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See detailGeneration of frustrated liquid crystal phases by mixing an achiral n–smc mesogen with an antiferroelectric chiral smectic liquid crystal
Lagerwall, Jan UL; Giesselmann, Frank; Selbmann, Christine et al

in Journal of Chemical Physics (2005), 122(14), 144906

By mixing the achiral liquid crystal HOAB, exhibiting a nematic (N)-smectic-C (SmC) mesophase sequence, with the chiral antiferroelectric liquid crystal (AFLC) (S,S)-M7BBM7, forming the antiferroelectric ... [more ▼]

By mixing the achiral liquid crystal HOAB, exhibiting a nematic (N)-smectic-C (SmC) mesophase sequence, with the chiral antiferroelectric liquid crystal (AFLC) (S,S)-M7BBM7, forming the antiferroelectric SmCa phase, at least seven different mesophases have been induced which neither component forms on its own: a twist-grain-boundary (TGB) phase, two or three blue phases, the untilted SmA phase, as well as all three chiral smectic-C-type ``subphases,'' SmCalpha, SmCbeta, and SmCgamma. The nature of the induced phases and the transitions between them were determined by means of optical and electro-optical investigations, dielectric spectroscopy, and differential scanning calorimetry. The induced phases can to a large extent be understood as a result of frustration, TGB at the border between nematic and smectic, the subphases between syn and anticlinic tilted smectic organization. X ray scattering experiments reveal that the smectic layer spacing as well as the degree of smectic order is relatively constant in the whole mixture composition range in which AFLC behavior prevails, whereas both these parameters rapidly decrease as the amount of HOAB is increased to such an extent that no other smectic-C-type phase than SmC/SmC exists. By tailoring the composition we are able to produce liquid crystal mixtures exhibiting unusual phase sequences, e.g., with a direct isotropic-SmCa transition or a temperature range of the SmCbeta subphase of about 50 K. (C) 2005 American Institute of Physics. [less ▲]

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See detailDifferences between smectic homo- and copolysiloxanes as a consequence of microphase separation
Rössle, Martin; Braun, L.; Schollmeyer, D. et al

in Liquid Crystals (2005), 32(5), 533-538

This paper compares smectic phases formed from LC-homo- and LC-co-polysiloxanes. In the homopolysiloxane, each repeating unit of the polymer chain is substituted with a mesogen, whereas in the ... [more ▼]

This paper compares smectic phases formed from LC-homo- and LC-co-polysiloxanes. In the homopolysiloxane, each repeating unit of the polymer chain is substituted with a mesogen, whereas in the copolysiloxanes mesogenic repeating units are separated by dimethylsiloxane units. Despite a rather similiar phase sequence of the homo- and co-polysiloxanes—higher ordered smectic, smectic C* (SmC*), smectic A (SmA) and isotropic—the nature of their phases differs strongly. For the copolymers the phase transition SmC* to SmA is second order and of the ‘de Vries’ type with a very small thickness change of the smectic layers. Inside the SmA phase, however, the smectic thickness decreases strongly on approaching the isotropic phase. For the homopolymer the phase transition SmC* to SmA is first order with a significant thickness change, indicating that this phase is not of the ‘de Vries’ type. This difference in the nature of the smectic phases is probably a consequence of microphase separation in the copolymer, which facilitates a loss of the tilt angle correlation between different smectic layers. This has consequences for the mechanical properties of LC- elastomers formed from homo- and co-polymers. For the elastomers from homopolymers the smectic layer compression seems to be rather high, while it seems to be rather small for the copolymers. [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 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 detailElectrolyte effects on the nematic-isotropic phase transition in lyotropic liquid crystals
Mukherjee, Prabir K.; Lagerwall, Jan UL; Giesselmann, Frank

in Liquid Crystals (2005), 32(10), 1301-1306

A phenomenological approach to the description of the electrolyte effect on the nematic– isotropic phase transition in lyotropic liquid crystals is proposed. The influence of the electrolyte is discussed ... [more ▼]

