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See detailMolecular model for de Vries type smectic-A–smectic-C phase transition in liquid crystals
Gorkunov, M. V.; Giesselmann, Frank; Lagerwall, Jan UL et al

in Physical Review. E : Statistical, Nonlinear, and Soft Matter Physics (2007), 75(6), 060701

We develop both phenomenological and molecular-statistical theory of smectic-A-smectic-C phase transition with anomalously weak smectic layer contraction. Using a general mean-field molecular model, we ... [more ▼]

We develop both phenomenological and molecular-statistical theory of smectic-A-smectic-C phase transition with anomalously weak smectic layer contraction. Using a general mean-field molecular model, we demonstrate that a relatively simple interaction potential suffices to describe the transition both in conventional and de Vries type smectics. The theoretical results are in excellent agreement with experimental data. The approach can be used to describe tilting transitions in other soft matter systems. [less ▲]

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See detailPartitioning and reorientational dynamics of phenylalcohols in SDS lyotropic liquid crystalline mesophases: An alc-μsr study
Martyniak, Aleksandra; Dilger, Herbert; Mckenzie, Iain et al

in Colloids and Surfaces A : Physicochemical and Engineering Aspects (2007), 309

Avoided level crossing muon spin resonance (ALC-􏰝SR) has been applied to monitor location and reorientational dynamics of two different sets of muoniated cyclohexadienyl radicals derived from 3 ... [more ▼]

Avoided level crossing muon spin resonance (ALC-􏰝SR) has been applied to monitor location and reorientational dynamics of two different sets of muoniated cyclohexadienyl radicals derived from 3-phenylpropan-1-ol and 5-phenylpentan-1-ol as tracer molecules in SDS–water–pentanol ternary mixtures and in SDS–water–pentanol–dodecane quaternary systems. The present results show a dependence of the phenylalcohol partitioning on the environment polarity and on the structure of the SDS dispersions. Both radicals tend to be incorporated within the mesophases at 25◦C. The low polarity sensed by the tracer molecule indicates that among all mesophases these species are immersed the deepest in the sponge phase. Alternatively, the effect could be explained by a dependence of the water gradient in the surfactant layer on the overall water versus hydrocarbon content of a system. The changing amplitudes, widths and shapes of the 􏰰1 resonances reveal a different extent of dynamics of three isomers of the muoniated 3-phenylpropan-1-ol radical in particular mesophases. The most extensive dynamics occurs in the hexagonal phase. The probe molecule chain length and the fractions of pentanol and dodecane are additional factors that affect partitioning. [less ▲]

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See detailOn the change in helix handedness at transitions between the sm-c* and sm-ca* phases in chiral smectic liquid crystals
Lagerwall, Jan UL; Giesselmann, Frank; Osipov, Mikhail A.

in Liquid Crystals (2006), 33(6), 625-633

Using a discrete model for the synclinic SmC* and the anticlinic SmC􏰀a phases we give a theoretical explanation for the fact that the helix twisting sense reverses at a transition between these phases ... [more ▼]

Using a discrete model for the synclinic SmC* and the anticlinic SmC􏰀a phases we give a theoretical explanation for the fact that the helix twisting sense reverses at a transition between these phases (direct transition or via the so-called chiral smectic C ‘subphases’) and we derive an explicit expression for the helical pitch in the SmC􏰀a phase. As the theory shows and as we also demonstrate experimentally, the reversal is of a different nature from helix inversions within a single phase, where the inversion is always coupled to a pitch divergence. At a clinicity change the common behaviour is instead pitch-shortening on approaching the phase transition and the associated helix twisting sense reversal. The phenomenon may be put to use in smart mixing in order to control the helix pitch, either for achieving long pitch for surface-stabilized ferroelectric and antiferroelectric liquid crystal displays; or a very short pitch, in the case of devices utilizing the deformed helix mode. [less ▲]

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See detailSimultaneous alignment and dispersion of carbon nanotubes with lyotropic liquid crystals
Lagerwall, Jan UL; Scalia, G.; Haluska, Miroslav et al

in Physica Status Solidi B. Basic Research (2006), 243(13), 3046-3049

We demonstrate that single-wall carbon nanotubes (SWCNTs) can be macroscopically aligned by means of templating in a lyotropic nematic liquid crystal (LC), a self-assembling anisotropic fluid with orienta ... [more ▼]

