Properties Of Liquid Crystal Elastomers Research Articles

A review of humidity-Responsive Actuation in Liquid Crystal elastomer films: Controlling reversibility and irreversibility through hydrogen bonding

Humidity-responsive actuation refers to the phenomenon where certain materials alter their shape, size, or color in response to fluctuations in humidity levels. This behavior is reminiscent of natural processes observed in plants, where seeds and flowers react to moisture. The potential applications of humidity-responsive materials span various fields, including soft robotics, smart sensors, biomimetic devices, and anti-counterfeiting measures. Liquid Crystal Elastomers (LCEs)are particularly noteworthy in this context, as they possess a unique combination of properties derived from both liquid crystals and elastomers. These materials exhibit significant changes in shape in response to external stimuli such as temperature, light, electric fields, and humidity. As a result, this paper focuses on the humidity-responsive properties of Liquid Crystal Elastomer (LCE) films, with a specific emphasis on the role of hydrogen bonding in this process. And this review covers the synthesis methods, underlying mechanisms, performance characteristics, challenges, and future prospects of humidity-responsive Liquid Crystal Elastomer (LCE) films.

Liquid Crystal Elastomer Based Thermal Microactuators and Photothermal Microgrippers Using Lateral Bending Beams

AbstractThe remarkable properties of liquid crystal elastomers (LCE) for spatially controlled actuation are utilized to perform complex tasks such as to mimic versatile motion of living organisms and cilia in soft robotics. As opposed to the common out‐of‐plane bending in flat LCE films, many planar micro‐electro‐mechanical and microfluidic systems typically operate based on the lateral movement of the functional parts. The challenge of implementing the in‐plane bending of LCE actuators is often associated with the limited capability for miniaturization of the existing techniques, along with the requirement for post‐fabrication assembly. In this paper a method for fabrication of thermally actuated LCE beams for in‐plane bending applications, which is based on direct laser writing and micromolding is reported. Mesogenic alignment is modified near the sidewalls of the channel structures patterned with vertical microgrooves. The method allows monolithic fabrication of LCE‐based microgrippers for micromanipulation and microassembly applications in life sciences and in manufacture of microsystems. It is shown that the photothermally driven microgrippers can perform soft grasping of micro‐objects, providing large gripping strokes with relatively low actuation stimuli. The fabrication method offers more design opportunities for LCE‐based microactuators and a useful route toward realization of gripping and cargo transportation functionality in microrobotics.

Manipulated and Improved Photoinduced Deformation Property of Photoresponsive Liquid Crystal Elastomers by Copolymerization.

Photoresponsive liquid crystal elastomers (LCEs) have aroused much attention due to their unique structures, properties, and potential applications. However, many reported photoresponsive LCEs show small photoinduced deformation with a slow deformation speed, which limits their application to a certain extent. In this article, multiple hydrogen bond component and biphenyl component was introduced into photoresponsive LCEs bearing cyanostilbene by copolymerization, and prepared a series of LCEs CSm-BPn (CS and BP means cyanostilbene and biphenyl component, respectively; m, n means the content ratio of CS and BP). All uniaxially oriented CSm-BPn fibers show photoinduced deformation behavior under 365nm UV light except for CS0-BP5. The introduction of hydrogen bond and the decrease of glass transition temperature realize large deformation and fast deformation speed. Besides, the properties of LCEs are successfully regulated by changing the content ratio of each component. What's more interesting is that the addition of appropriate amount of biphenyl can significantly improve and manipulate the deformation property of the CSm-BPn fibers. The maximum bending angle can reach 175° , and the whole photoinduced deformation process only takes 11 s. The photoresponsive LCEs with such large bending angle and fast deformation rate are rarely reported.

Understanding the effect of liquid crystal content on the phase behavior and mechanical properties of liquid crystal elastomers.

