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See detailChiral smectic C subphases induced by mixing a bistereogenic antiferroelectric liquid crystal with a non-chiral liquid crystal
Lagerwall, Jan UL; Giesselmann, Frank; Rauch, Sebastian et al

in Ferroelectrics (2005), 315

By mixing a bistereogenic antiferroelectric liquid crystal (AFLC) compound, exhibiting only the SmQ and SmCa mesophases, with the achiral N-SmC liquid crystal HOAB we could induce all three AFLC SmC-type ... [more ▼]

By mixing a bistereogenic antiferroelectric liquid crystal (AFLC) compound, exhibiting only the SmQ and SmCa mesophases, with the achiral N-SmC liquid crystal HOAB we could induce all three AFLC SmC-type subphases, SmCalpha, SmCbeta and SmCgamma. This seems to be in contradiction with two recent postulations regarding the subphase stability, one of which suggests that the subphases appear as a result of strong chiral interactions, the other that these phases require high smectic order something one would generally not expect in mixtures. We have studied the helical pitch, optical tilt angle, spontaneous polarization and the x-ray diffraction due to the smectic layering, as a function of mixing ratio in order to better understand the relation between phase sequence and mixture composition. The smectic layer spacing shows a strongly non-linear behavior suggesting that the basic structure of the pure AFLC substance is retained up to a HOAB content of about 75\%. [less ▲]

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See detailA study of a bistereogenic mesogen for the development of orthoconic antiferroelectric liquid crystal materials
Lagerwall, Jan UL; Yates, Chris; Rauch, Sebastian et al

in Ferroelectrics (2005), 315

The first orthoconic antiferroetectric liquid crystals (OAFLCs), i.e. smectics where the optical director changes direction by 90 degrees between neighboring layers, were mixtures of partially fluorinated ... [more ▼]

The first orthoconic antiferroetectric liquid crystals (OAFLCs), i.e. smectics where the optical director changes direction by 90 degrees between neighboring layers, were mixtures of partially fluorinated monostereogenic compounds. They have successfully demonstrated the orthoconic properties (orientation-independent dark state between crossed polarizers) but suffer from too high polarization and too short helical pitch, necessitating very thin samples. Using an (S,R) bistereogenic OAFLC we have obtained the orientation-independent dark state in rather thick samples, but several other problems arise with this compound. The strongly first-order SmA-SmCa transition produces defects leading to light leakage. In order to be switchable the sample must furthermore be mixed with chiral dopants, generally reducing the tilt angle as well as a shortening the helical pitch. Finally, a SmC phase often appears in the phase sequence of the mixture, strongly dominating over the desired SmCa phase in thin cells. [less ▲]

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See detail(-)-isopinocampheol substituted mesogens: An investigation of the effect of bulky terminal groups in chiral smectic liquid crystals
Yates, Chris; Lagerwall, Jan UL; Nobili, Maurizio et al

in Ferroelectrics (2004), 311

A ferroelectric liquid crystal with an end-substituted (-)-isopinocampheol (IPC) group has been studied in isolation and in mixtures with the antiferroelectric compound EHPOCBC. The samples were studied ... [more ▼]

A ferroelectric liquid crystal with an end-substituted (-)-isopinocampheol (IPC) group has been studied in isolation and in mixtures with the antiferroelectric compound EHPOCBC. The samples were studied with respect to electroclinic coefficients, tilt angles, X-ray layer spacing, dielectric permittivity, spontaneous polarisation and relative birefringence under an applied field. In the pure isopinocampheol substituted compound, the SmA-SmC phase transition appears strongly first order and is associated with a significant increase in effective birefringence. The smectic layer spacing decreases below the transition approximately with the optical tilt. On increased EHPOCBC concentration the change in birefringence across the transition diminishes, until the point at which the mixture adopts an anticlinic SmC(A) phase, and the expected decrease in birefringence is observed. There is very little change in birefringence with field for the IPC compound, whilst EHPOCBC shows a marked increase with increasing field, for a few degrees above the phase transition. The results are analyzed with respect to two models. In one it is proposed that the bulky IPC group disorders the SmA phase by introducing undulations in the smectic layer. These undulations may disappear in the SmC phase due to an antiparallel arrangement of neighbouring molecules. In the other model the bent shape of the mesogen as a whole plays a crucial role in changing the optical properties at the SmA-SmC transition, due to the biased rotation around the molecular axis of inertia. [less ▲]

