Andreas Hoischen

Hysteresis and memory factor of the Kerr effect in blue phases

The performance of a polymer-stabilized blue phase system based on a nematic host with large dielectric anisotropy and a chiral dopant with high helical twisting power is investigated and the influence of the reactive monomer composition on the electro-optic characteristics is studied. Field-induced birefringence with a Kerr coefficient greater than 1 nm V−2 can be achieved in a large temperature range from well below 20 °C to above 55 °C. The disturbing influences of electro-optic hysteresis and memory effects can be reduced by diligent choice of the composition and appropriate electric addressing.

Switching performance of a polymer-stabilized antiferroelectric liquid crystal based on bent-core molecules

Mixtures of a non-reactive and a photo-reactive liquid crystal, both consisting of bent core molecules, were used to prepare polymer-stabilized antiferroelectric liquid crystal cells by in-situ photo-polymerization. The mixtures exhibit a larger temperature range of the antiferroelectric phase than the pure compounds. The polymer network improves the ability of uniform alignment and thus enhances the switching contrast. In addition, the polymer-network reduces the switching times, in particular the relaxation time from the field-induced state to the ground state.

Submicrometer periodic patterns fixed by photopolymerization of dissipative structures

In photoreactive liquid crystals, spontaneous pattern formation can be used to fabricate phase gratings or polymer films with a periodic surface modulation. Typically, the periodicity of such structures is comparable to the sample thickness, and thus in the range between several micrometers and about 100 μm. Here, attempts to create and to investigate smaller structures are presented. For a chevron pattern, periodicities as small as 800 nm could be detected.

An Efficient Optical Method to Detect Phase Transitions in Liquid Crystals

ABSTRACT Phase transitions in liquid crystals can be detected by polarizing microscopy, even if the transition enthalpy is extremely low. We describe an efficient way of transforming microscopic video images to a scalar signal. The temperature dependence of this signal is compared to DSC measurements for two standard compounds. The results indicate that this kind of optical investigation is suitable as a standard tool to measure the transition temperatures of liquid crystals.

Optical storage effect due to photopolymerization of mesogenic twin molecules

The photopolymerization of diacrylate mixtures containing a nematic twin compound and a chiral dopant leads to perfectly stable, highly crosslinked cholesteric networks. The Bragg reflection spectra of the respective monomer and polymer mixtures were measured and the influences of the chemical structure, the temperature and the concentration of chiral dopant on the reflection wavelength was studied. Subsequent UV exposures of different parts of the sample at different temperatures are suitable to generate colored patterns. Deposition of several cholesteric films with different Bragg wavelengths results in additive color mixing.

Liquid Crystal/Polymer Composites: Kinetic Study of the Grating Formation in Holographic Polymer-Dispersed Liquid Crystals

Holographic polymer-dispersed liquid crystals are promising materials for switchable holograms, optical data storage, tunable beam deflectors and various diffractive optical elements, which can be addressed by electric fields. The present paper describes investigations on the grating formation in a model system. The results of holographic and calorimetric experiments are in good agreement with a reaction-diffusion model.

Asymmetric switching and storage effects in ferroelectric and antiferroelectric gels and polymers

Abstract Ferroelectric liquid crystal mixtures containing photochemical crosslinkers are found to exhibit asymmetric switching properties if the crosslinking process was performed under the influence of an electric field. This behavior can be attributed to the stabilizing effect of the polymer network on the ferroelectric state of the appropriate polarity. We report on the electro-optic properties of ferroelectric and antiferroelectric samples crosslinked with and without an applied electric field. Samples with sufficiently high content of the mesogenic crosslinker can be used for photo-optical storage effects which occur in both ferroelectric and antiferroelectric systems.

Electroconvection of liquid crystals : Tool for fabricating modulated polymer surfaces

Electrically induced dissipative structures in liquid crystals can be stored by the photocuring of reactive mesogens. Thus, spontaneous pattern formation can be used to fabricate phase gratings or polymer films with a periodic surface modulation from an initially uniform sample. Different patterns, both in the conductive and in the dielectric regimes, have been stored. Photopolymerization and subsequent investigation by atomic force microscopy provide a useful tool in analyzing dissipative structures, even if the lattice constants are smaller than 1 μm. This method indicates surface modulations with amplitudes of up to 12 nm and periodicities between 800 nm and several tens of micrometers. The influences of composition and curing conditions on the topography have been studied.

Bold relief fabrication by means of electroconvection: basic properties of a suitable mixture

Electroconvection of liquid crystals does not only lead to a large variety of spontaneously formed patterns, but these patterns can also be used as a template for the fabrication of polymer surfaces exhibiting a periodic bold relief. Here, a useful room-temperature nematic mixture consisting of two photostable liquid crystals and a photosensitive cross-linker is presented. Its stability diagram in the frequency-voltage plane, the influence of monomer concentration and the topography of the resulting polymer surfaces are described.