Chapter 1. General Introduction: Liquid Crystals

Transcription

1 Chapter 1 General Introduction: Liquid Crystals

2 1.1 Introduction: After 125 years of discovery, liquid crystals (LCs) are still a fascinating arena for the researchers with their multifunctional properties and application potentials [1]. Liquid crystals are a unique class of materials in a state that is strongly anisotropic in its properties and yet possesses a certain degree of fluidity in a word "anisotropic fluid". The elementary difference between the crystal and the ordinary liquid state is that the molecules in a crystal are much ordered, whereas in an ordinary liquid they are not. The order in crystal is usually both positional and orientational. i.e.. the molecules are constrained both to occupy specific sites in a lattice and to point their molecular axes in specific direction. In contrary to this, the molecules in ordinary liquids diffuse randomly with the molecular axes stumbling wildly. iiii/in^m% III! %#nnm lllll\l%#wf Figure 1.1: Molecular arrangements in (a) crystals (b) liquid crystals and (c) liquids. Remarkably there are many materials, in which the building blocks are anisotropic bodies, exhibit more complex phase arrangements. Particularly, as they are heated from the solid state at the melting point the subsequent possibilities exist: (i) both types of order (positional and orientational) disappear at the same time resulting a phase that will be an isotropic liquid possessing no symmetry, (ii) only orientational order disappears leaving the positional order intact and the corresponding phase is called a plastic crystal, (iii) the positional as well as the orientational order are partially disappears resulting a phase called as liquid crystal (LC) phase. More properly the name

3 would be mesomorphic phase (intermediate phase) or mesophase. These mesophases are thermodynamically stable phase. A liquid-crystalline molecule consists of a rigid moiety and one or more flexible parts. The rigid part of the molecules aligns themselves in one direction, whereas the flexible parts promote fluidity. The optimum balance of these two parts is essential to form liquid-crystalline materials. 1.2 History of liquid crystals: Austrian botanist Friedrich Reinitzer discovered the double melting behaviour in cholesteryl materials in Even though, history shows that the lyotropic liquid crystals were discovered 30 years before the thermotropic liquid crystals. Virchow ( ) a physician and pathologist described a soft, floating substance from nerve core in the early 1850's named it "MyeHn". In 1857, Matteneheimer ( ) discovered that myelin was birefringent. The traditional foundation of liquid crystal research is set with the work of Reinitzer. His experiments involved extracting of cholesterol from carrots in order to determine its chemical formula. Reinitzer found that cholesteryl benzoate exhibited two melting points. At C the solid melted into a cloudy viscous liquid. The cloudy liquid at C became clear. Unable to describe the strange behavior Reinitzer sought the help of Lehmann, a wellknown crystallographer. Otto Lehmann examined the intermediate cloudy fluid, and reported that he had seen crystallites. Lehmann concluded it by giving the name ''fliessende krystalw due to its shared characteristics of both liquid and crystal. And thus the name "liquid crystal" had born. The important point here is that these first observations of liquid crystals were a serendipitous by-product of an apparently unrelated and evidence lacking piece of research. Neither for the first nor for the last time, had nature sprung a surprise on an unprepared investigator? Table LI depicts some of the molecular structures of important discoveries in the area of liquid crystals.

4 Table 1.1: Important liquid crystal discoveries. \ / ^ First thermotropic liquid crystal Common abbreviation: Cholesterol Benzoate (CB), [1888].Cr N* Iso. H3CO N u \=/ C4H 4^9 First room temperature nematic liquid crystal Common abbreviation: MBBA, [1969], Cr N 48.0 Iso. C10H21O First ferroelectric liquid crystal Common abbreviation: DOBAMBC, [1977], Cr C* 95 A Iso. CsHiyO- CH, First anti-ferroelectric liquid crystal. Abbreviation: MHPOBC, [1989], Cr CA* 18.4 Cy C* Ca A Iso. o r> o C2H5O First banana shaped liquid crystal. Abbreviation: NOBOW, [2001], Cr Iso. N O N OC2H5

