4D Printed Shape-Memory Elastomer for Thermally Programmable Soft Actuators

Polymeric shape-memory elastomers can recover to a permeant shape from any programmed deformation under external stimuli. They are mostly cross-linked polymeric materials and can be shaped by three-dimensional (3D) printing. However, 3D printed shape-memory polymers so far only exhibit elasticity above their transition temperature, which results in their programmed shape being inelastic or brittle at lower temperatures. To date, 3D printed shape-memory elastomers with elasticity both below and above their transition temperature remain an elusive goal, which limits the application of shape-memory materials as elastic materials at low temperatures. In this paper, we printed, for the first time, a custom-developed shape-memory elastomer based on polyethylene glycol using digital light processing, which possesses elasticity and stretchability in a wide temperature range, below and above the transition temperature. Young's modulus in these two states can vary significantly, with a difference of up to 2 orders of magnitude. This marked difference in Young's modulus imparts excellent shape-memory properties to the material. The difference in Young's modulus at different temperatures allows for the programming of the pneumatic actuators by heating and softening specific areas. Consequently, a single actuator can exhibit distinct movement modes based on the programming process it undergoes.

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.

Regression analysis for predicting the elasticity of liquid crystal elastomers

It is highly desirable but difficult to understand how microscopic molecular details influence the macroscopic material properties, especially for soft materials with complex molecular architectures. In this study we focus on liquid crystal elastomers (LCEs) and aim at identifying the design variables of their molecular architectures that govern their macroscopic deformations. We apply the regression analysis using machine learning (ML) to a database containing the results of coarse grained molecular dynamics simulations of LCEs with various molecular architectures. The predictive performance of a surrogate model generated by the regression analysis is also tested. The database contains design variables for LCE molecular architectures, system and simulation conditions, and stress-strain curves for each LCE molecular system. Regression analysis is applied using the stress-strain curves as objective variables and the other factors as explanatory variables. The results reveal several descriptors governing the stress-strain curves. To test the predictive performance of the surrogate model, stress-strain curves are predicted for LCE molecular architectures that were not used in the ML scheme. The predicted curves capture the characteristics of the results obtained from molecular dynamics simulations. Therefore, the ML scheme has great potential to accelerate LCE material exploration by detecting the key design variables in the molecular architecture and predicting the LCE deformations.

Review of Thermoresponsive Electroactive and Magnetoactive Shape Memory Polymer Nanocomposites

Electroactive and magnetoactive shape memory polymer nanocomposites (SMCs) are multistimuli-responsive smart materials that are of great interest in many research and industrial fields. In addition to thermoresponsive shape memory polymers, SMCs include nanofillers with suitable electric and/or magnetic properties that allow for alternative and remote methods of shape memory activation. This review discusses the state of the art on these electro- and magnetoactive SMCs and summarizes recently published investigations, together with relevant applications in several fields. Special attention is paid to the shape memory characteristics (shape fixity and shape recovery or recovery force) of these materials, as well as to the magnitude of the electric and magnetic fields required to trigger the shape memory characteristics.

Muscle-Mimetic Highly Tough, Conductive, and Stretchable Poly(ionic liquid) Liquid Crystalline Ionogels with Ultrafast Self-Healing, Super Adhesive, and Remarkable Shape Memory Properties

Here, we report a simple method for preparing muscle-mimetic highly tough, conductive, and stretchable liquid crystalline ionogels which contains only one poly(ionic liquid) (PIL) in an ionic liquid via in situ free radical photohomopolymerization by using nitrogen gas instead of air atmosphere. Due to eliminating the inhibition caused by dissolved oxygen, the polymerization under nitrogen gas has much higher molecular weight, lower critical sol-gel concentration, and stronger mechanical properties. More importantly, benefiting from the unique loofah-like microstructures along with the strong internal ionic interactions, entanglements of long PIL chains and liquid crystalline domains, the ionogels show special optical anisotropic, superstretchability (>8000%), high fracture strength (up to 16.52 MPa), high toughness (up to 39.22 MJ/m³), and have ultrafast self-healing, ultrastrong adhesive, and excellent shape memory properties. Due to its excellent stretchability and good conductive-strain responsiveness, the as-prepared ionogel can be easily applied for high-performance flexible and wearable sensors for motion detecting. Therefore, this paper provides an effective route and developed method to generate highly stretchable conductive liquid crystalline ionogels/elastomers that can be used in widespread flexible and wearable electronics.

