Karl Siemensmeyer

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.

Photoconductivity in Discotic Liquid Crystals: A New Class of High-Mobility Materials

Abstract Using a time-of-flight technique, different transport mechanisms, deep trapping, multiple shallow trapping and Gaussian transport, can be observed in the different temperature and phase regions of the liquidcrystalline (LC) photoconductor hexapentyloxytriphenylene (HPT). Transient photocurrents and carrier mobilities for various temperatures, electric fields, and sample histories were examined. The ideal intrinsic Gaussian transport, observed for holes in the mesophase, puts HPT into a new class of highmobility materials with both hole mobilities on the order of 1.10−3cm2/Vs and a steplike current decay. These features result from the fact that there is obviously neither a positional disorder nor an energetic disorder, which would impair efficient charge transport. This is probably achieved by special molecular packing mechanisms in combination with the dynamical processes of the LC phase and due to favourable properties of the HPT molecule, which has a rigid and conjugated core, a symmetric stru...

ELECTRONIC PROPERTIES OF DISCOTIC LC-MATERIALS

Abstract We have made extensive studies of the charge-carrier mobilities in the discotic hexagonal mesophases of triphenylene-based discotic liquid crystals. Using the time-of-flight technique, transient photocurrents were measured yielding charge-carrier mobilities for various electric fields and temperatures. Starting from promising results obtained with the monomeric discotic liquid crystalline model compound hexapentyloxytriphenylene (H5T), we synthesized a “discotic twin” with two triphenylene units linked together by a suitable spacer. Additionally, we synthesized a discotic liquid crystalline oligomer consisting of four triphenylene units which are bound to a flexible cyclosiloxane ring. In the dimer and in the oligomer the discotic mesophase can be maintained in a discotic glass. Our data show that in both H5T as well as in the dimer and in the oligomer hole mobilities on the order of magnitude of 10−3cm2/Vs can be reached. Since the mobilities obtained in the oligomeric compounds are comparable t...

Comparative Dielectric Investigations of Liquid Crystalline Oligomers and Monomers

Abstract Dielectric relaxation times and the static dielectric behaviour of monomers and oligomers with definite chemical structure are compared with each other.