Liquid crystalline epoxy resin with improved thermal conductivity by intermolecular dipole–dipole interactions

To address tremendous needs for developing efficiently heat dissipating materials with lightweights, a series of liquid crystalline epoxy resins (LCEs) are designed and synthesized as thermally conductive matrix. All prepared LCEs possess epoxies at the molecular side positions and cyanobiphenyl mesogenic end groups. Based on several experimental results such as differential scanning calorimetry, polarized optical microscopy, and X‐ray diffraction, it is found that the LCEs exhibited liquid crystalline mesophases. When LCE is cured with a diamine crosslinker, the cured LCE maintains the oriented LC domain formed in the uncured state, ascribing to a presence of dipole–diploe and π–π interactions between cyanobiphenyl mesogenic end groups. Due to the anisotropic molecular orientation, the cured LCE exhibits a high thermal conductivity of 0.46 W m^?1 K^?1, which is higher than those of commercially available crystalline or amorphous epoxy resins. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 708–715     

Photomechanics of liquid-crystalline elastomers and other polymers

Muscle is a transducer that can convert chemical energy into mechanical motion. To construct artificial muscles, it is desirable to use soft materials with high mechanical flexibility and durability rather than hard materials such as metals. For effective muscle-like actuation, materials with stratified structures and high molecular orders are necessary. Liquid-crystalline elastomers (LCEs) are superior soft materials that possess both the order of liquid crystals and the elasticity of elastomers (as they contain polymer networks). With the aid of LCEs, it is possible to convert small amounts of external energy into macroscopic amounts of mechanical energy. In this Review, we focus on light as an energy source and describe the recent progress in the area of soft materials that can convert light energy into mechanical energy directly (photomechanical effect), especially the photomechanical effects of LCEs with a view to applications for light-driven LCE actuators.     

Shape recovery liquid crystal elastomer synthesized by melt‐polycondensation

A novel liquid crystal elastomer (LCE) synthesized by melt polymerization, which exhibits the capacity of shape memory, is reported here for the first time. The method of synthesize the shape memory LCE has been explored. A facile two‐step method to synthesize these anisotropic materials to realize reversible shape change behavior is reported. The first reaction is the addition of nematic liquid crystal molecules to form a kind of liquid crystal polymer. Subsequently, the polymer is crosslinked to trap the order of the liquid crystal into a crosslinked LCE. The LCE exhibits liquid crystalline behavior which has shape memory with excellent fixity and recovery. Its shape memory and actuating properties also have been studied. When reheating the LCE to 165 °C, the shape will recover. The main chains and crosslinked bonds of the LCE contain ester groups, which are sensitive to alkaline and acidic condition. It turns out that the LCE is intact under acidic condition, but it can be degraded under alkaline condition. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 389–394     

Nonpolar second‐order nonlinear and electrooptic materials: Axially ordered chiral polymers and liquid crystals

This article reviews our work on the development and optimization of chiral, nonpolar media with large second‐order nonlinear optical responses. We show how molecular engineering, theory, and measurements can be used to optimize this promising class of nonlinear optical materials. We describe how supramolecular alignment into easily processable materials takes advantage of the relevant molecular hyperpolarizabilities. A wide variety of techniques can be used to fabricate bulk materials belonging to the chiral nonpolar symmetry groups, D_∞ and D₂. The microscopic chromophore alignment schemes that optimize the nonlinear optical response in such materials are deduced from general symmetry considerations for both molecules and bulk. We also speculate on the possible applications of such materials as image‐plane modulators. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2744–2754, 2003     

Mechanotropic Elastomers

Liquid crystal elastomers (LCEs) are anisotropic polymeric materials. When subjected to an applied stress, liquid crystalline (LC) mesogens within the elastomeric polymer network (re)orient to the loading direction. The (re)orientation during deformation results in nonlinear stress‐strain dependence (referred to as soft elasticity). Here, we uniquely explore mechanotropic phase transitions in elastomers with appreciable mesogenic content and compare these responses to LCEs in the polydomain orientation. The isotropic (amorphous) elastomers undergo significant directional orientation upon loading, evident in strong birefringence and x‐ray diffraction. Functionally, the mechanotropic displacement of the elastomers to load is also nonlinear. However, unlike the analogous polydomain LCE compositions examined here, the isotropic elastomers rapidly recover after deformation. The mechanotropic orientation of the mesogens in these materials increase the toughness of these thiol‐ene photopolymers by nearly 1300 % relative to a chemically similar elastomer prepared from wholly isotropic precursors.     

