Christian Ohm

Liquid Crystalline Elastomers as Actuators and Sensors

This review collects recent developments in the field of liquid crystalline elastomers (LCEs) with an emphasis on their use for actuator and sensor applications. Several synthetic pathways leading to crosslinked liquid crystalline polymers are discussed and how these materials can be oriented into liquid crystalline monodomains are described. By comparing the actuating properties of different systems, general structure-property relationships for LCEs are obtained. In the final section, how these materials can be turned into usable devices using different interdisciplinary techniques are described.

A Continuous Flow Synthesis of Micrometer‐Sized Actuators from Liquid Crystalline Elastomers

Adv. Mater. 2009, 21, 4859–4862 2009 WILEY-VCH Verlag G T IO N Liquid crystalline elastomers (LCEs) are weakly crosslinked polymers that contain shape-anisotropic moieties (mesogens) that self organize into liquid crystalline phases. It was proposed by de Gennes in 1975 that these materials would undergo a shape change during the phase transition from the liquid crystalline to the isotropic state, if all mesogens were ordered into the same direction, forming a monodomain. The ability to change shape on application of a certain external stimulus led to the fabrication of actuators based on these ‘‘intelligent’’ materials. These are mainly macroscopic actuators, having sizes of millimeters or centimeters. However, in recent years there has been a growing interest in the preparation of micrometer sized actuators, as these are interesting for novel fields of science such as micromechanics and robotics. Here we present the use of a microfluidic setup to prepare monodisperse, monodomainic, and micrometer-sized liquid-crystalline elastomer beads that show a strong and rapid shape change of about 70% in length during the phase transition. We show that the particle size as well as the quality of the monodomain can be controlled by the operating parameters of the microfluidic setup. The key step in preparing LCE-based actuators is the overall orientation of the liquid crystalline director (ideally the formation of a monodomain) before the material is crosslinked. Methods used in the previous cited works are mainly the stretching of pre-crosslinked films, the drawing of fibers from a polymer melt and the use of electric or magnetic fields. All these methods have in common that they are complex multistep processes that are difficult to automate and not suitable for the continuous preparation of a large number of actuators. In addition, they are problematic for preparing samples in the micrometer size region. Using microfluidics on the other hand allows the continuous preparation of a large number of monodisperse micro-particles with a minimum of time and effort. In addition we argue, that the polymerization of the droplets while they are flowing through a tube increases the tendency of the mesogens to adopt a monodomainic director field configuration, thus giving the particles actuation properties. In our approach a liquid crystallinemonomer wasmixed with a crosslinker and a photoinitiator, melted into the isotropic phase and injected through a thin needle into a co-flowing stream of immiscible silicone oil. The resulting droplets were cooled into the liquid-crystalline phase and polymerized by irradiation with UV-light while flowing through a piece of thin tubing. To achieve this, we constructed a novel microfluidic setup, which is based on earlier works of Serra and Zhang. In this approach the mixing of monomer and continuous phase is carried out via a fused silica capillary in a T-junction. The main challenge for the new setup was temperature control. As all known LC-monomers are solid at room temperature, we placed the T-junction as well as the tube containing the monomer mixture in a heat bath, which was set above the monomer clearing temperature. Two syringe pumps were used, one providing the flow of the continuous phase, the other one for pushing a low viscous oil into the monomer tube, while also providing flow. An uplight microscope was used to observe droplet formation at the end of the needle. The tube containing the monomer droplets continued out of the heat bath and was rolled onto a hot plate with its temperature set to that of the liquid crystalline phase of the monomer. UV-light was shone on the tube, initiating radical polymerization and crosslinking inside the droplets in the LC-phase. Several aspects were considered for choosing an appropriate LC material for this project. Polymeric materials were excluded because their viscosities are too high to be pumped through thin capillaries while in melt. In order to induce an orientation of the mesogens, the material has to be processed (crosslinked) in the liquid crystalline phase, thus making the use of solvents to reduce viscosity impossible. Therefore we needed a monomer that was already liquid crystalline and could easily and rapidly be polymerized and crosslinked in the flow setup. In addition, a strong coupling between the mesogen and the resulting polymer is important, as this is a prerequisite for a strong shape change of the actuator. Finally, the transition temperature for the material must occur in a temperature range to which the whole reactor setup can be heated. The choice fell on a three-core mesogen with a polymerizable acrylate group attached laterally over a flexible spacer (see Fig. 1 for chemical structure), which was described earlier by Patrick Keller. It has a nematic phase between 72 and 98 8C and has been used for actuator applications before. For the preparation of crosslinked polymer particles this LC-monomer was mixed with 10mol % of hexanedioldiacrylate (chemical structure also in Fig. 1) and 2wt % of the photoinitiator Lucirin TPO. The mixture was melted and injected to the monomer tube of the setup. In a microfluidic setup the particle size is controlled mainly by two parameters: the viscosity of

Bioinspired Actuated Adhesive Patterns of Liquid Crystalline Elastomers

Gecko-inspired arrays of micropillars made of a liquid crystalline elastomer display thermoswitchable adhesive behavior as a consequence of elongation changes caused by reorientation of the mesogens at the nematic-isotropic (N-I) phase transition.

