Indian Journ al or Pure & Applied Ph ysics Vol. 40. October 2001. pp 694-700

. "

Induced ~wist grain boundary phase in the binary mixtures of cholesteryl caprylate and !!onyloxybenzoic acid

; I RavindrcqDhar \ - - - , - ,

Physics Department. Ewing Chri stian Co li 'gc, Allahabad 2 11 003 . - - - ,

(Email : dr_rav indra_dh~r@ hotma i I com)

and

A K S ri vastava & V K Agrawal

Ph ys ics Departmcnl. All ahahad Un iversity, Allahabad 2 11 002

Received 18 April 1002; rev ised 3 1 May 2002: accepted 16 July 2002

~ /?in ary mixtures or cholestcry l caprylatc (ChC) and nonyloxyhenzo ic acid (NOBA) having dirfe rcnt mol e rati os have been stud ied usi ng dirrere nti al scanning calorimeter (DSC) and polari zing mi croscope (PM ). S:hC has nionotw pic chiral _ nematic , (N*)\ me~ophase in the coo ling cycle whereas. NOBA has enanti otropi c nemat ic, (N) ;and slllec ti c C (S mC) mesophases. Transi tion temperat ures and enthalpi es or these mixtu res have been determined with the help or DSC, whereas difrerent mesophascs have bcen identili ed by their typi cal tex tures as viewed under polari zing microscope. <\ddition or NO BA in ChC induces slllectic A (SmA ) and twist grain houndary A (TGBA ) phases hetween two critical co ncc n t rati()n s~

1 Introduction

Study o f liquid crys tal mi xtures is important no t onl y from application po int of view ' but also to understand the di fferent kind s o f mo lec ular inte raction s responsible for the induction o f new mesophases2

-.J. A recent topic of study is the variety of tw ist g rain boundary (TGB ) phases in the binary mi xtures of liquid c rys tal s l

!, . If a chiralliquid c rystal mate ri a l (or a mi xtme of c hiral and non -chiral components) has laye red smec ti c phase then, in such a sys te m. molec ules have a tendency to fo rm cho leste ric like he li cal direc to r field as well as layered structure. Howe ver, it is imposs ible to rea li ze a continuo us structure that may ex hibi [ both , a choleste ri c direc to r f ie ld and a smectic-Iaye red structure at the same time. The competi t ion be tween these two struc tura l fea tures ma y result in fru strated structure containing a regular latti ce of g rain boundari es , whi ch in turn consists of sc rew d islocation s. Thi s de fec t struc ture phase is now popularl y known as twi st g rain boundary (TGB) phase".7 . On the bas is o f analogy between liquid c rysta ls and supe rconductors proposed by P G de Gennes, TGB phases are sa id to be analogous to Shubnikov pha se/ Abrikosov vo rtex lattice of type II supe rconductors"'

As early as in 1972 . de Gennes predi cted that tw ist or bend di s torti on can be incorporated into a laye red smectic structure by the presence of an array of screw or edge di sloca tion ss. A theore tical mode l for such an array of di storti ons for smectic A (SmA) phase of I iquid c rysta ls was proposed by Re nn & Lubensky in 1988 and was named SmA* phase9

S uch a theoretically predicted SmA* phase was ex pe rimentall y realized fo r the first time by Goodby ef al. 1O in pure components ( in 1989) and by Lavrentovich er al. 13 in binary mixtures of liquid c rystal s (i n 1990). Since then, la rge var iety of TGB phases (TGBA, TGBC, TGBC*, TGB2q and UTG BC*) have bee n observed in pure liquid c rys tal as well as in binary liquid crystal mi xtures, but thi s field is still open at leas t fo r the prese nt decade and many more TGB phases are expected to be di scovered6

!1 . Re-entrant phe no menon of TGB phases ha s a lso been reported in pure compound s as we ll as in the binary mi xtures l2

.