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

The 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 (T_i) 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     

Effects of Structural Variations on the Cellular Response and Mechanical Properties of Biocompatible,Biodegradable, and Porous Smectic Liquid Crystal Elastomers

The authors report on series of side‐chain smectic liquid crystal elastomer (LCE) cell scaffolds based on star block‐copolymers featuring 3‐arm, 4‐arm, and 6‐arm central nodes. A particular focus of these studies is placed on the mechanical properties of these LCEs and their impact on cell response. The introduction of diverse central nodes allows to alter and custom‐modify the mechanical properties of LCE scaffolds to values on the same order of magnitude of various tissues of interest. In addition, it is continued to vary the position of the LC pendant group. The central node and the position of cholesterol pendants in the backbone of ε‐CL blocks (alpha and gamma series) affect the mechanical properties as well as cell proliferation and particularly cell alignment. Cell directionality tests are presented demonstrating that several LCE scaffolds show cell attachment, proliferation, narrow orientational dispersion of cells, and highly anisotropic cell growth on the as‐synthesized LCE materials.

     

A Novel Side-Chain Liquid Crystal Elastomer Exhibiting Anomalous Reversible Shape Change

Liquid crystalline elastomers (LCEs) have been actively investigated as stimuli-controlled actuators and soft robots. The basis of these applications is the ability of LCEs to undergo a reversible shape change upon a liquid crystalline (LC)-isotropic phase transition. Herein, we report the synthesis of a novel LCE based on a side-chain liquid crystalline polymer (SCLCP). In contrast to known LCEs, this LCE exhibits a striking anomalous shape change. Subjecting a mechanically stretched monodomain strip to LC-disorder phase transition, both the length and width of the strip contract in isotropic phase, and both elongate in LC phase. This thermally induced behaviour is the result of a subtle interplay between the relaxation of polymer main chain oriented along the stretching direction and the disordering of side-group mesogens oriented perpendicularly to the stretching direction. This finding points out potential design of LCEs of this peculiar type and possible applications to exploit.     

Highly thermal conductive benzoxazine‐epoxy interpenetrating polymer networks containing liquid crystalline structures

A diamine‐based benzoxazine monomer (Bz) and a liquid crystalline epoxy monomer (LCE) are synthesized, respectively. Subsequently, a benzoxazine‐epoxy interpenetrating polymer network (PBEI) containing liquid crystalline structures is obtained by sequential curing of the LCE and the Bz in the presence of imidazole. The results show that the preferential curing of LCE plays a key role in the formation mechanism of liquid crystalline phase. Due to the introduction of liquid crystalline structures, the thermal conductivity of PBEI increases with increasing content of LCE. When the content of LCE is 80 wt %, the thermal conductivity reaches 0.32 W m^?1 K^?1. Additionally, the heat‐resistance of PBEI is superior to liquid crystalline epoxy resin. Among them, PBEI55 containing equal weight of Bz and LCE has better comprehensive performance. Its thermal conductivity, glass transition temperature, and the 5 % weight loss temperature are 0.28 W m^?1 K^?1, 160 °C, and 339 °C, respectively. By introducing boron nitride (BN) fillers into PBEI55, a composite of PBEI/BN with the highest thermal conductivity of 3.00 W m^?1 K^?1 is obtained. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55 , 1813–1821     

A Novel Side‐Chain Liquid Crystal Elastomer Exhibiting Anomalous Reversible Shape Change

Liquid crystalline elastomers (LCEs) have been actively investigated as stimuli‐controlled actuators and soft robots. The basis of these applications is the ability of LCEs to undergo a reversible shape change upon a liquid crystalline (LC)‐isotropic phase transition. Herein, we report the synthesis of a novel LCE based on a side‐chain liquid crystalline polymer (SCLCP). In contrast to known LCEs, this LCE exhibits a striking anomalous shape change. Subjecting a mechanically stretched monodomain strip to LC‐disorder phase transition, both the length and width of the strip contract in isotropic phase, and both elongate in LC phase. This thermally induced behaviour is the result of a subtle interplay between the relaxation of polymer main chain oriented along the stretching direction and the disordering of side‐group mesogens oriented perpendicularly to the stretching direction. This finding points out potential design of LCEs of this peculiar type and possible applications to exploit.     

