Supramolecular Chalcogen‐Bonded Shape Memory Actuators

Supramolecular liquid crystal elastomers (LCEs) incorporating chalcogen bonds (ChBs) have been synthesized and investigated for shape memory and actuation properties. LCEs featuring strong Se⋯N ChBs (-40 to -43kJ mol-1) exhibit both shape memory effects (SME) and reversible actuation, while those with weaker S⋯N ChBs (-32 to -34kJ mol-1) do not. The Se⋯N-based LCEs transition from one-way to two-way SME after three days at room temperature, enabling programmable 2D/3D shape transformations and rewritable surface patterns. They also enable the fabrication of light-powered crawling robots as well as temperature-responsive knot-like actuators. These results highlight the critical role of the ChB strength and specificity in designing functional materials. Solid-state NMR, Raman spectroscopy, and density functional theory calculations provide further molecular-level insights into ChB interactions, laying the groundwork for advanced supramolecular actuators and shape-morphing LCEs.

This dissertation demonstrates the design of reversibly self-morphing novel liquid crystalline elastomer (LCE) antennas that can dynamically change electromagnetic performance in response to temperature. This change in performance can be achieved by programming the shape change of stimuli-responsive (i.e., temperature-responsive) LCEs, and using these materials as substrates for reconfigurable antennas. Existing reconfigurable antennas rely on external circuitry such as Micro-Electro-Mechanical-Systems (MEMS) switches, pin diodes, and shape memory alloys (SMAs) to reconfigure their performance. Antennas using MEMS or diodes exhibit low efficiency due to the losses from these components. Also, antennas based on SMAs can change their performance only once as SMAs response to the stimuli and is not reversible. Flexible electronics are capable of morphing from one shape to another using various techniques, such as liquid metals, hydrogels, and shape memory polymers. LCE antennas can reconfigure their electromagnetic performance, (e.g., frequency of operation, polarization, and radiation pattern) and enable passive (i.e., battery-less) temperature sensing and monitoring applications, such as passive radio frequency identification device (RFID) sensing tags. Limited previous work has been performed on shape-changing antenna structures based on LCEs. To date, self-morphing flexible electronics, including antennas, which rely on stimuli-responsive LCEs that reversibly change shape in response to temperature changes, have not been previously explored. Here, LCE antennas will be studied and developed. Also, the metallization of LCEs with different metal conductors and their fabrication process, by either electron beam (E-Beam) evaporation or optical gluing of the metal film will be observed. The LCE material can have a significant impact on sensing applications due to its reversible actuation that can enable a sensor to work repeatedly. This interdisciplinary research (material polymer science and electrical engineering) is expected to contribute to the development of morphing electronics, including sensors, passive antennas, arrays, and frequency selective surfaces (FSS).

Synthesis-Processing-Property Relationships in Thermomechanics of Liquid Crystal 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.

Liquid crystal elastomer (LCE) is a rubbery material composed of polymer chains and liquid crystals (LC). LCE is well known to undergo a shape transformation from the isotropic to the anisotropic phase. This shape transformation is caused by the nematic transition of the LC included in the LCE. However, the mechanism of this transformation is unknown because the interaction of LC with the bulk polymers is too complex. In this presentation, we extend the two-dimensional Finsler geometry model for membranes to a three-dimensional model for LCE. The Finsler geometry model for LCE is a coarse grained one: the Guassian bond potential S1 is obtained by extending the one for membranes, which is originally obtained by a simple extension of the Guassian bond potential for the linear chain polymer model. The continuous Hamiltonian, which contains S1 and the curvature energy S2, is discretized using a three-dimensional rigid sphere composed of tetrahedrons. We study the shape transformation as a phase transition between the isotropic and anisotropic phases and report the results of the transition order, obtained by the Monte Carlo simulations.

A method for fabricating long, soft, and reversibly actuatable liquid crystal elastomer (LCE) fibers by using direct ink write (DIW) printing was developed. Here, the LCE was produced based on a two-stage thermal-photo curingreaction between a difunctional acrylate monomer and thiol. The LCE ink, mixed with nanoclay to increase the viscosity, was extruded through a nozzle onto a rotating mandrel to obtain a long fiber. After printing, the fiber was first thermally cured on the mandrel, then mechanically stretched, and photocured to achieve liquid crystal chain alignment for stress-free reversible activation. Upon optimizing the ink viscosity and DIW printing parameters, long fibers (up to 1.5 m long from the laboratory) were obtained. The resulting fiber had a modulus of 2 MPa, 51% actuation strain, and a failure strain of well over 100%. The potential of these fibers for applications was demonstrated. The LCE fibers were knit, sewn, and woven to form a variety of smart textiles. The fiber was also used to mimic bicep muscles with both large activation force and activation strain. By incorporating further intelligent characteristics, such as conductivity and biosensing into a single fiber, the LCE fibers could be potentially used for smart clothing, soft robotics, and biomedical devices.