Binary mi xtures of compou nds ex hibiting chol esteric and ne matic phases are ex pec ted to show TGB phases . In fact , the first mixtu re showing the TGB A phase (mixture of cho leste ryl pclargonatc and nonyl oxybe nzoi c ac id ) was a lso a cho les te ri c ­ne matic mixture i3

, for which ex tens ive e lec tri ca l.

DH AR el al .: BINARY MIXT URES 695

optical and the rmodynami c studies,.j·'5 have been

made. The binary mi xtures of cho leste ryl es te rs w ith nemati cs are ex pec ted to show TG B phases because of the tw isting power or cholestero l but, a ll these mi xtures do not show l'G1.3 phases '('. Ex perimenta l data (spec ia ll y ca lorimetr ic ) on TGB phases are still very scarce. In the present work are reported the opti cal and the rmodyna mic studies o n the binary mi xtures of cho les te ryl caprylate (ChC) and nonyloxybenzoic ac id (NOBA). C hC is di ffe rent fro m C hP (cholesteryl pe la rgonate) as it has onl y c hira l nematic phase and that too is monotropi c.

2 Experimental Techniques

Pure sa mples of cho leste ry l capry late (ChC) and nonyloxybenzoic ac id (NOS A) have been proc ured from In sti tute of Phys ics, Academy of Sc ie nces of Ukra ine, Kiev (Ukra ine). S ixteen binary mi xtures of different mo le ratios of ChC and NOB A have been prepared by weighing o ut pure samples using e lectro-balance of Ca hn (mode l C-33) hav ing an

acc uracy of 1 J..lg. Before taking the meas urements, these mi xtures have been ho mogeni zed by adequate stirring in the ir isotropi c liquid phase.

Diffe rent mesophase transition tempe ratures and transition entha lpies have been dete rmined by using a Diffe renti a l Scanning Ca lorimete r of Perkin -Elmer (mode l DSC-7). DSC has been a ll owed to run at

di ffe rent scanning rates be tween 1 and 15 °C/min . It has been observed that fo r most of the trans iti o ns, pea k transItIon temperatures (Tr) and o nset temperatures (TON) vary linearl y w ith the sca nning rates. Us ing least square fit pl ot of Tp with sca nning

rate ( in °C/min ), ex trapo lated transItI on

temperatures at the scanning rate of 0 °C/min have been dete rmined and they have been take n as true transiti on temperatures l 7 Tr and TON have been dete rmined with an uncertainty of ±O. I 0c, whereas

transiti on entha lpies (1'1H) have been de te rmined with the max imum uncerta inty of 5% 1'0 1' full y­grown pea ks. However, fo r weak peaks, these uncerta inti es a re large.

Us ing a po lari z ing mi croscope of Censico mode l Izumi-7626 di ffe rent mesophases of the mi xtures could be identified . Temperature of the samples kept between two cover slips of glass under the microscope has been controlled with the he lp of a hot stage of In stec with temperature acc uracy o f ±0.003 0c. However, mesophase trans It IOn te mperatures recorded by observing c hange in

tex tures are uncertain by more than ± 0. 1 °C and

c rys ta lli za ti on te mperatures have uncerta inty of ± I 0c. Samples under the po lari zing mi croscope have

been scanned at the rate of 1.0 °C/min . Tex tures have been photographed with a Yas hi ca mode l FX­D quartz ca mera with a magnificatio n o r 100x .

3 Results and Discussion

Extrapo lated transiti on te mperatures at the

scanning rate o f 0 °C/min obta ined from DSC data show the fo ll owing phase sequence for cho les te ryl capryla te (ChC) and nonyloxybenzoic ac id (NOBA).

