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Miscibility and DSC studies of some ferroelectric liquidcrystals

J. Billard, A. Dahlgren, K. Flatischler, S.T. Lagerwall, B. Otterholm

To cite this version:J. Billard, A. Dahlgren, K. Flatischler, S.T. Lagerwall, B. Otterholm. Miscibility and DSCstudies of some ferroelectric liquid crystals. Journal de Physique, 1985, 46 (7), pp.1241-1248.<10.1051/jphys:019850046070124100>. <jpa-00210067>

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Miscibility and DSC studies of some ferroelectric liquid crystals

J. Billard (+), A. Dahlgren, K. Flatischler, S. T. Lagerwall and B. Otterholm (++)

Department of Physics and Department of Organic Chemistry (++),Chalmers University of Technology, S-412 96 Göteborg, Sweden

(Reçu le 18 février 1985, accepté le 21 mars 1985)

Résumé. 2014 Des mésophases smectiques C chirales thermodynamiquement stables à la température ambiante sontsouhaitables pour les recherches sur les dispositifs électrooptiques rapides. La suppression de la torsion par dessurfaces traitées peut être évitée par l’emploi de mélanges smectiques C chiraux sans torsion. Pour trouver dessolutions binaires convenables quelques matériaux sont réétudiés : mesures des températures des transitionset des variations d’enthalpie molaire aux transitions, identifications de phases et déterminations des sens de torsion.Un nouvel exemple de solutions liquides, smectiques A et C, smectiques I métastables, et solides, thermodynami-quement parfaites, est donné. Des exemples de solutions binaires smectiques C chirales sans torsion, thermody-namiquement stables à basses températures, sont présentés.

Abstract. 2014 Chiral smectic C mesophases thermodynamically stable at room temperature are desirable for thedevelopment of high-speed electro-optic devices. The elastic unwinding by treated surfaces can be obliviated byuse of untwisted chiral smectic C mixtures. In order to find convenient binary solutions the properties of some of theinteresting Schiff base materials have been revisited : measurements of temperatures and molar enthalpy changesat the transitions, phase identifications and determination of twist sense have been performed A new example ofthermodynamically perfect (ideal) liquid, smectic A and C, monotropic smectic I and solid solutions of enantiomersis given. Examples of untwisted chiral smectic C binary mixtures thermodynamically stable at low temperaturesare presented

J. Physique 46 (1985) 1241-1248 JUILLET 1985,

Classification

Physics Abstracts61.30E

1. Introduction.

The phenomenon of a certain kind of ferroelectricityin the chiral smectic C and similar liquid crystalphases [1, 2] and the possibility to use these materialsto develop high-speed electro-optic devices [3-5] havestimulated research for compounds exhibiting this

ferroelectricity at room temperature and also forconvenient broad-temperature mixtures [6]. Availablematerials exhibit twisted C and I phases and theunwinding is obtained elastically by the use of treatedsurfaces. However, by mixing two non-enantiomericcomponents a chiral smectic C phase without twistcan be obtained [7]. To explore this way, suggested byMeyer et al. [1] in the first study on ferroelectric liquidcrystals, we have investigated some new aspects ofsome of the classic Schiff base materials together withsome recently synthesized ones.

(+) Permanent address : Laboratoire de Physique de laMati6re condens6e (Equipe de Recherche associ6e au

C.N.R.S. no. 542), College de France, 75231 Paris Cedex 05,France.

2. Methods and materials.

Ten of the studied compounds are listed in the table.The temperatures and the molar enthalpy changes atthe transitions are measured with a Mettler TA 3000differential scanning calorimeter at increasing tem-peratures. (2 °C/min in the absence of a contradictoryindication.) The textures are observed with a polariz-ing microscope equipped with a Mettler FP 52 hotstage. The liquid crystal phases are identified by themiscibility method. The binary isobaric phase dia-grams are constructed from microscopic observationsof contact preparations [8]. The twist sense of the chiralphases are deduced from the observations of the dis-placement versus the rotation of the analyser of theisochromatic lines in binary mixtures of the contactpreparations at areas exhibiting gradients of compo-sition [9] or from the observation of textures ofuntwisted mixture between two solutions havingopposite twist senses [10].The thermodynamic phases of compounds 1 and 2

(Table) first identified by isomorphy as smectic A, Cand B [ 11 ] (also supported by an X-ray study [12] on 1),

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019850046070124100

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Table. - Studied esters of 4-n-alkoxybenzylidene-4’-amino-cinnamic acids. X and L are our phase designations forcrystalline and isotropic liquid, respectively. The transition temperatures are given in degreesCelsiusand(underlined)the molar enthalpy changes in kJ/mole. The values in parenthesis concern monotropic or virtual transitions. Theletters S (virtual for sinister) and R ( for rectus) indicate the absolute molecular configurations [15]. The letters I and ras index indicate respectively left and right handed twist sense.