Transient response and domain formation in electrically deforming liquid crystal networks

Recently, three distinct, well-separated transient regimes were discovered in the dynamics of the volume expansion of shape-shifting liquid crystal network films in response to the switching on of an alternating electric field [Van der Kooij et al., Nat. Commun., 2019, 10, 1]. Employing a spatially resolved, time-dependent Landau theory that couples local volume generation to the degree of orientational order of mesogens that are part of a viscoelastic network, we are able to offer a physical explanation for the existence of three time scales. We find that the initial response is dominated by overcoming the impact of thermal noise, after which the top of the film expands, followed by a permeation of this response into the bulk region. An important signature of our predictions is a significant dependence of the three time scales on the film thickness, where we observe a clear thin-film-to-bulk transition. The point of transition coincides with the emergence of spatial inhomogeneities in the bulk of the film in the form of domains separated by regions of suppressed expansion. This ultimately gives rise to variations in the steady-state overall expansion of the film and may lead to uncontrolled patterning. According to our model, domain formation can be suppressed by (1) decreasing the thickness of the as-prepared film, (2) increasing the linear dimensions of the mesogens, or (3) their degree of orientational order when cross-linked into the network. Our findings provide a handle to achieve finer control over the actuation of smart liquid crystal network coatings.

Liquid Crystalline Elastomers Based on Click Chemistry

Liquid crystalline elastomers (LCEs) have emerged as an important class of functional materials that are suitable for a wide range of applications, such as sensors, actuators, and soft robotics. The unique properties of LCEs originate from the combination between liquid crystal and elastomeric network. The control of macroscopic liquid crystalline orientation and network structure is crucial to realizing the useful functionalities of LCEs. A variety of chemistries have been developed to fabricate LCEs, including hydrosilylation, free radical polymerization of acrylate, and polyaddition of epoxy and carboxylic acid. Over the past few years, the use of click chemistry has become a more robust and energy-efficient way to construct LCEs with desired structures. This article provides an overview of emerging LCEs based on click chemistries, including aza-Michael addition between amine and acrylate, radical-mediated thiol-ene and thiol-yne reactions, base-catalyzed thiol-acrylate and thiol-epoxy reactions, copper-catalyzed azide-alkyne cycloaddition, and Diels-Alder cycloaddition. The similarities and differences of these reactions are discussed, with particular attention focused on the strengths and limitations of each reaction for the preparation of LCEs with controlled structures and orientations. The compatibility of these reactions with the traditional and emerging processing techniques, such as surface alignment and additive manufacturing, are surveyed. Finally, the challenges and opportunities of using click chemistry for the design of LCEs with advanced functionalities and applications are discussed.

Application and Development of Shape Memory Micro/Nano Patterns

Shape memory polymers (SMPs) are a class of smart materials that change shape when stimulated by environmental stimuli. Different from the shape memory effect at the macro level, the introduction of micro-patterning technology into SMPs strengthens the exploration of the shape memory effect at the micro/nano level. The emergence of shape memory micro/nano patterns provides a new direction for the future development of smart polymers, and their applications in the fields of biomedicine/textile/micro-optics/adhesives show huge potential. In this review, the authors introduce the types of shape memory micro/nano patterns, summarize the preparation methods, then explore the imminent and potential applications in various fields. In the end, their shortcomings and future development direction are also proposed.