C hC

K---{ 1 06. 8)-7 I---{92.2)-7N *- (7 1.6)-7K

NOB A

K---{92. 3)-7 S mC---{ 111. 7)-7 N---{ 136.6)-7

I---{ 1 36.5)-7N---{ Ill .8)-7SmC---{ 104.6)-7

K,---{88.7)-7K

where K, I, N, N* and SmC represent c rysta l, isotropic liquid , ne mati c, chiral nemat ic (c ho les teri c) and smectic C phases , respecti ve ly. K , is a mi xed phase of c rys ta l and S mC phases. Data in

parentheses a re transiti on temperatures in 0c. These trans it ion temperatures agree with the transition te mperatures o bta ined fro m tex ture study under po lari z ing microscope except c rysta llizat ion te mperatures, whic h have been fo und to be s light ly higher than those shown by DSC.

Phase di agrams (transiti on te mperatures wi th mole f racti on of compo nents) fo r the hea ting and coo ling cyc les of C hC-NOBA binary system have been drawn. Phase di ag ram fo r the cooling cyc le is shown in Fig . I. It shows that. additi o n of ChC in NOB A destabili zes SmC phase and it gets suppressed on the additi o n of 10 mo le per cent of C hC as observed for ChP-NOBA syste m a lso l.j . SmA and TGBA phases a re induced on adding 5-36 mo le pe r cent of NOB A in ChC. Beyond 36 mole pe r cent of NOB A, these mesophases are suppressed (as shown by broken lines in F ig. I ), leav ing a mi xed mesophase of N* and S mA up to about 60 mo le pe r cent of NOB A. In between NOBA concentrations of 40 and 90 mo le pe r cent , these

mi xtures show 1-7BP-7N* transItIons. Phase di agram of the heating cyc le shows eutectic compos iti on o f the sys tem at 70 mo le pe r cent of NOS A. N * phase appears in the heating cycle also

696 INDIAN J PURE & APPL PHYS, VOL 40, OCTOBER 2002

for NOBA concentration above 30 mole per cent whereas, SmA and TGBA phases are not observed in the heating cycles . Thermodynamic data of mixtures showing SmA and TGBA phases are discussed below in detail.

\

U I

of: K

~L ~~ . ...ll:----,-_ -;;' -----"------~ JUo . 20 LO 60 80 100

~le percellt NOBA

Fig. I - Phase diagram (transition temperature in °C with mole percent of NOBA ) for the ChC-NOBA binary system in the

cooling cycle. K' is the mixed phase

3.1 Mixtures showing TGBA phases

SmA and TGBA phases have been observed between two critical concentration that is 5 and 36 mole per cent of · NOBA. N*-~TGBA and TGBA---7SmA transition peaks of the mixtures having critical concentrations are very weak and unstable, and therefore, it has not been possible to calculate their transition enthalpies. For the mixtures having NOBA concentration 13.7 and 30.0 mole per cents fully-grown stable N*---7TGBA and TGBA---7SmA transition peaks are observed as shown in Figs 2 and 3. Texture identification by polarizing mjcroscope and extrapolated transition temperatures at the scanning rate of 0 DC/min obtained from DSC show following phase sequences for the mixture having NOBA concentration 13 .7 and 30.0 mole per cent, respective ly.

K-{86.0)---7K2-{ 103.1)---7I-{93.4)-7N*-{88.2) ---7 TGBA-{80.2 )---7SmA-{ 46.8)---7 K

K-{85 .5 )---7 K)-{99.4)-7 I-{99. 3)-7 N*-{87 .5)---7 TGBA--{80.2)---7SmA---7( 40.8)---7 K

where K2 and K) are the mixed phases of crystal with isotropic liquid and crystal with chiral nematic , respectively. A typical TGBA phase texture of un­a ligned sample of mixture having NOB A concentration 13.7 mole per cent is shown in Fig. 4 and resembles with the textures of TGBA phase as observed by other workers7.I O.13.18.20. This phase

shows high degree of fluidity as sample between two cover slips spil ls from one place to another showing long oily streaks (see Fig. 4). Presence of oily streaks is another signature for TGBA phase6

.2 1

•

[.00'-. ---___ _ . ____ _ _ i -'--l

~ ii: 0.50 ~ '0 • !

