References of "Lagerwall, Jan 50002154"
     in
Bookmark and Share    
Full Text
Peer Reviewed
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 ▲]

Detailed reference viewed: 59 (1 UL)
Full Text
Peer Reviewed
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 ▲]

Detailed reference viewed: 49 (1 UL)
Full Text
Peer Reviewed
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 ▲]

Detailed reference viewed: 76 (1 UL)
Full Text
Peer Reviewed
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 ▲]

Detailed reference viewed: 70 (2 UL)
Full Text
Peer Reviewed
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 Phys. Rev. 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 ▲]

Detailed reference viewed: 85 (0 UL)
Full Text
Peer Reviewed
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 ▲]

Detailed reference viewed: 70 (1 UL)
Peer Reviewed
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 ▲]

Detailed reference viewed: 211 (0 UL)
Full Text
Peer Reviewed
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 ▲]

Detailed reference viewed: 66 (0 UL)
Full Text
Peer Reviewed
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 ▲]

Detailed reference viewed: 89 (0 UL)
Full Text
Peer Reviewed
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 ▲]

Detailed reference viewed: 74 (1 UL)
Full Text
Peer Reviewed
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 ▲]

Detailed reference viewed: 67 (1 UL)
Full Text
Peer Reviewed
See detailElectrooptic and dielectric spectroscopy measurements of binary chiral-dopant antiferroelectric mixtures
Parghi, D. D.; Lagerwall, Jan UL; Heppke, Gerd

in Molecular Crystals & Liquid Crystals (1999), 351

Recent examinations on binary chiral-dopant antiferroelectric liquid crystalline mixtures revealed an unusual switching behaviour. In order to investigate the switching processes that take place in the ... [more ▼]

Recent examinations on binary chiral-dopant antiferroelectric liquid crystalline mixtures revealed an unusual switching behaviour. In order to investigate the switching processes that take place in the mixtures, both in the bulk and as influenced by surfaces, further electrooptic and dielec- tric spectroscopy studies have been carried out on two model binary mix- tures in the induced SmA*, SmC* and SmCA* phases. The dielectric spectra feature the different modes typical for these phases, but of varying purity, indicating a more complex phase behaviour than expected. The electrooptic behaviour in the antiferroelectric phase varied from clearly antiferroelectric to near hysteresis-free switching directly between ferro- electric states. We conclude that the phase sequence, and consequently switching behaviour, is a function of both the type of dopant used and of the strength of the surface interactions (cell thickness). In thin cells syn- clinic (ferroelectric) ordering appears to be favoured. [less ▲]

Detailed reference viewed: 74 (1 UL)
Full Text
Peer Reviewed
See detailThe case of thresholdless antiferroelectricity: Polarization-stabilized twisted SmC* liquid crystals give V-shaped electro-optic response
Rudquist, Per; Lagerwall, Jan UL; Buivydas, M. et al

in Journal of Materials Chemistry (1999), 9(6), 1257-1261

We have studied the three-component liquid crystal mixture reported to exhibit ‘thresholdless antiferroelectricity’ [Inui et al., J. Mater. Chem, 1996, 6, 671]. We find that the thresholdless or V-shaped ... [more ▼]

We have studied the three-component liquid crystal mixture reported to exhibit ‘thresholdless antiferroelectricity’ [Inui et al., J. Mater. Chem, 1996, 6, 671]. We find that the thresholdless or V-shaped switching is obtained in thf absence of antiferroelectricity. This analog electro-optic response is due to the field-induced switching of a twisted smectic C* structure stabilized by polar surface interactions and by electrostatic bulk polarization charge interactions. The latter confine the director twist to thin surface regions leaving the bulk of the cell uniform, which gives good extinction at zero field. In sufficiently thin cells, such thresholdless switching can in fact be followed down to much lower temperatures, where the bulk would be antiferroelectric, but is maintained in the cells in the ferroelectric state by hysteresis from surface action. [less ▲]

Detailed reference viewed: 164 (7 UL)
Full Text
Peer Reviewed
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 ▲]

Detailed reference viewed: 80 (1 UL)