Surface electroclinic effect in ferroelectric liquid crystal

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Surfce electroclinic effect in ferroelectric liquid crystl A. Birdr, S. Bw, Knchn Sxen, Subhs Chndr

Journl de Physique II, EDP Sciences, 1993, 3 (12), pp.1787-1793. <10.1051/jp2:1993103>. <jp-00247938> HAL Id: jp-00247938 https://hl.rchives-ouvertes.

they re published or not. The documents my come from teching nd reserch institutions in Frnce or brod, or from public or privte reserch centers.

1 Surfce electroclinic effect in ferroelectric liquid crystl A. Birdr, S. Bw, Knchn Sxen, Subhs Chndr To cite this version: A. Birdr, S. Bw, Knchn Sxen, Subhs Chndr. Surfce electroclinic effect in ferroelectric liquid crystl. Journl de Physique II, EDP Sciences, 1993, 3 (12), pp < /jp2: >. <jp > HAL Id: jp Submitted on 1 Jn 1993 HAL is multi-disciplinry open ccess rchive for the deposit nd dissemintion of scientific reserch documents, whether they re published or not. The documents my come from teching nd reserch institutions in Frnce or brod, or from public or privte reserch centers. L rchive ouverte pluridisciplinire HAL, est destinée u dépôt et à l diffusion de documents scientifiques de niveu recherche, publiés ou non, émnnt des étblissements d enseignement et de recherche frnçis ou étrngers, des lbortoires publics ou privés.

2 ferroelectric liquid crystl hs been studied by dielectric relxtion nd electro-opticl method in homogeneously ligned, thin smples in the frequency rnge of 400 Hz to loo khz t different of the smple which hs been confirmed by dielectric nd opticl observtions. The strong surfce in the surfce lyers is lso presented by pplying dc bis field. nchoring nemtic IN * phse of FLCS hve been scrcely studied [5, 6]. The electroclinic effect occurs of ferroelectric liquid crystl molecules is generlly chieved by properly treting Alignment surfces of the two substrtes in the fbriction of FLC cells or devices. Berremn [7] first the this problem by rubbing the glss substrtes with dimond pste nd creting studied He clculted the elstic distortion due to the grooves nd found tht the lowest microgrooves. configurtion ws for ll the molecules to lie long the grooves, creting n uniformly energy cell long the rubbing direction. Gery, Goodby, Kmetz nd Ptel [8] hve proposed ligned different mechnism of the lignment of liquid crystl molecules on the polymer coted rubbed with cloth. Rubbing of the films orients the polymer chins long substrtes preferred nd the moleculr interction between the liquid crystl (LC) molecules nd the direction stretched polymer induces the lignment. It hs been found [9] tht lignment is either due to J. Phys, if Fince 3 (1993) DECEMBER 1993, PAGE 1787 Clssifiction Phj,.<ic.s Ah.<trct.I Surfce electroclinic effect in ferroelectric liquid crystl A. M. Birdr, S. S. Bw, Knchn Sxen nd Subhs Chndr Ntionl Physicl Lbortory, K. S. Krishnn Rod, New Delhi, l10012, Indi (Received 23 April J993, iei,ised 27 July J993, ccepted 3J A~lgu.<t J993) Abstrct. Surfce electroclinic effect in chirl nemtic IN") phse of high-tilt-ngle The electroclinic effect in the N* phse observed in the surfce lyers is due to the tempertures. of surfce nchoring nd smectic-like ordering of the molecules t the rubbed boundry effect 1. Introduction. The electroclinic effect ner Smectic A-Smectic C* (Sm A-Sm C*) phse trnsition is well- ii in the ferroelectric liquid crystls (FLCS). A lrge number of dt hs been reported known SmA nd in SmC* phses in homogeneously ligned thin cells from the opticl in observtions [2] nd dielectric mesurements [3, 4]. The electroclinic effect in the chirl when n electric field pplied prllel to the smectic lyers induces moleculr tilt reltive to the lyer norml. formtion or interction between the LC molecules nd the stretched polymer. The grooves nchoring energy in thin cells is strongly dependent on the rubbing strength. Severl surfce surfce tretments hve been successfully employed, but the phj~sicl mechnisms ffecting

