H. Skupin

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.

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.

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.

Reorientation behaviour of bent core molecules in response to an external electric field as detected by time-resolved FTIR spectroscopy

Time-resolved polarized Fourier transform infrared spectroscopy (FTIR) is employed to analyse the segmental orientation and mobility of achiral bent core molecules in response to an external electric field. By shearing the substance between ITO coated CaF2 windows two types of domain, racemic and homochiral, are formed in the high temperature B2 phase. Each of these domains is characterized by two spontaneous symmetry-breaking instabilities which yield a symmetric and an antisymmetric electro-optical response, respectively. Taking advantage of the specificity of IR spectroscopy, this switching behaviour is analysed on a molecular level for the moieties of the bent core liquid crystal materials. In this way, the electrically induced reorientation of the different segments on a cone and the suppression of the antiferroelectric structure at higher frequencies can be followed in detail. Furthermore the biased rotation of the two carbonyl groups around the molecular long axis is determined. It is shown that al...

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.

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.

Segmental motion of ferroelectric liquid crystal polymer and elastomer during electro-optical switching

Application of both the conservative rapid scan and a novel step-scan Fourier-transform spectroscopy enables us to follow the intra- and intermolecular motion of ferroelectric liquid crystalline polymers and elastomers during electro- optical switching on a time scale from seconds down to microseconds. It has been detected that although all molecular parts move synchronously in course of the switching, the amplitude of such motions is different. The obtained results are discussed in terms of the role played by the molecular segments during the process of reorientation.

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.

Time-Resolved Polarized FTIR-Spectroscopy on Structure and Mobility in Banana-Shaped Liquid Crystal Molecules

Abstract By use of time-resolved polarized Fourier-transform infrared (FTIR-) spectroscopy the molecular structure and mobility of B2 phase LC banana-shape molecules is investigated. Two samples are investigated: an achiral banana-shaped (B14) system and a mixture of achiral (B14) and chiral (B7*) banana-shaped molecules. The molecules are oriented by shearing the substance between ITO-coated CaF2 windows. The measurements give detailed information about a) the average orientation, b) the orientational order, c) the reorientation time, d) the reorientation angle and e) the phase relation between the different segments for the different molecular moieties. The experiments are performed in dependence on frequency and strength of the external electric field. The corresponding molecular response of the different moieties is recorded by means of step-scan FTIR spectroscopy with a time resolution of 5 microseconds.

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.