We demonstrate that single-wall carbon nanotubes (SWCNTs) can be macroscopically aligned by means of templating in a lyotropic nematic liquid crystal (LC), a self-assembling anisotropic fluid with orienta- tional but no translational order. The CNTs spontaneously adopt the alignment of the host, as we verify by means of resonant Raman spectroscopy. The aqueous LC host, based on the surfactant SDS, simultane- ously keeps the nanotubes well dispersed over time scales of months or longer. The LC can be loaded with CNTs to almost the same extent as the standard isotropic 1% surfactant solutions normally used for dispersing CNTs without any optically visible bundling occurring. [less ▲]

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See detailThe peculiar optic, dielectric and x-ray diffraction properties of a fluorinated de vries asymmetric-diffuse-cone-model ferroelectric liquid crystal
Lagerwall, Jan UL; Coleman, D.; Körblova, E. et al

in Liquid Crystals (2006), 33(1), 17-24

A new semi-fluorinated chiral smectic liquid crystal, W504, is investigated by electro-optic, dielectric and X-ray scattering experiments. It exhibits a huge dielectric soft mode response, strong ... [more ▼]

A new semi-fluorinated chiral smectic liquid crystal, W504, is investigated by electro-optic, dielectric and X-ray scattering experiments. It exhibits a huge dielectric soft mode response, strong electroclinic effect and a birefringence which increases considerably with the director tilt angle theta; typical characteristics of a SmA - SmC transition following the de Vries asymmetric diffuse cone (ADC) model in which the non-zero director tilt in SmC arises through an ordering of tilting directions rather than an actual increase in average molecule tilt <theta(mol)>. In W504 a small increase in <theta(mol)> of about 4 degrees is however detected in the SmC phase. Although the increase in molecule inclination is much less than the increase in director tilt h, saturating close to 30 degrees, it leads to a shrinkage of the smectic layers by about 1 angstrom, a result of the large initial molecule tilt in the SmA phase, <theta(mol)>(SmA) approximate to 30 degrees. The tilting transition in W504 is thus mainly an ADC model disorder - order transition, but it also has a component of a structural transition. The semi- fluorinated molecular structure of W504 leads to a very weak electron density modulation along the layer normal, giving a vanishing form factor in bulk samples which exhibit no (001) X-ray scattering peak. In thin films the (001) peak is however observed, indicating that the electron density modulation is enhanced by the breaking of the head - tail symmetry of the liquid crystal phase at the LC - air interface. [less ▲]

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See detailCurrent topics in smectic liquid crystal research
Lagerwall, Jan UL; Giesselmann, Frank

in Chemphyschem : A European Journal of Chemical Physics and Physical Chemistry (2006), 7(1), 20-45

Interest in the smectic liquid-crystalline state of matter received a substantial boost with the discovery by Meyer in the mid-1970s that a chiral smectic C (SmC*) phase exhibits a spontaneous elec- tric ... [more ▼]

Interest in the smectic liquid-crystalline state of matter received a substantial boost with the discovery by Meyer in the mid-1970s that a chiral smectic C (SmC*) phase exhibits a spontaneous elec- tric polarization, and with the subsequent demonstration by Clark and Lagerwall of the surface-stabilized SmC* ferroelectric liquid crystal at the beginning of the 1980s. Since then, chiral smectic phases and their plethora of polar effects have dominat- ed the research in this field, which today has reached a mature state where the first commercial microdisplay applications are now shipping in millions-per-year quantities. In this Review we discuss some of the topics of highest interest in current smectic liquid crystal research, and address application-relevant research (de Vries-type tilting transitions without defect generation and high-tilt antiferroelectric liquid crystals with perfect dark state) as well as more curiosity-driven research (the nature and origin of the chiral smectic C subphases and their intermediate frustrated states between ferro- and antiferroelectricity). [less ▲]

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See detailBook review: Crystals that flow
Lagerwall, Jan UL

in Technikgeschichte (2006), 73

Ein bekanntes Zitat von Mark Twain lautet "ein Klassiker ist ein Werk, das jeder gelesen ha- ben möchte, aber niemand lesen will". Mit dem ausgezeichneten Buch Crystals that flow - classic papers from the ... [more ▼]

