Main-chain Liquid Crystal Elastomers Research Articles

Coupling between viscoelasticity and soft elasticity in main-chain nematic Liquid Crystal Elastomers

Liquid crystal elastomers (LCEs) are a class of smart elastomers exhibiting unusual mechanical behavior, including large energy dissipation and soft elasticity under uniaxial tensile loading. LCEs are composed of liquid crystal molecules, called mesogens, linked by a network of polymer chains. During deformation, the mesogens orient in the direction of the loading, leading to soft elasticity, which is an increase in strain at constant stress. The combination of mesogen rotation and intrinsic polymer viscoelasticity leads to a nonlinear viscoelastic soft elastic behavior. The aim of this paper is to investigate the coupling between the viscoelastic mechanisms and soft elasticity in main chain LCEs. We propose a rheological model in which the mesogen rotation during deformation is represented by a reversible slider while viscoelastic relaxation mechanisms are modeled as series of Maxwell elements coupled or decoupled with mesogen rotation. Fitting this model to experimental data demonstrate that the coupling between polymer chain viscoelasticity and mesogen rotation is partial, i.e. the long-time relaxation mechanisms are coupled and the short-time relaxation mechanisms are decoupled from mesogen rotation. Furthermore, we show that the viscosity of mesogen rotation is not necessary to properly predict the elastic modulus during the soft elasticity but it is needed to properly predict the initiation of the phenomenon.

Thermomechanical response of liquid crystal elastomers: role of crosslinker density

In this work thermomechanical properties of main-chain liquid crystal elastomers (MC-LCEs) with different degrees of crosslinking were investigated, and gradual loss of thermomechanical response was observed on repetitive measurements. Specifically, six samples of MC-LCEs were prepared, with crosslinker-to-mesogen relative concentration ranging from 5% to 10% in steps of 1%. The obtained results were then compared to thermomechanical response of side-chain liquid crystal elastomers (SC-LCEs). Additionally, thermomechanical response of polymer dispersed main-chain liquid crystal elastomers (MC-PDLCEs) was investigated. Results indicate that in MC-LCEs the concentration of crosslinker defines thermomechanical response and affects stability of the system. The loss of thermomechanical response is negligible in the case of crosslinker to mesogen ratio being the smallest, namely in 5% sample, and it is unaffected by glasslike to nematic phase transition. SC-LCEs do not show any sign of such behaviour and remain stable after several cycles of thermomechanical measurements.

Modeling nematic phase main-chain liquid crystal elastomer synthesis, mechanics, and thermal actuation via coarse-grained molecular dynamics.

This paper presents a coarse-grained molecular dynamics simulation study of the synthesis, mechanics, and thermal actuation of nematic phase main-chain liquid crystal elastomers (LCEs), a type of soft, temperature-responsive, polymeric actuating material. The simulations herein model the crosslinking, mechanical stretching, and additional crosslinking synthesis process, following which, the simulated LCE exhibits a direction-dependent thermal actuation and mechanical response. The thermal actuation response shows good qualitative agreement with experimental results, including the variation of a global order parameter that describes the orientation of the mesogen domains comprising the LCE. The mechanical response due to applied deformation shows less agreement, but manifests the key features observed in experiments on LCEs, namely soft strain and hyperelasticity that is present when loaded perpendicularly and in-line, respectively, to the mesogen alignment direction. We also present a topological analysis of the simulated LCEs, which, in conjunction with the simulated thermomechanical responses, allows us to infer the relative contribution of entanglements and chemical crosslinks on those responses. We suggest that the model proposed herein will help enable improved LCE formulations via mechanistic insights that can be gained via the use of such a relatively computationally inexpensive coarse-grained molecular dynamics model, which may be of further value to application areas including soft robotics, bio-mimicking devices, artificial muscles, and adaptive materials.

On the Effects of Different trans and cis Populations in Azobenzene Liquid Crystal Elastomers: A Monte Carlo Investigation.