腰接型侧链液晶弹性体研究进展

液晶弹性体是交联型液晶大分子,兼具液晶取向有序性和交联聚合物熵弹性等特性,在传感器、触发器、微流体装置和仿生器件等方面具有很好的应用前景.制备液晶弹性体的微结构,探索其独特的刺激响应性,是目前液晶弹性体研究的重要方向.侧链液晶弹性体的液晶相态类型取决于其液晶基元和主链的连接方式.腰接型侧链液晶弹性体倾向于形成向列型液晶相,具有较快的响应速度和形变程度,是一类独特的液晶弹性体.本文重点介绍腰接型液晶弹性体微结构(如微米柱、微米线等)的制备;利用金纳米粒子的光热转换效应,实现液晶弹性体光响应性的新途径;以及腰接型侧链液晶弹性体仿生微结构的功能性等.同时还对该领域的发展前景进行了展望.     

One-Step Manufacturing of Supramolecular Liquid-Crystal Elastomers by Stress-Induced Alignment and Hydrogen Bond Exchange

Liquid-crystal elastomers (LCEs) capable of performing large and reversible deformation in response to an external stimulus are an important class of soft actuators. However, their manufacturing process typically involves a multistep approach that requires harsh conditions. For the very first time, LCEs with customized geometries that can be manufactured by a rapid one-step approach at room temperature are developed. The LCEs are hydrogen bond (H-bond) crosslinked main chain polymers comprising flexible short side chains. Applying a stretching/shear force to the LCE can simultaneously induce mesogen alignment and H-bond exchange, allowing for the formation of well-aligned LCE networks stabilized by H-bonds. Based on this working principle, soft actuators in fibers and 2D/3D objects can be manufactured by mechanical stretching or melt extrusion within a short time (e.g. <1 min). These actuators can perform reversible macroscopic motions with large, controlled deformations up to 38 %. The dynamic nature of H-bonds also provides the actuators with reprocessability and reprogrammability. Thus, this work opens the way for the one-step and custom manufacturing of soft actuators.     

可多次使用的液晶型类玻璃高分子

通过高温下酯交换反应的进行,含酯键的液晶型类玻璃高分子(liquid crystalline vitrimer),能够通过简单拉伸进行取向,获得随温度变化可逆伸缩的智能材料.在目前已报道的此类主链型高分子中,酯交换剧烈发生需要的临界温度(Tv),与液晶弹性体发生可逆形变的温度(Ti,即液晶相-各向同性相转变温度)相隔较近,导致材料的使用温度范围比较窄,而且多次升降温后,取向及可逆形变会消失.为解决此问题,本文在原来体系的基础上,通过共聚合另外一种液晶基元,有效地降低了Ti,从而拓宽Ti与Tv之间的距离.这不仅使材料的使用次数明显增加,还能延长此类液晶弹性体的使用期限.     

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

Despite 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     

Simulation-based design of thermally-driven actuators using liquid crystal elastomers

Liquid crystal elastomers (LCEs) are a class of soft functional materials which exhibit complex mechanical responses to external stimuli. Their promise for technological applications is difficult to realise in practice due to the complexity of design, fabrication and performance quantification of these materials. In order to address these issues, simulation-based methods are necessary to both enhance and accelerate the design process, compared to traditional experimentation alone. This work presents such an approach using a hyperelastic solid mechanics model and experimental measurement of material parameters for a thermotropic LCE. The simulation method is validated using existing experimental data of the thermomechanical response of an LCE-based cantilever resulting from a hybrid-aligned nematic texture imposed during crosslinking. The validated method is then used to perform a proof-of-concept design process of an LCE multilegged gripper in order to determine optimal design parameters for gripper performance. The simulation method and results presented in this work represent a significant step towards simulation-based design of LCE materials, which has the potential to overcome the complexity and cost of the LCE design process.     