Controlling permeation in electrically deforming liquid crystal network films: A dynamical Landau theory

Liquid crystal networks exploit the coupling between the responsivity of liquid crystalline mesogens, e.g., to electric fields, and the (visco)elastic properties of a polymer network. Because of this, these materials have been put forward for a wide array of applications, including responsive surfaces such as artificial skins and membranes. For such applications, the desired functional response must generally be realized under strict geometrical constraints, such as provided by supported thin films. To model such settings, we present a dynamical, spatially heterogeneous Landau-type theory for electrically actuated liquid crystal network films. We find that the response of the liquid crystal network permeates the film from top to bottom, and illustrate how this affects the timescale associated with macroscopic deformation. Finally, by linking our model parameters to experimental quantities, we suggest that the permeation rate can be controlled by varying the aspect ratio of the mesogens and their degree of orientational order when crosslinked into the polymer network, for which we predict a single optimum. Our results contribute specifically to the rational design of future applications involving transport or on-demand release of molecular cargo in liquid crystal network films.

Self-healing and shape memory functions exhibited by supramolecular liquid-crystalline networks formed by combination of hydrogen bonding interactions and coordination bonding

We here report a new approach to develop self-healing shape memory supramolecular liquid-crystalline (LC) networks through self-assembly of molecular building blocks via combination of hydrogen bonding and coordination bonding. We have designed and synthesized supramolecular LC polymers and networks based on the complexation of a forklike mesogenic ligand with Ag+ ions and carboxylic acids. Unidirectionally aligned fibers and free-standing films forming layered LC nanostructures have been obtained for the supramolecular LC networks. We have found that hybrid supramolecular LC networks formed through metal-ligand interactions and hydrogen bonding exhibit both self-healing properties and shape memory functions, while hydrogen-bonded LC networks only show self-healing properties. The combination of hydrogen bonds and metal-ligand interactions allows the tuning of intermolecular interactions and self-assembled structures, leading to the formation of the dynamic supramolecular LC materials. The new material design presented here has potential for the development of smart LC materials and functional LC membranes with tunable responsiveness.

Liquid crystalline networks based on photo-initiated thiol-ene click chemistry

Photo-initiated thiol-ene click chemistry is used to develop shape memory liquid crystalline networks (LCNs). A biphenyl-based di-vinyl monomer is synthesized and cured with a di-thiol chain extender and a tetra-thiol crosslinker using UV light. The effects of photo-initiator concentration and UV light intensity on the curing behavior and liquid crystalline (LC) properties of the LCNs are investigated. The chemical composition is found to significantly influence the microstructure and the related thermomechanical properties of the LCNs. The structure-property relationship is further explored using molecular dynamics simulations, revealing that the introduction of the chain extender promotes the formation of an ordered smectic LC phase instead of agglomerated structures. The concentration of the chain extender affects the liquid crystallinity of the LCNs, resulting in distinct thermomechanical and shape memory properties. This class of LCNs exhibits fast curing rates, high conversion levels, and tailorable liquid crystallinity, making it a promising material system for advanced manufacturing, where complex and highly ordered structures can be produced with fast reaction kinetics and low energy consumption.

Two-Way and Multiple-Way Shape Memory Polymers for Soft Robotics: An Overview

Shape memory polymers (SMPs) are smart materials capable of changing their shapes in a predefined manner under a proper applied stimulus and have gained considerable interest in several application fields. Particularly, two-way and multiple-way SMPs offer unique opportunities to realize untethered soft robots with programmable morphology and/or properties, repeatable actuation, and advanced multi-functionalities. This review presents the recent progress of soft robots based on two-way and multiple-way thermo-responsive SMPs. All the building blocks important for the design of such robots, i.e., the base materials, manufacturing processes, working mechanisms, and modeling and simulation tools, are covered. Moreover, examples of real-world applications of soft robots and related actuators, challenges, and future directions are discussed.

High shear capillary rheometry of cellulose nanocrystals for industrially relevant processing

A microcapillary rheometer was employed to study the rheological characteristics of CNCs at temperatures between 15 °C and 50 °C and aqueous concentrations between 1.5 wt% and 12.1 wt%, at rates up to 8 × 10⁵ s^-1. Time-temperature and time-concentration superposition were applied to analyze the data. A master curve of shear rate sweeps at temperatures between 15 °C and 50 °C was successfully generated to a reference temperature of 25 °C with the shift factor plot suggesting an Arrhenius relationship over the entire measured temperature range. Concentration-dependent data indicate a high shear Newtonian plateau at the limit of low concentration. Repeated testing of the same sample volume was implemented to represent extended times at elevated stress, with repeated experiments leading to a permanent decrease in viscosity. Atomic force microscopy (AFM) suggests sensitivity of the CNC geometry to moderate stress in a flow field.