5 1.3 Classification of liquid crystals: Liquid crystals are divided into two broad classes: Thermotropic liquid crystals and lyotropic liquid crystals. Thermotropic liquid crystals are those for which the liquid crystalline behaviour is temperature dependent. Contrary lyotropic liquid crystalline phases show mesomorphic behaviour by the change in the concentration of the solution. Thermotropic liquid crystal can further be subdivided on the basis of molecular architecture or geometrical structure of the molecule as rod like, bow like or bent-core or banana shaped and disc like or discotic. The "classical" liquid crystals are derived from rod like molecules called "calamitic" (derived from the Greek word colomos = rod). This class of liquid crystalline compounds is highly investigated and most important with respect to practical applications. Liquid crystals derived from disc like molecules were discovered independently by Chandrasekhar et al. [2] in 1977 and Billard et al. [3]. The synthesis of bent-core molecules reported for the first time at Halle University during [4, 5] and was not identified at that time.these classes of compounds are also called "banana" or "bent-core" or "V"-shaped liquid crystals. 1.4 Calamitic liquid crystals: An immense majority of thermotropic liquid crystals are composed of rod-like molecules. A common structural feature of calamitic mesogens is a fairly rigid core, often incorporating phenyl or biphenyl groups and two flexible end chain, often alkyl or alkoxy chains. They display a variety of mesophases with the variation in temperature. In 1922, Friedel, a French scientist [6-8] proposed a classification system based on the structural arrangement where he named different liquid crystal systems: nematic, smectic and cholesteric. The molecular structure of the two most prominent representatives of rod-like molecules, synthesized in 20"^century, that exhibits liquid crystalline phase at room temperature, is shown in Figure 1.2. N-(4-methoxybenzylidene)-4'-n-

6 butylaniline (MBBA) [9(a)] and 4-n-pentylcyanobiphenyl (ncb) [9(b)] are the first room temperature liquid crystals reported in 1969 and 1970 respectively. 5 A H3CO-H^^^C=N^^J^C4H9 5 A CgH 11 V /"A/ -C=N j 25 A 20 A MBBA:Cr 22 C Nematic 47 C Iso 5CB:Cr 18 C Nematic 36 C Iso Figure 1.2: Rod-like liquid crystalline molecules. 1.4a. Nematic phase: The simplest and most applicable liquid crystalline phase is Nematic (N) phase. The word "nematic" is derived from the Greek word for "vfifxa [nema)" stand for "thread" because thread like textures were observed under the polarizing microscope [9]. This has least order among all other mesophases and resembles more liquid like rather than solid. In this phase, the molecules prefer to orient in a specific direction as they diffuse throughout the sample, but there is no positional order as shown in Figure 1.3a. There are different types of microscopic textures observed in compounds exhibiting nematic phase viz. nematic droplet, schlieren texture, marble texture, planar texture. 1.4b. Smectic phase: The word "smectic" is derived from the Latin word "smecticus". meaning cleaning, or having soap like properties. Molecules in this phase show additional degrees of positional order not present in the nematic phase. In the smectic phase, the molecules maintain the general orientational order of nematics, but also tend to align themselves in layers; with a well-defined interlayer spacing, which can be measui-ed by X-ray diffraction [10]. a Nematic Smectic A Smectic C Figure 1.3: Orientation of molecules in (a) nematic and (b, c) smectic phase.

7 1.4c. Different classes of smectic phases: Smectic A/Smectic A*: In smectic A phase the molecules are oriented along the layer normal. Smectic A is the simplest among the smectic liquid crystal and belongs to the symmetry group D IT This phase can be classified as an orientationally ordered liquid on which a 1-D density wave is superimposed. Smectic A* is the chiral smectic phase. Smectic B: In smectic-b mesophase the molecules orient with the director perpendicular to the smectic plane, but the molecules are arranged into a network of hexagons within the layer. Two distinct types of smectic B phase had been identified, (a) Crystal B, 3-D crystal having hexagonal lattice and weak interlayer ordering, (b) Hexatic B, stack of interacting hexatic layers with short-range in plane ordering and 3-D six-fold bond orientational ordering. Smectic C/ Smectic C*: In the smectic-c mesophase, molecules are arranged as like smectic-a mesophase, but the director is at a constant tilt angle measured normally to the smectic plane. Chiral smectic C with twist axis normal to the layers. Smectic D: Cubic lattice with micelle type molecular arrangement. This phase is usually observed between Sm A and Sm C or between Sm C and isotropic phase. Smectic E: 3-D orthorhombic crystal with interlayer herringbone structure. Smectic F: Titled hexatic structure with C centered monoclinic (a>b) lattice with in-plane short-range positional correlation and weak or no interlayer positional correlations are considered to be SmF phase. Smectic G: 3-D crystal with C-centered monoclinic (a>b). Smectic H: 3-D crystal with C-centered monoclinic (a>b) and having herringbone structure. Smectic I: Titled hexatic C-centered monoclinic (b>a) with slightly greater in plane correlation than Sm F are considered to be SmI phase. Smectic J: 3-D crystal with C centered mono clinic (b>a) structure. Smectic K: 3-D monoclinic (b>a) structure with herringbone structure.