We investigate main-chain liquid crystal elastomers (LCEs) formed by photoresponsive azobenzene units with different populations of trans and cis conformers (from fully trans to fully cis). We study their macroscopic properties as well as their molecular organization using extensive Monte Carlo simulations of a simple coarse-grained model where the trans and cis conformers are represented by soft-core biaxial Gay-Berne particles with size and interaction energy parameters obtained by fitting a bare bone azobenzene moiety represented at atomistic level. We find that increasing the fraction of cis conformers, as could be obtained by near-UV irradiation, shifts the nematic-isotropic transition to a lower temperature, consistently with experiment, while generating internal stress in a clamped sample. An analysis of pair distributions shows that the immediate surroundings of a bent cis molecule are slightly less dense and more orientationally disordered in comparison with that of a trans conformer. Comparing nematic and smectic LCEs, actuation in the smectic phase proved less effective, disrupting the smectic layers to some extent but preserving orientational order of the azobenzene moieties.

Programmable Complex Shape Changing of Polysiloxane Main-Chain Liquid Crystalline Elastomers.

Liquid crystal elastomers (LCEs) are shape-morphing materials whose large and reversible shape transformations are caused by the coupling between the mobile anisotropic properties of liquid crystal (LC) units and the rubber elastic of polymer networks. Their shape-changing behaviors under certain stimuli are largely directed by the LC orientation; therefore, various strategies have been developed to spatially modulate the LC alignments. However, most of these methods are limited as they require complex fabrication technologies or have intrinsic limitations in applicability. To address this issue, programmable complex shape changes in some LCE types, such as polysiloxane side-chain LCEs, thiol-acrylate main-chain LCEs, etc., were achieved by using a mechanical alignment programming process coupled with two-step crosslinking. Here, we report a polysiloxane main-chain LCE with programmable 2- and 3D shape-changing abilities that were created by mechanically programming the polydomain LCE with two crosslinking steps. The resulting LCEs exhibited a reversible thermal-induced shape transformation between the initial and programmed shapes due to the two-way memory between the first and second network structures. Our findings expand on the applications of LCE materials in actuators, soft robotics, and smart structures where arbitrary and easily programmed shape morphing is needed.

Impact of molecular architectures on mesogen reorientation relaxation and post-relaxation stress of liquid crystal elastomers under electric fields

Applying electric fields to liquid crystal elastomers (LCEs) has attracted attention as a fast and energy-efficient actuation method for soft robotics. It is helpful to predict the minimum electric field strength that can drive the mesogen reorientation and the time required for the reorientation of LCEs with specific molecular architecture to optimize LCE electrical actuation. In this study, molecular dynamics (MD) simulations of main- and side-chain LCE systems under an applied electric field were performed, and the relaxations of stress and orientational order were observed. The minimum electric field strength that can drive the reorientation for each LCE molecular system was estimated. The time required for reorientation follows a power law of the electric field strength, and the reorientation time for side-chain LCEs is much shorter than that for main-chain LCEs. The behavior of the relaxation value of stress with respect to the electric field strength differed markedly between the main- and side-chain LCEs. These results suggest that designing LCEs at the molecular architecture level and using different LCE molecules in different actuation sites are essential to more precise soft robotics.

Polymer-dispersed liquid crystal elastomers as moldable shape-programmable material

The current development of soft shape-memory materials often results in materials that are typically limited to the synthesis of thin-walled specimens and usually rely on complex, low-yield manufacturing techniques to fabricate macro-sized, solid three-dimensional objects. However, such geometrical limitations and slow production rates can significantly hinder their practical implementation. In this work, we demonstrate a shape-memory composite material that can be effortlessly molded into arbitrary shapes or sizes. The composite material is made from main-chain liquid crystal elastomer (MC-LCE) microparticles dispersed in a silicone polymer matrix. Shape-programmability is achieved via low-temperature induced glassiness and hardening of MC-LCE inclusions, which effectively freezes-in any mechanically instilled deformations. Once thermally reset, the composite returns to its initial shape and can be shape-programmed again. Magnetically aligning MC-LCE microparticles prior to curing allows the shape-programmed artefacts to be additionally thermomechanically functionalized. Therefore, our material enables efficient morphing among the virgin, thermally-programmed, and thermomechanically-controlled shapes.

Rate-dependent stress-order coupling in main-chain liquid crystal elastomers.

