Walter Lehmann

Giant lateral electrostriction in ferroelectric liquid-crystalline elastomers

Mechanisms for converting electrical energy into mechanical energy are essential for the design of nanoscale transducers, sensors, actuators, motors, pumps, artificial muscles, and medical microrobots. Nanometre-scale actuation has to date been mainly achieved by using the (linear) piezoelectric effect in certain classes of crystals (for example, quartz), and ‘smart’ ceramics such as lead zirconate titanate. But the strains achievable in these materials are small—less than 0.1 per cent—so several alternative materials and approaches have been considered. These include grafted polyglutamates (which have a performance comparable to quartz), silicone elastomers (passive material—the constriction results from the Coulomb attraction of the capacitor electrodes between which the material is sandwiched) and carbon nanotubes (which are slow). High and fast strains of up to 4 per cent within an electric field of 150 MV m-1 have been achieved by electrostriction (this means that the strain is proportional to the square of the applied electric field) in an electron-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymer. Here we report a material that shows a further increase in electrostriction by two orders of magnitude: ultrathin (less than 100 nanometres) ferroelectric liquid-crystalline elastomer films that exhibit 4 per cent strain at only 1.5 MV m-1. This giant electrostriction was obtained by combining the properties of ferroelectric liquid crystals with those of a polymer network. We expect that these results, which can be completely understood on a molecular level, will open new perspectives for applications.

The inverse electromechanical effect in mechanically oriented S-C*-elastomers examined by means of an ultra-stable Michelson interferometer

Abstract An ultra-stable Michelson interferometer is described which enables to measure periodic displacements (of a mirror in one interferometer arm) with a resolution of less than 100 fm. This setup is employed to measure the (linear) inverse piezoelectric and the (quadratic) electrostrictive effect in macroscopically oriented ferroelectric liquid crystalline elastomers (FLCE). A pronounced temperature dependence of the electromechanical effects is observed. Superposition of a DC-bias field leads to a strong enhancement of the inverse piezoelectric effect being well comparable in its strength with classical piezoceramics (BaTiO3, PZT, etc.). The molecular mechanisms of the observed effects are discussed.

Direct and inverse electromechanical effect in ferroelectric liquid crystalline elastomers

Ferroelectric liquid crystalline elastomers (FLCEs) form a novel class of materials, in which the properties of macroscopically oriented ferroelectric liquid crystals can be combined with those of elastomers. The (direct and the inverse) piezoelectric effect in these systems is measured and analyzed on a molecular level. Backed also by x-ray measurements, a model is suggested that interprets the observed piezoelectricity caused by electrically or mechanically induced motions of smectic layers which are inclined with respect to the sample surface. The strength of the observed electromechanical effects compares well with (or exceeds) that of classical piezoelectric materials like barium titanate, lead zirconate titanate or poled polymers like poly(vinylidene fluoride), making FLCEs an interesting candidate for applications in microsystems technology.

Piezoelectricity in ferroelectric liquid crystalline elastomers

The direct and inverse piezoelectricity in single crystal ferroelectric liquid crystalline elastomers was measured by means of purpose-made experimental setups. As expected the observed effects depended strongly on temperature and the strength of the applied alternating electric field; additionally they could be strongly enhanced by a superimposed direct current electric field. The latter resulted from a molecular amplification of the polarization vector of the single smectic layers by inducing a rotational bias of the lateral distribution of the polar groups of the mesogens. This resulted in a pronounced magnification of the (macroscopic) piezoelectric effect. Backed also by X-ray measurements, a model is suggested, which interprets the observed piezoelectricity as caused by a field-induced change of the inclination of the tilted smectic layers. The strength of the observed electromechanical effects compares well with or exceeds that of classical materials, such as barium titanate or lead–zirconate–titanate or the polymer polyvinylidene fluoride.

Piezoelectric and Pyroelectric Investigations on Microtomized Sections of Single-Crystalline Ferroelectric Liquid Crystalline Elastomers (SC-FLCE)

Abstract Microtomized sections of single-crystalline ferroelectric liquid crystalline elastomers (SC-FLCE) have been prepared. The piezoelectric (and electrostrictive) responses in these sections have been measured by means of an interferometric setup and the pyroeletric properties have been determined using the laser intensity modulation method (LIMM).

Inverse Piezoelectric and Electrostrictive Response in Freely Suspended FLC Elastomer Film as Detected by Interferometric Measurements

We report the electric field induced thickness variations of homeotropically oriented free standing films of a smectic (C* or A*) FLCE prepared from cross - linkable ferroelectric polysiloxanes. The changes in optical path length in free standing ferroelectric liquid crystal elastomer films have been detected by means of interferometric measurements at both the first and second harmonic of the exciting electric field (ω=33 Hz). The measured electrostrictive strain is above 2.7% (in the thickness direction) at a electric field around 1.5 MV /m. Our experiment reveal that the inverse piezoelectric and electrostrictive response increases sharply near the Sm-C* - Sm-A* phase transition temperature. Also X-ray reflection measurements on a spin cast FLCE film reveal the constriction of smectic layers.

Electromechanical effects in free standing FLC elastomer films as determined by interferometric measurements

Abstract The electrically induced thickness variations of homeotropically oriented free standing films of a smectic (C* and A*) ferroelectric liquid crystalline elastomer (FLCE) have been examined. For this purpose an interferometric setup has been built with a resolution of 3 pm at 133 Hz AC excitation frequency. The laser beam has been focussed by means of two “long distance” microscope objective lenses in order to study small homogeneous areas in the film. The changes in optical pathlength through the sample were measured at the first (ω), second (2ω) and fourth (4ω) harmonic of the AC-electric excitation voltage (ω=133 Hz). The measured thickness (optical pathlength) modulation at the 2nd harmonic is in the range of some % and thus stronger than expected from FTIR-measurements on the electroclinic effect in CaF2 cells filled with this sample. Nevertheless, the strength of this quadratic effect and also of the linear effect is greatly increased at the phase transition Sm-C*-Sm-A*, which indicates a correlation of the thickness variation with the electroclinic effect.

Nanomotoren aus Flüssigkristallen: Elektrostriktion

Herkommliche Hochleistungs-Piezomaterialien erreichen bei Anlegen elektrischer Spannung einen Hub von maximal 0, 1 % ihrer Langen. Arbeitsgruppen der Universtaten Leipzig und Mainz haben ein neues, ferroelektrisches Elastomer entwickelt, das auf Flussigkristallen basiert. Die Flussigkristallmolekule sind so in das Polymernetzwerk eingebunden, dass sie sich wie mikroskopische Hebel bewegen und Krafte ubertragen konnen. Bei einem elektrischen Feld von nur 1, 5 kV/mm andert ein frei tragender Film des neuen Materials seine Dicke um bis zu 4 %. Das ist Weltrekord und eroffnet vollig neue Felder fur technische Anwendungen.