(j The calorimetric data have been taken at 5 degrees per minute(b) [a]589 = + 4.46 grad CM3 g-1 dm-1 (c = 0.040, benzene)0 [0e] 219 = - 4.05 grad CM3 g-1 dm-1 (c = 0.0395, benzene)(d) [a] 219 = - 18.3 grad cm3 g-1 dm-1 (c = 0.060, benzene).

have lately been shown to be smectic A, C and I [13].From our calorimetric measurements and microscopicobservations the compound 2 exhibits a thermodyna-mically stable smectic I phase in disaccordance withpreviously published data [33]. To confirm the exis-tence of this mesophase we have established the phasediagram of 2 and 1, figure 1. In the chiral compound 3

(DOBAMBC) [14] used for the first experiments onferroelectricity in mesomorphic phases [1] the smectic Iphase has been deduced by X-ray studies [16]. Thesematerials have been used as reference compounds inour study. The enantiomer 4 corresponding to 3 andtheir racemic mixture 5 have been synthesized at

Chalmers [17].

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Fig. 1. - Phase diagram of the mixtures of the compoundsI (A) and 2 (B).

Fig. 2. - Phase diagram of the mixtures of the compounds3 (A), 5 (in the middle) and 4 (B) (see Table).

The calorimetric data from this work measured on 3and 5 and given in the table support the ideality of thephase diagram in figure 2 (deduced with the aid of 3and 5). The DOBAMBC compound pair (3 and i)

is the sixth known example of a thermodynamicallyperfect liquid, liquid crystal and solid solution ofmesogenic enantiomers [7]. In these mixtures it is

possible to adjust the chiral properties without anychange in the nonchiral properties [18]. The twistsense was determined on 3 which was shown to have aright-handed helicity, cf table.The phase identifications for 5 in this work were

made from the phase diagrams of the racemic (5)with 1 and 2 as shown in figure 3 and 4.

Fig. 3. - Phase diagram of the mixtures of the compounds5 (on the left) and 1 (on the right). The solubilities are

calculated with the Le Chatelier-Schr6der relations [31-32].

Fig. 4. - Phase diagram of the mixtures of the compounds2 (A) and 5 (B).

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The compound 5 can then be used to identify thephases of the racemic mixture 6 (calorimetric data forthe C-A transition obtained by heating at 100/min)cf figure 5. The compound 7 first synthesized byGray [19], has been assigned the phases A, C* and G*from texture observations [ 14]. From our observationson the mixtures of 7 with the reference substance 1we now conclude, firstly, from the phase diagram(Fig. 6) that the phase sequence is A, C* and I* and,secondly, that the C* and I* phases both have a right-handed helicity.The phases of the HOBACPC (compound 8) have

been described successively as

from texture examinations [20] and

from X-ray studies [16]. We have here used the recentlygiven [21] denomination smectic J in the phasesequence. Our calorimetric measurements are inaccordance with the ones recently published by Jainand Wahl [22] and stated in the table. No opticalevidence for a twist in the J phase has been observedon this compound, in accordance with other find-ings [16, 22]. By mixing 8 with 4 we have been able toconfirm, according to the phase diagram of figure 7,

Fig. 5. - Phase diagram of the mixtures of 5 (A) and 6 (B).

Fig. 6. - Phase diagram of the mixtures of 7 (A) and 1 (B).

Fig. 7. - Phase diagram of the mixtures of 4 (A) and 8 (B).

the nature of the A, C* and I* states for HOBACPC.In these mixtures, moreover, untwisted but chiralsmectic C and I phases appear between the corres-ponding twisted C* and I* of opposite helicity. Thus,the twist sense for the chiral smectic C and I phasesof 8 is rightThe right-handed twist senses of the monotropic C*

phases of the cyano-substituted compounds [14] 9and 10 are similarly deduced from the phase diagramsof the mixtures of 9 with 5 (Fig. 8) and the 10 with 9(Fig 9).

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Fig. 8. - Phase diagram of the mixtures of 9 (A) and 5 (B). Fig 9. - Phase diagram of the mixtures of 10 (A) and 9 (B).

A new examination has been made on the compound 11

first described by Helfrich and Oh [23] as having thetransitions

In fact, the X-C* transition lies at 43.6 OC, but super-cooling of the C* phase is pronounced and can easilybe misjudged. Our calorimetric data are :

A phase identification was made with the aid of thecompound [24, 25] 2-chloro-(bis-4-n-decyloxybenzy-lidene)-1.4-phenylenediamine (DOBCP)

A new observation of the mixtures confirms the phasediagram [26] and shows that the N* and the C* statesof 11 are left-handed.

