Micron-Sized Main-Chain Liquid Crystalline Elastomer Actuators with Ultralarge Amplitude Contractions

A calamitic mesogenic near-infrared absorbing croconaine dye/liquid crystalline elastomer composite

In this work, we report the first example of a calamitic mesogenic near-infrared (NIR) absorbing organic dye, made by functionalizing a thiophene-croconaine chromophore rigid core with two symmetric long flexible alkyl chains. The liquid crystal (LC) NIR dye YHD796 exhibits a sharp and intense NIR absorption band with a maximum absorption peak at 796 nm. Taking advantage of the improved solubility of YHD796 dispersed in mesogenic molecules, a homogeneously-aligned mono-domain liquid crystalline elastomer (LCE)/YHD796 composite film is successfully prepared by applying the classical LC-cell-alignment method and in situ photo-polymerization of photocurable LC monomer mixtures. This LCE/YHD796 composite film performs a fully reversible contraction/expansion response towards NIR light stimulus due to the photo-thermal heating effect induced by the YHD796 dye well-dispersed in the LCE matrix.

Free-form Light Actuators - Fabrication and Control of Actuation in Microscopic Scale

Liquid crystalline elastomers (LCEs) are smart materials capable of reversible shape-change in response to external stimuli, and have attracted researchers' attention in many fields. Most of the studies focused on macroscopic LCE structures (films, fibers) and their miniaturization is still in its infancy. Recently developed lithography techniques, e.g., mask exposure and replica molding, only allow for creating 2D structures on LCE thin films. Direct laser writing (DLW) opens access to truly 3D fabrication in the microscopic scale. However, controlling the actuation topology and dynamics at the same length scale remains a challenge.

In this paper we report on a method to control the liquid crystal (LC) molecular alignment in the LCE microstructures of arbitrary three-dimensional shape. This was made possible by a combination of direct laser writing for both the LCE structures as well as for micrograting patterns inducing local LC alignment. Several types of grating patterns were used to introduce different LC alignments, which can be subsequently patterned into the LCE structures. This protocol allows one to obtain LCE microstructures with engineered alignments able to perform multiple opto-mechanical actuation, thus being capable of multiple functionalities. Applications can be foreseen in the fields of tunable photonics, micro-robotics, lab-on-chip technology and others.

Better Actuation Through Chemistry: Using Surface Coatings to Create Uniform Director Fields in Nematic Liquid Crystal Elastomers

Controlling the molecular alignment of liquid crystal monomers (LCMs) within nano- and microstructures is essential in manipulating the actuation behavior of nematic liquid crystal elastomers (NLCEs). Here, we study how to induce uniformly vertical alignment of nematic LCMs within a micropillar array to maximize the macroscopic shape change using surface chemistry. Landau-de Gennes numerical modeling suggests that it is difficult to perfectly align LCMs vertically in every pore within a poly(dimethylsiloxane) (PDMS) mold with porous channels during soft lithography. In an untreated PDMS mold that provides homeotropic anchoring of LCMs, a radially escaped configuration of LCMs is observed. Vertically aligned LCMs, a preferred configuration for actuation, are only observed when using a PDMS mold with planar anchoring. Guided by the numerical modeling, we coat the PDMS mold with a thin layer of poly(2-hydroxyethyl methacrylate) (PHEMA), leading to planar anchoring of LCM. Confirmed by polarized optical microscopy, we observe monodomains of vertically aligned LCMs within the mold, in agreement with modeling. After curing and peeling off the mold, the resulting NLCE micropillars showed a relatively large and reversible radial strain (∼30%) when heated above the nematic to isotropic transition temperature.

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

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

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

Homeotropically-aligned main-chain and side-on liquid3crystalline elastomer films are reported. Their thermal conductivities in the normal direction are both dramatically higher than those along the horizontal direction.

An entropy-driven ring-opening metathesis polymerization approach towards main-chain liquid crystalline polymers

In this manuscript, an entropy-driven ring-opening metathesis polymerization (ED-ROMP) approach is applied for the first time in producing main-chain liquid crystalline polymers by polymerizing a macrocyclic olefin monomer.

Biomimetic Submicroarrayed Cross-Linked Liquid Crystal Polymer Films with Different Wettability via Colloidal Lithography

Photoresponsive cross-linked liquid crystal polymer (CLCP) films with different surface topographies, submicropillar arrays, and submicrocone arrays were fabricated through colloidal lithography technique by modulating different types of etching masks. The prepared submicropillar arrays were uniform with an average pillar diameter of 250 nm and the cone bottom diameter of the submicrocone arrays was about 400 nm, which are much smaller than previously reported CLCP micropillars. More interestingly, these two species of films with the same chemical structure represented completely different wetting behavior of water adhesion and mimicked rose petal and lotus leaf, respectively. Both the submicropillar arrayed film and the submicrocone arrayed film exhibited superhyrophobicity with a water contact angle (CA) value of 144.0 ± 1.7° and 156.4 ± 1.2°, respectively. Meanwhile, the former demonstrated a very high sliding angle (SA) greater than 90°, and thus, the water droplet was pinned on the surface as rose petal. On the contrary, the SA of the submicrocone arrayed CLCP film consisting of micro- and nanostructure was only 3.1 ± 2.0°, which is as low as that of lotus leaf. Furthermore, the change on the wettability of the films was also investigated under alternating irradiation of visible light with two different wavelengths, blue light and green light.

