grab regular and irregular objects. This work provides a feasible scheme for fabricating complex 3D-mLCEs with reversible changes in shape.

Effects of network structure of main-chain liquid crystal elastomer on its thermal actuation performance

The effect of the network structure on the thermal actuation of the main-chain liquid crystal elastomers (LCEs) was studied by controlling the strand length and crosslinking density of the network. The LCEs were prepared with the thiol-terminated oligomers synthesized via a thiol-acrylate Michael addition reaction between dithiol and diacrylate monomers. The LCE network structure was formed by crosslinking the oligomers using the pentaerythritol tetraacylate crosslinkers and coupling between the thiol end groups in the oligomer to form disulfide linkages. The monodomain LCE (MLCE) films were fixed by the dynamic exchange reaction between disulfide linkages through ultraviolet irradiation after drawing the polydomain LCE films. The maximum degree of actuation was obtained when 50% of the available thiol groups were crosslinked with the acrylate groups of the crosslinkers. The degree of actuation increased with the oligomer length for sufficiently high crosslinking density. However, the degree of actuation decreased with the oligomer length after reaching the maximum especially for low crosslinker density because crosslinking with long oligomers did not provide enough crosslinking sites and disulfide linkages. Thus, the oligomer length and crosslinking density of the MLCE are important control factors during LCE preparation to obtain the best actuation performance of the MLCE film.

Stress-Induced Self-Alignment Monodomain Liquid Crystal Elastomer Fiber

Stress-Induced Self-Alignment Monodomain Liquid Crystal Elastomer Fiber

Three‐state Structural Switching and Selective Molecular Interactions of Cylindrical Concentric Monodomain Liquid Crystal Elastomer

AbstractA cross‐linked poly(acrylate) containing an ethynylhelicene (P)‐pentamer was synthesized by radical polymerization between two horizontally aligned films with a parallel arrangement in trifluoromethylbenzene. The resulting liquid crystal elastomer swollen in bromobenzene showed the Maltese cross, which indicated a cylindrical concentric monodomain structure of the millimeter‐order size as determined by polarized optical microscopy (POM) analysis. In toluene, a homogeneously bright POM image was obtained, which reversibly changed in intensity upon heating and cooling. Addition of an (M)‐tetramer to the liquid crystal elastomer in toluene produced a hetero‐double‐helix exhibiting the Maltese cross, whose intensity did not change upon heating and cooling. The three states can be switched by addition and removal of an (M)‐tetramer and solvents.

Soft elasticity optimises dissipation in 3D-printed liquid crystal elastomers

Soft-elasticity in monodomain liquid crystal elastomers (LCEs) is promising for impact-absorbing applications where strain energy is ideally absorbed at constant stress. Conventionally, compressive and impact studies on LCEs have not been performed given the notorious difficulty synthesizing sufficiently large monodomain devices. Here, we use direct-ink writing 3D printing to fabricate bulk (>cm3) monodomain LCE devices and study their compressive soft-elasticity over 8 decades of strain rate. At quasi-static rates, the monodomain soft-elastic LCE dissipated 45% of strain energy while comparator materials dissipated less than 20%. At strain rates up to 3000 s−1, our soft-elastic monodomain LCE consistently performed closest to an ideal-impact absorber. Drop testing reveals soft-elasticity as a likely mechanism for effectively reducing the severity of impacts – with soft elastic LCEs offering a Gadd Severity Index 40% lower than a comparable isotropic elastomer. Lastly, we demonstrate tailoring deformation and buckling behavior in monodomain LCEs via the printed director orientation.