3 lignment. techniques [9-11] hve been employed to study the surfce induced lignment due Different report the electrodinic effect due to the interction of LC molecules in chole~teric phse we *) with the polymide treted rubbed substrtes in thin cells of high tilt ngle ferroelectric (N liquid crystls by dielectric nd electro-opticl observtions. Corportion, Jpn) used in this study hs the following phse sequence Cryst -- Sm-C * -- N * The mteril ws introduced in the cell by mens of cpillry ction t elevted tempertures to to the plne of the electrodes for this geometry. Electric field is pplied prllel perpendiculr the smectic lyers. to dielectric mesurements hve been crried out using HP 4192A Impednce Anlyzer The the frequency rnge 400 Hz to 100 khz. The dielectric relxtion in N * phse t different in mode is most commonly observed in Sm A phse s soft mode (which corresponds to n in N * phse. The criticl frequencies of this dielectric mode hve been computed temperture~ the position of the mxim on the plot which is lmost constnt with temperture. As from is wekly nchored. A high dc electric field is pplied to unwind the twisted structure to see 2 shows tn 6 s function of frequency t different pplied dc bis field in N* phse. Figure dielectric relxtion in N* phse in this study is not due to flexoelectric effect which The ppers on wek nchoring surfces or due to the breking of the cell symmetry (more thn one 1788 JOURNAL DE PHYSIQUE II N 12 the surfce induced bulk lignment re still not well understood. For homogeneous bulk the specilly treted surfces often used re rubbed polymer coted substrtes. As lignment induced effect, how the first monolyer of LC molecules t the interfce is oriented by surfce the surfce moleculr interction is fundmentl to our understnding of homogeneous to the rubbing of the polymer coted substrtes in ferroelectric liquid cry~tls. In this rticle, 2. Experimentl. conducting indium tin oxide (ITO) glss pltes were used for electrodes. The distnce Highly the pltes ws kept round lo ~m nd this distnce cn be considered s thin between with the helicl pitch vlue (15 ~m in Sm C* nd 23 ~m in N* phse) of the compred The high tilt ngle (44 ) ferroelectric liquid crystl mteril CS-2004 (Chisso mteril. -- Iso 9 C 62 C 71 C ensure tht the filling took plce in its isotropic (Jso) phse. Homogeneous lignment of the FLC smple is obtined with one surfce buffed, which hs been pretreted with dhesion nd nylon nd well defined monodomin smple ws obtined by pplying n c promoter A detiled procedure of the lignment nd the dielectric studies in the Sm C* phse of field. this mteril hs been reported elsewhere [12]. The smectic lyers re ssumed to be hs been studied by pplying dc electric field (± 0 to 30 V/10 ~m. The tempertures effect in N* phse hs been further, nlyzed by observing under polrizing electroclinic microscope (Olympus BH-2). 3. Results nd discussion. The electroclinic effect in N* phse hs been nlysed by dielectric relxtion method. This electroclinic effect). Figure I shows the tn 6 (loss fctor)»ersiis logjo v plot t different described erlier one of the glss pltes used in the cell is coted with polymer nd rubbed wheres other plte is without ny surfce tretment for ligning the FLC molecules. This mens tht the surfce which is rubbed with polymer is highly nchored [13] nd other surfce whether the dielectric mode in N* phse comes due to surfce or bulk effect of the cell.

4 ~ o-lo [ vw o.5 v i]i']] osc. ~ o vn v ~ Bis O.05 3.O 4.O 5.O 2.O log jo~ (Hz) N 12 SURFACE ELECTROCLINIC EFFECT 1789 D.63.OC "~ x.s9.o.c log j~~ (Hz) Fig, I. Frequency dependence of tn 3 (loss fctor) mesured t different temperture~ in N± phse. o.<5 O ~OV ~5V x ~<OV ~45V. o-lo tn 6 o.05 2.O 3.O 4.O 5.O Fig. 2. Frequency dependence of tn 3 mesured t different bising voltges t 69 C in N± phse. [14] in N* phse. Recently Lee nd Ptel [15] hve observed the flexoelectric effect in domin) phse where the difference between nchoring strength on the two surfces is very nemtic smll. In the present investigtions the possibility of flexoelectric effect is ruled out becuse I) the nchoring energy in polymer rubbed surfce is very strong [13], it) the lignment is uniform nd single domin is checked under polrizing microscope. The dielectric relxtion

17] tht the FLC mteril, which hs Sm C* phse fter the Sm A phse is likely to form Sm-C* phse without pssing through the Sm A phse would not form firm lyer structure in thin smples.