Programmable Liquid Crystal Elastomers Prepared by Thiol-Ene Photopolymerization

The spontaneous conversion of a flat film into a 3-D shape requires local programming of the mechanical response. Historically, the ability to locally program the mechanical response of high strain (>30%) liquid crystalline elastomers (LCEs) has been limited to magnetic or mechanical alignment techniques, which limits spatial resolution. Recently, we reported on the preparation of LCEs capable of 55% strain with spatial control of the mechanical response at scales as small as 0.01 mm². Here, we report a distinct formulation strategy to realize programmable stimulus-response in LCEs. Photopolymerization of thiol-ene/acrylate formulations yields materials that exhibit large reversible strain up to 150%. The photopolymerization reaction is extremely rapid, reducing preparation time from days to minutes. The mechanical behavior of these materials can be tuned by varying cross-link density. Spatial and hierarchical programming of the director profile is demonstrated, enabling 3-D shape change, including twisting ribbons and localized Gaussian curvature.

Poly(vinyl benzoate)-backbone mesogen-jacketed liquid crystalline polymers

This work describes a new system of MJLCPs with poly(vinyl benzoate)-backbone, which are efficiently synthesized by the bulk photo-polymerization method. Furthermore, homogeneous- or homeotropic-aligned cross-linked MJLCP (xMJLCP) films are for the first time successfully prepared.

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.

Humidity-responsive liquid crystalline polymer actuators with an asymmetry in the molecular trigger that bend, fold, and curl

We show a versatile method for the preparation of a variety of humidity-responsive actuators based on a single sheet of a hydrogen-bonded, uniaxially aligned liquid crystal polymer network. In this approach, the asymmetry in the molecular trigger in the anisotropic polymer film plays a dominant role leading to programmed deformation events. The material is locally treated with a potassium hydroxide solution to create the asymmetry in the responsiveness toward humidity, which allows to prepare actuators that bend, fold, or curl.

Mouldable liquid-crystalline elastomer actuators with exchangeable covalent bonds

Liquid-crystal elastomers (LCEs) are a class of actively moving polymers with remarkable practical potential for converting external stimuli into mechanical actuation. However, real-world applications of LCEs are lacking because macroscopic orientation of liquid-crystal order, which is required for reversible actuations, is hard to achieve in practice. Here we show that the processing bottleneck of LCEs can be overcome by introducing exchangeable links in place of permanent network crosslinks, a concept previously demonstrated for vitrimers. Liquid-crystal elastomers with exchangeable links (xLCEs) are mouldable, allow for easy processing and alignment, and can be subsequently altered through remoulding with different stress patterns, thus opening the way to practical xLCE actuators and artificial muscles. Surprisingly, instead of external-stress relaxation through the creep of non-liquid-crystal transient networks with exchangeable links, xLCEs develop strong liquid-crystal alignment as an alternative mechanism of mechanical relaxation.

Stimuli-responsive topological change of microstructured surfaces and the resultant variations of wetting properties

It is now well established that topological microstructures play a key role in the physical properties of surfaces. Stimulus-induced variations of topological microstructure should therefore lead to a change in the physical properties of microstructured responsive surfaces. In this paper, we demonstrate that roughness changes alter the wetting properties of responsive organic surfaces. Oriented nematic liquid crystalline elastomers (LCEs) are used to construct the microstructured surfaces via a replica molding technique. The topological microstructure of the surfaces covered with micropillars changes with temperature, due to the reversible contraction of the LCE pillars along the long axis at the nematic-to-isotropic phase transition. This is directly observed for the first time under environmental scanning electron microscopy (E-SEM). A high boiling point liquid, glycerol, is used to continuously monitor the contact angle change with temperature. The glycerol contact angle of the microstructured surfaces covered with small pillars decreases from 118° at room temperature to 80° at 140 °C, corresponding to a transition from Cassie state to Wenzel state.

Liquid-crystalline ordering as a concept in materials science: from semiconductors to stimuli-responsive devices

While the unique optical properties of liquid crystals (LCs) are already well exploited for flat-panel displays, their intrinsic ability to self-organize into ordered mesophases, which are intermediate states between crystal and liquid, gives rise to a broad variety of additional applications. The high degree of molecular order, the possibility for large scale orientation, and the structural motif of the aromatic subunits recommend liquid-crystalline materials as organic semiconductors, which are solvent-processable and can easily be deposited on a substrate. The anisotropy of liquid crystals can further cause a stimuli-responsive macroscopic shape change of cross-linked polymer networks, which act as reversibly contracting artificial muscles. After illustrating the concept of liquid-crystalline order in this Review, emphasis will be placed on synthetic strategies for novel classes of LC materials, and the design and fabrication of active devices.