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See detailSurface- and field-induced AFLC structures detected by dielectric spectroscopy
Lagerwall, Jan UL; Rudquist, Per; Lagerwall, Sven T. et al

in Ferroelectrics (2002)

In order to better understand which features in dielectric spectra of antiferroelectric liquid crystals (AFLCs) are due to the bulk director geometry and which are due to surface-induced structures, we ... [more ▼]

In order to better understand which features in dielectric spectra of antiferroelectric liquid crystals (AFLCs) are due to the bulk director geometry and which are due to surface-induced structures, we have performed dielectric spectroscopy measurements with simultaneous texture monitoring on two SmC*-exhibiting AFLC homologues (11- and 12F1M7), at varying cell gap. Such AFLCs are strongly affected by surface action even in fairly thick cells (d≈15µm), with heavy supercooling of the SmC* phase as the most obvious result. We show that the supercooled structure can be removed by AC-field treatment in the SmCa* phase, but some domains may stay in a polar geometry, as reflected in both texture and dielectric signature. On heating from the antiferroelectric SmC?* subphase into SmC*, meta-stable non-helical domains may form at cell gaps much larger than the helical pitch. These domains give rise to a lowfrequency dielectric absorption not seen in bulk SmC* samples. [less ▲]

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See detailFreezing of Nano-Scaled Fractal Molecules: Dendrimers
Krüger, Jan-Kristian UL; Veith, M.; Elsässer, R. et al

in Ferroelectrics (2001), 259(1-4), 27-36

Within this paper we present mono-disperse dendrimers as a new class of ideal canonical glass formers where the frustration mechanism originates from the self similar intramolecular order. Because of ... [more ▼]

Within this paper we present mono-disperse dendrimers as a new class of ideal canonical glass formers where the frustration mechanism originates from the self similar intramolecular order. Because of their fractal intramolecular structure these dendrimers are hardly able to build up a three dimensional translation symmetry necessary to form the crystalline state. Instead they show an unconventional dynamic and static freezing behavior at rather low temperatures. Based on temperature modulated differential scanning calorimetry (TMDSC) and Brillouin spectroscopy (BS) we give further evidences for the existence of an ideal primary glass transition with a thermal glass transition temperature which increases with the generation index of the fractal molecule. However, the primary glass transitions of higher generations do not completely erase the memory of the fractal intramolecular mobility but allow the survival of dynamic processes significant for the freezing of the constituting molecular unit. [less ▲]

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See detailOptic, electrooptic and dielectric properties of novel antiferroelectric liquid crystal compounds
Jägemalm, P.; Lagerwall, Jan UL; Dahlgren, A et al

in Ferroelectrics (2000)

The optic, electrooptic and dielectric spectroscopy characterisation of novel four-ring thiophene-based chiral esters comprising suitably 5,5’-disubstituted 2,2’-bithiophene (antiferroelectrics) is ... [more ▼]

The optic, electrooptic and dielectric spectroscopy characterisation of novel four-ring thiophene-based chiral esters comprising suitably 5,5’-disubstituted 2,2’-bithiophene (antiferroelectrics) is reported. The two compounds exhibit interesting hysteresis in their phase sequences, and we show that even whole phases (e.g. their relatively wide ferri-phases) may be suppressed by the surface interactions in thin cells. [less ▲]

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See detailOn the coexistence of SmC* and SmCa* phases in binary chiral-dopant antiferroelectric mixtures
Lagerwall, Jan UL; Parghi, D. D.; Heppke, Gerd et al

in Ferroelectrics (2000), 244

We present the results from investigations on a series of binary chiral- dopant liquid-crystalline mixtures. The racemic host materials possess tilted smectic phases with varying strengths of synclinic ... [more ▼]

We present the results from investigations on a series of binary chiral- dopant liquid-crystalline mixtures. The racemic host materials possess tilted smectic phases with varying strengths of synclinic and anticlinic ordering (SmC and SmCalt phases respectively). The dopants used were either a strongly synclinic-favouring non liquid-crystalline chiral material (commonly used in FLC chiral-dopant mixtures) or a strongly anticlinic (antiferroelectric) material ((S)-MHPOBC). The electrooptic and dielectric properties of the mixtures were investigated in test cells of different thickness and the influence of each mixture component on the observed physical properties is discussed. [less ▲]