8 1.4d. Cholesteric phases: The cholesteric phase, being the chiral version of the nematic phase, is incorporated with a chiral molecule then the resulting structure can be visualized as a stack of very thin 2-D nematic layers with the director in each layer twisted with respect to those above and below. This phase possesses solely orientational order of the long moleculai- axis. This induces a helical director configuration in which the director rotates through the material. Hence, the name twisted nematic (Nt) is assigned to this class of liquid crystals. The molecules first to form this phase were derivatives of cholesterol, so they are also called cholesteric liquid crystals. An important characteristic of the cholesteric mesophase is its pitch (Figure 1.4). The pitch, p, is defined as the distance it takes for the director to rotate one full turn in the helix [9]. Figurel.4: (a) molecular arrangement in cholesteric phase (b) pitch of cholesteric phase. 1.4e, Twist Grain Boundary (TGB) phase: Twist grain boundary (TGB) phases are frustrated phases. It is generally located between the cholesteric and the respective smectic phase. As like the blue Phases, it can only be observed in chiral materials with large twisting power. In 1974, de Gennes [11] predicted a topologically defect-stabilized mesophase in analogy with superconductors. When the intrinsic twisting power of the material is high, the smectic A structure breaks down into periodic stacks of layers, with a finite twist distortion. These layers are mediated by regular array of screw dislocation. Renn and Lubensky [ 12] proposed a model for this

9 mesophase and named it as TGBA phase. Similarly a SmC phase can give rise to a TGBC phase. 1.4f. Discotic Mesophase: Until 1977 thermotropic liquid crystal, was predominantly reign by rod-like molecules but Chandrasekhar et al. [13] and Billard et al. [14] demonstrated that disk shaped molecules such as hexa-n-alkanoyloxybenzenes exhibit mesomorpliism. There was evidence that mesomorphic phases were formed from disc-like molecules in the year 1960 (Brooks and Taylor, 1968); however the first identification of a discotic phase was by Chandrasekhar et al. in 1977 [13]. Since then, a large number of discotic compounds have been synthesized and a variety of discotic mesophases discovered. Structurally, most of them fall into two distinct categories; nematic and columnar. Both the phases of the disklike molecules are shown in Figures 1.5. Nematic phase is the simplest discotic phase, because there is orientational order of the discs without any long-range translational order. The molecules move around quite randomly, but on average the axis perpendicular to the plane of each molecule tends to orient along the director. Unlike the classical nematic phase of rod-like molecules, this phase is optically negative. a b Figure 1.5: Molecular arrangement of (a) discotic nematic and (b) columnar mesophase

10 1.5. Banana shaped liquid crystals: Bent-core or banana-shaped or "V"-shaped liquid crystals named after the molecular architecture of the constituent molecules whose molecular shapes resemble to the bananas or a bow with a bend at the centre of the molecules and also exhibiting the optical textures similar to banana leaf like. A vast majority of the bent-core molecules link through an aromatic 1,3-disubstituted phenyl ring (Figure 1.6) or 2,7-disubstitued naphthalene moiety. The angle between two linking arms ranges from 100 to 140. The first bent-core liquid crystal was synthesized in the research group of Vorlander by Schroder in 1923 [15], but the mesophase properties of the compound were not reported. In 2001 Pelzl reported in Liquid Crystal journal about the mesophase in original Vorlander compounds [4] synthesized in Matsunaga [16] synthesized new mesogenic compounds with "bent-core" or "banana-shaped" or V-shaped molecular structures and reported as smectic phases. The observation of mesophases in bent-core compounds appeared in literature for the first time in In 1996 bent-core or banana-shaped molecules exhibiting mesomorphism get attention at 6th ILCC at Kent, after Takezoe presentation of the first mesophase [17] with antiferroelectricity formed by achiral bent-core molecules reported by Matsunaga et al. [16]. This phase was later known as the B2 phase (tilted polar smectic phase). This discovery is the beginning of interest in these new mesophases which are distinctly different in physical properties from that of smectic phases exhibited by calamitic (rod-like) molecules except layer ordering. However, most of the research efforts had been focused on the bent-core smectics phases because nematic phases are uncommon and rarein bent-core liquid crystal due to their bent molecular architecture that does not allow the molecules to rotate freely along their molecular axis Different classes of banana mesophases: After the Berlin workshop on 1997 [18], liquid crystal phases of bent-core molecules have been named as the 'Bz' (/ is an integer number) phase in the chronological order of their discovery. After the discovery of Bl to B8 phases till date [19], still there are several different molecular structural arrangements 10