Liquid crystal elastomers are stimuli-responsive, shape-shifting materials. They typically require high temperatures for actuation which prohibits their use in many applications, such as biomedical devices. In this work, we demonstrate a simple and general approach to tune the order-to-disorder transition temperature (TODT) or nematic-to-isotropic transition temperature (TNI) of LCEs through variation of the overall liquid crystal mass content. We demonstrate reduction of the TNI in nematic LCEs through the incorporation of non-mesogenic linkers or the addition of lithium salts, and show that the TNI varies linearly with liquid crystal mass content over a broad range, approximately 50 °C. We also analyze data from prior reports that include three different mesogens, different network linking chemistries, and different alignment strategies, and show that the linear trend in TODT with liquid crystal mass content also holds for these systems. Finally, we demonstrate a simple approach to quantifying the maximum actuation strain through measurement of the soft elastic plateau and demonstrate applications of nematic LCEs with low TODTs, including the first body-responsive LCE that curls around a human finger due to body heat, and a fluidic channel that directionally pumps liquid when heated.

Numerical simulation and experimental validation of bending and curling behaviors of liquid crystal elastomer beams under thermal actuation

The exceptional actuation properties of liquid crystal elastomers (LCEs) have made these materials highly attractive for various emerging applications such as soft robotics and artificial muscles. The large strain gradients occurring under thermal stimuli induce bending and curling of initially flat LCE films. Due to the complex physics behind the spontaneous deformation in nematic liquid crystal elastomers, there is no single universal finite element-based method for the simulation of the behaviors of LCE actuators. In this work, we developed a simple layered 2D model for modeling and simulation of the bending and curling characteristics of LCE beams based on the gradient of the temperature-dependent equivalent thermal expansion. The appropriate parameters were derived by measuring the radius of curvature of the LCE film aligned unidirectionally at one surface produced on a rubbed Kapton film. It was found that in a large range of thicknesses (12–134 μm) of the LCE beams, the equivalent thermal expansion coefficients tend to approach a similar value. It was demonstrated and experimentally validated that the thermal expansion model is very effective in predicting the nonlinear curling behavior of LCE beams of various thicknesses. Remarkably, the model is also capable of simulating the rolling behavior of LCE beams with tapered thickness variation. The proposed method offers good flexibility in terms of the geometric shape and expansion parameters, computational efficiency, and accuracy.

Thermomechanically active electrodes power work-dense soft actuators.

The effect of chain extender structure and composition on the thermomechanical properties of liquid crystal elastomers (LCE) synthesized using thiol-acrylate Michael addition is presented. The intrinsic molecular stiffness of the thiol chain extender and its relative molar ratio to acrylate-based host mesogens determine the magnitudes of the thermomechanical strains, temperatures at which they are realized and the mechanical work-content. A non-linear structure-property relationship emerges, wherein higher concentrations of flexible extenders first magnify the thermomechanical sensitivity, but a continued increase leads to weaker actuation. Understanding this interplay leads to a composite material platform, enabling a peak specific work production of ∼2 J kg-1 using ∼115 mW of electrical power supplied at 2 V. Composites of LCE with eGaIn liquid metal (LM) are prepared, which act as heaters, while being capable of actuation themselves. The thermomechanically active electrodes convert the electrical power into Joule heat, which they efficiently couple with the neat LCE to which they are bound. This system harnesses the nascent responsiveness of the LCE using electrodes that work with them, instead of fighting against them (or passively standing in the way). Specific work generated increases when subjected to increasing levels of load, reaching a peak at loads ∼260× the actuator weight. These ideas are extended to tri-layered actuators, where LCE films with orthogonal molecular orientations sandwich LCE-LM composite heaters. Torsional actuation modes are harnessed to twist under load.

Fabrication and Characterization of Highly Deformable Artificial Muscle Fibers Based on Liquid Crystal Elastomers

AbstractArtificial muscles have important applications in areas ranging from robotics to prosthetics and medical devices. In this study, highly deformable artificial muscle fibers that utilize superior actuating properties of liquid crystal elastomers and liquid-like deformability of liquid metal are reported. An effective and low-cost fabrication approach using screen printing technique is developed. The actuating properties of the artificial muscle fibers, including the dependence of temperature, contraction strain, and pulling force of the artificial muscle fiber on electric heating current and heating time, are characterized. The results could provide important guidance to design and for development of soft systems that utilize the actuating mechanisms of liquid crystal elastomers.