5 17] tht the FLC mteril, which hs Sm C* phse fter the Sm A phse is likely to form Sm-C* phse without pssing through the Sm A phse would not form firm lyer structure in thin smples. The electroclinic effect rises from interction of the liquid crystl pronounced with the locl electric field t the rubbed boundry surfces in N* phse. This locl molecules 10 V/10 ~m in Fig. 2) the dielectric losses or conductnce due to electroclinic effect ws not in N* phse which is shown by the error brs in the figure nd hence t higher consistent field the dielectric relxtion frequency could not be computed. bising cycle (Iso-N*-Sm C*) this effect ws not observed. This mens tht when the phse cooling is from Iso-N*-Sm C* then the LC molecules in N* phse re not ligned long the trnsition When it is further cooled in SmC* phse the lyer structure lso disppers surfce. cell) which hs been confirmed by opticl microscopic observtions. Recently (multidomin Lee, Ptel nd Goodby [18] hve lso reported surfce electroclinic phenomenon in the found tht the moleculr tilt exists in bout C bove the trnsition to the bulk isotropic hve nd predicted smectic-like ordering within the surfce lyer which contributes to the phse At zero voltge the surfce molecules nd bulk molecules re in the sme phse. voltges. uniform texture is ~een under the microscope (Fig. 3A). The increse in the dc Therefore, which gives rise to n induced tilt of the surfce molecules with respect to the bulk voltge re~ults into the chnge in the texture of the smple (Figs. 38, C, D nd El. molecules, into the complete switching in the N* phse. Figure 4 shows opticl microgrph by resulting positive or negtive bis voltge to the rubbed surfce. When 0 to + 10 V bis is pplying be stressed here tht in the whole temperture rnge the surfce electroclinic effect ws 1790 JOURNAL DE PHYSIQUE II N 12 in N* phse (Fig, I) ppers due to strong nchoring effect on the polymer rubbed surfce which preserves the smectic-like ordering ner the rubbed surfce lyers. As it is known [16, firm lyer structure in the Sm C* phse. Contrry to this sitution, the FLC mteril tht h~ nd consequently the direction of the molecules ner the surfce is nchored rther tightly due to the nture of rubbed films (grooved formtion) nd this nchoring effect would be more field origintes from the polr nchoring of the few liquid crystl monolyers on the rubbed surfce [10]. The nchoring effect t the rubbed surfce is so strong tht even t the dc bis field of 10 VII 0 ~m the electroclinic effect in N* phse ws observed (Fi g. 2). This mens tht high bis field is needed to nullify the nchoring effect on the rubbed surfce so tht the surfce nd bulk molecules behve identiclly. However, bove this bising field (bove The smectic-like ordering in N* phse is confirmed due to the fct tht the electroclinic effect ws observed in N* phse during the heting cycle (Sm C*-N*-Iso). Wheres in the direction resulting into the rndomly lying (isotropiclly) the molecules on the surfce rubbing therefore, there would not be ny surfce nchoring of the LC molecules on rubbed nd phse of n FLC on rubbed polymer surfce nd interpreted s n electric field isotropic moleculr tilt in chirl liquid crystl which hs direct Iso-Sm A trnsition. They induced electroclinic effect. Further, the electroclinic effect in the present investigtion hs been confirmed by electricl method by inducing tilt ngle in molecules by pplying high electric field in N* ~witching Figure 3 shows the chnge in the texture of the smple t different pplied sttic dc phse. However, if squre pulse is pplied t low frequency complete switching in the smple is observed which is due to the fct tht surfce molecules nd bulk molecules re in unison. Thi~ mens tht the switching strts t the rubbed surfce molecules nd percolte in the bulk to rubbed surfce smectic-like ordering is clerly seen (Fig. 4A) which is due to the pplied tht the polymide surfce cts like positively chrged surfce, s suggested by Ptel nd fct [19]. When negtive bis is pplied cholesteric type texture is seen (Fig. 48). The Goodby response nd polriztion rever~l current due to electroclinic effect in N* phse is switching lso shown in figure 5 by pplying squre nd tringulr pulses t 45 Hz frequency. It should

6 r = A B c o E 4. Microphotogrph in N* phse t 69 C by pplying either positive or negtive voltge to the Fig. plte. (A) 0 to + lo V nd (B) 0 to lo V. rubbed N 12 SURFACE ELECTROCLINiC EFFECT 1791 Fig. 3. Microphotogrphs of lo ~m cell in N* phse t 69 C t different pplied dc voltges (Al 0 V, (B) v 8 V, (Cl c 12 V, (D) v 17 V nd (E) v 24 V. A B

; ~,_.~ j j[. ~ i /~ &44~W~W,~ Fig. 5. Polriztion reversl current nd switching response to tringulr nd squre wve pulses for isotropic phse, in the FLC mteril possessing Sm C*-N*-Iso phse trnsition.