Unusual stress and strain concentration behaviors at the circular hole of a large monodomain liquid crystal elastomer sheet

Liquid crystal elastomers combine the hyperelasticity of elastomers with the multi-functionality of liquid crystals and have emerged as an important class of soft active materials. Monodomain liquid crystal elastomers under loading exhibit the soft elastic behavior due to the stress induced director rotation and the resulting spontaneous strain. Here, we numerically study their stress and strain concentration behavior by considering the classical example, a large sheet with a small circular hole in the middle under uniaxial loading. The concentration behavior is found to be very different from regular elastomers. Firstly, the concentration factors are much bigger at both small and large strains. Secondly, the locations of the strain concentration may not coincide with that of the stress concentration. Detailed analysis of the director rotation and the resulting spontaneous strain around the free edge of the hole are shown to be the main causes for the unusual concentration behavior. Moreover, under a given strain, the stress level of the LCE sample, and therefore the free energy, is slightly lower than that of the neo-Hookean material, while the local free energy density on the hole edge is much bigger due to the severer concentrations. By considering material parameters obtained from various samples of polysiloxane and polyacrylate side-chain nematic elastomers, we find that their stress and strain concentration behaviors are qualitatively similar but quantitatively quite different. As a result, at a prescribed strain, the samples with a larger maximal spontaneous strain has severer stress and strain concentrations. The stress concentration factor difference scaled by a coupling constant that combines the effect of the two material parameters, r and a, increases as the semi-soft coefficient a decreases. Similar results are found for the scaled maximal spontaneous strains and the scaled strain concentration factor difference at small strains.

Programmable Mechanical Energy Absorption and Dissipation of Liquid Crystal Elastomers: Modeling and Simulations

Liquid crystal elastomer (LCE) is a special polymer material that combines the properties of rubbers and liquid crystals. It can exhibit soft elasticity with stress plateau and hysteresis due to the mechanical‐director coupling and may be utilized in mechanical energy absorption and dissipation. Herein, some numerical results based on the continuum model of LCE are presented. Using COMSOL, the highly anisotropic loading–unloading behavior of monodomain LCE samples is studied in some detail. When the loading axis is close to the initial LC alignment, the mechanical behavior of the LCE is similar to regular rubber, hyperelastic with weak rate dependences. However, if the loading axis is far from the initial LC alignment, stripe domains appear with apparent stress plateau and a large loading–unloading hysteresis. Consequently, rather big residual strains and stresses can be observed after unloading. Such samples have much stronger rate dependence, much higher efficiency of energy absorptions, and much bigger mechanical energy damping capacity. Thus, it is important to program the initial LC alignment according to the external loading directions to maximize the mechanical energy absorption and dissipation of LCEs.

Liquid crystal elastomers wrinkling

When a liquid crystal elastomer layer is bonded to an elastic layer, it creates a bilayer with interesting properties that can be activated by applying traction at the boundaries or by optothermal stimulation. Here, we examine wrinkling responses in three-dimensional nonlinear systems containing a monodomain liquid crystal elastomer layer and a homogeneous isotropic incompressible hyperelastic layer, such that one layer is thin compared to the other. The wrinkling is caused by a combination of mechanical forces and external stimuli. To illustrate the general theory, which is valid for a range of bilayer systems and deformations, we assume that the nematic director is uniformly aligned parallel to the interface between the two layers, and that biaxial forces act either parallel or perpendicular to the director. We then perform a linear stability analysis and determine the critical wave number and stretch ratio for the onset of wrinkling. In addition, we demonstrate that a plate model for the thin layer is also applicable when this is much stiffer than the substrate.

Large-Size Honeycomb-Shaped and Iris-Like Liquid Crystal Elastomer Actuators

Large-Size Honeycomb-Shaped and Iris-Like Liquid Crystal Elastomer Actuators

Body‐temperature shape‐shifting liquid crystal elastomers

AbstractNematic monodomain liquid crystal elastomers (LCEs) undergo efficient temperature‐induced reversible shape‐shifting around the nematic‐isotropic transition temperature (Tni) due to the presence of the liquid‐crystalline order of mesogens. Usually, the Tni of nematic LCEs is much higher than the human body temperature, and therefore LCEs are not often considered for biomedical applications. This study describes an LCE system where the Tni is tuned by substitution of the rigid mesogens RM257 with a flexible backbone PEGDA250. By systematically substituting the RM257 with PEGDA250, the Tni of LCEs was observed to decrease from 66°C to 23°C. A rate‐optimized LCE material was fabricated with 10 mol % rigid mesogens substituted with a flexible backbone that demonstrated Tni at 32°C, in‐between the room temperature of 20°C and the body temperature of 37°C. The Tni allowed the programmed shape at room temperature, quick shape‐shifting upon exposure to body temperature, and before‐programmed shape when kept at body temperature. This LCE material displayed reversible length change of 23%, opacity change, and shape change between room temperature and body temperature.