Effect of Isomeric Amine Chain Extenders and Crosslink Density on the Properties of Liquid Crystal Elastomers.

Among the various types of shape changing materials, liquid crystal elastomers (LCEs) have received significant attention as they can undergo programmed and reversible shape transformations. The molecular engineering of LCEs is the key to manipulating their phase transition, mechanical properties, and actuation performance. In this work, LCEs containing three different types of butyl groups (n-, iso-, and sec-butyl) in the side chain were synthesized, and the effect of isomeric amine chain extenders on the thermal, mechanical, and actuation properties of the resulting LCEs was investigated. Because of the considerably low reactivity of the sec-butyl group toward the diacrylate in the LC monomer, only a densely crosslinked LCE was synthesized. Most interestingly, the mechanical properties, actuation temperature, and blocking stress of the LCEs comprising isobutyl groups were higher than those of the LCEs comprising n-butyl groups. This difference was attributed to the presence of branches in the LCEs with isobutyl groups, which resulted in a tighter molecular packing and reduced the free volume. Our results suggest a facile and effective method for synthesizing LCEs with tailored mechanical and actuation properties by the choice of chain extenders, which may advance the development of soft actuators for a variety of applications in aerospace, medicine, and optics.

Mechanical deformations of a liquid crystal elastomer at director angles between 0° and 90°: Deducing an empirical model encompassing anisotropic nonlinearity

ABSTRACTDespite the wealth of studies reporting mechanical properties of liquid crystal elastomers (LCEs), no theory can currently describe their complete mechanical anisotropy and nonlinearity. Here, we present the first comprehensive study of mechanical anisotropy in an all‐acrylate LCE via tensile tests that simultaneously track liquid crystal (LC) director rotation. We then use an empirical approach to gain a deeper insight into the LCE's mechanical responses at values of strain, up to 1.5, for initial director orientations between 0° and 90°. Using a method analogous to time–temperature superposition, we create master curves for the LCE's mechanical response and use these to deduce a model that accurately predicts the load curve of the LCE for stresses applied at angles between 15° and 70° relative to the initial LC director. This LCE has been shown to exhibit auxetic behavior for deformations perpendicular to the director. Interestingly, our empirical model predicts that the LCE will further demonstrate auxetic behavior when stressed at angles between 54° and 90° to the director. Our approach could be extended to any LCE; so it represents a significant step forward toward models that would aid the further development of LCE theory and the design and modeling of LCE‐based technologies. © 2019 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 1367–1377

Study of the optical, thermal, and mechanical properties of nematic liquid crystal elastomers

ABSTRACTIn the present study, the optical, thermal, and mechanical properties of liquid crystal elastomers (LCEs) were investigated using various techniques. The presence of functional groups in LCE was studied using Fourier transform infrared spectroscopy. The phase transition temperatures were confirmed via polarizing optical microscopy and Fabry–Perot scattering studies. The differential thermal analysis was used for investigating the thermal behavior. A dynamic mechanical analysis was used to study the mechanical properties of LCE. The significant mechanical changes with a considerable reversible effect were observed for this soft material. The changes in the mechanical shape with the temperature are attributed to the change in the phase of the LCE material.

