Martin Brehmer

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.

Tailored Semiconducting Polymers: Living Radical Polymerization and NLO-Functionalization of Triphenylamines

This paper describes the preparation of various polymers with triarylamine side groups. High molecular weight materials were obtained by free radical polymerization utilizing the gel effect. Polymers with a marrow polydispersity and a predetermined molecular weight could be prepared by living radical polymerization. The T g could, thereby, be controlled between 50 and 140°C either by using different monomers or by varying the molecular weight. Living radical polymerization allowed in addition the preparation of block copolymers. The triarylamine side groups could be transformed into NLO-chromophores by reaction with tetracyanoethylene. This leads to the incorporation of tricyanoethylene. This leads to be incorporation of tricyanoethylene acceptor groups. As this reaction can be performed selectively on one block in block copolymers, microphase separated structures are accessible, which possess charge transport moieties in one phase and NLO-chromophores in the other phase.

Ferroelectric liquid‐crystalline elastomers

Oriented liquid-crystalline (LC) elastomers (polar monodomains, concerning the direction of polarisation) have been prepared from polar monodomains of ferroelectric LC-polysiloxanes by a radical photocrosslinking process. Attempts to perform ferroelectric switching lead in these soft elastomers to an elastic stress which prohibits—for low voltages—a complete reorientation of the polar axis (ferroelectric switching). A ferroelectric switching can, however, be observed for high voltages. The loop of hysteresis of this switching is asymmetric concerning the zero point of the driving voltage. Piezoelectric measurements show that these elastomers combine an elastic memory for one polar state with enough flexibility to allow a reorientation of the polar axis.

Coupling of liquid crystalline and polymer network properties in LC-elastomers

Abstract Electromechanical and electro-optical properties which result from an interplay of ferroelectricity and elasticity in slightly crosslinked ferroelectric liquid crystalline elastomers are studied in the vicinity of the smectic C-smectic A phase transition. In the chiral smectic C phase, the formation of the network during the crosslinking procedure stabilizes the polar orientation which is present. On heating the smectic C network into the chiral smectic A phase, an induced tilt is observed (mechanoclinic effect). The internal mechanical stress induced by the network formation appears to act like an external electric bias-voltage. The results are discussed in terms of a simple Landau model.

Liquid Crystalline Elastomers—Characterization as Networks

Abstract This paper summarises the properties of archiral and chiral liquid crystalline (LC) elastomers, the latter of which have found interest due to their piezoelectric properties. In addition the formation of new LC elastomers by a radical polymerization of acrylate groups covalently linked to LC polymers was investigated. This was done concerning the progress of the crosslinking reaction, concerning the influence of the crosslinking on the LC phases and concerning the network properties (swelling ratio and elastic modulus) of the resulting elastomers. It turns out that the networks prepared in this way are very soft and their crosslinking density is low. This can be explained assuming that the radical polymerization of the acrylates leads only to short oligomers, due to steric restrictions. Photo crosslinking initiated by a photoinitiator in the LC phase creates a memory of the orientation before crosslinking.

LCD components obtained by patterning of chiral nematic polymer layers

An overview is given of the methods for preparation of birefringent films with patterned optical properties and their applications in liquid crystal displays. A versatile method of obtaining patterned films is based on manipulation of the helical twisting power of chiral dopants by photoisomerization reactions. Because of their transparency to visible light, high helical twisting power and large changes of the helical twisting power during UV irradiation, menthone derivatives are very suitable dopants for this process. A number of potential applications of the principles are discussed. Patterning of chiral nematic films with regions with different twisting angles is a suitable method for the preparation of patterned polarization rotators. Patterning of chiral nematic liquid crystal layers can also be used to control the reflection color and state of polarization of the reflected light. Such layers are useful for the preparation of reflective color filter arrays and broad band reflective polarizers.

Modification of the twist angle in chiral nematic polymer films by photoisomerization of the chiral dopant

A method for the production of polarization sensitive recordings in liquid crystalline polymers is presented. The system is based on local modification of the twist angle of chiral nematic polymer films. The twist angle of the polymer film is varied by modifying the chemical structure of the chiral dopant. Here a photoisomerizable menthone derivative is used that has a fivefold difference in helical twisting power between the E and the Z isomer. The twist angle of the film can be varied between −90° and 0° by introducing a nonphotosensitive chiral dopant with opposite twisting sense. Complex pictures with gray scales can be recorded in the films with black and white contrasts higher than 20.

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.

New Developments in Soft Lithography

Abstract The burgeoning area of soft lithography is reviewed with special emphasis on developments within the past three years. Applications in electronics have driven such developments, but more recently, other kinds of device structures and 3D prototyping have also found application, in part, through soft lithography. Microcontact printing (μCP), “lift off” μCP nano transfer printing (nTP), micromolding in capillaries (MIMIC), solvent assisted micromolding (SAMIM), replica molding (REM), and microtransfer molding are the main soft lithography schemes discussed.

Simple chiral urea gelators, (R)- and (S)-2-heptylurea: Their gelling ability enhanced by chirality

We present the first report on the synthesis of chiral ureas, (R)- and (S)-2-heptylurea, and their gelling behaviors. The ureas were prepared by the reactions of chiral amines and phenyl carbamate in the presence of triethylamine. On cooling from homogeneous solutions, the chiral ureas form gels in water and various nonpolar organic solvents, such as cyclohexane, toluene, and tetrachloromethane, while the racemate gelatinize only toluene and tetrachloromethane among the solvents we examined. The gelling ability of the enantiomeric urea is higher than the racemate, as the critical gelling concentrations in toluene, for example, were 0.2% and 0.7% (wt/wt), respectively. The enhanced gelling ability of the enantiomeric ureas is due to the 1D supramolecular structure formed during gelation. In contrast, the racemate crystallizes into two-dimensional lamellae, where the (R)- and (S)-2-heptylurea exist alternatingly in a plane (P2(1)/c space group). Powder X-ray diffraction pattern of the enantiomeric urea showed that it has a different crystal lattice from that of the racemate, implying that the steric effect by the methyl group at the chiral center prevents the pure enantiomers from having 2D hydrogen bonding networks, which lead to sheet-like structures for the racemate and the achiral analog. Thus the pure enantiomers self-organize into one-dimensional fibrous structures. The simplicity and the ambidextrous gelling behaviors of the chiral ureas in forming both hydrogels and organogels present numerous possibilities for future applications.