Shape-Memory Metallopolymer Networks Based on a Triazole-Pyridine Ligand

Shape memory polymers represent an interesting class of stimuli-responsive polymers. With their ability to memorize and recover their original shape, they could be useful in almost every area of our daily life. We herein present the synthesis of shape-memory metallopolymers in which the switching unit is designed by using bis(pyridine-triazole) metal complexes. The polymer networks were synthesized via free radical polymerization of methyl-, ethyl- or butyl-methacrylate, tri(ethylene glycol) dimethacrylate and a methacrylate moiety of the triazole-pyridine ligand. By the addition of zinc(II) or cobalt(II) acetate it was possible to achieve metallopolymer networks featuring shape-memory abilities. The successful formation of the metal-ligand complex was proven by Fourier transform infrared (FT-IR) spectroscopy and by 1H NMR spectroscopy. Furthermore, the shape-recovery behavior was studied in detailed fashion and even triple-shape memory behavior could be revealed.

Design of Ethylene-Vinyl Acetate Copolymer Fiber with Two-Way Shape Memory Effect

One-dimensional shape memory polymer fibers (SMPFs) have obvious advantages in mechanical properties, dispersion properties, and weavability. In this work, a method for fabricating semi-crystallization ethylene-vinyl acetate copolymer (EVA) fiber with two-way shape memory effect by melt spinning and ultraviolet (UV) curing was developed. Here, the effect of crosslink density on its performance was systematically analyzed by gel fraction measurement, tensile tests, DSC, and TMA analysis. The results showed that the crosslink density and shape memory properties of EVA fiber could be facilely adjusted by controlling UV curing time. The resulting EVA fiber with cylindrical structure had a diameter of 261.86 ± 13.07 μm, and its mechanical strength and elongation at break were 64.46 MPa and 114.33%, respectively. The critical impact of the crosslink density and applied constant stress on the two-way shape memory effect were analyzed. Moreover, the single EVA fiber could lift more than 143 times its own weight and achieve 9% reversible actuation strain. The reversible actuation capability was significantly enhanced by a simple winding design of the single EVA fiber, which provided great potential applications in smart textiles, flexible actuators, and artificial muscles.

Phase behavior of main-chain liquid crystalline polymer networks synthesized by alkyne-azide cycloaddition chemistry

Liquid crystalline polymer networks (LCNs) couple polymer chain organization to molecular ordering, the switching of which has been shown to impart stimuli-responsive properties, including actuation and one-way shape memory, to the networks. While LCNs have long been proposed as artificial muscles, recent reports have also suggested potential as dynamic biomaterial substrates. In contrast to many existing LCNs synthesized using hydrophobic spacers, this work investigates networks synthesized using more hydrophilic spacers to promote interaction with water. A challenge with such materials is liquid crystalline phases could be disrupted in hydrated networks. This work thus investigates the impact of polyether spacers and mesogen composition on the phase behavior of LCNs. Main-chain LCNs were synthesized using alkyne-azide cycloaddition ("click" chemistry), where two different mesogens (5yH and 5yMe) and a non-LC monomer (5yTe) were coupled with one of two different polyether spacers, poly(ethylene glycol) and poly(propylene glycol), and a crosslinker. The chemistry led to high gel fraction materials, the workup of which resulted in networks that displayed no difference in cellular toxicity due to leachable components compared to tissue culture plastic control. Calorimetric analysis, dynamic mechanical analysis, and X-ray scattering revealed the LC microstructure and temperature-responsive properties of the networks. The use of low molecular weight polyether spacers was found to prevent their crystallization within the LC network, and adjusting mesogen composition to enhance its LC phase stability allowed the use of spacers with larger molecular weights and pendant groups. Hydrated networks were found to rearrange their structure compared to dry networks, while maintaining their LC phases. Like other crosslinked LC materials, the networks display shape changes (actuation) that are tied to changes in LC ordering. The result is a new synthetic approach for polydomain networks that form stable LC phases that are tailorable using polyether spacers and may enable future application as hydrated, stimuli-responsive materials.

