Main-chain Liquid Crystalline Elastomers Research Articles

Versatile Mechanochromic Sensor based on Highly Stretchable Chiral Liquid Crystalline Elastomer.

A mechanochromic strain sensor that is capable of distinguishing the orientation, the location, and the degree of deformation based on the highly stretchable membrane of main-chain chiral liquid crystalline elastomer (MCLCE) is proposed. The MCLCE film is designed to exhibit uniform and significant color shift upon the small strain by using step-growth polymerization of liquid crystal (LC) oligomer and its phase-stabilization in solvent mesogen. As conformally placed on the bottom elastomer sheet, the MCLCE film shows multimodal, instantaneous color change for sensing arbitrary in-plane deformation, out-of-plane bending, and nonzero Gaussian deformation. Based on high freedom in the device design, it is also demonstrated that this sensor can display color patterns or encrypted images in response to the localized weight or strain. The simple and straightforward concept proposed here can be applicable in the fields of wearable devices, displays, and soft robotics.

Actuation performances of catkin fibers reinforced thiol-acrylate main-chain liquid crystalline elastomer

ABSTRACT Liquid crystalline elastomers (LCEs) have been utilized as an important class of smart actuator materials. However, the modest actuation mechanical and robustness performances remain a challenge. Inspired by the specific structures, well mechanical properties and physico-chemical characteristics of some natural plant fibers, a composite of thiol-acrylate main-chain LCE matrix incorporated with catkin fibers is designed and developed. The catkin fibers build a network as reinforcement phase, and demonstrate effective compatibility and integration property with the matrix, their high flexibility can be adapted to the large deformational performance of LCE matrix. The prepared LCE composite demonstrates strong mechanical actuation properties. The modulus and driving force triggered by the stimuli are obviously increased. The tensile strength and fatigue failure resistant property under high loadings and repeated cycles of thermal actuation or photothermal actuation are greatly enhanced. While the stimulus response deformation rate, phase transition temperature and liquid crystal phase structure of the LCE matrix, and so on, do not weaken or change. This work promotes the LCE materials’application potential and broadens the application value of natural plant fibers.

Broadband and pixelated camouflage in inflating chiral nematic liquid crystalline elastomers.

Living organisms such as fishes1, cephalopods2 and clams3 are cryptically coloured with a wide range of hues and patterns for camouflage, signalling or energy regulation. Despite extensive efforts to create colour-changing materials and devices4, it is challenging to achieve pixelated structural coloration with broadband spectral shifts in a compact space. Here, we describe pneumatically inflating thin membranes of main-chain chiral nematic liquid crystalline elastomers that have such properties. By taking advantage of the large elasticity anisotropy and Poisson's ratio (>0.5) of these materials, we geometrically program the size and the layout of the encapsulated air channels to achieve colour shifting from near-infrared to ultraviolet wavelengths with less than 20% equi-biaxial transverse strain. Each channel can be individually controlled as a colour 'pixel' to match with surroundings, whether periodically or irregularly patterned. These soft materials may find uses in distinct applications such as cryptography, adaptive optics and soft robotics.

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.

Ultrasonics Sonochemistry Assisted Preparation of Polysiloxane Main‐Chain Liquid‐Crystalline Elastomers

AbstractFor the first time, an ultrasonics sonochemistry method is developed to promote the one‐pot hydrosilylation polyaddition polymerization and crosslinking reaction in the preparation of polysiloxane main‐chain liquid‐crystalline elastomers (MC‐LCEs). Due to the extraordinary effect of acoustic cavitation, the polyaddition polymerization and crosslinking reaction can be successfully carried out in an ordinary laboratory ultrasonic cleaner at room temperature, and generates the LCE matrix network in about 30 min. The prepared MC‐LCEs demonstrate good quality, good properties, and stimuli‐actuation performances. Compared to the traditional thermal processing methods for preparing polysiloxane MC‐LCEs, this method exhibits superior properties rapidly, with high convenience and efficiency, and can be a path for batch fabrication at low cost. The work also demonstrates that the ultrasonics sonochemistry method is effective in generating linear main‐chain liquid crystal polymers through hydrosilylation polyaddition and polysiloxane side‐chain LCE matrix through hydrosilylation crosslinking reaction, thus confirming the high availability of ultrasonics sonochemistry in the processes of hydrosilylation polymerization, crosslinking reactions, or synchronous polymerization and crosslinking reactions.

Effect of Block Immiscibility on Strain-Induced Microphase Segregation and Crystallization of Model Block Copolymer Elastomers

Main-chain liquid crystalline elastomers (LCE) of ABA block copolymer chains are simulated using a polymer bead–spring model to understand how their sawtooth tensile behavior is affected by changes...