____ TGSA N t' _____ tl1'l(liC

S!T\fctic A

',"L ___ --L _ __ ...L. J

60 III 80 90 --~IOO

Tempera[ure I'e)

Fig. 2 - DSC thermogram for the mi xture with 13.7 mole % of NOB A in the cooling cycle at the scanning rate of 5.0 °C/min

Un-aligned sample of mixture with 30 mole per cent of NOBA has shown two different types of domains in TGBA phase. In one type of domains, some thin lines are observed in complete dark patches similar to TGBA texture of (S)-I-methylheptyl 4' -(4-n­dodecyloxybenzoyloxy) bi-phenyl-4-carboxylate in homeotropic alignmene. In the other type of domains, spiral SmA fans as shown in Fig. 5 have

DHAR et al.: BINARY MIXTURES 697

l o u: 0.30

"2 I

Chirot Nematic

Fi g. :1 - DSC thermogram for the mixtu re with 30.0 mole % of NOBA in the cooling cycle at the scann ing rate 01"7.5 °C/min

Fig. 4 - Optical texture of TGBA phase in the cooling cycle of mi xture with 13.7 mole % of NOB A (un-aligned sample) at 8 1.6 °C with magnification 100x

Fig. 5 - Optical texture of TGBA ph ase in the coo ling cycle of mixture with 30.0 mole % of OBA (un-aligned sample) at 85.6 °C with magnilicalion 100x

Fig. 6 - Opti cal texture of TGBA phase in the cooli ng cycle of mixture wi th 30.0 mole % of NOBA (homeotropic al igned sample) at 8 1.3 °C with magnification 100x

been observed. Spiral nature of fan s suggests he lical nature of this phase as observed by several other workers22

.23

. Chen et al .24 and Hsu et al. 25 have observed similar textures for the TGBA phase of

698 INDI AN] PURE & APPL PHYS, VOL 40, OCTOBER 2002

Table I - Ex trapolated peak and onset transit ion temperatures at the scanning rate (SR) of 0 °e/min (Tp and TON in °e ), peak wi dth (~T=2 ( Tp - TON) in °e ), transi tion enthalpy (6.1-1 in k]lmole) and transiti on entropy (~S in J/mole- K) for Che- NOBA binary mixlU res showing TGBA phase

Transiti ons Tp (Oe ) TON ~T MI (kJ/mole) 6S Remark (Oe) (Oe )

Mi xture with 13.7 mole % of NOB A K-) K2 86.0 83.0 6.0 6 32 K2-)1 103. 1 92.9 20.4 23. 1 I-)N* 93.4 94.4 2.0 0. 182 N*-)TG BA 88.2 89.6 2.8 0. 108 TGBA-)SmA 80.2 76.2 8.0 0.033 SmA-) K 46.8 53.5 13.4 7.85

Mi xtu re with 30.0 mole % of NOB A K-) K, 85.5 83.7 3.6 15.7 K,-) I 99.0 90.6 16.8 18.4 I-)N* 99.6 100.4 1.6 0.974 N*-)TG BA 87.5 0001 8 TG BA-)SmA 80.2 83.8 7.2 0.084 SIl1A-) K 40.8 42.5 3.4 23.6

ferroe lectric liquid crystals (FLCs). Colours of SmA fa ns continuously change with change in te mperature. Thi s phenomenon a lso indicates that, phase is he lical and change of the colours is due to change of the pitch of the he li x, due to temperature va ri ati on. It is important to mention that, the sa mples used in thi s study are un-aligned, kept between two cover slips. Hence, differe nt types of doma in s are not surpri sing due to different types of a lignment at different places. The authors have also tried to get homeotropic al ignment of the molecules by coa ting the inner surfaces of the cover slips with the lec ithin . Under thi s conditi on, striped SmA fans have been observed, as shown in F ig. 6. It seems that, perfect homeotropic al ignment could not be achieved even with the lec ithin coating, which probabl y is due to non-uniform thic kness of the samples between the cover slips. All these textures sugges t thi s phase to be he lical SmA (TGBA) phase.