Liquid crystal elastomers (LCEs) exhibit significant viscoelasticity. Although the rate-dependent stress-strain relation of LCEs has already been widely observed, the effect of the intricate interplay of director rotation and network extension on the viscoelastic behavior of main-chain LCEs remains inadequately understood. In this study, we report real-time measurements of the stress, director rotation, and all strain components in main-chain nematic LCEs subjected to uniaxial tension both parallel and tilted to the initial directors at different loading rates and relaxation tests. We find that both network extension and director rotation play roles in viscoelasticity, and the characteristic relaxation time of the network extension is much larger than that of the director rotation. Interestingly, the gradual change of the director in a long-time relaxation indicates the director reorientation delay is not solely due to the viscous rotation of liquid crystals but also arises from its coupling with the highly viscous network. Additionally, significant rate-dependent shear strain occurs in LCEs under uniaxial tension, showing non-monotonic changes when the angle between the stretching and the initial director is large enough. Finally, a viscoelastic constitutive model, only considering the viscosity of the network by introducing multiplicative decomposition of the deformation gradient, is utilized to manifest the relation between rate-dependent macroscopic deformation and microscopic director rotation in LCEs.

Enhanced photocontrollable dynamic adhesion of nematic elastomers on rough surfaces

Polymers with switchable adhesion properties are important for efficient catch-and-release processes in robotic hand applications and handgrips with on-demand adhesion control. However, counter objects often have a rough surface topography, which requires a dynamic adhesion system that balances adhesion and detachment performances. Here, we demonstrate that a photosensitive nematic main-chain liquid crystal elastomer (LCE) has suitable photoswitchable adhesion properties on surfaces with micrometer-scale roughness, resulting in a large adhesion change. The nematic LCE exhibits the so-called “soft elasticity,” wherein it can deform at a certain strain without increasing the elastic energy. Such deformation can be maintained owing to arrested relaxation, enabling the LCE to deform along with a rough surface upon light pressure. This increases the contact area and consequently enhances adhesion ability. On the other hand, the LCE exhibits rubber elasticity after proper light irradiation owing to a phase transition from the nematic to the isotropic phase. As rubber elasticity limits deformation, the contact area on the rough surface, and thus the adhesion, is greatly reduced. This LCE with photoswitchable elasticity has great merit as a dynamic adhesive pad for treating objects with rough surfaces, which may expand the versatility of stimuli-responsive adhesive systems.

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.

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.

Effects of photo-isomerizable side groups on the phase and mechanical properties of main-chain nematic elastomers

A series of main-chain nematic liquid crystal elastomers containing various photo-isomerizable side groups branching from the main chain were synthesized. The effects of the side groups on the thermal phase and mechanical properties were explored.

Molecular architecture dependence of mesogen rotation during uniaxial elongation of liquid crystal elastomers

Macroscopic deformation of liquid crystal elastomers (LCEs) has been assumed to result from the coupling dynamics of the elastic polymer network and liquid-crystalline mesogenic units. However, the microscopic dynamics of LCE molecules are extremely difficult to directly observe from experiments. In this work, in silico experiments of uniaxial elongation of LCEs were performed by molecular dynamics simulations. The molecular systems of main- and side-chain-type LCEs were constructed using coarse-graining techniques. The stress–strain curves as the response to an external force were obtained, and the orientational order parameter of the mesogens along the elongation axis was observed as evidence of symmetry breaking. For the main-chain LCEs, soft elasticity was clearly observed regardless of the details of the molecular architecture. A plateau region of the stress–strain curves appeared, corresponding to drastic symmetry breaking of the orientational order. In contrast, for the side-chain LCEs, the presence/absence of soft elasticity was highly dependent on the details of the molecular architecture. In some cases, rotation of the mesogens was inhibited by a local steric constraint by the main chain and crosslinked particles, which prevented the onset of soft elasticity.

