H. Poths

FROM MONOMERIC TO POLYMERIC FERROELECTRIC LIQUID-CRYSTALS - A COMPARATIVE-STUDY OF FERROELECTRIC PROPERTIES

Abstract Two new ferroelectric oligosiloxanes, a cyclic tetramer and a twin, have been synthesized. By a comparative study with their corresponding monomer and side chain polysiloxanes, the influence of oligo- and polymerization on the liquid crystalline and ferroelectric properties have been investigated. Polymerization leads to a stabilization of LC phases through increase of the clearing temperatures and suppression of crystallization. Oligomerization also leads to mesophase broadening, but, due to the low degree of polymerization, the effect is inferior to the linear polysiloxanes. The low viscosity of the oligosiloxanes ensures response times in the microsecond region, thus being comparable with their monomer and conventional LMWFLCs. It is found that polymerization increases the spontaneous polarization P s. This is attributed to the density increase after polymerization, enhancing the inter-mesogenic interactions. The collective and local dynamics of the OFLCs are influenced differently with respec...

Structure–property relationships of ‘diluted’ ferroelectric polysiloxanes

Abstract A series of new ferroelectric copolysiloxanes with systematically varied comonomer content (‘dilution’) has been synthesized. Good planar alignment could be achieved for all copolysiloxanes and they were studied with respect to their mesomorphic and ferroelectric properties. Broad enantiotropic S*C phases and spontaneous polarizations up to 286 nC cm−2 are found. X-ray diffraction experiments show a linear increase of the smectic layer spacing by ‘dilution’. This points to a microphase separated structure of mesogenic groups and siloxane chains. It is found that the fixation of a mesogen to homopolysiloxane leads to an increase of P s, whereas the ‘dilution’ of the mesogens with dimethylsiloxane units decreases P s again. Based on the microphase separated model, it can be shown that the decrease of P s is not only due to the decrease of the vol % of mesogenic groups. The coupling between different mesogens mediated by the polymer chain, has additionally to be taken into consideration. A remarkabl...

LC-Polymers with Axial Chirality

Abstract Liquid crystalline polymers with axial chirality are prepared. These polymers are chiral, not due to a single asymmetric carbon atom, but due to a larger molecular fragment, which is chiral as a whole. They exhibit cholesteric and chiral smectic C* phases. Dielectric spectroscopy proves strong ferroelectric properties in the chiral smectic C* phases of these polymers.

Anomalous current and electrooptic response in a polyacrylate ferroelectric liquid crystal with large spontaneous polarization

Abstract A side-chain polymer exhibiting a chiral smectic phase was investigated. Due to its anomalous current and electrooptic responses, we discuss the possibility of antiferroelectric order in the polymer, and compare its behaviour with a low-molar mass antiferroelectric substance. In particular, we study the birefringence modulation resulting from the field induced antiferroelectric-to-ferroelectric transition, and determine the induced polarization by the field reversal method.

Influence of order in thin smectic polymer films on the structure at the surface

Abstract Vertical and lateral structures of thin films (6–200 nm thick) of a combined liquid crystalline polymer with mesogenic groups in the main and side chains were investigated by X-ray reflectometry and scanning tunneling microscopy. The liquid crystalline polymer exhibits a cholesteric, a smectic C * and a smectic A mesophase. Films prepared by spin-coating onto glass substrates were investigated as a function of temperature in the different mesophases. The main interest was focussed on the smectic C * phase, in which the polymer chains show different orientations depending on the film thickness. In films thicker than about 10 nm, chains are oriented perpendicular to the substrate due to interface effects. Film thickness constraints in thinner films force chains to lie predominatly flat on the substrate and films partially show dewetting. X-ray reflection was used for the determination of structural parameters like layer thickness, chain orientation, phase sequence, perturbations in the layered structure and laterally averaged surface roughnesses, which partially depend on film thickness. Direct information about lateral surface structures on the nanometer length scale was obtained from scanning tunneling microscopy. In films thinner than about 10 nm after annealing holes of height of the initial film thickness are observed. In thicker films extended terraces are observed separated by steps of height of single layers. Defects in the layered structure are assumed to be responsible for the formation of such a surface structure. The surface profile could quantitatively be analyzed by a recent theoretical treatment of edge dislocation in thin liquid crystalline films.

Collective and molecular relaxations in polymeric ferroelectric liquid crystals and experimental proof of piezoelectricity in chiral smectic C-elastomers

Abstract Broadband dielectric spectroscopy (10−1 Hz — 109 Hz) was used to study the collective and molecular dynamics in polymeric side-chain FLC. In the frequency window under study collective (Goldstone-mode) and molecular (β-relaxation) relaxations are observed. By crosslinking combined main-chain side-group polymers an elastomeric FLC can be prepared which exhibits a Goldstone-mode as well. The piezoelectricity in these elastomeric materials is experimentally proven.

Direct observation of the main-chain conformation of a combined liquid-crystal polymer

Abstract The conformation of the main-chain (backbone) of a combined main-chain/side-chain liquid crystalline polymer has been qualitatively determined by small angle neutron scattering in the oriented nematic, the smectic A and the smectic C phases. The polymer backbone presents only a weak anisotropy, of prolate shape, in the nematic and the smectic C phases. A stronger reorientation of the backbones in the direction of the applied magnetic field is measured for the SA phase. However, this anisotropy remains small compared to the stretching of a main-chain liquid crystal polymer and the smectic structure results apparently from side-chain ordering. On the other hand, hydrodynamic measurements show that the combined polymer, in solvent, is as flexible as a polystyrene chain. This result is compatible with an explanation for the weak observed anisotropy.

New fluorinated chiral side-chain polymers with very large tilt angles in the smectic C* phase

Abstract A fluorinated chiral side-chain polysiloxane with very large tilt angles in the ferroelectric phase was studied. The optical response on an applied square wave could be analyzed in detail by an optical/electrical model for the cell, only assuming book-shelf geometry and negligible dielectric director torques. The model yields molecular tilt angles to a high accuracy by a curve-fitting procedure. Comparisons to other mesogenic substituents were also made, and indications were found that small (monatomic) substituents favor large tilt angles.

BROAD-BAND DIELECTRIC-SPECTROSCOPY ON FERROELECTRIC LIQUID-CRYSTALLINE POLYMERS

The collective and molecular dynamics of ferroelectric liquid crystalline side group polasiloxanes has been investigated be use of broadband dielectric spectroscopy (10 -2 Hz-10 9 Hz). In the lower freqeuncy range (<10 6 Hz) two collective relaxations, Goldstone - and soft - mode, could be observed. They are assigned to the fluctuations of the phase and the amplitude of the helical superstructure, respectively. In the high frequency regime (10 6 Hz - 10 9 Hz) one dielectric relaxation, the β-relaxation, is found

Elastic deformations in a magnetic field of a liquid-crystalline polymer with mesogenic groups in the main and side chains

Abstract Orientational elastic deformations in a magnetic field and phase transition temperatures of a thermotropically mesogenic “combined” aromatic polyester with mesogenic groups, both in the main and in the side chains, have been investigated in the range of 900 to 10,200 mol. wt. It was shown that in this molecular weight range the birefringence of a completely oriented nematic and correspondingly, the degree of its orientational order S are independent on molecular weight. When the relative temperature ΔT changes from −2 to −30°C the orientational order parameter increases from 0.35 to 0.55. The bend elasticity constants K 3 coincide in the order of magnitude with those for high and low molecular weight nematics investigated previously and their dependence on molecular weight has not been detected.