Trial Manufacture of Nanofibers Made from a Main-Chain Liquid-Crystalline Elastomer Composed of Bibenzoate Mesogens

Trial Manufacture of Nanofibers Made from a Main-Chain Liquid-Crystalline Elastomer Composed of Bibenzoate Mesogens

Surface Aligned Main-Chain Liquid Crystalline Elastomers: Tailored Properties by the Choice of Amine Chain Extenders

A promising way to induce shape transformation in soft materials is via spatial variation in the orientation of the alignment of liquid crystalline elastomers (LCEs). Here, we improve the nascent thermomechanial shape transformation in main-chain LCEs prepared via aza-Michael addition reactions. Specifically, increasing the alkyl length in the n-alkylamine chain extender effectively reduces the actuation temperature by destabilizing the nematic phase as well as reduces the glass transition temperature (Tg) by increasing the free volume. In addition, incorporating a hydroxyl end-group in the amine chain extender (i.e., n-alkanolamine) increases the actuation strain and improves the film quality by preventing side-chain aggregates of n-alkylamine-functionalized LCEs. Interestingly, uniaxially aligned n-alkanolamine-functionalized LCEs exhibit an unprecedentedly large elongation and an enhanced toughness even along the loading direction likely due to hydrogen bonding between chains. Thus, our study highlight...

Influence of Cross-Linker Concentration on Physical Properties of Main-Chain Liquid Crystalline Elastomers

We studied physical properties of main-chain liquid crystalline elastomers (MCLCEs) to observe the influence of variations of cross-linker concentrations on properties such as spontaneous deformation, elastic free energy, and order parameter. Cross-linker concentrations of four MCLCE samples were 8%, 12%, 14%, and 16%. Samples with higher cross-linker concentrations underwent spontaneous deformation to a greater extent than those with lower concentrations, indicated by a smaller aspect ratio of length from the end to the beginning of heating. We also reformulated the elastic free energy expression to observe the dependence of the physical quantity on temperature variation. It showed that the maximum relative elastic free energy of the MCLCEs occurred at different temperatures, dependent on cross-linking density, where higher density caused the maximum relative free energy to decrease. The cross-linker concentration also significantly influenced the order parameter of the samples, where the higher the cross-linker concentration, the higher the maximum order parameter.

PB19 Bibenzoateをメソゲン基とする主鎖型液晶エラストマーの変形挙動と配向秩序パラメータ(液晶化学材料,ポスター発表,2015年日本液晶学会討論会)

PB19 Bibenzoateをメソゲン基とする主鎖型液晶エラストマーの変形挙動と配向秩序パラメータ(液晶化学材料,ポスター発表,2015年日本液晶学会討論会)

Thiol‐acrylate main‐chain liquid‐crystalline elastomers with tunable thermomechanical properties and actuation strain

ABSTRACTThe purpose of this study was to investigate the influence of cross‐linking on the thermomechanical behavior of liquid‐crystalline elastomers (LCEs). Main‐chain LCE networks were synthesized via a thiol‐acrylate Michael addition reaction. The robust nature of this reaction allowed for tailoring of the behavior of the LCEs by varying the concentration and functionality of the cross‐linker. The isotropic rubbery modulus, glass transition temperature, and strain‐to‐failure showed strong dependence on cross‐linker concentration and ranged from 0.9 MPa, 3 °C, and 105% to 3.2 MPa, 25 °C, and 853%, respectively. The isotropic transition temperature (Ti) was shown to be influenced by the functionality of the cross‐linker, ranging from 70 °C to 80 °C for tri‐ and tetra‐functional cross‐linkers. The magnitude of actuation can be tailored by controlling the amount of cross‐linker and applied stress. Actuation increased with increased applied stress and decreased with greater amounts of cross‐linking. The maximum strain actuation achieved was 296% under 100 kPa of bias stress, which resulted in work capacity of 296 kJ/m3 for the lowest cross‐linked networks. Overall, the experimental results provide a fundamental insight linking thermomechanical properties and actuation to a homogenous polydomain nematic LCE networks with order parameters of 0.80 when stretched. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 157–168

Electrocaloric and elastocaloric effects in soft materials.