A phenomenological approach to the description of the electrolyte effect on the nematic– isotropic phase transition in lyotropic liquid crystals is proposed. The influence of the electrolyte is discussed by varying the coupling between the concentration variables and the orientational order parameter. The analysis shows that the discontinuity in the first order nematic–isotropic phase transition as measured by TNI{T0 increases as a function of the NI weight fraction of the electrolyte. Here TNI is the first order nematic–isotropic phase transition temperature and TN0I is the extrapolated supercooling limit. The electrolyte dependence of the Cotton–Mouton coefficient and the non-linear dielectric effect in the isotropic phase above the nematic–isotropic phase transition are calculated. The theoretical predictions are found to be in good agreement with experimental results. [less ▲]

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See detailDemonstration of the antiferroelectric aspect of the helical superstructures in SmC*, SmC$_α$* and SmC$_a$* liquid crystals
Lagerwall, Jan UL

in Physical Review. E : Statistical, Nonlinear, and Soft Matter Physics (2005), 71(5), 051703

We show that the helical superstructure in chiral smectic-C-type liquid crystal phases can, if the pitch is short enough, give rise to the tristate switching characteristic of antiferroelectrics even if ... [more ▼]

We show that the helical superstructure in chiral smectic-C-type liquid crystal phases can, if the pitch is short enough, give rise to the tristate switching characteristic of antiferroelectrics even if the local commen- surate order is synpolar as in the ordinary Sm-C* phase. Since the field-induced helix unwinding exhibits a distinct threshold, in contrast to mere helix distortion, two unwinding / rewinding peaks per half cycle of an applied triangular wave voltage can be seen in the current response. By considering this antiferroelectric aspect * of the helical modulation we give a simple explanation of why the ultrashort-pitch Sm-C􏰄 phase exhibits antiferroelectric switching although its dielectric spectroscopy response is qualitatively identical to that of the synpolar Sm-C* phase. Using data from the compound MHPOCBC we show that the Sm-C*􏰄 dielectric re- sponse is well described by continuum theory. We also demonstrate that, if the pitch is very short as in MHPOCBC, helix unwinding / rewinding leave characteristic traces in the electrooptic response even in the commensurately antiferroelectric 􏰐antipolar􏰋 Sm-C*a phase, distinguishable from the switching beween the antipolar and synpolar states of this phase. [less ▲]

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See detailOn the origin of high optical director tilt in a partially fluorinated orthoconic antiferroelectric liquid crystal
Lagerwall, Jan UL; Giesselmann, Frank; Saipa, Alexander et al

in Liquid Crystals (2004), 31(9), 1175-1184

We have investigated the orthoconic antiferroelectric liquid crystal mixture W107 by means of optical, X-ray and calorimetry measurements in order to assess the origin of the unusally high tilt angle ... [more ▼]

We have investigated the orthoconic antiferroelectric liquid crystal mixture W107 by means of optical, X-ray and calorimetry measurements in order to assess the origin of the unusally high tilt angle between the optic axis and the smectic layer normal in this material. The optical birefringence increases strongly below the transition to the tilted phases, showing that the onset of tilt is coupled with a considerable increase in orientational order. The layer spacing in the smectic A (SmA) phase is notably smaller than the extended length of the molecules constituting the mixture, and the shrinkage in smectic C (SmC) and smectic C-a (SmCa) is much less than the optical tilt angle would predict. These observations indicate that the tilting transition in W107 to a large extent follows the asymmetric de Vries diffuse cone model. The molecules are on average considerably tilted with respect to the layer normal already in the SmA phase but the tilting directions are there randomly distributed, giving the phase its uniaxial characteristics. At the transition to the SmC phase, the distribution is biased such that the molecular tilt already present in SmA now gives a contribution to the macroscopic tilt angle. In addition, there is a certain increase of the average tilt angle, leading to a slightly smaller layer thickness in the tilted phases. Analysis of the wide angle scattering data show that the molecular tilt in SmCa is about 20degrees larger than in SmA. The large optical tilt (45degrees) in the SmCa phase thus results from a combination of an increased average molecule tilt and a biasing of tilt direction fluctuations. [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 detailA chameleon chiral polar liquid crystal: Rod-shaped when nematic, bent-shaped when smectic
Lagerwall, Jan UL; Giesselmann, Frank; Wand, Michael D. et al

in Chemistry of Materials (2004), 16(19), 3606-3615

The first antiferroelectric liquid crystal (AFLC) exhibiting a (chiral) nematic phase, a combination which has long been the goal of synthetic chemists working with polar liquid crystals but which at the ... [more ▼]