7 ; ~,_.~ j j[. ~ i /~ &44~W~W,~ Fig. 5. Polriztion reversl current nd switching response to tringulr nd squre wve pulses for isotropic phse, in the FLC mteril possessing Sm C*-N*-Iso phse trnsition. terms of polriztion nd response current in the electroclinic geometry. in is seen in the figures I nd 2 the electrodinic effect observed by dielectric relxtion in As phse of high tilt ngle FLC mteril is wek due to the fct tht the pitch vlue in N* phse N* quite lrge nd lso the tilt ngle nd polriztion vlue in the Sm C* phse of this mteril is is thin plnr unwound smples. Lrge electroclinic effect hs recently been observed [6] using the FLC mteril where the helicl pitch vlue of N* phse is quite smll (0.5 ~m). in 1792 JOURNAL DE PHYSIQUE II N 12 ' ~ 'f 15 V in N* phse (ii pplied tringulr pulse (2) polriztion reversl current due to electroclinic V~~ effect (3) pplied squre pulse nd (4) switching response due to electroclinic effect in thin (10 ~m) smple. observed in N* phse for the first time, even in the blue phse which is observed just below the The electrodinic effect ppers due to the strong polr nchoring [10] nd smectic-like [18] of the molecules within the surfce lyers of polymer rubbed smples in N* ordering One should expect spontneous two-dimensionl electric polriztion which resides phse. close to the surfce nd is oriented perpendiculr to the director. In order to probe the very effect in terms of polriztion, n c electric field (E) of frequency v is pplied electroclinic to the director, thereby inducing smll rottion of the surfce molecules. This perpendiculr motion then propgtes elsticlly into the smples interior by combintion of surfce-driven bend nd twist elsticity resulting into n overll motion of the bulk smples which is observed lmost independent of temperture. These temperture independent vlues of tilt ngle nd re retined within the surfce lyers of the rubbed surfce even in N* phse. This polriztion been confirmed by opticl observtions (Figs. 3 nd 4). It is worth mentioning here tht in hs thick smple (25 ~m) very wek electroclinic effect ws observed very close to the trnsition in N.~ phse which indictes tht the electroclinic effect due to surfce lyers is temperture in this smples. Similr results hve been observed in [5, 16, 17] by opticl method dominnt

8 In summry, surfce electroclinic effect induced in polymide coted rubbed surfces ws by dielectric relxtion nd electricl switching method in N* phse of high tilt ngle observed liquid crystl which hs phse sequence of Sm-C*-N*-Iso in thin smple. It is ferroelectric Birdr A. M., Wrobel S, nd Hse W., Phj,s. Ret'. A 39 (1989) [4] Li Zili, Lisi G. A. D., Petschek R. G, nd Rosenbltt C., Phys. Rei~. A 41(1990) [5] Komitov L.. Lgerwll S. T., Stebler B., Andersson A. nd Fltischler, Ferioelecfi.ic.s 114 (1991) [17] 167. N 12 SURFACE ELECTROCLINIC EFFECT Conclusions. concluded tht the surfce electroclinic effect in N* phse is due to the strong nchoring nd smectic-like ordering of the molecules within the surfce lyers of rubbed smples. Acknowledgments. The uthors sincerely thnk Professor E. S. R. Gopl, Director, Ntionl Physicl Lbortory for continuous encourgement nd interest in this work. References Groff S. nd Meyer R. B., Phys. Ret,. Lett. 38 (1977) 848. II Andersson G., Dhl I., Kuczynski W., Lgerwll S. T., Skrp K, nd Stebler B., Feiioelecfi.ics 84 [2] 285. (1988) Pvel J. nd Glogrov M., Feiioelectric.I 84 (1988) 241. [3] [6] Legrnd C., lsert N., Hmine J., Buisine J. M., Prneix J. P., Nguyen H. T. nd Destrde C., Feiioelectiic,I Berremn D. W., Phi<.<. Ret,. Lett. 28 (1972) [7] Gery M., Goodby J. W., Kmetz A. R, nd Ptel J. S., J. Appl. Phj,s. 62 (1987) [8] Yng Y. B., Bng T., Mochizuki A, nd Kobyshi S., Ferioelectric.I121 (1991) 113. [9] Xue J, nd Clrk N. A., Phy.I. Ret,. Lent. 67 (1990) 307. [10] I] Chen W., Ouchi Y., Moses T., Shen Y. R, nd Yng K. H., Ph>'s. Ret. Left. 68 (1992) Birdr A. M., Bw S. S. nd Chndr Subh~, Phys. Ret,. A 45 (1992) [12] Birdr A. M., Bw S. S., Shrm C. P. nd Chndr Subhs, Jpfi J. Appl. Phj,s. 30 (1991) [13] [14] de Gennes P. G., The Physic~ of Liquid Crystls (Clrendon Pre~s, Oxford, 1974). 5] Lee Sin-Doo nd Ptel J. S., Phys. Ret~. Lent. 65 (1990) 56. II Li Z., Petschek R. G, nd Ro~enbltt C., Ph>'.I. Rev. Lent. 62 (1989) 796. [16] 8] Lee Sin-Doo, Ptel J. S. nd Goodby J. W., Phj's. Ret,. A 44 (1991) II Ptel J. S. nd Goodby J. W., J. Appl. Phvs. 59 (1986) [19]

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