Materials with large caloric effect have the promise of realizing solid-state refrigeration which has potential to be more efficient and environmentally friendly compared with current cooling technologies. Recently, the focus of caloric effects investigations has shifted towards soft materials. An overview of recent direct measurements of the large electrocaloric effect (ECE) in a composite mixture of a liquid crystal and nanoparticles (NPs) and large elastocaloric (eC) effect in main-chain liquid crystal elastomers is given. In mixtures of 12CB liquid crystal with functionalized CdSSe NPs, an ECE exceeding 5 K was found in the vicinity of the isotropic to smectic A phase transition. It is shown that the NPs smear the isotropic to smectic coexistence range in which a large ECE is observed due to latent heat enhancement. NPs acting as traps for ions reduce the moving-ion density and consequently the Joule heating. Direct eC measurements indicate that the significant eC response can be found in main-chain liquid crystalline elastomers, but at a fraction of the stress field in contrast to other eC materials. Both soft materials could play a significant role as active cooling elements or parts of thermal diodes in development of new cooling devices.This article is part of the themed issue 'Taking the temperature of phase transitions in cool materials'.

Large spontaneous deformation of uniaxially oriented main-chain liquid–crystalline elastomers composed of BB-n mesogens

ABSTRACTWe observed the spontaneous shape change of a uniaxially oriented liquid–crystalline elastomer composed of smectic main-chain liquid–crystalline polyesters in a cyclic heating–cooling process. Although the elastomer contracted by about 650% on heating up to the isotropic phase, the sample length recovered by about 450% on cooling to room temperature in the first heating–cooling process. In contrast, the elastomer exhibited completely reversible deformation in the second heating–cooling process. The shape change occurred with almost no change in the orientational order of mesogens in the temperature region of the liquid–crystalline phase. The mechanism of the spontaneous thermotropic shape change is discussed.

Synthesis of Programmable Main-chain Liquid-crystalline Elastomers Using a Two-stage Thiol-acrylate Reaction

This study presents a novel two-stage thiol-acrylate Michael addition-photopolymerization (TAMAP) reaction to prepare main-chain liquid-crystalline elastomers (LCEs) with facile control over network structure and programming of an aligned monodomain. Tailored LCE networks were synthesized using routine mixing of commercially available starting materials and pouring monomer solutions into molds to cure. An initial polydomain LCE network is formed via a self-limiting thiol-acrylate Michael-addition reaction. Strain-to-failure and glass transition behavior were investigated as a function of crosslinking monomer, pentaerythritol tetrakis(3-mercaptopropionate) (PETMP). An example non-stoichiometric system of 15 mol% PETMP thiol groups and an excess of 15 mol% acrylate groups was used to demonstrate the robust nature of the material. The LCE formed an aligned and transparent monodomain when stretched, with a maximum failure strain over 600%. Stretched LCE samples were able to demonstrate both stress-driven thermal actuation when held under a constant bias stress or the shape-memory effect when stretched and unloaded. A permanently programmed monodomain was achieved via a second-stage photopolymerization reaction of the excess acrylate groups when the sample was in the stretched state. LCE samples were photo-cured and programmed at 100%, 200%, 300%, and 400% strain, with all samples demonstrating over 90% shape fixity when unloaded. The magnitude of total stress-free actuation increased from 35% to 115% with increased programming strain. Overall, the two-stage TAMAP methodology is presented as a powerful tool to prepare main-chain LCE systems and explore structure-property-performance relationships in these fascinating stimuli-sensitive materials.

Synthesis of Programmable Main-chain Liquid-crystalline Elastomers Using a Two-stage Thiol-acrylate Reaction

This study presents a novel two-stage thiol-acrylate Michael addition-photopolymerization (TAMAP) reaction to prepare main-chain liquid-crystalline elastomers (LCEs) with facile control over network structure and programming of an aligned monodomain. Tailored LCE networks were synthesized using routine mixing of commercially available starting materials and pouring monomer solutions into molds to cure. An initial polydomain LCE network is formed via a self-limiting thiol-acrylate Michael-addition reaction. Strain-to-failure and glass transition behavior were investigated as a function of crosslinking monomer, pentaerythritol tetrakis(3-mercaptopropionate) (PETMP). An example non-stoichiometric system of 15 mol% PETMP thiol groups and an excess of 15 mol% acrylate groups was used to demonstrate the robust nature of the material. The LCE formed an aligned and transparent monodomain when stretched, with a maximum failure strain over 600%. Stretched LCE samples were able to demonstrate both stress-driven thermal actuation when held under a constant bias stress or the shape-memory effect when stretched and unloaded. A permanently programmed monodomain was achieved via a second-stage photopolymerization reaction of the excess acrylate groups when the sample was in the stretched state. LCE samples were photo-cured and programmed at 100%, 200%, 300%, and 400% strain, with all samples demonstrating over 90% shape fixity when unloaded. The magnitude of total stress-free actuation increased from 35% to 115% with increased programming strain. Overall, the two-stage TAMAP methodology is presented as a powerful tool to prepare main-chain LCE systems and explore structure-property-performance relationships in these fascinating stimuli-sensitive materials.