11 to be explored to realize a number of phases. Also, as the detailed structures of these new phases are in progress, many other phases having very similar but slightly different structures are reported. Bl mesophase The first compound (Figure 1.6) exhibiting the Bl mesophase was reported by Watanabe et al. [20] and has the following chemical structure. The Bl phase exhibits two periodicities like a columnar phase (Figure 1.7 (d)-(e)), i.e. smectic layer ordering and another periodicity in a plane of smectic layers. Three different structures have been reported [21, 22] as shown in Figure 1.7 (a)-(c). All of these phases do not exhibit macroscopic spontaneous polarization [23] and do not respond to applied electric field, though there are several reports about the field induced transition from B1 phase to the Sm-CP state [24,25]. Further they are yet to be confirmed by X-ray studies. N N CeHisO Figure 1.6: The first compound exhibiting the Bl mesophase. OCQH^'^»),j» miii^h /M m» «^(^^^ Figure: 1.7: (a-c) Three proposed structures of the Bi phase (d) Bl texture (Renfan Shao, ILCS picture gallery) (e) Bl (columnar) phase (Md Lutfor Rahman, ILCS picture gallery) 11

12 B2 mesophase The electro-optically switchable B2 phase is also known as the Sm-CP phase, in which molecules forming a smectic layer are tilted with respect to the layer normal (Sm-Q and the smectic layer exhibits macroscopic spontaneous polarization perpendicular to the molecular tilt plane (Sm-CP). This mesophase is less viscous than Bl phase and is generally observed for the higher homologues. XRD studies on oriented samples of this mesophase have been reported [26-28]. The reflections in the small angle region are in the ratio of 1:1/2:1/3, indicating a lamellar periodicity. The wide-angle diffuse peak indicates the absence of in-plane ordering within the smectic layers. The first order reflection obtained is less than the full molecular length indicating a tilt of the molecules within the layer. The measured tilt angle from XRD studies varies between 35 to 50" depending on the chemical nature of the constituent molecules and the temperature. A lot of attention has been paid to this mesophase mainly because of its response to an applied electric field. Figure: 1.8:Texture of B2 phase. B3 mesophase In the B3 phase [Figure: 1.9], molecules in a smectic layer are not tilted from the layer normal and having positional order in layer plane. Although this phase was considered under B-phases, careful XRD studies on this phase 12

13 revealed the crystalline nature. This phase is a lower temperature phase with respect to the B2 mesophase [19]. Figure. 1.9: Texture B3 phase of P-8-0PIMB. B4 mesophase The B4 phase [Figure. 1.10] is one of the phases of "under investigation". Detailed XRD studies on this phase revealed the crystalline structure and it exhibits some interesting optical features. This phase was observed on cooling from either a B2 or a B3 phase [19]. The unique textural feature of this phase was the observation of a blue colour with domains of opposite handedness. Hence this phase has been named as smectic blue phase [29, 30]. The atomic force microscope experiments show that the phase has a helical superstructure. Sckine et al. [29] have reported a TGB-like structure for this phase. Figure. l.lortexture B4 phase of P-S-OPIMB 13