Mechanotropic Elastomers

Liquid crystal elastomers (LCEs) are anisotropic polymeric materials. When subjected to an applied stress, liquid crystalline (LC) mesogens within the elastomeric polymer network (re)orient to the loading direction. The (re)orientation during deformation results in nonlinear stress‐strain dependence (referred to as soft elasticity). Here, we uniquely explore mechanotropic phase transitions in elastomers with appreciable mesogenic content and compare these responses to LCEs in the polydomain orientation. The isotropic (amorphous) elastomers undergo significant directional orientation upon loading, evident in strong birefringence and x‐ray diffraction. Functionally, the mechanotropic displacement of the elastomers to load is also nonlinear. However, unlike the analogous polydomain LCE compositions examined here, the isotropic elastomers rapidly recover after deformation. The mechanotropic orientation of the mesogens in these materials increase the toughness of these thiol‐ene photopolymers by nearly 1300 % relative to a chemically similar elastomer prepared from wholly isotropic precursors.     

Instant Locking of Molecular Ordering in Liquid Crystal Elastomers by Oxygen‐Mediated Thiol–Acrylate Click Reactions

Liquid crystal elastomers (LCEs) with intrinsic anisotropic strains are reversible shape‐memory polymers of interest in sensor, actuator, and soft robotics applications. Rapid gelation of LCEs is required to fix molecular ordering within the elastomer network, which is essential for directed shape transformation. A highly efficient photo‐cross‐linking chemistry, based on two‐step oxygen‐mediated thiol–acrylate click reactions, allows for nearly instant gelation of the main‐chain LCE network upon exposure to UV light. Molecular orientation from the pre‐aligned liquid crystal oligomers can be faithfully transferred to the LCE films, allowing for preprogrammed shape morphing from two to three dimensions by origami‐ (folding‐only) and kirigami‐like (folding with cutting) mechanisms. The new LCE chemistry also enables widely tunable physical properties, including nematic‐to‐ isotropic phase‐transition temperatures (T_N‐I), glassy transition temperatures (T_g), and mechanical strains, without disrupting the LC ordering.     

Effect of substituents on the curing of liquid crystalline epoxy resin

The synthesis of an aromatic ester based liquid crystalline epoxy resin (LCE) with a substituent in the mesogenic central group is described. Chlorine and methyl groups were introduced as substituents. The curing behaviors of three epoxy resins were investigated using diaminodiphenyl ester as the curing agent. The curing rate and heat of curing of LCE were measured with dynamic and isothermal DSC. The chlorine substituent accelerated the curing of LCE, while the methyl substituent decelerated the curing of LCE. The heat of curing of substituted LCE was diminished compared to LCE with no substituent. Glass transition temperature and elastic modulus of LCE decreased with increasing the size of the substituent. Three liquid crystalline epoxy resins based on aromatic ester mesogenic groups formed a liquid crystalline phase after curing, and the liquid crystalline phase was stable up to the decomposition temperature. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 911–917, 1998     

Self-aligning liquid crystals in polymer composite systems

Soft composite materials combined with a holographic photopolymerization process are used for realizing an innovative switchable periodic structure made of slices of almost pure polymer alternated with films of well aligned nematic liquid crystals named POLICRYPS. It exhibits negligible scattering losses, while the effect of the spatial modulation of the refractive index (from polymer to nematic liquid crystal) can be switched on and off by applying a low (few V/μm) electric field. The diffractive properties of the POLICRYPS structure are characterized in terms of cell thickness, impinging probe angle and wavelength revealing a strong correlation between the diffraction efficiency and all the above mentioned parameters. These results are very attractive for many applications such as switchable Bragg gratings for telecom devices, phase modulators, and displays. Other advantages of the technology include absence of an alignment layer, absence of haze, robust structure, and inexpensive manufacturing. In addition, no special alignment layers are required. This is a unique opportunity and a big advantage compared to conventional liquid crystal devices. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 158–162     

Liquid‐Crystalline Soft Actuators with Switchable Thermal Reprogrammability

Thermal reprogrammability is essential for new‐generation large dry soft actuators, but the realization sacrifices the favored actuation performance. The contradiction between thermal reprogrammability and stability hampers efforts to design high‐performance soft actuators to be robust and thermally adaptable. Now, a strategy has been developed that relies on repeatedly switching on/off thermal reprogrammability in liquid‐crystalline elastomer (LCE) actuators to resolve this problem. By post‐synthesis swelling, a latent siloxane exchange reaction can be induced in the common siloxane LCEs (switching on), enabling reprogramming into on‐demand 3D‐shaped actuators; by switching off the dynamic network by heating, actuation stability is guaranteed even at high temperature (180 °C). Using partially black‐ink‐patterned LCEs, selectively switching off reprogrammability allows integration of completely different actuation modes in one monolithic actuator for more delicate and elaborate tasks.