3. Untwisted smectic C phase with large spontaneouspolarization.

In the chiral smectic C state the component P of thelocal spontaneous electric polarization perpendicularto the plane containing the normal i to the smecticlayers and the long axis n of the averaged molecules canfundamentally have either of two directions, oppositeto one another (cf Fig. 10). Which possibility is chosenby nature in any single case (compound) cannot beguessed and has only been determined experimentallyfor some of the used compounds [27]. Arbitrarily [28]we give the sign + to the case a in figure 10. With thisconvention the sign for compounds 3 and 8 is negative.Consequently, the enantiomer 4 corresponding to 3is positive. Untwisted smectic C phases can be obtainedby mixing two compounds with C* states of oppositehelicity. The spontaneous electric polarization of the untwisted solution can be considerable if these two

components of opposite twist have the same ferro-electric sign. To our knowledge no such pairs have yetbeen demonstrated, although there is no reason whythey should not exist If the ferroelectric sign deter-minations of reference [27] were all correct, the sub-stances 4 9 and 4 10 would constitute pairs of the de-scribed kind, but according to our observations 4 10

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Fig. 10. - In the chiral smectic C mesophase the two

possible choices of the local polarization are perpendicularto the plane containing the normal z to the smectic layersand the long axis n of the averaged molecules.

has also a negative sign on P. Thus, among the des-cribed compounds, the polarization P is always partlycancelled in binary mixtures at the same time as thehelix is unwound The interesting case 4 8 (or theircorresponding antipodes - the authors do not statethe configuration) has been previously studied [29, 30]but without the phase diagram given here (Fig. 7).Effectively untwisted smectic C solutions exist butonly above 62 °C. The measurements in reference [30]made 20 °C under the C*-A transition for certainconcentrations probably concern the I* phase. In themixtures of 4 and 9 (Fig. 11) the untwisted chiralsmectic C state is stable approximately between 59°and 76 OC.

4. Other stable low temperature untwisted C* statebinary solutions.

In absence of information about the ferroelectric signwe have examined binary solutions exhibiting un-twisted low-melting C* states. If the ferroelectric signsof the two components are opposite the spontaneouspolarization of these solutions is lower than that ofeither pure compound but such a chiral solution isimportant for studying other phenomena. In additionto C* states of opposite twist the primarily interestingmaterials should have a C*-I* transition at sufficientlylow temperature. Otherwise the appearance of thestable smectic I phase will drastically change the ferro-electric, and thereby the electro-optic properties at theonset of the I* phase as for example in the case offigure 12. The inherent advantages for switching inthe I*-phase itself are considerable (enhanced bista-bility-memory) but not considered here. Convenientmixing components having virtual (or monotropic)C*-I* transitions at sufficiently low temperatures arefor example 3 and S-4-(6-mcthyl)octyl-rcsorcylidene-4’-hexylaniline [6] (MORA 6) with the binary diagram

Fig. 11. - Phase diagram of the mixtures of 9 (A) and 4 (B).

Fig.12. - Phase diagram of the mixtures of 3 (A) with S-4-(6-methyl)octyl-resorcylidene-4’-octylaniline (B).

shown in figure 13 : the resulting untwisted chiral Cstate is stable in a considerable temperature rangeincluding ambient temperature. This phase diagramshows some principally very attractive possibilitiesthat will be explored as soon as new syntheses resultin a larger variety of available starting materials. The

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Fig. 13. - Phase diagram of the mixtures of (3 (A) and S-4-( 6-methyl)octyl-resorcylidene-4’ -hexylaniline (B).

MORA 6 (to the right of the diagram) has a C* phaseas a pure compound with a considerable temperaturerange but it has two inconveniences in regard to elec-tro-optic use (except for its weak polarization). The

first is that it lacks an A phase which is much easier toalign than the C phase and thus helpful for preparinglarge homogeneous monocrystals. The other inconve-nience is the presence of a helical twist, a feature thatis shared by all C* compounds. As we see, however,addition of about 12 % of DOBAMBC ( +) not onlystabilizes the C* phase down to below room tempe-rature and unwinds the helix, giving an « infinitepitch » material, but at the same time introduces thedesired A phase in a small but sufficient temperatureinterval on top of the C* phase.

5. Conclusion.

After revisiting previously partially studied materialsuntwisted chiral smectic C binary solutions thermo-dynamically stable at low temperature are found. Forexample, as shown in figure 13, the broad temperaturerange of MORA 6 can be even broadened (from about20-82 OC to about 10-84 °C) gaining a ferroelectricphase with higher polarization and without twist, bymixing with a small amount of DOBAMBC Q). Suchmixtures possessing important electric spontaneouspolarization can be used for the potential high-speedelectro-optic devices and will facilitate the unwindingthe helix by elastic interaction with the surfaces.

Acknowledgments.

This work was supported by the National SwedishBoard for Technical Development

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