Thermomechanical properties of monodomain nematic main-chain liquid crystal elastomers

Two-stage thiol-acrylate Michael addition reactions have proven useful in programming main-chain liquid crystal elastomers (LCEs). However, the influence of excess acrylate concentration, which is critical to monodomain programming, has not previously been examined with respect to thermomechanical properties in these two-stage LCEs. Previous studies of thiol-acrylate LCEs have focused on polydomain LCEs and/or variation of thiol crosslinking monomers or linear thiol monomers. This study guides the design of monodomain LCE actuators using the two-stage methodology by varying the concentration of mesogenic acrylate monomers from 2 mol% to 45 mol% in stoichiometric excess of thiol. The findings demonstrate a technique to tailor the isotropic transition temperature by 44 °C using identical starting monomers. In contrast to expectations, low amounts of excess acrylate showed excellent fixity (90.4 ± 2.9%), while high amounts of excess acrylate did not hinder actuation strain (87.3 ± 2.3%). Tensile stress-strain properties were influenced by excess acrylate. Linear elastic behavior was observed parallel to the director with modulus increasing from 1.4 to 6.1 MPa. The soft elastic plateau was observed perpendicular to the director with initial modulus and threshold stresses increasing from 0.6 MPa to 2.6 MPa and 14 kPa to 208 kPa, respectively. Overall, this study examines the influence of excess acrylate on mechanical properties of LCE actuators.

Liquid-crystal order during synthesis affects main-chain liquid-crystal elastomer behavior

This study presents the first direct comparison of the influence of liquid-crystal order during synthesis on the thermo-mechanical behaviors of main-chain liquid-crystal elastomers (LCEs) in thiol-acrylate networks. Six polydomain nematic elastomer (PNE) chemistries were compared directly by synthesizing with the mesogens in either an isotropic state (i-PNE) or a nematic state (n-PNE). The i-PNE networks were created in the presence of solvent, which disrupted any liquid-crystal order during network formation. Conversely, the n-PNE networks were created without the presence of solvent below the isotropic transition (T_NI). Differential scanning calorimetry (DSC) was first performed, and it showed that i-PNE networks experienced a clearly defined nematic-to-isotropic transition upon heating, whereas the transition in n-PNE networks was unable to be identified, which may be the result of a nematic-to-paranematic phase transition. Dynamic mechanical analysis (DMA) tests revealed that while both networks maintained elevated loss tangent in the nematic region, only i-PNE networks prominently displayed dynamic soft elasticity behavior. The two-way shape switching behaviors of LCE networks were examined using actuation tests under a 100 kPa bias stress. It showed that the strain amplitude strongly depends on synthesis history; it ranges from 66% to 126% in i-PNE samples and 3% to 61% in n-PNE samples. To help interpret the different actuation strain behaviors between i-PNEs and n-PNEs, wide-angle X-ray scattering (WAXS) was then performed where the LCE samples were strained to 40%. The results showed that order parameter (S) in n-PNE samples (ranging from 0.37 to 0.50) is lower than that in i-PNE samples (0.54 for all cases), and the parameter decreased as the cross-linking density increased. The stress-strain behaviors of the LCE networks measured from uniaxial tension tests revealed that all i-PNE samples had a lower soft-elasticity plateau during loading compared to the n-PNE samples. Finally, free-standing strain recovery of LCE samples after being strained to 100% was investigated. Immediately after removing stress on the samples, i-PNE and n-PNE samples recovered 14% to 38% and 27% to 73% of strain, respectively. We discuss the advantages and disadvantages of the different synthetic histories on LCE design.