Mechano-Actuated Light-Responsive Main-Chain Liquid Crystal Elastomers

Herein, a highly mechano-actuated light-responsive main-chain liquid crystal elastomer (LCE) was prepared using the two-stage thiol–acrylate Michael addition–photopolymerization (TAMAP) method with a reactive mesogenic mixture. This reactive mixture included a newly synthesized azobenzene-containing diacrylate reactive mesogen 4,4′-di(4-(acrylolylhexyloxy) benzoyloxy) azobenzene (D6OA), which exhibited a long mesogenic structure around the azobenzene bond. The monodomain main-chain azobenzene-containing LCE (M azo-LCE) film was prepared by mechanical drawing and visible-light curing the polydomain azo-LCE (P azo-LCE) film synthesized using the TAMAP method. M azo-LCE film showed approximately 30% contraction in length because of the trans-to-cis isomerization caused by ultraviolet (UV) irradiation, but it did not exhibit response to visible light because the long mesogenic structure of D6OA has a high energy barrier with respect to the cis-to-trans isomerization caused by visible light. However, the simple mechanical drawing of the M azo-LCE film induced the cis-to-trans isomerization. The alternating UV irradiation and mechanical drawing resulted in reversible trans-to-cis and cis-to-trans isomerizations for more than 10 cycles. The cis-to-trans/trans-to-cis isomerization and structural changes during UV irradiation and mechanical drawing were investigated via Raman spectroscopy and wide- and small-angle X-ray scatterings using a synchrotron radiation source. The azobenzene-containing chains in the layer were changed such that the staggered chains with extended trans isomers in the tilted layer (smectic C) were transformed into the chains with kinked cis isomers in the parallel-packed layer via UV irradiation, which were back to the original staggered extended chains with extended trans isomers in the tilted layer through mechanical drawing. The cis-to-trans isomerization via mechanical drawing can be attributed to the location of the azobenzene linkage in the main-chain backbone, but this has been rarely reported. The high actuation and simple recovery of the main-chain M azo-LCEs provide many application possibilities in various fields such as robotics, prosthetics, microfluids, data storage, and surface relief gratings.

Magnetic polydomain liquid crystal elastomers – synthesis and characterisation

ABSTRACT Although liquid crystal elastomers (LCE) are a fascinating class of materials with interesting thermomechanical properties of their own, the aim is to enhance their performance or add new features, e.g. response to external stimuli. The generally weak response of organic materials can be significantly intensified by mixing nano-sized magnetic particles into the host polymer matrix. An alternative approach is chemically coupling the nanoparticles to the elastomer. We achieved this by bonding functionalised magnetic nanoplatelets to the backbone of a main-chain LCE and obtained polydomain magnetic liquid crystal elastomers. We measured the magnetisation curves in samples, which were exposed to either small or large magnetic fields – their response being a consequence of partial particle reorientation or magnetic moment flipping. In contrast to the samples, which were exposed to small magnetic field and in which the remanent magnetisation can be reset to zero by heating the sample, the samples with flipped magnetisation within the platelets cannot be reversed into the original state. Coupling of magnetic and mechanical properties shows a slight magneto-elastic response at elevated temperatures and a significant inverse magneto-elastic effect: the magnetisation caused by mechanical stretching is almost equal to the magnetisation caused by an external magnetic field.

Effect of stretching angle on the stress plateau behavior of main-chain liquid crystal elastomers.

The equilibrium nonlinear stress-stretch relationships for a monodomain main-chain nematic elastomer (MNE) are investigated by varying the angle between the stretching and initial director axes (θ0). Angle θ0 has pronounced effects on the ultimate elongation as well as on the width of the low stress plateau regime (Λp) during director rotation, whereas θ0 has no appreciable effect on the plateau stress (σp). In the stretching normal to the initial director (θ0 = 90°), the plateau end exceeds 200% strain. At oblique angles of 90° > θ0≥ 35°, Λp decreases with decreasing θ0, whereas the definite plateau regime vanishes at θ0 < 24°. Wide-angle X-ray scattering and polarized optical microscopy measurements reveal that the director rotates uniformly in the biased direction for the MNE of θ0°≪ 90°, whereas directors rotating clockwise and counterclockwise are coexistent for θ0 = 90°. Over the entire plateau regime, the MNEs exhibit pure shear deformation characterized by a Poisson's ratio of zero in the direction of the rotation axis. The Λp for the corresponding polydomain NE (PNE) undergoing a transition to the monodomain alignment is smaller than that of the MNE of θ0 = 90°, while the σp values for both NEs are almost similar. The semi-soft elasticity concept satisfactorily explains the effects of θ0 on Λp, and the Λp value of the PNE, using a single anisotropy parameter which is evaluated from the degree of thermally induced deformation of MNEs.

