Liquid Crystalline Elastomers Based on Click Chemistry

Enhancing Thermo-Mechanical Properties of Liquid Crystal Elastomers through Chain Entanglements

In this study, we present an approach to enhance the thermo-mechanical performance of liquid crystal elastomers (LCEs) by inducing chain entanglements through mechanical kneading. This process creates a network of highly entangled polymer chains, significantly improving the mechanical properties of LCEs, over a wide range of strain rates and temperatures. Mechanical kneading also improves the actuation performance, resulting in higher actuation stresses, greater contraction, and increased tolerance to self-rupture at elevated temperatures. Chain entanglements can also serve as a crucial enabler for the fabrication of monodomain LCEs. Using entanglements as the initial cross-linking step provides sufficient elasticity to LCEs, enabling the synthesis of aligned LCEs. This work demonstrates the benefits of chain entanglements, offering a pathway for the design and fabrication of high-performance LCE-based actuators for advanced applications.

Facile Integration of Bacterial Cellulose with Liquid Crystal Elastomers Enables Robust Biomimetic Helical Yarn Actuators

Inspired from helical structures in nature, liquid crystal elastomer (LCE) fiber actuators are developed for soft robotics and smart wearables. However, the facile development of robust LCE yarn actuators remains challenging due to the lightly cross-linked networks of LCE with the inherently poor mechanical properties. Here, the bionic helical yarn actuator is constructed through integrating the shape-morphing LCE fiber as the actuation phase and the highly ordered orientation biomass bacterial cellulose (BC) macrofibers as the reinforcement phase by a facile twisting and two-step cross-linking strategy. Thanks to the 3D nanofiber network inside BC macrofibers and biomimetic helical structure, the mechanical strength (43.9 MPa) and the creep phenomenon of the resulted yarn have been significantly improved, which are obviously better than the reported LCE fiber actuators (1.4-30.8 MPa). The designed LCE/BC helical yarn actuators demonstrate high work capacity (304.1 J kg-1) and reliable reusability. As a proof-of-concept, this work constructs micro rolling device with customizable speed, soft gripper for grasping and moving heavy objects and passive micro motor with a speed of 7.7 rad s-1. The findings of this work are expected to provide insights into the development of high-performance and durable smart yarn actuators through biomimetic engineering strategies.

Exploiting photopolymerization to modulate liquid crystalline network actuation

Liquid Crystalline Networks (LCNs) are widely investigated to develop actuators, from soft robots to artificial muscles. Indeed, they can produce forces and movements in response to a plethora of external stimuli, showing kinetics up to the millisecond time-scale. One of the most explored preparation technique involves the photopolymerization of an aligned layer of reactive mesogens. Following this approach, side-chain polymers are widely described, while a detailed comparison of light-responsive LCNs with different architectures is not properly addressed. In this paper, two synthetic approaches are exploited leading to photoresponsive LCNs with different architectures. Mixed main-chain/side-chain LCNs are obtained in one-pot through a thiol-acrylate chain-transfer reaction, while main-chain LCNs are achieved by a two-step approach involving an aza-Michael addition followed by acrylate crosslinking. Comparison among the two materials highlighted the superior performances in terms of tension developed upon light-activation of the former one, showing muscle-like force production comparable to standard side-chain LCNs combined with the greater ability to contract from common main-chain LCNs.

Smiles-Rearrangement-Based One-Pot Synthesis of Diarylacetylenes from Benzylic Sulfones and Methyl Benzoates Mediated by LiN(SiMe3)2/KN(SiMe3)2

The one-pot synthesis of diphenylacetylene by the reaction of methyl benzoate with 1-(benzylsulfonyl)-3,5-di(trifluoromethyl)benzene was developed. The combination of LiN(SiMe3)2 and KN(SiMe3)2 is key to promoting the reaction. Simply combining methyl benzoate, 1-(benzylsulfonyl)-3,5-di(trifluoromethyl)benzene, LiN(SiMe3)2, and KN(SiMe3)2 can produce a variety of diaryl acetylenes (28 examples, 18-70% yields).

3D Printing of Thermally Responsive Shape Memory Liquid Crystalline Epoxy Networks

A two-component liquid crystalline epoxy network (LCEN) with shape memory behavior was developed and evaluated as a candidate material for 3D printing. The cure kinetics of the uncured material and the shape memory properties of the cured LCEN were investigated by using parallel plate rheology and dynamic mechanical analysis, respectively. A commercially available fumed silica additive was introduced to the neat, uncured material to improve the rheological properties for 3D printing. The addition of fumed silica was found to increase the yield stress, shear-thinning behavior, and toughness of the uncured epoxy ink. Polarized light microscopy, differential scanning calorimetry, and wide-angle X-ray scattering measurements between the neat and additive-modified LCEN suggested a reduction in liquid crystalline alignment in the modified LCEN, owing to interactions between crystalline domains and fumed silica, which in turn influenced the mechanical behavior. Overall, the additive was found to be successful in preserving the shape memory properties of LCEN while improving its printability.