Fabrication of Liquid Crystalline Polyurethane Networks with a Pendant Azobenzene Group to Access Thermal/Photoresponsive Shape-Memory Effects

Herein, we report a novel thermal/photoresponsive shape-memory polyurethane network with a pendant azobenzene group by utilizing its anisotropic-isotropic phase transitions and photoresponsive feature concurrently. To achieve this goal, the side-chain liquid crystalline polyurethane networks based on the pendant azobenzene group [SCLCPU(AZO)-Ns] were developed in a well-defined architecture. The smectic C nature of an LC phase in the polyurethane networks was confirmed by differential scanning calorimetry, polarized optical microscopy, and one-dimensional and two-dimensional wide-angle X-ray diffraction. The well-defined architecture-made SCLCPU(AZO)-N displays two distinct transition temperatures (T_trans) (T_g and T_cl), with a difference of about 40 °C. Consequently, the excellent triple-shape-memory effect in this network was demonstrated by cyclic thermomechanical analysis. By making full use of the trans-cis photoisomerization of azobenzene, the reversible bending and unbending behaviors were realized under the light irradiation with wavelengths of 450 and 550 nm, respectively.

Photoinduced Plasticity in Cross-Linked Liquid Crystalline Networks

Photoactivated reversible addition fragmentation chain transfer (RAFT)-based dynamic covalent chemistry is incorporated into liquid crystalline networks (LCNs) to facilitate spatiotemporal control of alignment, domain structure, and birefringence. The RAFT-based bond exchange process, which leads to stress relaxation, is used in a variety of conditions, to enable the LCN to achieve a near-equilibrium structure and orientation upon irradiation. Once formed, and in the absence of subsequent triggering of the RAFT process, the (dis)order in the LCN and its associated birefringence are evidenced at all temperatures. Using this approach, the birefringence, including the formation of spatially patterned birefringent elements and surface-active topographical features, is selectively tuned by adjusting the light dose, temperature, and cross-linking density.

Thermo-responsive shape and optical memories of photonic composite films enabled by glassy liquid crystalline polymer networks

We propose a novel shape and optical memories of a photonic composite film based on a silica opal photonic crystal (PC) template and a liquid crystal polymer network (LCN). Here, the photonic composite film was fabricated by introducing a LCN precursor into a silica opal PC template, followed by UV photo-polymerization and then by the removal of the template. The obtained bilayer-structure photonic film was found to spontaneously form a three-dimensional (3D) temporary bending shape in response to heating, and thus the corresponding reflection color of the photonic composite film shows a blue shift during bending deformation. The inherent mechanisms of these two observations could be attributed to the variations of the LC molecule orientation and the light reflection in the photonic composite film during the thermal process. More intriguingly, the resulting temporary bending shape was fixed by applying mechanical force during slowly cooling down to the room temperature or autonomously fixed by a rapid cooling in liquid nitrogen. Additionally, this temporary state could restore back to the permanent flat shape when the film is cooled from the heat source without an external force. Finally, more complex 3D shape-memory samples could also be achieved by simply controlling the LC alignment or designing the sample geometry. This work opens up a new way to develop a novel shape-memory polymer photonic film.

The Effect of 4-Octyldecyloxybenzoic Acid on Liquid-Crystalline Polyurethane Composites with Triple-Shape Memory and Self-Healing Properties

To better understand shape memory materials and self-healing materials, a new series of liquid-crystalline shape memory polyurethane (LC-SMPU) composites, named SMPU-OOBAm, were successfully prepared by incorporating 4-octyldecyloxybenzoic acid (OOBA) into the PEG-based SMPU. The effect of OOBA on the structure, morphology, and properties of the material has been carefully investigated. The results demonstrate that SMPU-OOBAm has liquid crystalline properties, triple-shape memory properties, and self-healing properties. The incorporated OOBA promotes the crystallizability of both soft and hard segments of SMPU, and the crystallization rate of the hard segment of SMPU decreases when the OOBA-content increases. Additionally, the SMPU-OOBAm forms a two-phase separated structure (SMPU phase and OOBA phase), and it shows two-step modulus changes upon heating. Therefore, the SMPU-OOBAm exhibits triple-shape memory behavior, and the shape recovery ratio decreases with an increase in the OOBA content. Finally, SMPU-OOBAm exhibits self-healing properties. The new mechanism can be ascribed to the heating-induced "bleeding" of OOBA in the liquid crystalline state and the subsequent re-crystallization upon cooling. This successful combination of liquid crystalline properties, triple-shape memory properties, and self-healing properties make the SMPU-OOBAm composites ideal for many promising applications in smart optical devices, smart electronic devices, and smart sensors.