The monodomain nematic liquid crystal elastomer (LCE) is a nonporous material with one-dimension (1D) negative thermal expansion (NTE) along the direction of orientation. This stimuli-responsive size change is comparable with the contractile response of a muscle fibre. Recently, LCEs have become more and more popular and increasingly investigated as a full-fledged class of stimuli-responsive materials. However, the nematic LCE is limited to a single, unidirectional deformation unless complex hybrid architectures are made. We develop here a special LCE based on a liquid crystal polymer with an extraordinary phase transition sequence, “re-entrant nematic (NRe) – smectic A (SmA) – nematic (N) – isotropic (I) phase”, along with “prolate – oblate – weakly oblate – spherical” chain conformation evolution. In the aligned LCE film, these conformational changes of polymer chain are observed with the naked eye through their macroscopic translations into a unique sequence of “contraction – expansion – second expansion” deformations. Notably, a switch from NTE to positive thermal expansion (PTE) occurs upon heating. Moreover, a bilayer actuator composed of aligned and non-aligned LCE layers can perform 2D to 3D shape transformation with “curling – uncurling – second uncurling” actuation sequence. This LCE capable of multiple deformations in response to a single stimulus paves the way to multimodal single-material actuators. It provides a new strategy for the development of advanced materials with adjustable and multidirectional deformations.

Intelligent Eucommia ulmoides rubber (EUR) and ionomer Surlyn resin (SR) blends were prepared and studied in this manuscript. This is the first paper to combine EUR with SR to prepare blends with both the shape memory effect and self-healing capability. The mechanical, curing, thermal, shape memory and self-healing properties were studied by a universal testing machine, differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA), respectively. Experimental results showed that the increase in ionomer content not only improved mechanical and shape memory properties but also endowed the compounds with excellent self-healing ability under the appropriate environmental conditions. Notably, the self-healing efficiency of the composites reached 87.41%, which is much higher than the efficiency of other covalent cross-linking composites. Therefore, these novel shape memory and self-healing blends can expand the use of natural Eucommia ulmoides rubber, such as in special medical devices, sensors and actuators.

Liquid crystal elastomers (LCEs) are a class of smart materials that combine the anisotropic properties of liquid crystals with the elasticity of polymers, enabling reversible metamorphosis (i.e., shape transformation) in response to external stimuli. This reversible metamorphosis makes them ideal for many applications including soft robotics, artificial muscles, sensors, actuators, responsive coatings, etc. Prior studies have designed LCE surfaces with superhydrophobicity (i.e., extreme repellency to high surface tension liquid like water), but the combination of reversible metamorphosis of LCEs with superomniphobicity (i.e., extreme repellency to both high and low surface tension liquids) is unexplored. In this work, we developed LCE-based superomniphobic surfaces with reversible metamorphosis by laser texturing followed by low surface energy surface modification. Our LCE-based superomniphobic surfaces display extreme repellence to both aqueous and organic liquids as well as reversible metamorphosis due to nematic-isotropic transition of LCE. Utilizing these properties, we demonstrated loss-less manipulation of aqueous and organic liquid droplets, enabling merging, mixing, chemical reaction and microfluidic gating. We envision that our LCE-based superomniphobic surfaces with reversible metamorphosis will pave the way towards a wide range of applications including microfluidic reactors, lab-on-chip technologies, adaptive liquid-handling devices, controlled drug delivery systems etc.

Liquid crystal elastomers (LCEs) with reversible thermal actuation are promising platforms for multifunctional flexible electronics. Herein, we present a PVDF/LM-LCE (PVDF, polyvinylidene fluoride; LM, liquid metal) composite in which PVDF is polymerized in situ within the LCE matrix to achieve seamless mechanical coupling and efficient stress transfer. LM nanodroplets enhance mechanical robustness, charge transport, and photothermal conversion, enabling LCEs to serve as photothermally driven transducers that amplify the piezoelectric and pyroelectric outputs in these flexible systems. The optimized composite achieves a pyroelectric coefficient of -4.81 nC·cm- 2·K- 1, 1.8 times higher than conventional PVDF films. Furthermore, the composite device powers two LEDs and digital sensors using low-grade photothermal fluctuations. This LCE-based light-driven thermomechanical-to-electrical conversion strategy offers a generalizable pathway for high-performance, flexible pyroelectric and piezoelectric energy-harvesting materials.