The first antiferroelectric liquid crystal (AFLC) exhibiting a (chiral) nematic phase, a combination which has long been the goal of synthetic chemists working with polar liquid crystals but which at the same time represents a fundamental contradiction in terms of translational order, was recently reported by Nishiyama and co-workers. We have investi- gated this chiral twin dimer by optic, electrooptic, and dielectric methods and conclude that it is not an ordinary AFLC material, but one where the peculiar properties of bent-core smectics are combined with those of ordinary rod-shaped liquid crystals. The compound exhibits a new type of nematic-smectic phase transition, connected with a change of molecule conformation from rod- to bent-shaped. This also has an important impact on the chiral interactions in the system. Toward the high-temperature end of the smectic phase, the energy balance between bent conformation smectic and straight conformation nematic can be shifted by an electric field such that the transition to the nematic phase with stretched-out molecules can be field-induced. [less ▲]

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See detailPolarity-directed analog electrooptic switching in a low-polarization chiral smectic liquid crystal with positive dielectric anisotropy
Lagerwall, Jan UL; Kane, Alexander; Clark, Noel A. et al

in Physical Review. E : Statistical, Nonlinear, and Soft Matter Physics (2004), 70(3), 031703

We describe an analog electro-optic (EO) switching mechanism occurring in thin cells filled with a low- polarization ferroelectric liquid crystal mixture with positive dielectric anisotropy. The mixture ... [more ▼]

We describe an analog electro-optic (EO) switching mechanism occurring in thin cells filled with a low- polarization ferroelectric liquid crystal mixture with positive dielectric anisotropy. The mixture is composed of an achiral nonpolar smectic-C 􏰐Sm-C􏰋 host doped with a small amount of a commercially available unichiral compound. The switching mechanism provides analog EO behavior, and thus could be attractive for informa- tion display applications. The process is polarization-driven for weak fields, while for higher field strength the dielectric coupling dominates the process. [less ▲]

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See detailFerroelectric polysiloxane liquid crystals with ‘de vries’-type smectic a* -smectic c* transitions
Rössle, Martin; Zentel, Rudolf; Lagerwall, Jan UL et al

in Liquid Crystals (2004), 31(6), 883-887

We report preliminary results of optical and small angle X-ray scattering (SAXS) experiments on the smectic A*2smectic C* transition in two ferroelectric liquid crystalline polysiloxanes. Although the ... [more ▼]

We report preliminary results of optical and small angle X-ray scattering (SAXS) experiments on the smectic A*2smectic C* transition in two ferroelectric liquid crystalline polysiloxanes. Although the optical tilt angle in the SmC* phases reaches values up to 30‡, temperature-dependent SAXS measurements clearly reveal that the smectic layer spacing is basically conserved during the A*–C* transition as well as in the subsequent C* phase. Connected with the A*–C* transition we further observed a significant increase in bire- fringence, hence reflecting an increase of orientational order. The practical absence of layer shrinkage and the enhanced orientational ordering are consistent with the de Vries diffuse cone model of smectic A2smectic C transitions. [less ▲]

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See detailOn the phase sequence of antiferroelectric liquid crystals and its relation to orientational and translational order
Lagerwall, Jan UL; Rudquist, Per; Lagerwall, Sven T. et al

in Liquid Crystals (2003), 30(4), 399-414

The substance MHPOBC is the oldest and still most important reference antiferroelectric liquid crystal (AFLC). There is still considerable controversy concerning the correct phase designations for this ... [more ▼]

