E. Gebhard

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.

Ferroelectric liquid crystalline elastomers, 1. Variation of network topology and orientation

In order to evaluate structure-property relations in ferroelectric LC-elastomers concerning netpoint topology and netpoint density, three different elastomers were investigated. As far as the netpoint topology is concerned systems with a crosslinking within the smectic layers (intra-layer) and between different layers (inter- layers) behave differently. Only the inter-layer systems (elastomer E2) are able to stabilize the polar order of the smectic-C* phase. Increasing the crosslinking density by stepwise crosslinking leads to a continuous shift of the ferroelectric hysteresis. Two switching times with and against the elastic field of the network are observed. The difference between these switching times increases with increasing crosslinking density. From this difference an electric field can be calculated, which is necessary for a compensation of the elastic field of the network. Crosslinking of elastomer E2 in the smectic A-phase leads to a stabilization of a macroscopically untilted state. If a tilt is induced in the crosslinked smectic A-phase by application of an electric field (electroclinic effect) the network keeps a memory of the polar state present during crosslinking.

Segmental orientation and mobility of ferroelectric liquid crystal polymers

IR spectroscopy was used to study the orientation and mobility of different molecular segments in a side chain ferroelectric liquid crystalline polymer (FLCP) in the book-shelf geometry. It was directly shown that the tilt angles for the mesogenic units and the spacers are different. The data obtained allowed us to construct a detailed model of segmental orientation in the SC phase for this FLCP. This model is consistent with the ‘zigzag’ model for tilted smectic phases. The rotational bias of carbonyl bonds is also confirmed and a possible orientation function for the carbonyl group is discussed. Time-resolved step-scan FTIR spectroscopy enabled us to follow the intra- and inter-molecular response of the FLCP to an external electric field with a time resolution of 5 mus. It was detected that mesogenic moiety, spacer and backbone take part in the reorientation process. The time responses of different molecular segments are similar on the time scale of a few hundred microseconds.

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.

Atomic force microscopic studies of the influence of stretching on thin oriented films of ferroelectric liquid crystalline elastomers

Abstract To analyze the impact of the molecular structure on the network formation in ferroelectric liquid crystalline polymers, two copolymers are studied, which are identical except for the molecular position of the cross-linkable group: (i) Polymer P1 with cross-linkable groups attached to the backbone via a short spacer and (ii) Polymer P2 with the cross-linkable group in terminal position of a mesogenic side group. When mechanical stress is imposed on thin films in homeotropic orientation through stretching, the two elastomers react differently to the deformation, as seen by AFM imaging of the surface topology: For Polymer P1, “intra-layer” cross-linking results in two dimensional networks in each backbone layer, separated by liquid-like FLC sidegroup layers. As there are practically no vertical connections in this intralayer network, no vertical distortions occur. In Polymer P2 a three dimensional, “inter-layer” network is formed; the system reacts with a distortion of the smectic layering.

Time-Resolved Polarized FTIR-Spectroscopy on the Molecular Structure and Mobility of Ferroelectric LC Elastomers (FLCE)

Abstract Time-resolved FTIR-spectroscopy is employed to study structure and mobility in ferroelectric LC polymers and elastomers (temperature range 20°C…130°C, dynamic range 0.1 Hz… 105 Hz). Due to its specifity the analysis of the (polarized) IR-spectra enables to determine the average orientation, the orientational order and the mobility in response to an electric field for the different molecular moieties (phenyl group, the polar groups, the methylene spacer and the polymer backbone). Furthermore the phase relationship in the motion of the different molecular groups can be extracted.

Structure and mobility in ferroelectric liquid crystalline elastomers as studied by time-resolved FTIR spectroscopy

Abstract The molecular structure and reorientation of ferroelectric liquid crystalline elastomers (FLCE) in response to an external electric field is studied on a microsecond scale with time-resolved Fourier transform infrared (FTIR) spectroscopy. In order to analyze the influence of the network on the molecular structure and mobility in FLCE, three similar FLC polysiloxanes are under study that differ just in their crosslinking architecture: besides the uncrosslinked polymer we obtain by photocrosslinking FLCE in which the backbones of either adjacent smectic layers (“interlayer”) or of the same smectic layer (“intralayer”) are preferably crosslinked. It is shown that the crosslinking leads to a slowing down of the molecular mobility which is stronger for the inter-than for the intralayer FLCE. Asymmetries in the reorientation times and/or in the reorientation angles are observed (elastic memory effect). The intralayer crosslinking causes a “locomotive effect”: the reorientation of the mesogenic cores precedes that ofthe backbones.

Temperature Dependent AFM on Ultrathin Oriented Films of FLC Elastomers

Crosslinked ferroelectric liquid crystalline polymers are studied by atomic force microscopy. Polysiloxane copolymers have been synthesized with mesogenic and photo-crosslinkable side groups, the l...

Liquid Crystalline Polymers under Uniaxial Mechanical Stress as Observed with Modulated Waveguide Spectroscopy

Abstract The reorientation of a liquid crystalline statistical side chain polymer under application of a dynamic uniaxial stress is studied using mechanical modulated waveguide spectroscopy (MMWS). It is found that a tilt angle of the order of 5[ddot] is induced in the SmA phase by compressions of 19 nm of the originally homeotropic orientation with a pronounced oricntationa! profile normal to the cell. The dependence of this mechanical reorientalion process on the thickness modulation amplitude and frequency is discussed.