Positive/Negative Phototropism: Controllable Molecular Actuators with Different Bending Behavior

Herein, a series of molecular actuators based on the crystals of (E)-2-(4-fluorostyryl)benzo[d]oxazole (BOAF4), (E)-2-(2,4-difluorostyryl)benzo[d]oxazole (BOAF24), (E)-2-(4-fluorostyryl)benzo[d]thi...

Effect of gold and graphene oxide nanoparticles on the thermo- and photo-actuation of monodomain liquid crystal elastomers

Monodomain liquid crystal elastomers (m-LCEs) were synthesized from polymethylhydrosiloxane (PMHS), two distinct vinyl-functionalized mesogens (benzoate and azobenzene) and one crosslinking monomer, using the hydrosilylation reaction. Gold or amino-functionalized graphene oxide nanoparticles (NP) were in-situ incorporated into the m-LCEs, aiming to enhance their response to thermal and optical stimuli. All m-LCEs exhibited a degradation temperature of ~310 °C and a mesomorphic behavior over a wide temperature range (clearing temperature around 100 °C). The inclusion of nanoparticles stiffened the m-LCEs, although they induced a highly randomized ordering of mesogens at the nematic to isotropic transition, resulting in a higher specimen contraction; a schematic model is proposed in this concern. The photo-actuation (bending) of the m-LCEs/NP composites resulted rather diminished as they became stiffer than the neat m-LCEs, although the increasing content of azobenzene units allowed higher bending angles that could be suitable for remote actuation applications.

Biocompatible liquid-crystal elastomers mimic the intervertebral disc

The hierarchical and anisotropic mechanical behavior requirement of load-bearing soft tissues limits the utility of conventional elastomeric materials as a replacement for soft-tissue materials. Liquid-crystal elastomers (LCEs) have the potential to excel in this regard owing to its unique combination of mesogenic order in an elastomeric network. In this study, the mechanical behavior of the LCEs relevant to load-bearing biomedical applications was explored. LCEs with different network orientations (i.e., mesogen alignments) were investigated by fabricating the LCEs with polydomain and monodomain configurations. The polydomain and monodomain LCEs with the same degree of network crosslinking demonstrated diverse mechanical behavior, ranging from highly stiff and elastic nature to high damping capacity, depending on the loading direction with respect to the network alignment. The LCEs were also capable of matching the anisotropic mechanical behavior of an intervertebral disc. Additional studies were conducted on the in vivo biological response of LCEs upon subcutaneous implantation, as well as on the effect of the exposure to an in vitro simulated physiological environment on the mechanical behavior. The LCEs’ mechanical response was negligibly affected when exposed to biomedically relevant conditions. Furthermore, the solid and porous LCEs did not show any adverse effect on the surrounding tissues when implanted subcutaneously in rats. The biological response allows for tissue ingrowth and helps illustrate their utility in implantable biological devices. Finally, the utility of LCEs to mimic the mechanical function of biological tissue such as intervertebral disc was demonstrated by fabricating a proof of concept total disc replacement device.

Liquid Crystal Elastomer Waveguide Actuators.

While most photomechanical materials developed to date have relied on free-space illumination to drive actuation, this strategy fails when direct line-of-site access is precluded. In this study, waveguided light is harnessed by liquid crystal elastomer (LCE) nanocomposites to drive actuation. Using photo-chemical reduction of gold salts to plasmonic nanoparticles, prescription of photoresponsive regions within fibers of mono-domain LCEs is demonstrated with control over both the location along the fiber axis, as well as in the azimuthal direction. Due to localized photothermal heating provided by plasmonic absorption of waveguided light and resulting inhomogeneous thermally induced deformation of the LCE, reversible bending along multiple axes is demonstrated.