The substance MHPOBC is the oldest and still most important reference antiferroelectric liquid crystal (AFLC). There is still considerable controversy concerning the correct phase designations for this material and, in particular, about the presence or absence of SmC* in its phase sequence. By means of dielectric spectroscopy and polarizing microscopy, we show that whereas the pure compound lacks the SmC* phase, this phase rapidly replaces the SmC*b subphase through the reduced purity resulting from temperature-induced chemical degradation which is hard to avoid under standard experimental conditions. X-ray investi- gations furthermore show that this change in phase sequence is coupled to a decrease in translational order. This explains the large variations in the reported phase sequence and electro-optic behaviour of MHPOBC, in particular concerning the SmC*b phase which has been said to exhibit ferro-, ferri- as well as antiferroelectric properties. It is likely that the sensitivity of the AFLC phase sequence to sample purity is a general property of AFLC materials. We discuss the importance of optical and chemical purity as well as tilt and spontaneous polarization for the observed phase sequence and propose that one of the key features determining the existence of the different tilted structures is the antagonism between orientational (nematic) and translational (smectic) order. The decreased smectic order (increased layer interdigitation) imposed by chemical impurities promotes the synclinic SmC* phase at the cost of the AFLC phases SmC*a , SmC*b , SmC*c and SmC*a . We also propose that the SmA* phase in FLC and AFLC materials may actually have a somewhat different character and, depending on its microstructure, some of the tilted phases can be expected to appear or not to appear in the phase sequence. AFLC materials exhibiting a direct SmA*–SmC*a transition are found to be typical ‘de Vries smectics’, with very high orientational disorder in the SmA* phase. Finally, we discuss the fact that SmC*b and SmC*c have two superposed helical superstructures and explain the observation that the handedness of the large scale helix may very well change sign, while the handedness on the unit cell level is preserved. [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 detailTilt plane orientation in antiferroelectric liquid crystal cells and the origin of the pretransitional effect
Rudquist, Per; Lagerwall, Jan UL; Meier, Johann G. et al

in Physical Review. E : Statistical, Nonlinear, and Soft Matter Physics (2002), 66(6), 061708

The optic, electro-optic, and dielectric properties of antiferroelectric liquid crystals 􏰐AFLCs􏰋 are analyzed and discussed in terms of the local tilt plane orientation. We show that the so-called ... [more ▼]

The optic, electro-optic, and dielectric properties of antiferroelectric liquid crystals 􏰐AFLCs􏰋 are analyzed and discussed in terms of the local tilt plane orientation. We show that the so-called pretransitional effect is a combination of two different electro-optic modes: the field-induced antiphase distortion of the antiferroelectric structure and the field-induced reorientation of the tilt plane. In the presence of a helix, the latter corresponds to a field-induced distortion of the helix. Both electro-optic modes are active only when the electric field has a component along the tilt plane. Thus, by assuring a horizontal surface-stabilized condition, where the helix is unwound by surface action and the tilt plane is everywhere parallel to the cell plates, the pretransitional effect should be suppressed. We also discuss the dielectrically active modes in AFLCs and under which circum- stances they contribute to the measured dielectric permittivity. [less ▲]

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See detailOptical and x-ray evidence of the de vries sm-A*-sm-C* transition in a non-layer shrinkage ferroelectric liquid crystal with very weak interlayer tilt correlation
Lagerwall, Jan UL; Giesselmann, Frank; Radcliffe, M. D.

in Physical Review. E. (2002), 66(3), 031703

A non-layer-shrinkage fluorinated ferroelectric liquid crystal compound, 8422@2F3\#, has been characterized by means of optical, x-ray, and calorimetric methods. The orientational distribution within ... [more ▼]

A non-layer-shrinkage fluorinated ferroelectric liquid crystal compound, 8422@2F3\#, has been characterized by means of optical, x-ray, and calorimetric methods. The orientational distribution within macroscopic volumes determined through wide-angle x-ray scattering and birefringence measurements, was found to be identical in the Sm-A* and helical Sm-C* phases. Together with the absence of layer shrinkage, this constitutes strong evidence that the second-order Sm-A*–Sm-C* transition in this material is well described by the diffuse cone model of de Vries. The absolute values of the layer spacing show that the molecules aggregate to antiparallel pairs. The molecular interaction across the layer boundaries will then occur only between fluorine atoms, leading to unusually weak interlayer tilt direction correlation. This explains the experimental observations of a very easily disturbed Sm-C* helix and a peculiar surface-stabilized texture. Tilt angle and birefringence values as a function of field and temperature have been evaluated in the Sm-A* and Sm-C* phases and the results corroborate the conclusions from the x-ray investigations. [less ▲]