Ein bekanntes Zitat von Mark Twain lautet "ein Klassiker ist ein Werk, das jeder gelesen ha- ben möchte, aber niemand lesen will". Mit dem ausgezeichneten Buch Crystals that flow - classic papers from the history of liquid crystals haben die drei Autoren Timothy J. Sluckin, David A. Dunmur und Horst Stegemeyer, vielen technik- und wissenschaftsinteressierten Le- sern das Klassikerlesen wesentlich erleichtert, da sie in diesem Buch 46 klassische Artikel aus der Geschichte der Erforschung und technischen Anwendung von Flüssigkristalle zu- sammengestellt haben. Sie erzählen damit die Geschichte, wie eine ‚akademische Kuriosität’, entdeckt Ende des neunzehnten Jahrhunderts, erst unsere Vorstellung von den Aggregatzu- ständen kondensierter Materie verändert hat, und danach zu einer Schlüsseltechnologie ent- wickelt wurde, die die Grundlagen der heutigen Multimillionen-Euro-Industrie der Flachbild- schirme liefert (LCD = Liquid Crystal Display, also Flüssigkristallanzeige). ... [less ▲]

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See detailEffect of phenyl rings in liquid crystal molecules on swcnts studied by raman spectroscopy
Scalia, G.; Lagerwall, Jan UL; Haluska, Miroslav et al

in Physica Status Solidi B. Basic Research (2006), 243(13), 3238-3241

Carbon nanotubes can be aligned by dispersing them in a liquid crystalline matrix. To control and opti- mize the obtained alignment it is important to understand the interactions between the molecules of ... [more ▼]

Carbon nanotubes can be aligned by dispersing them in a liquid crystalline matrix. To control and opti- mize the obtained alignment it is important to understand the interactions between the molecules of the liquid crystal host phase and the carbon nanotubes. To this end we have carried out resonant Raman spec- troscopy investigations of dispersions of single-wall carbon nanotubes (SWCNTs) in a liquid crystal com- pound comprising molecules with a biphenyl rigid core structure. We detect a distinct wavenumber shift of the radial breathing modes, confirming that the carbon nanotubes interact with the surrounding liquid crystal molecules, most likely through aromatic interactions (π-stacking). The interactions between liquid crystal host and nanotube guests are also evident from a polarizing microscopy study of the liquid crys- tal – isotropic phase transition in the proximity of bundles of nanotubes. The ordered liquid crystal phase is stable up to higher temperatures around the bundles than in areas without visible signs of CNTs. Con- versely, the transition from the disordered isotropic phase to the liquid crystal phase on cooling always nucleates at the carbon nanotube bundles. [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 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 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 detailFrustration between syn- and anticlinicity in mixtures of chiral and non-chiral tilted smectic-c-type liquid crystals
Lagerwall, Jan UL; Heppke, Gerd; Giesselmann, Frank

in European Physical Journal E -- Soft Matter (2005), 18(1), 113-121

We study the effects of mixing ferroelectric and antiferroelectric liquid-crystal compounds (FLCs and AFLCs) when the former are strictly synclinic and the latter strictly anticlinic, i.e. one mixture ... [more ▼]

We study the effects of mixing ferroelectric and antiferroelectric liquid-crystal compounds (FLCs and AFLCs) when the former are strictly synclinic and the latter strictly anticlinic, i.e. one mixture component exhibits only SmC* and the other only SmCa* as tilted phase. Three different paths between syn- and anticlinicity were detected: transition directly between SmC* and SmCa*, transition via the SmCβ* and SmCγ* subphases, or by “escaping” the clinicity frustration by reducing the tilt to zero, i.e. the SmA* phase is extended downwards in temperature, separating SmC* from SmCa* in the phase diagram. The most common path is the one via the subphases, demonstrating that these phases appear as a result of frustration between syn- and anticlinic and, consequently, between syn- and antipolar order. For assessing the role of chirality, we also replaced the FLC with non-chiral synclinics. With one of the AFLCs, the route via supbhases was detected even in this case, suggesting that chirality —although necessary— does not have quite the importance that has previously been attributed to the appearance of the subphases. The path chosen in the mixture study seemed to be determined mainly by the synclinic component, the subphase induction occurring only when the SmA*-SmC* transition was second order. [less ▲]

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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 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 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 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 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 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 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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