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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 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 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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