Applications of Liquid Crystalline Elastomers

This chapter focuses on recent developments in the field of liquid crystalline elastomers (LCEs) that bring these materials closer to the world of real applications, concentrating on their actuation properties. First, we briefly introduce different LCE materials that show actuation behavior and explain how they can be synthesized. In the second part, we focus on materials in which a shape change is triggered by a phase transition. In particular, we discuss how the chemistry of the polymeric material influences the strength and direction of the shape change. We review the efforts made to trigger the actuation event by stimuli other than temperature variation. Subsequently, we summarize preparation techniques for various sample geometries of aligned LCEs that all show actuation properties and assign them to particular applications. A short summary is given of devices that have been built in this way. In the third part, we concentrate on actuators that show deformation in an electric field without any phase transition. We start with a short introduction to ferroelectric liquid crystalline elastomers (FLCEs) and discuss molecules exhibiting these phases. Subsequently, we show how the electroclinic effect of FLCEs can be utilized to induce macroscopic deformations by an electric field.

Preparation of actuating fibres of oriented main-chain liquid crystalline elastomers by a wetspinning process

We present a versatile method to prepare oriented fibres with a defined thickness from main-chain liquid crystalline elastomers. A microfluidic setup is utilized to inject a solution of a photocrosslinkable smectic A main-chain polymer into a co-flowing stream of silicone oil. Diffusion of the solvent into the oil yields solid polymer filaments that are crosslinked in a continuous way by UV-irradiation. The obtained fibres are highly oriented and show a reversible and significant contraction during the liquid crystals phase transition.

Nanosized Shape-Changing Colloids from Liquid Crystalline Elastomers

A method to prepare shape-changing nanospheres from liquid crystalline elastomers is reported. The nanosized colloids are prepared by a miniemulsion process. During this process, colloids are prepared from a liquid crystalline (LC) main-chain polyester and subsequently crosslinked into a nanometer-sized LC elastomer. The ability of these LC elastomers to change their shape at the phase transition temperature from the smectic A to the isotropic phase was detected by temperature-dependent transmission electron microscopy. The phase transition-induced shape change leads to strongly shape anisotropic nanosized elastomer particles.

Engineering Polymer Microparticles by Droplet Microfluidics

Capillary-based flow-focusing and co-flow microsystems were developed to produce sphere-like polymer micro-particles of adjustable sizes in the range of 50 to 600 μm with a narrow size distribution (CV < 5%) and different morphologies (core–shell, janus, and capsules). Rod-like particles whose length was conveniently adjusted between 400 μm and few millimeters were also produced using the same microsystems. Influence of operating conditions (flow rate of the different fluid, microsystem characteristic dimensions, and design) as well as material parameters (viscosity of the different fluids and surface tension) was investigated. Empirical relationships were thus derived from experimental data to predict the microparticle’s overall size, shell thickness, or rods length. Besides morphology, microparticles with various compositions were synthesized and their potential applications highlighted: drug-loaded microparticles for new drug delivery strategies, composed inorganic–organic multiscale microparticles for sensorics, and liquid crystalline elastomer microparticles showing an anisotropic reversible shape change upon temperature for thermal actuators or artificial muscles.

Mechanical and optical properties of continuously spun fibres of a main-chain smectic A elastomer

Oriented smectic liquid crystal elastomer fibres are prepared with a special wet-spinning technique. The continuous spinning process in principle allows the preparation of fibres with arbitrary length. In comparison to ordinary rubbers, they have unique mechanical properties that qualify them as potential candidates for mechanical actuator applications. We demonstrate that these fibres show a remarkable contraction and extension at the transition from the ordered smectic to the disordered isotropic phase. We characterise their most relevant physical properties, viz. the thermally driven shape changes, stress–strain relations and optical birefringence, by optical and mechanical measurements.

Preparation of cholesteric particles from cellulose derivatives in a microfluidic setup

A microfluidic setup was used to process lyotropic cholesteric liquid crystalline mixtures of cellulose derivatives into spherical particles in the micrometre scale. By the method of co-flowing injection, monodisperse droplets of the liquid crystal, dispersed in an aqueous carrier fluid, were prepared. Polymerization of the acrylic solvent with UV-light fixed the orientation obtained by the flowing motion. The resulting particles were characterized by polarizing optical microscopy.

Micro-actuators prepared from liquid crystalline elastomers in a microfluidic setup

In this paper we demonstrate the preparation of monodisperse particles from a liquid crystalline elastomer with a preferred director orientation. For this we use a microfluidic setup to create droplets from a liquid crystalline monomer which are polymerized in the liquid crystalline phase while flowing. When the obtained particles are heated above the transition temperature of the nematic phase, they show a reversible shape change from spherical to cigar-like. We show, that we can control the size as well as the polydispersity of these microactuators. If the particles are prepared in non-flowing conditions, no or an undefined shape change is observed. Particles polymerized in the isotropic phase show no shape deformation as well. Additionally we provide an experimental proof for the stability of these structures versus acidic and basic conditions.