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See detailElectrooptic and dielectric properties of new antiferroelectric liquid crystal mixtures
Andersson, G.; Dabrowski, R.; Drzewinski, W. et al

in Ferroelectrics (2000), 244

Two new antiferroelectric liquid crystal mixtures based on optically active hydroxyesters have been studied with respect to their electro-optic and dielectric behavior. The mixtures exhibit only a SmA ... [more ▼]

Two new antiferroelectric liquid crystal mixtures based on optically active hydroxyesters have been studied with respect to their electro-optic and dielectric behavior. The mixtures exhibit only a SmA* phase above SmCa* and show a stable antiferroelectric order over a broad temperature range (≈100°C). Electro-optic as well as dielectric studies suggest that the SmCa*-SmA* transition is second order. In thin cells signs of a sur- face-induced SmC* phase has also been observed. We present the rele- vant characteristics for three different electro-optic modes which could be exploited in these materials, including the very fast linear mode for sub-threshold voltages. [less ▲]

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See detailAntiferroelectric liquid crystals with 45° tilt - a new class of promising electro-optic materials
D’havé, K.; Dahlgren, A; Rudquist, Per et al

in Ferroelectrics (2000), 244

Antiferroelectric liquid crystals with a tilt angle of 45 degrees have very interesting optical properties, which seem to have been overlooked so far perhaps because such materials have hardly been ... [more ▼]

Antiferroelectric liquid crystals with a tilt angle of 45 degrees have very interesting optical properties, which seem to have been overlooked so far perhaps because such materials have hardly been available. We have prepared a four-component mixture of partially fluorinated compounds with a SmC/sub n/* phase in the interval between 27.4 degrees C and 121.6 degrees C, in which the tilt angle theta saturates at 45 degrees for T<or=80 degrees C, and we investigate the optical properties, theoretically and experimentally. One of the surprising features of 45 degree materials is that they permit a remarkably high contrast by virtue of an excellent dark-state, in spite of the fact that AFLC materials are notoriously difficult to align. This is because a 45 degrees AFLC turns out to be (negatively) uniaxial instead of biaxial. We describe these properties and propose a number of potentially interesting new applications, including a polarizer-free display mode and a three-level ``phase-only'' modulator. [less ▲]

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See detailThe dependence on the helical pitch of the antiferroelectric dielectric modes
Lagerwall, Jan UL; Fütterer, T.; Moro, D. et al

in Ferroelectrics (1999), 244

In an attempt to determine the physical nature of the low-frequency dielectric mode in antiferroelectric liquid crystals, we have studied the dielectric response in thick planar-oriented samples of ... [more ▼]

In an attempt to determine the physical nature of the low-frequency dielectric mode in antiferroelectric liquid crystals, we have studied the dielectric response in thick planar-oriented samples of Smectic C/sub A/* compounds undergoing a helix inversion. If the mode is collective there ought to be a minimum in the dielectric absorption due to the mode, at the temperature at which the helical pitch diverges. Although the effect is not large, we find minima for both antiferroelectric modes at this point, indicating that both modes are related to the helical director structure. A corresponding experiment has also been performed on a binary mixture which shows a helix inversion for a certain mixing ratio. However, because the effect in this case is overshadowed by unavoidable variations in other parameters, no minimum was detected. [less ▲]

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See detailPhases and phase transitions of KPF6
Huber, P.; Krummeck, H.; Baller, Jörg UL et al

in Ferroelectrics (1997), 203(1), 211-219

Potassium hexafluorophosphate KPF6 has been investigated by X-ray powder diffraction, calorimetry and Brillouin spectroscopy. With decreasing temperature, the crystal shows a sequence of phases: NaCl-type ... [more ▼]

Potassium hexafluorophosphate KPF6 has been investigated by X-ray powder diffraction, calorimetry and Brillouin spectroscopy. With decreasing temperature, the crystal shows a sequence of phases: NaCl-type cubic-monoclinic-monoclinic with two-phase coexistence regions in between. In the cubic phase the elastic constants have been determined. c44 and c12 show a slight softening. [less ▲]

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