Anhydride-Based Reconfigurable Shape Memory Elastomers

Soft shape memory polymers typically embody a permanently memorized geometry that cannot be altered, and therefore a new sample must be fabricated each time a new structure is required. We present a shape memory elastomeric composite featuring thermoplastic fibers as a fixing phase and a polyanhydride-based elastomer as the permanent, elastic phase. Interestingly, dynamic covalent exchange reactions at elevated temperatures (T > 50 °C) among the network chains of the elastomer allow near-complete reconfiguration of the permanent shape in the solid state. Together, these features combine to create a shape memory elastomer capable of arbitrary programming of both temporary and permanent shapes.

Deuteron NMR resolved mesogen vs. crosslinker molecular order and reorientational exchange in liquid single crystal elastomers

²H NMR spectroscopy can resolve differences in orientational order and reorientational dynamics between crosslinker and mesogen molecules in liquid single crystal elastomers.

Programmable and adaptive mechanics with liquid crystal polymer networks and elastomers

Liquid crystals are the basis of a pervasive technology of the modern era. Yet, as the display market becomes commoditized, researchers in industry, government and academia are increasingly examining liquid crystalline materials in a variety of polymeric forms and discovering their fascinating and useful properties. In this Review, we detail the historical development of liquid crystalline polymeric materials, with emphasis on the thermally and photogenerated macroscale mechanical responses--such as bending, twisting and buckling--and on local-feature development (primarily related to topographical control). Within this framework, we elucidate the benefits of liquid crystallinity and contrast them with other stimuli-induced mechanical responses reported for other materials. We end with an outlook of existing challenges and near-term application opportunities.

Mechanically programmed shape change in laminated elastomeric composites

Soft, anisotropic materials, such as myocardium in the heart and the extracellular matrix surrounding cells, are commonly found in nature. This anisotropy leads to specialized responses and is imperative to material functionality, yet few soft materials exhibiting similar anisotropy have been developed. Our group introduced an anisotropic shape memory elastomeric composite (A-SMEC) composed of non-woven, aligned polymer fibers embedded in an elastomeric matrix. The composite exhibited shape memory (SM) behavior with significant anisotropy in room-temperature shape fixing. Here, we exploit this anisotropy by bonding together laminates with oblique anisotropy such that tensile deformation at room temperature - mechanical programming - results in coiling. This response is a breakthrough in mechanical programming, since non-affine shape change is achieved by simply stretching the layered A-SMECs at room temperature. We will show that pitch and curvature of curled geometries depend on fiber orientations and the degree of strain programmed into the material. To validate experimental results, a model was developed that captures the viscoplastic response of A-SMECs. Theoretical results correlated well with experimental data, supporting our conclusions and ensuring attainability of predictable curling geometries. We envision these smart, soft, shape changing materials will have aerospace and medical applications.

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.

Photo-Mediated Copper(I)-Catalyzed Azide-Alkyne Cycloaddition (CuAAC) "Click" Reactions for Forming Polymer Networks as Shape Memory Materials

The formation of polymer networks polymerized with the Copper (I) - catalyzed azide - alkyne cycloaddition (CuAAC) click reaction is described along with their accompanying utilization as shape memory polymers. Due to the click nature of the reaction and the synthetic accessibility of azide and alkyne functional-monomers, the polymer architecture was readily controlled through monomer design to manipulate crosslink density, ability for further functionalization, and the glass transition temperature (55 to 120°C). Free strain recovery is used to quantify the shape memory properties of a model CuAAC network resulting in excellent shape fixity and recovery of 99%. The step growth nature of this polymerization results in homogenous network formation with narrow glass transitions ranges having half widths of the transition close to 15°C for these materials resulting in shape recovery sharpness of 3.9 %/°C in a model system comparable to similarly crosslinked chain growth polymers. Utilization of the CuAAC reaction to form shape memory materials opens a range of possibilities and behaviors that are not readily achieved in other shape memory materials such as (meth) acrylates, thiolene, thiol-Michael, and poly(caprolactone) based shape memory materials.