14 B5 mesophase The B5 phase is the Sm-CP phase with the in-plane positional ordering. This mesophase has in-plane periodicity within the layers and reflections in the wide-angle region can be indexed to a centered rectangular lattice. Electrooptic response in this phase is similar to that in the B2 phase [31]. From x-ray scattering measurements, the phase is known to have a positional ordering in the smectic layer. W. Weissflog et al. reported three distinct B5 phases in which electrooptic response and x-ray scattering profile were the same but distinguishable only in DSC measurements [32] [Figure. 1.11]. This phase does not exhibit 2-dimensional ordering which easily determined by x-ray scattering experiments. Figure Texture B5 phase. B6 mesophase The B6 phase [Figure 1.12] is sometimes observed in an upper temperature range of the Bl phase [33]. Because dipole moments of molecules are cancelled out each other, no electro-optic response is observed except a dielectric response. I!^l^l Figure 1.12: (a) Photomicrograph of the B6 phase, 60Am5Am06 in a 6- micron thick cell (M. Nakata thesis) and (b) molecular orientation in the phase. X4

15 B7 mesophase The B7 mesophase shows the most beautiful and optical unique textures [Figure 1.13]. This phase is also one of Sm-CP phases [34]. The mesophase has CI symmetry and exhibits helical superstructure. The detailed structure of B7 phase is recently investigated by means of x-lay scattering and observations of freeze fracture techniques [35]. The B7 phase was first defined by texture observed using optical microscope, which exhibits huge variation of spiral domains, helical ribbon like domains, focal conies and leaf-like domains [27a]. From high-resolution x-ray measurements, several different structures that show the same texture under an optical microscope are known to exist. Hence the name "B7" is often used as the name of texture not of phase and the phase is called as the polarization modulated phase. B8 mesophase Among all the banana mesophase this B8 phase was discovered quite recently in a symmetric bent-core compound as shown in Figure 1.14 [36]. The homologues of this compound with shorter end chains did not exhibit the B8 phase. The texture of the B8 phase are shown in Figure 1.14 a, b and c. Upon steady cooling from the isotropic phase, spiral domains grow as like in the B7 phase. By further cooling, a fan-shaped texture is also observed, in which the extinction direction is parallel to the layer normal, and antiferroelectiic-type switching is observed. The X-ray diffraction suggests that the B8 phase has a bilayer structure. This is the first and only report on the B8 phase thus far, and further investigations are in progress. 15

16 h ^^^U^m Figure: 1,13: (a) Structure of the undulated smectic phase proposed as a model of layer structure of the B7 phase (b, c) Texture of B7 phase. a C10H21' o o N'^^ " ^ ^^ N Cryst SmO Sml Iso C10H21 Figure 1.14: (a) Chemical structures of compounds exhibiting B8 phase (b, c) texture of 88 phase Molecular Order In the crystalline state or solid state the molecules usually have near-perfect orientational order. In the mesophase this degree of orientational order is partially lost though not completely, as the molecules show highly dynamic behavior and on an average point in the same direction. In fact, the molecules point preferably along a common orientation axis than in any other direction sufficient enough to create orientational order. This preferred direction is called the director (n. Figure 1.15). The degree of order is described by the order parameter (S), which is a measure for the average angle 6 between the director and the long axes of the mesogens. For an isotropic sample, S = 0, whereas for a perfectly aligned crystal S = I. For a typical liquid crystal, the value of S lies 16

17 between 0.3 and 0.8, and this value generally decreases due to higher mobility and disorder as the temperature is raised. 2/ S=(3cos"e-l)/2 Figure: 1.15: Model structure of the nematic phase and definition of the nematic director n. The angle 6 denotes the deviation of the long axis of an individual mesogens from the director. 17