Extrapo lated trans iti on te mperatures at the scanning rate of 0 DC/min , average trans ition entha lpies and entropies a long with the width of the tra ns iti ons obtained from DSC are shown in Table 1 for the mi xtures hav ing NOB A concentrations of 13.7 and 30.0 mole pe r cent. Quantar ive data of

entha lpies (tlH) and entropies (tlS) for N *-TGB A and TGB A-S mA transiti ons are very scarce in

(J/mole-K )

17.6 61.3 0.495 0300 0.093 24.5 Data o f SR 5.0 °e/min

48 9 49.4 263 0.0049 Very weak peak 0.238 75 .2 Data of SR 1.0 °e/min

literature, but the author's data on the binary mix tures of cho lesteryl pe largonate and nonyloxybenzoic ac id l4 are ava il able for these transitions and these are of the same order as observed here. Prasad et al. 26 ha ve a lso observed very weak TGBA-SmA peak in the bi nary mi xtures of 4-(2'-metylbutyl) phenyl 4'-n-octyl biphenyl-4-carboxylate (CE-8) and 4-n-dodecyloxy biphenyl-

4' -(2' -methyl butyl) benzoate (C 12) but tlH values have not been quantified ,

ChC-NOBA mi xtures show age ing effec t, where mesophase trans iti on te mperatures dec rease with time. Thi s could be due to the presence of highly reac tive NOBA, which deteriorates the mixture with time. Appearance of most of the mesophase textures also shows hysteres is with te mperature. C rystalli zati on temperatures for these mixtures under pol ari zing microscopic studies are found to be hi gher than those observed in DSC experiments. Thi s is a common observation for a ll the materi als, but these mi xtures have been found to be highl y sensiti ve to thi s effec t. Thi s effect happens due to the contact of the samples with the atmosphere while polari zing microscopic studies,

OHAR et al.:BINARY MIXTURES 699

4 Conclusions

Range of TGBA phase In ChC-NOBA binary mixtures lying between two critical concentrations (5 and 36 mole per cent of NOBA) is 8-10 0c. Although, TGB phases are observed for small ranges but Kuczynski & Stegemeyer27 have observed TGBAIC phases in the range of about 40 °C in the binary mixtures of di-heptyloxybenzene with cholesteryl myri state and cholesteryl benzoate and also have observed TGB phases between two critical concentrations. It seems that , presence of a small amount of NOBA (5 mole %) in the ChC matri x may introduce layered SmA phase and the twi sting power of ChC results in TGBA phase. For more than 36 mole per cent of NOBA, layered SmA phase (and hence TGBA) di sappears due to competition between ChC molecules (to form SmA) and NOBA molecules (to form SmC). It is interesting to note that, mixtures lying in NOBA concentration range of 40-90 mole per cent show blue phase and thi s is the range where, no smectic phase is being observed . The packing model suggested 14 for ChP-NOBA system seems applicable here also, as molecular length of ChC is smaller than the length -of ChP only by one alkyl chain . According to thi s mode l, ChC molecules are expected to fill the space between the dimers of NOBA. IR and dielectric studies are needed to strengthen one's understanding of the molec ular packing for TGB phases in these mixtures, which is In progress.

Acknowledgement

The authors thank Prof M V Kurik , Institute of Physics, Ukraine Academy of Sciences, Kiev (Ukraine) for providing liquid crystal materi als. They express the ir sincere thanks to Prof S L Srivastava, Coordinator and also to K Banerjee, Centre for Atmospheric and Ocean Studies, Allahabad University, All ahabad for their expert opinion and guidance on the present work. The authors also wish to thank the University Grants Commission and Department of Science and Technology, New Delhi for financial assistance under major research proj ects. One of the authors (AKS), thanks the Department of Science and Technology, New Delhi , for a fellowship under the research project.

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