Programmable, Self-Healable, and Photochromic Liquid Crystal Elastomers Based on Dynamic Hindered Urea Bonds for Biomimetic Flowers

Recently, researchers have been exploring the use of dynamic covalent bonds (DCBs) in the construction of exchangeable liquid crystal elastomers (LCEs) for biomimetic actuators and devices. However, a significant challenge remains in achieving LCEs with both excellent dynamic properties and superior mechanical strength and stability. In this study, a diacrylate-functionalized monomer containing dynamic hindered urea bonds (DA-HUB) is employed to prepare exchangeable LCEs through a self-catalytic Michael addition reaction. By incorporating DA-HUB, the LCE system benefits from DCBs and hydrogen bonding, leading to materials with high mechanical strength and a range of dynamic properties such as programmability, self-healing, and recyclability. Leveraging these characteristics, bilayer LCE actuators with controlled reversible thermal deformation and outstanding dimensional stability are successfully fabricated using a simple welding method. Moreover, a biomimetic triangular plum, inspired by the blooming of flowers, is created to showcase reversible color and shape changes triggered by light and heat. This innovative approach opens new possibilities for the development of biomimetic and smart actuators and devices with multiple functionalities.

Fundamental Aspects of Stretchable Mechanochromic Materials: Fabrication and Characterization

Mechanochromic materials provide optical changes in response to mechanical stress and are of interest in a wide range of potential applications such as strain sensing, structural health monitoring, and encryption. Advanced manufacturing such as 3D printing enables the fabrication of complex patterns and geometries. In this work, classes of stretchable mechanochromic materials that provide visual color changes when tension is applied, namely, dyes, polymer dispersed liquid crystals, liquid crystal elastomers, cellulose nanocrystals, photonic nanostructures, hydrogels, and hybrid systems (combinations of other classes) are reviewed. For each class, synthesis and processing, as well as the mechanism of color change are discussed. To enable materials selection across the classes, the mechanochromic sensitivity of the different classes of materials are compared. Photonic systems demonstrate high mechanochromic sensitivity (Δnm/% strain), large dynamic color range, and rapid reversibility. Further, the mechanochromic behavior can be predicted using a simple mechanical model. Photonic systems with a wide range of mechanical properties (elastic modulus) have been achieved. The addition of dyes to photonic systems has broadened the dynamic range, i.e., the strain over which there is an optical change. For applications in which irreversible color change is desired, dye-based systems or liquid crystal elastomer systems can be formulated. While many promising applications have been demonstrated, manufacturing uniform color on a large scale remains a challenge. Standardized characterization methods are needed to translate materials to practical applications. The sustainability of mechanochromic materials is also an important consideration.

Shape programming of liquid crystal elastomers

Liquid crystal elastomers (LCEs) are shape-morphing materials that demonstrate reversible actuation when exposed to external stimuli, such as light or heat. The actuation's complexity depends heavily on the instilled liquid crystal alignment, programmed into the material using various shape-programming processes. As an unavoidable part of LCE synthesis, these also introduce geometrical and output restrictions that dictate the final applicability. Considering LCE's future implementation in real-life applications, it is reasonable to explore these limiting factors. This review offers a brief overview of current shape-programming methods in relation to the challenges of employing LCEs as soft, shape-memory components in future devices.

Copolymerization Involving Sulfur-Containing Monomers

Incorporating sulfur (S) atoms into polymer main chains endows these materials with many attractive features, including a high refractive index, mechanical properties, electrochemical properties, and adhesive ability to heavy metal ions. The copolymerization involving S-containing monomers constitutes a facile method for effectively constructing S-containing polymers with diverse structures, readily tunable sequences, and topological structures. In this review, we describe the recent advances in the synthesis of S-containing polymers via copolymerization or multicomponent polymerization techniques concerning a variety of S-containing monomers, such as dithiols, carbon disulfide, carbonyl sulfide, cyclic thioanhydrides, episulfides and elemental sulfur (S8). Particularly, significant focus is paid to precise control of the main-chain sequence, stereochemistry, and topological structure for achieving high-value applications.