Tunability of the elastocaloric response in main-chain liquid crystalline elastomers

ABSTRACT Materials exhibiting a large caloric effect could lead to the development of a new generation of heat-management technologies that will have better energy efficiency and be potentially more environmentally friendly. The focus of caloric materials investigations has shifted recently from solid-state materials towards soft materials, such as liquid crystals and liquid crystalline elastomers. It has been shown recently that a large electrocaloric effect exceeding 6 K can be observed in smectic liquid crystals. Here, we report on a significant elastocaloric response observed by direct elastocaloric measurements in main-chain liquid crystal elastomers. It is demonstrated that the character of the nematic to paranematicisotropic transition can be tuned from the supercritical regime towards the first-order regime, by decreasing the density of crosslinkers. In the latter case, the latent heat additionally enhances the elastocaloric response. Our results indicate that a significant elastocaloric response is present in main-chain liquid crystalline elastomers, driven by stress fields much smaller than in solid elastocaloric materials. Therefore, elastocaloric soft materials can potentially play a significant role as active cooling/heating elements in the development of new heat-management devices.

Liquid-Crystal-Elastomer-Based Dissipative Structures by Digital Light Processing 3D Printing.

Digital Light Processing (DLP) 3D printing enables the creation of hierarchical complex structures with specific micro- and macroscopic architectures that are impossible to achieve through traditional manufacturing methods. Here, this hierarchy is extended to the mesoscopic length scale for optimized devices that dissipate mechanical energy. A photocurable, thus DLP-printable main-chain liquid crystal elastomer (LCE) resin is reported and used to print a variety of complex, high-resolution energy-dissipative devices. Using compressive mechanical testing, the stress-strain responses of 3D-printed LCE lattice structures are shown to have 12 times greater rate-dependence and up to 27 times greater strain-energy dissipation compared to those printed from a commercially available photocurable elastomer resin. The reported behaviors of these structures provide further insight into the much-overlooked energy-dissipation properties of LCEs and can inspire the development of high-energy-absorbing device applications.

A copper(i)-catalyzed azide–alkyne click chemistry approach towards multifunctional two-way shape-memory actuators

Here we report a copper(i)-catalyzed azide–alkyne click chemistry approach towards the fabrication of main chain liquid crystal elastomers.

The effect of alignment on the rate-dependent behavior of a main-chain liquid crystal elastomer

This study investigated the effect of alignment on the rate-dependent behavior of a main-chain liquid crystal elastomer (LCE). Polydomain nematic LCE networks were synthesized from a thiol-acrylate Michael addition reaction in the isotropic state. The polydomain networks were stretched to different strain levels to induce alignment then crosslinked in a second stage photopolymerization process. The LCE networks were subjected to dynamic mechanical tests to measure the temperature-dependent storage modulus and uniaxial tension load-unload tests to measure the rate-dependence of the Young's modulus, mechanical dissipation, and characteristics of the soft stress response. Three-dimensional (3D) digital image correlation (DIC) was used to characterize the effect of domain/mesogen relaxation on the strain fields. All LCE networks exhibited a highly rate-dependent stress response with significant inelastic strains after unloading. The Young's modulus of the loading curve and hysteresis of the load-unload curves showed a power-law dependence on the strain rate. The Young's modulus increased with alignment and larger anisotropy and a smaller power-law exponent was measured for the Young's modulus and hysteresis for the highly aligned monodomains. The polydomain and pre-stretched networks loaded perpendicular to the alignment direction exhibited a soft stress response that featured a rate-dependent peak stress, strain-softening, and strain-stiffening. The 3D-DIC strain fields for the polydomain network and programmed networks stretched in the perpendicular direction were highly heterogeneous, showing regions of alternating higher and lower strains. The strain variations increased initially with loading, peaked during the strain softening part of the stress response, then decreased during the strain stiffening part of the stress response. Greater variability was measured for lower strain rates. These observations suggest that local domain/mesogen relaxation led to the development of the heterogeneous strain patterns and strain softening in stress response. These findings improved understanding of the kinetics of mesogen relaxation and its contributions to the rate-dependent stress response and mechanical dissipation.