Homeotropically-aligned main-chain and side-on liquid crystalline elastomer films with high anisotropic thermal conductivities.

Homeotropically-aligned main-chain and side-on liquid crystalline elastomer films are prepared by using LC thiol-ene and acrylate systems respectively. Evaluated by laser flash analysis, the room temperature thermal conductivities of these two LCP films in the film normal direction are both dramatically higher than those along the horizontal direction.

Controlled Shape Memory Behavior of a Smectic Main-Chain Liquid Crystalline Elastomer

A smectic main-chain liquid crystalline elastomer (LCE), with controlled shape memory behavior, is synthesized by polymerizing a biphenyl-based epoxy monomer with an aliphatic carboxylic acid curing agent. Microstructures of the LCEs, including their liquid crystallinity and cross-linking density, are modified by adjusting the stoichiometric ratio of the reactants to tailor the thermomechanical properties and shape memory behavior of the material. Thermal and liquid crystalline properties of the LCEs, characterized using differential scanning calorimetry and dynamic mechanical analysis, and structural analysis, performed using small-angle and wide-angle X-ray scattering, show that liquid crystallinity, cross-linking density, and network rigidity are strongly affected by the stoichiometry of the curing reaction. With appropriate structural modifications it is possible to tune the thermal, dynamic mechanical, and thermomechanical properties as well as the shape memory and thermal degradation behavior of LCEs.

Tailorable and programmable liquid-crystalline elastomers using a two-stage thiol–acrylate reaction

A methodology is introduced to synthesize main-chain liquid-crystalline elastomers (LCEs) using a thiol–acrylate-based reaction. This method can program an aligned LCE monodomain and offer spatio-temporal control over liquid-crystalline behavior.

Reversible actuation in main-chain liquid crystalline elastomers with varying crosslink densities

Main-chain smectic-C liquid crystalline elastomers (LCEs) with varying crosslink densities were prepared using a two stage hydrosilylation reaction scheme, in which diene mesogens were first polymerized with hydride-terminated poly(dimethylsiloxane) and subsequently crosslinked using a tetravinyl molecule. Adjustment of prepolymer molecular weight afforded control over network crosslink density. The LCEs exhibit two thermal transitions that are determined by the mesogen composition and are independent of crosslink density: the mesogen glass transition (∼−30 °C) and isotropization (∼40 °C). Thermal cycling around the isotropization transition under tensile load leads to reversible extension (cooling) and contraction (heating) of the sample, a phenomenon called two-way shape memory or “actuation”. Actuation strains reached up to 30%, a substantial amount for a polydomain LCE. Creep experiments revealed different dynamics between the smectic and isotropic phases, and comparison to actuation results indicated that actuation involves more than simply a transition between isotropic and smectic rheological behavior. Instead, the phase transition itself plays an important role. Wide-angle X-ray scattering analysis revealed that samples strained to the same level, whether by creep or actuation, showed different orientation levels. This indicates that microstructure is not a unique property of the deformed state and supports the hypothesis that cooling to the liquid crystalline phase under stress is important to achieving the large strains associated with actuation.

Evolution of microstructure during shape memory cycling of a main-chain liquid crystalline elastomer

The field of shape memory polymers (SMPs) has been dominated by polymeric systems whose fixing mechanism is based on crystallization or vitrification of the constituent chains, rendering such systems stiff in comparison to elastomers, gels, and living tissues. Previously, we reported the synthesis and characterization of main-chain, segmented liquid crystalline elastomers (LCEs) that exhibit both bulk and surface shape memory effects. These LCEs have excellent shape fixing and recovery characteristics with compositionally-dependent transition temperatures that determine the fixing and recovery critical temperatures. Synthesis of the soft shape memory LCEs proceeded by hydrosilylation-linking of poly(dimethylsiloxane) oligomers with mesogenic dienes of two compositions and a tetravinyl crosslinker. The present report describes microstructural changes during ex situ shape memory deformation and recovery of one such LCE. Wide-angle x-ray scattering showed that, once a critical deformation stress was reached, the microstructure of the fixed, oriented LCEs was independent of the stress applied above the clearing (isotropization) transition. Stepped recovery of the fixed, oriented LCE showed additional intermediate microstructures, however. Recovery was shown to proceed through changes in both the smectic layer thickness and chevron architecture, while mesogen tilt angle remained unchanged. The mechanical and microstructural studies described herein give deeper insight to shape memory fixing and recovery mechanisms of these unique materials, which offer potential for exploitation in areas such as cell and tissue culture, microcontact printing, and microfluidics.