S. U. Vallerien

Broadband dielectric spectroscopy on side group liquid crystal polymers

Abstract Dielectric studies on three liquid-crystalline side group polymers were performed in the temperature range from 100 K to 450 K and in the frequency range from 10−1 Hz to 107 Hz. All samples exhibit an α relaxation with a WLF dependence of the activation energy as well as a β relaxation, which are comparable in the glassy state. Their mean relaxation times, their relaxation time distribution, in its asymmetry and half-width, and their activation energies are nearly unaffected by the spacer length, terminal group and/or main chain. These findings are supported by N.M.R. measurements. A third relaxation process occurring close to the clearing point is also observed and assigned to the cooperative rotation of the side group around the main chain.

Effect of functionalization on glass formation by columnar mesophases of substituted triphenylene mesogens

Abstract A new kind of discotic liquid crystal has been investigated where one out of the six ether side chains of a triphenylene core was replaced by an ester side chain. We have found greatly increased clearing temperatures and suppressed crystallization thus leading to glass forming discotics. Here we report the synthesis, X-ray and dielectric studies concerning the nature of the glass transition. Furthermore we give arguments as to why a ‘mixed’ substitution can lead to such drastic effects.

Field dependent soft and goldstone mode in a ferroelectric liquid crystal as studied by dielectric spectroscopy

Abstract Broadband dielectric spectroscopy (frequency range 10 -1 –10 7 Hz and temperature range 100 to 450 K) was used to analyze in detail the frequency and temperature dependence of the soft and the Goldstone mode in a ferroelectric liquid crystal.

Molecular dynamics and the glass transition in a columnar liquid crystal formed by a chiral discotic mesogen

Abstract A new type of chiral discotic liquid crystal is described. It is based on a triphenylene core but carries only one asymmetric side chain per mesogen. This system displays a columnar liquid-crystalline mesophase with a helical superstructure and a pitch of 3·0 nm over a temperature range of 227 K. Upon cooling it forms a glassy state. By broadband dielectric and 2H NMR spectroscopy two motional processes are detected. The axial rotation of the discs around the column axis exhibits non-Arrhenius behaviour and is directly related to the glass transition. The second process is ascribed to localized side chain motions involving the ester linkages.

Softmode at the ferroelectric/non ferroelectric phase-transition in a liquid crystalline polymer

Abstract Broadband dielectric spectroscopy covering the frequency range from 10−1 to 107 Hz was used to analyse the softmode at the phase transition from a ferroelectric highly ordered and tilted smectic phase to the smectic A—phase in a combined side group main chain liquid crystalline polymer.

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.

The molecular origin of ferroelectricity in FLC: A challenge for dielectric spectroscopy at microwave frequencies

Abstract Broadband dielectric spectroscopy delivers in the frequency regime from 10 Hz to 10 GHz two collective relaxation processes (soft— and Goldstone—mode) and one molecular relaxation (β-relaxation). The latter does not split or broaden at the phase transition SmA*/SmC*. Its dielectric strength does not decline - in pronounced contrast to the (generalized) Landau theory of the phase transition SmA*/SmC* in FLC. Our experimental findings contradict the common explanation of the spontaneous polarization which is based on the existence of a free rotation inside the SmA* phase and its strong hindrance in the ferroelectric SmC* phase.

Comparative dielectric investigations of two liquid-crystalline side chain polymers

Abstract Dielectric relaxation measurements of two isomeric siloxane polymers with opposite direction of the carboxyl group in the liquid crystal part of the side chain were carried out in the frequency and temperature range from 10 Hz to 10 MHz and 150 K to 400 K. For every sample two intensive absorption ranges (∊″ >0·1) and two dielectric absorptions with lower intensities (∊″ <0·05) were detected. The first two are interpreted as reorientation processes of the side group as a whole around the short and long molecular axes, respectively. The less intensive absorptions can be observed below the glass temperature. They are discussed with respect to the different structures as the reorientation of the carboxyl electric dipoles in the middle part of the side group and of the terminal methoxy group, respectively.

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.

Broadband dielectric spectroscopy on ferroelectric liquid-crystalline side group polymers

Abstract Broadband dielectric spectroscopy (10−1 to 107Hz, 100 to 450 K) has been applied, for the first time, to investigate the molecular dynamics of recently synthesized fast switching ferroelectric side group polymers. The softmode could be studied in detail in the Sc∗ phase and in the S1∗ phase. At the transition a pronounced increase of the relaxational strength was found whereas its relaxational frequency remains constant The softmode is the molecular basis for the electroclinic effect which has high application potential for ferroelectric liquid crystal polymers.