Stimulus-response properties of liquid crystal elastomers and gels with controlled director configurations

Stimulus-response properties of liquid crystal elastomers and gels with controlled director configurations

Evaluation of the Important Properties of Liquid Crystal Elastomers (LCEs)

Liquid Crystal Elastomers (LCEs) have been the object of growing interest in recent years due to the combination of mesogenic ordering and rubber elasticity which allows them to respond to thermal stimuli by changing their shape, size and optical properties. The use of LCEs thus makes it possible to convert small quantities of external energy to mechanical energy which makes it easy to be used as energy harvesting. This paper will firstly focus on thermal and mechanical properties of LCEs it will be seen that they will be affected by elasticity of the polymer backbone and the nature of mesogenic monomer. Following this, the focus will turn to the electric field, light, and thermal energy that can be converted into mechanical energy induces thermomechanical, electromechanical and electrooptical effects by such materials. It will be observed that the shape of LCE materials can be changed through changing orientation of LC. Additionally, swollen samples can minimize threshold field by a factor of 200 compared with unswollen samples. Then, the emphasis will go to the impact of mixing nanoparticles with the LCE samples on its behaviour. This is done by increasing crosslinking agent content of the sample and then its impact is shown by the strain-stress curve which leads to reduction in the degree of nematic phase and increase in Young’s modules. The paper then gives a future direction to expand the work for instance its application in photovoltaic systems to convert solar energy into electricity.

Synthesis and Characterization of Liquid Crystal Elastomer

The thermal and mechanical properties of Liquid Crystal Elastomers (LCEs) were characterized using various techniques for understanding of their physical behavior. The material used for investigation was synthesized by us, using Finklemann procedure, with proper cross linking density in nematic phase. The material is found to have unique coupling between anisotropicorder of liquid crystal component and elasticity of polymer network. The chemical structures were confirmed by Fourier Transform Infrared (FTIR) Spectroscopy and Scanning Electron Microscopy (SEM). Fabry Perot Scattering Studies (FPSS), Thermo gravimetric Analysis (TGA) and Differential Scanning Calorimatory (DSC) were used to study thermal properties. The mechanical properties were studied using force sensor. Our investigation shows that this synthesized Liquid Crystal Elastomer has ability of spontaneous change as a function of temperature and mechanical force, which shows it as a unique class of soft material.

Tack energy and switchable adhesion of liquid crystal elastomers

The mechanical properties of liquid crystal elastomers (LCEs) make them suitable candidates for pressure-sensitive adhesives (PSAs). Using the nematic dumbbell constitutive model, and the block model of PSAs, we study their tack energy and the debonding process as could be measured experimentally in the probe-tack test. To investigate their performance as switchable PSAs we compare the tack energy for the director aligned parallel, and perpendicular to the substrate normal, and for the isotropic state. We find that the tack energy is larger in the parallel alignment than the isotropic case by over a factor of two. The tack energy for the perpendicular alignment can be 50% less than the isotropic case. We propose a mechanism for reversibly switchable adhesion based on the reversibility of the isotropic to nematic transition. Finally we consider the influence of several material parameters that could be used to tune the stress-strain response.

Coupling between mesogenic units and polymer backbone in side-chain liquid crystal polymers and elastomers

The levels of alignment of the mesogenic units and of the polymer backbone trajectory for polyacrylate based nematic side-chain liquid crystal polymers and elastomers were evaluated by using wide angle X-ray and small angle neutron scattering procedures. The X-ray scattering measurements show that substantial levels of preferred orientation of the mesogenic units may be introduced through magnetic fields for uncrosslinked polymers and through mechanical extension for liquid crystal elastomers. Small angle neutron scattering measurements show that for highly aligned samples an anisotropic polymer backbone trajectory is observed in which the envelope is slightly extended by ∼ 10% in the direction parallel to the axis of alignment of the mesogenic units. The sense of this coupling differs from that recorded for other uncrosslinked side-chain liquid crystal polymers. Possible mechanisms to account for this anisotropy and its relationship to the properties of liquid crystal elastomers are discussed. The observed deformation behaviour of the liquid crystal elastomer is non-affine and this appears to confirm the dominating influence of the liquid crystal order of the side chains on the mechanical properties of these novel networks.

The synthesis and properties of liquid crystal elastomers

Liquid crystal elastomers have been synthesised in which up to 10 mol% of cross-link units may be introduced without completely disrupting the nematic liquid crystal phase, although the introduction of cross-linking has a marked effect upon the temperatures of the phase transitions.