Flexible energy harvester based on aligned PZT/SMPU nanofibers and shape memory effect for curved sensors

In this article, we proposed a new way to achieve monostable and bistable characteristics of composite layers based on liquid crystal elastomer (LCE). A smart trilayer composite structure is fabricated using LCE and acrylic elastomer, which can have several morphologies. It keeps flat at room temperature and can deform into a monostable saddle or bistable cylinder surface in response to simple temperature changes. The reversible deformation can be controlled through two parameters including geometrical size and actuation strain. The LCE can be programmed to generate different actuation strains by different formulas during synthesis or different mechanical stretches during UV radiation. The deformed morphology for different sample sizes and actuation strain is calculated using Finite element simulation. By comparison with the experimental results, we confirm that the phenomena can be captured through numerical simulations. Furthermore, to have a quantitative understanding, we use numerical simulation to calculate the deformation of the composite structure by tuning these two parameters and give a morphological portrait illustrating the relationship between the deformed shape and control parameters.

The advent of additive manufacturing (AM), commonly known as 3D printing, has enabled the rapid fabrication of complex structures previously unrealizable with traditional manufacturing techniques. Current approaches, however, are limited to single materials or single methodologies greatly limiting the potential scope of manufacturable products and components. Recently, our group has developed a novel multi-material multi-method (m4) 3D printer which integrates four AM technologies and two complementary technologies into one single platform. This allows for the fabrication of complex devices able to provide a wide range of functionalities ranging from stretchable electronics to self-sensing devices. To demonstrate these functionalities in the realm of printable electronics, multiple proof of concept printed circuit boards (PCBs) were fabricated which solve issues commonly encountered in 3D printed electronics such as high resolution or vertically integrated access (VIA) circuits. In addition, 3D printed smart structures able to respond to external stimulus, such as light or heat, have become highly desirable for applications ranging from soft robotics to implantable medical devices. Recently, our group has turned to liquid crystal elastomers (LCE), a class of active material able to generate large, rapid, and reversible actuations. Therefore, using the m4 3D printer, LCE-based smart structures requiring complex electronics were fabricated which can change their shape in response to an applied current. To demonstrate this, a smart, reconfigurable radio frequency (RF) antenna was 3D printed which can change its shape and operating frequency as a function of the applied current. These examples demonstrate the vast potential of m4 3D printing for creating smart, reconfigurable, and multi-functional structures.

The ability for materials to adapt their shape and mechanical properties to the local environment is useful in a variety of applications, from soft robots to deployable structures. In this work, we integrate liquid crystal elastomers (LCEs) with multistable structures to allow autonomous reconfiguration in response to local changes in temperature. LCEs are incorporated in a kirigami-inspired system in which squares are connected at their vertices by small hinges composed of LCE-silicone bilayers. These bend and soften as the temperature increases above room temperature. By choosing geometric parameters for the hinges such that bifurcation points in the stability exist, a transition from mono- or tristability to bistability can be triggered by a sufficient increase in temperature, forcing rearrangements of the structure as minima in the energy landscape are removed. We demonstrate temperature-induced propagation of transition waves, enabling local structural changes to autonomously propagate and affect other parts of the structure. These effects could be harnessed in applications in interface control, reconfigurable structures, and soft robotics.

PurposeThe purpose of this paper is to evaluate the influence of layer thickness and post-curing temperature on shape memory properties in components manufactured by stereolithography.Design/methodology/approachLayer thicknesses of 20 and 100 µm and 22 and 45°C for post-curing temperature were selected following the design of experiments approach. Tensile and bending tests were applied for quantitative evaluation of the shape memory effect (SME). Qualitative analysis was performed using complex geometries and computed tomography as a measurement tool. Additionally, the degree of photopolymerization and glass transition temperature (Tg) were evaluated.FindingsThe tensile test resulted in fixity and recovery ratio values close to 100%. In bending, they varied between 97%–111% for fixity and 88%–95% for recovery. The layer thickness was found to have a higher influence on the SME. In complex structures, SME was dependent on geometry and less sensitive to variation in process parameters. The post-curing temperature had a higher influence on the photopolymerization and Tg. Average Tg of 77.5°C was achieved at 45°C, compared to 73.1°C at 22°C.Originality/valueIn the current state of the art in the processing of shape memory polymers with vat photopolymerization typically, the chemical composition or the thermal and deformation patterns are studied. The effect of the processing parameters is, however, not explored. This paper aims to close the research gap and facilitate the process optimization towards high fixing and recovery characteristics.