Improvement of Intracellular Interactions through Liquid Crystalline Elastomer Scaffolds by the Alteration of Topology

Preparation of inherently bioactive scaffolds has become a challenging issue owing to their complicated synthesis and nonrobust modified cell-actuating property. Liquid crystalline elastomers (LCEs), due to their combined specialties of liquid crystals and elastomers as well as their ability to respond to various kinds of stimuli, have reversibly led to the design of a new class of stimuli-responsive tissue-engineered scaffolds. In this line, in the first stage of this research work, synthesis and evaluation of acrylate-based LCE films (LCEfilm) encompassing mesogenic monomers are carried out. In the second step, the design of an affordable electrospinning technique for preparing LCE nanofibers (LCEfiber) as a problematic topic, thanks to the low molecular weight of the mesogenic chains of LCEs, is investigated. For this purpose, two approaches are considered, including (1) photo-cross-linking of electrospun LCEfiber and (2) blending LCE with poly(ε-caprolactone) (PCL) to produce morphologically stable nanofibers (PCL-LCEfiber). In the following, thermal, mechanical, and morphological evaluations show the optimized crosslinker (mol)/aliphatic spacer (mol) molar ratio of 50:50 for LCEfilm samples. On the other hand, for LCEfiber samples, the appropriate amounts of excessive mesogenic monomer and PCL/LCE (v/v) to fabricate the uniform nanofibers are determined to be 20% and 1:2, respectively. Eventually, PC12 cell compatibility and the impact of the liquid crystalline phase on the PC12 cell dynamic behavior of the samples are examined. The obtained results reveal that PC12 cells cultured on electrospun PCL-LCEfiber nanofibers with an average diameter of ∼659 nm per sample are alive and the scaffold has susceptibility for cell proliferation and actuation because of the rapid increase in cell density and number of singularity points formed in time-lapse cell imaging. Moreover, the PCL-LCEfiber nanofibrous scaffold exhibits a high performance for cell differentiation according to detailed biological evaluations such as gene expression level measurements. The time-lapse evaluation of PC12 cell flow fields confirms the significant influence of the reprogrammable liquid crystalline phase in the PCL-LCEfiber nanofibrous scaffold on topographical cue induction compared to the biodegradable PCL nanofibers.

Photopolymerization of 1D photonic structures induced by nematic-isotropic phase transition in liquid crystal

In this paper, two types of polymer-stabilized periodic structures created by photopolymerization of a nematic liquid crystal confined in a cylindrical structure are presented. Both types of structures were induced by nematic-isotropic phase transition in liquid crystal doped with gold nanoparticles. The first type of structure was created by stabilizing periodic phase separation at the nematic-isotropic phase transition temperature. As a result, a periodic structure with two distinct molecular orientations of nematic liquid crystal was achieved. The period of this structure was equal to the period induced by nematic-isotropic phase separation. The second type of structure, also related to the phase transition, was created due to an induced periodic density change of gold nanoparticles in the sample volume. Through photopolymerization it was possible to preclude the dispersion of gold nanoparticles while preserving the periodicity. An increased concentration of gold nanoparticles caused periodic defects in molecular orientation of the liquid crystal. Both types of structures were stable at room temperature. Consequently, two types of 1D photonic structures stabilized by photopolymerization are presented.

Design of Surface-Aligned Main-Chain Liquid-Crystal Networks Prepared under Ambient, Light-Free Conditions Using the Diels-Alder Cycloaddition

Surface-aligned liquid-crystal networks (LCNs) offer a solution for developing functional materials capable of performing a range of tasks, including actuation, shape memory, and surfaces patterning. Here we show that Diels-Alder cycloaddition can be used to prepare the backbone of planar aligned LCNs under mild ambient conditions without the addition of additives or UV irradiation. The mechanical properties of the networks have robust viscoelastic modulus and stiffness with a reversible local free volume change upon physical aging. This study shows new opportunities to design surface-aligned LCNs based on additive free step-growth Diels-Alder polymerization and enables the potential to incorporate a wider range of photochromic materials into LCNs.

Optical wavelength selective actuation of dye doped liquid crystalline elastomers by quasi-daylight

We explore obtaining different photo responses of liquid crystalline elastomer (LCE) materials through modulating the optical wavelengths in order to promote the development of precise photocontrol on LCE actuators, and thus study the effect of light-absorbing dyes with different absorption bands on the selective actuation of LCE materials. The dye-doped LCEs were prepared by incorporating special visible absorber dyes into thiol-acrylate main chain LCE (MC-LCE) matrices. The dyes showed photo actuation performance to LCEs due to the photothermal effects. But, every dye-doped LCE could be effectively actuated by light irradiation whose wavelength was inside its absorption band, but could not be effectively actuated by the light whose wavelength was beyond its absorption band. Wavelength selective actuation effects, no matter actuating deformation or actuating force, could be remarkably demonstrated by these dye-doped LCEs through filtering the same quasi-daylight source to be different wavelength bands. Our work opens up a significant way for the precise and convenient photo actuation of LCE actuators, while expanding the utilization potential of quasi-daylight, and further natural sunlight.

A Convergent Approach to 1,3-Dithiolan-2-ones and an Unexpected Synthesis of Lactones

A convergent route to 1,3-dithiolan-2-ones based on the radical addition of xanthates to allylic acetates is described. The process is modular, uses inexpensive starting materials and reagents, and is atom economical, since both sulfur atoms of the xanthate end up in the products. With adducts derived from xanthates bearing an ester group, an unexpected transformation leading to lactones was uncovered.