18 References: [I] T. Geelhaar, K. Griesar, B. Reckmann, Angew. Chem. Int. Ed. 2013, 52, 2. [2] S. Chandrasekhar, B. K. Sadashiva, K. A. Suresh, Pramana, 1977, 9, 471. [3] J. Billard, J. C. Dubois, T. H. Nguyen, A. Zann, Nouveau. J. Chim. 1978,2,535. [4] D. Vorlander, A. Apel, Ber. Dtsch. Chem. Ges. 1932, 65, [5] D. Vorlander, Ber. Dtsch. Chem. Ges.\929, 62, [6] G. Freidel,^««. Physique.\922,18, 273. [7] D. Demus, L. Richter, Textures of Liquid Crystals. VerlagChemie, New York, [8] A. Cifferi, W. R. Krigbaum, R. B. Meyer, Polymer Liquid Crystals. Academic Press, New York, [9] Ingo Dierking, Textures of Liquid Crystals. WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, [10] G. Vertogen, W. H. De Jeu, Thermotropic Liquid Crystals. Fundamentals, Springer-Verlag, Berlin, [II] P. G. de Gennes, Solid State Commun. 1972,10, 753. [12] S. R. Renn, T. C Lubensky, Phys. Rev. A. 1988,38, [13] S. Chandrasekhar, B. K. Sadashiva, K. A. Suresh, Pramana.1977, 9, 471. [14] J. Billard, J. C. Dubois, T H. N. [15] D. Vorlander, Z Phys. Chem. Stoechiom. Verwandtschaftsl. 1923, 105,211. [16] H. Matsuzaki, Y.Matsunaga, Liq. Cryst. 1993,14, 105. [17] T. Niori, T. Sekine, J. Watanabe, T. Furukawa, H. Takezoe, J. Mat. Chem. 1996,6, [18] G. Pelzl, W. Weissflog, S. Diele, 6th European conference on Liquid Crystals, Halle, Germany, Workshop on banana-shaped liquid crystals: Chirality by achiral molecules, Berlin, December [19] G. Pelzl, S. Diele, W. Weissflog, Adv. Mater. \999,11,

19 [20] H. Takezoe, Y. Takanishi, Jpn. J. Appl. Phys. 2006, 45, 597. [21] J. P. Bedel, J. C. Rouillon, J. P. Marcerou, M. Laguerre, H. T. Nguyen, M. F. Achard, J. Mater. Chem. 2002,12, [22] J. C. Rouillon, J. P. Marcerou, M. Laguerre, H. T. Nguyen, M. F. Achard, J. Mater. Chem. 2001,11, [23] W. Weissflog, I. Wirth, S. Diele, G. Pelzl, H. Schmalftiss, Liq. C/75/.2001,28, [24] G. Heppke, G. Moro, Science.\9n, 11, [25] H. Nadasi, W. Wessiflog, A. Eremin, G. Pelzl, S. Diele, B. Das, S. Grande, J. Mater. Chem. 2002,12, [26] B. K. Sadashiva, V. A. Raghunathan, R. Pratibha, Ferroelectrics. 2000, 243, 249. [27] (a) G. Pelzl, S.Diele, C. Lischka, I. Wirth, W. Weissflog, Liq. Cryst. 1999, 26, 135. (b) R. Deb, R. K. Nath, M. K. Paul, N. V. S. Rao, F. Tuluri, Y. Shen, R. Shao, D. Chen, C. Zhu, 1.1. Smalyukh, N. A. Clark, J. Mater. Chem. 2010, 20, [28] R. Amaranatha Reddy, B. K. Sadashiva, Liq. Cryst.2002, 29, [29] T. Sekine, T. Niori, M. Sone, J. Watanabe, S. W. Chori, Y. Takanishi, H. Takezoe, Jpn. J. Appl. Phys. 1997, 36, [30] G. Heppke, D. Kruerke, C. Lohning, D. Lotzsch, S. Ranch, N. K. Sharma, Frieburger Arbeitstagung Flussige Kristalle, Freiburg, Germany, 1997, P70. [31] S. Diele, S. Grande, H. Kruth, C. Lischka, G. Pilzl, W. Weissflog, Wirth, Ferroelectrics. 1998,212,169. [32] G. Pelzl, W. Weissflog et al. 19th International Liquid Crystal Conference, Edinburgh, U.K [33] D. Shen, S. Diele, G. Pelzl, I. Wirth, C. Tschierske, J. Mater. Chem.\999,9,66\. [34] D. M. Walba, E. Korblova, R. Shao, J. E. Maclennan, D. R. Link, M. A. Glaser, N. A.Clark, Science. 2000, 288, [35] D.A.. Coleman, J. Femsler, N. Chattham, M. Nakata, Y. Takanishi, D. R. Link, R.-F. Shao, W.G. Jang, J.E. Maclennan, E. Korblova, O. 19

20 Mondain, C. Boyer, W. Weissflog, G. Pelzl, L.C. Chien, D.M. Walba, J. Zasadzinski, J. Watanabe, H. Takezoe, N. A. Clark, Science. 2003, 301, [36] J. P. Bedel, J. C. Rouling, J. P. Marcerou, M. Laguerre, H. T. Nguyen, M. F. Achard, Liq. Cryst. 2001, 28,