Finsler geometry modeling and Monte Carlo study of liquid crystal elastomers under electric fields

The shape transformation of liquid crystal elastomers (LCEs) under external electric fields is studied through Monte Carlo simulations of models constructed on the basis of Finsler geometry (FG). For polydomain side-chain-type LCEs, it is well known that the external-field-driven alignment of the director is accompanied by an anisotropic shape deformation. However, the mechanism of this deformation is quantitatively still unclear in some part and should be studied further from the microscopic perspective. In this paper, we evaluate whether this shape deformation is successfully simulated, or in other words, quantitatively understood, by the FG model. The FG assumed inside the material is closely connected to the directional degrees of freedom of LC molecules and plays an essential role in the anisotropic transformation. We find that the existing experimental data on the deformations of polydomain LCEs are in good agreement with the Monte Carlo results. It is also found that experimental diagrams of strain versus external voltage of a monodomain LCE in the nematic phase are well described by the FG model.

A cut-and-paste strategy towards liquid crystal elastomers with complex shape morphing

In this work, monodomain liquid crystal elastomer films with exchangeable disulfide crosslinkers are cut into pieces and pasted together through dynamic disulfide exchange to form versatile shaped soft actuator materials.

Thermomechanical properties of monodomain nematic main-chain liquid crystal elastomers.

Two-stage thiol-acrylate Michael addition reactions have proven useful in programming main-chain liquid crystal elastomers (LCEs). However, the influence of excess acrylate concentration, which is critical to monodomain programming, has not previously been examined with respect to thermomechanical properties in these two-stage LCEs. Previous studies of thiol-acrylate LCEs have focused on polydomain LCEs and/or variation of thiol crosslinking monomers or linear thiol monomers. This study guides the design of monodomain LCE actuators using the two-stage methodology by varying the concentration of mesogenic acrylate monomers from 2 mol% to 45 mol% in stoichiometric excess of thiol. The findings demonstrate a technique to tailor the isotropic transition temperature by 44 °C using identical starting monomers. In contrast to expectations, low amounts of excess acrylate showed excellent fixity (90.4 ± 2.9%), while high amounts of excess acrylate did not hinder actuation strain (87.3 ± 2.3%). Tensile stress-strain properties were influenced by excess acrylate. Linear elastic behavior was observed parallel to the director with modulus increasing from 1.4 to 6.1 MPa. The soft elastic plateau was observed perpendicular to the director with initial modulus and threshold stresses increasing from 0.6 MPa to 2.6 MPa and 14 kPa to 208 kPa, respectively. Overall, this study examines the influence of excess acrylate on mechanical properties of LCE actuators.

Adaptable liquid crystal elastomers with transesterification-based bond exchange reactions.

Adaptable liquid crystal elastomers (LCEs) have recently emerged to provide a new and robust method to program monodomain LCE samples. When a constant stress is applied with active bond exchange reactions (BERs), polymer chains and mesogens gradually align in the strain direction. Mesogen alignment is maintained after removing the BER stimulus (e.g. by lowering the temperature) and the programmed LCE samples exhibit free-standing two-way shape switching behavior. Here, a new adaptable main-chain LCE system was developed with thermally induced transesterification BERs. The network combines the conventional properties of LCEs, such as an isotropic phase transition and soft elasticity, with the dynamic features of adaptable network polymers, which are malleable to stress relaxation due to the BERs. Polarized Fourier transform infrared measurements confirmed the alignment of polymer chains and mesogens after strain-induced programming. The influence of the creep stress, temperature, and time on the strain amplitude of two-way shape switching was examined. The LCE network demonstrates an innovative feature of reprogrammability, where the reversible shape-switching memory of programmed LCEs is readily deleted by free-standing heating as random BERs disrupt the mesogen alignment, so LCEs are reprogrammed after returning to the polydomain state. Due to the dynamic nature of the LCE network, it also exhibits a surface welding effect and can be fully dissolved in the organic solvent, which might be utilized for green and sustainable recycling of LCEs.