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See detailAntiferroelectric liquid crystal mixture without smectic layer shrinkage at the direct sma* - smca* transition
Giesselmann, Frank; Lagerwall, Jan UL; Andersson, G. et al

in Physical Review. E : Statistical, Nonlinear, and Soft Matter Physics (2002), 66(5), 051704

We report results of x-ray, optic, electro-optic, and dielectric investigations on an antiferroelectric liquid- crystal mixture exhibiting a direct second-order phase transition between the Sm-A* and Sm ... [more ▼]

We report results of x-ray, optic, electro-optic, and dielectric investigations on an antiferroelectric liquid- crystal mixture exhibiting a direct second-order phase transition between the Sm-A* and Sm-Ca* phases with virtually no shrinkage in the smectic layer spacing. The birefringence measurements and texture observations suggest that the phase transition follows the diffuse cone model of Adrian de Vries, which explains the constant layer spacing. The antiferroelectric nature of the tilted phase is verified by the presence of twin polarization reversal peaks in the current response and by the absence of strong absorptions in the dielectric spectrum. The threshold for switching this phase to the synclinic, ferroelectric state is sharp and occurs at a very low voltage. [less ▲]

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See detailPhases, phase transitions and confinement effects in a series of antiferroelectric liquid crystals
Lagerwall, Jan UL; Parghi, Deven D.; Krüerke, Daniel et al

in Liquid Crystals (2002), 29(2), 163-178

Using a variety of optical and electrooptical techniques as well as dielectric spectroscopy, we have investigated three homologues in the chiral liquid crystal series nF1M7, where n denotes the length of ... [more ▼]

Using a variety of optical and electrooptical techniques as well as dielectric spectroscopy, we have investigated three homologues in the chiral liquid crystal series nF1M7, where n denotes the length of the unbranched side-chain. The main focus of the study is the series of smectic C subphases, i.e. SmCa*, SmC1/3* and SmC1/4*. During switching in the SmCa* phase, a peculiar redirection of the plane of biaxiality, distinguishing this phase from SmA* and SmC*, was observed. We present a simple explanation for this behaviour which correlates well with the clock model description of the SmCa* phase. We found a zero mesoscopic polarisation for the SmC1/4* phase and a non-zero mesoscopic polarisation for SmC1/3*, observations which are consistent with a distorted clock model. The dielectric spectroscopy investigations, performed at several different cell gaps, clearly show that the dielectric response in these materials is easily dominated by surface-induced structures if the cell gap is reduced, and thus reflects the bulk thermodynamic phase in very thick cells only. [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 detailDielectric investigations of a chiral monomer and side chain polymer
Gouda, F.; Kutub, Abdul A; Fodor-csorba, K. et al

in Molecular Crystals & Liquid Crystals (2000), 352

The dynamics of different collective and non-collective modes in a chiral liquid crystalline monomer and its side-chain polymer have been studied by dielectric spectroscopy. The monomer system enabled us ... [more ▼]

The dynamics of different collective and non-collective modes in a chiral liquid crystalline monomer and its side-chain polymer have been studied by dielectric spectroscopy. The monomer system enabled us to study the temperature dependence of the dielectric permittivity when crossing the transition from the SmA* phase directly to the crystal phase. Although the polymer system has a SmA* to SmC* transition, we found that the temperature dependence of the dielectric permittivity is radically different from the normally observed behaviour in these phases. The contributions from the soft and Goldstone modes, respectively, are extremely low, a result which is related to the very low value of the spontaneous polarization of the material. At lower temperatures in the SmC* phase, a very weak dielectric absorption located around 50 kHz is observed and discussed in terms of the liberational motion of side chains around the polymer main chain. [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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