Actuators based on liquid crystalline elastomer materials

Liquid crystalline elastomers (LCEs) exhibit a number of remarkable physical effects, including the unique, high-stroke reversible mechanical actuation when triggered by external stimuli. This article reviews some recent exciting developments in the field of LCE materials with an emphasis on their utilization in actuator applications. Such applications include artificial muscles, industrial manufacturing, health and microelectromechanical systems (MEMS). With suitable synthetic and preparation pathways and well-controlled actuation stimuli, such as heat, light, electric and magnetic fields, excellent physical properties of LCE materials can be realized. By comparing the actuating properties of different systems, general relationships between the structure and the properties of LCEs are discussed. How these materials can be turned into usable devices using interdisciplinary techniques is also described.

Photo-Responsive Shape-Memory and Shape-Changing Liquid-Crystal Polymer Networks

"Surrounding matters" is a phrase that has become more significant in recent times when discussing polymeric materials. Although regular polymers do respond to external stimuli like softening of material at higher temperatures, that response is gradual and linear in nature. Smart polymers (SPs) or stimuli-responsive polymers (SRPs) behave differently to those external stimuli, as their behavior is more rapid and nonlinear in nature and even a small magnitude of external stimulus can cause noticeable changes in their shape, size, color or conductivity. Of these SRPs, two types of SPs with the ability to actively change can be differentiated: shape-memory polymers and shape-changing polymers. The uniqueness of these materials lies not only in the fast macroscopic changes occurring in their structure but also in that some of these shape changes are reversible. This paper presents a brief review of current progress in the area of light activated shape-memory polymers and shape-changing polymers and their possible field of applications.

Synthesis and phase behavior of chiral liquid crystalline polymeric networks derived from menthol

The synthesis of new chiral monomer 4-(menthyloxyacetoxy- benzoyloxy)biphenyl-4′-(2-(undec-10-e noyloxy)ethoxy)benzoate (ML), crosslinking agent 4-(undec-10-enoyloxy)biphenyl-4′-(2-(undec-10-enoyloxy)ethoxy)benzoate (CA), and liquid crystal polymer networks (E1−E5) containing menthyl group is presented. Their chemical structures and phase behavior were characterized with Fourier transform infrared (FT-IR), proton nuclear magnetic resonance (1H-NMR), elemental analyses, polarizing optical microscopy, differential scanning calorimetry, thermogravimetric analysis (TGA), and X-ray diffraction. The selective reflection of light for ML was investigated with ultraviolet/visible/near infrared (UV/Visible/NIR). By inserting a flexible spacer between the mesogenic core and the terminal menthyl groups, MLcould form mesophase and show a chiral smectic C phase, cholesteric phase and cubic blue phase. CA displayed a smectic A phase and nematic phase. The polymer networks containing less than 12 mol% of the crosslinking units showed reversible cholesteric phase transition, wide mesophase temperature range, and excellent thermal stability. With increasing the content of crosslinking unit, the corresponding Tg increased, the Ti decreased, and the mesophase temperature range narrowed for E1−E5. TGA showed that the Td(5%) was greater than 330°C for E1−E5.

Stimuli-responsive supramolecular polymeric materials

Supramolecular materials, dynamic materials by nature, are defined as materials whose components are bridged via reversible connections and undergo spontaneous and continuous assembly/disassembly processes under specific conditions. On account of the dynamic and reversible nature of noncovalent interactions, supramolecular polymers have the ability to adapt to their environment and possess a wide range of intriguing properties, such as degradability, shape-memory, and self-healing, making them unique candidates for supramolecular materials. In this critical review, we address recent developments in supramolecular polymeric materials, which can respond to appropriate external stimuli at the fundamental level due to the existence of noncovalent interactions of the building blocks.

Wrinkled liquid-crystalline microparticle-enhanced photoresponse of PDLC-like films by coupling with mechanical stretching

Photoresponsive behaviors are studied in hybrid liquid-crystalline (LC) films prepared with light-responsive LC polymer microparticles as dopants using photoinert polymers as a host material. Upon mechanical stretching, both topological shape change and mesogenic alignment occur in the LC polymer microparticles, enabling the polymer-dispersed LC (PDLC)-like films to bend toward a light source upon UV irradiation. The rough morphologies of the hydrophobic LC microparticles enhance their interactions with hydrophilic polymer substrates. The bilayer-like structures of the hybrid film formed in the fabrication processes are responsible for the photomechanical behavior, which is reversibly controlled by combing light irradiation with the stretching processes.