Inci i<l ll .l olln1:il or Purc & Appli ed Pll y~ic~ Vol. :'7. Dccemher I ()')l) . pp. Xl) I -X0X

Study of liquid crystalline phases from temperature dependent dielectric constant

S L Srivastava & Ra vinclra Dhar

Phy~ ic ~ Dcpartmcnt. AII :lhahad U ll ivcr~ it y . A llah<lbad 2 11 002

Reccived 24 Augu ~ t 1900: :lcccptcd .'i Octohcr 1000

Dcri v:llive oj" the ~ t : llic dielcct ric con~tan t (E) with in ver~c temperature ( l i T) i ~ a sens iti vc tcchnique to detcrmine the liquid

cry~t.illin e pllase~. Accordi ng to M aicr and Meier theory (I'vhlier & M eicr. Z Naru r/iJ rsc/1. 16A ( I<)() I ) 2(2). E ean he IVrill cn <IS

A + f]i T. Plots or /J = dEld( I IT) I wh ich is the runction or molccular dcnsity (N). order parameter (S) and dipo le-dipolc corrclation

factor (g)llVit h T show pcaks at the tr:lnsitionlelllper<ltures. and the va lucs of /J give an estimatc or the naturc and magn itudc

of dipo lar c:orrelation in differeIll me~ophasc~. The techniquc has hccn .Ipp licd to detcrminc thc tr:lnsit ion t e mperalU re~ of six

liquid c ry~tal lin c matcri .il s hy thcir tcmpcrature depencicnt die lectri c data.

I ntrod uction

Liquid crys tal materia ls possess diffe rent meso­phases between so lid crystal (most ordered) and iso­tropi c li qu id (most disordered) phases . Arrangement of molcc ul es in liquid crys ta ls is different in different mesophases with different degrees of freedom. When tem perature of t he I iqu id material is gradual I y increased , it passc:s from crysta l to di fferent mesophases and then fi na ll y to isot rop ic liquid phase. The practical appli ca­tion or a liquid crysw lline material depends upon the kind of mesophase (S) present and its temperature range. The hasic characterization of I iq u id crystal I ine material. therefore demands the identification of the mesophase (s) and dete rmination of the tllesophase tran siti on tem­peratures.

Differential sca nning cal orimeter (DSC) helps to de­term ine the transition temperatures at whi ch material s transrortll rro m one Illeso phase to anot her with the change in enth alpy at tran siti ons l

. Genera ll y the change in enthalpy rrom crys tal to Illesophase transit ion is large

( - .:'i -~() Id /mo le), whereas meso to mesophase or me~o­

phase to isotropi c liquid transiti ons in volve slllall

chan ge in enthal py (~ I kJ/ lllo le). There are certai n tran sitions such ,tS nem,ltic or slllecti c A (S mA) to .~ Illectic C (S IllC) and smecti c F (S Ill F) to smect ic G (SIllG) where etl lh alpy cha nges are ver:- small « I k.l /lllol c) and may go undetected. Hot stage polarizing microscope is used to identiry di ffe rent mesophases hy their characte ri sti c opti cal t ex tures ~ . Another technique tn deterrni ne the t r ,lIl ~ it ion temperature is wh ite li ght

transmittance (WL T ) where chan ge of the whi te light

intensity transmitted through the material is observed at

the meso phase transiti ons3

Anisotropy orthe die lec tri c consta nt of liquid crysta l

materi ;1i is a necessary property for the ir poss ibl e use in

any device application . Variation of the di e lectric con­

stant with temperature in differeilt mesophases may also

be used to study the phase tran sitions . S ince the degree

of molecular orderi ng and their dipolar correlation"

changes while goi ng from one mesophase to the other.

the dielectric properti e~ of di fferent mesophases Illay

show abrupt changes at the transiti ons. In mnst of the

work on dielectric study of liquid crysta ls, di sconti nui­

ties of the di electric constant (E) are observed at the

phase tran si ti o n~5 Somerton(' has used the vari ati on or

temperature dependent di e lec tric constan t of unal igned

samples to determine the tran siti on temperature or liqu id

c rys tal s. In the present paper we report the temperatu re

dependence of dielectric constant of pbnar and hOllleOT­

ropic a li gned samples to determine the tr,lIlsition tl:tn­

pe rature of three pure materials viz . c hole steryl

pe largonate (C hP), c lt ole~te ryl myris tate (C hM), alld

nonyloxy benzo ic ac id (NOSA) ; two y-irradi ateclmate­

rial s (cho lesrery l myristate and cho lestery l pelargonate)

and bicomponent eutect ic mixture of cholesteryl pelar­

gonate and nonyloxybenzoic acid (ChP-N08A). Tht'

transition temperature thu s obtained have been com­

pared wit h those obtained from DSC.

INDIA N J PURE APPL PHYS. VOL 7>7 . DECEMBER 199<)

2 Dielectric theory fOt, the Mesophases Dielec tri c constant parallel to long axes of the mole-

cules (longitudinal component , £/) and normal to long

ax es of the molecul es (transverse component , £1) of the

nematic (N) phase are given by Maier and Meier equ a­

ti ons7 as

l · rJ'I I\ +~ St..CX +yf!.IN J~~ T' 1- ( I - JCOS~ ~)Sl } ... ( I )

NHy [= I +--, ,

Co

1'0.111" +* S t.. CX + Y g llv J ~~2 T ' I + ~ ( I - J cos" ~ ) S ,}

.. . (2)

where N is the number density of molec ul es, S the order

parameter, t..cx the anisot ropy of the polari zabi I it y, CXIII

the average polarizabi I ity, £" (= 8.85 pF/m) the electrica l

permittivity of the vacuum, m the resultant dipole mo­

ment of the molecule and ~ the angle between dipole

mom e nt and lon g axis of th e mo lec ul e, h =

J£(O)/(2£(())+ I ). The feed back factor y of the reactor

field is given by: (£(00) + 2)(2£ ( 0 )+ I )

y = J ( 2 £ ( 0 ) + £ ( 00 »

where £(0) and £(00) are the low and hi gh frequency

limi ting average dielec tric constants and giN and f!" N are the angu lar correlati on factors in N-phase introduced by

K Madhusudana and Chandrasekhar' . Eqs ( I) and (2) re-

duce to yield dielectri c constant (£/) of the isotropic

phase as

N h Y ( ~~ 1 £, = I + ~ ·CXIll · + Y J k T ... (3)

de Jeu el a /. 'J and Bengui gu i 10 ex tended the Maier and

Meier equati ons to SmA, 5mB and SmC phases intro­

ducing correlation factor (g). General forms of (£1 and

£,) in SmA phase are give n as:

3 £ ( () ) N ~L~ cos" ~ £ / = £1 ( ex> ) + k T ( I + g/~ )

2£( O) +£(ex» £0 .

... (4)

and

3£(0) N ~ '1 s i n " ~ £, = £, ( 00 ) + , T ( I + f!. IS )

2£(0)+£( 00 ) 2 £11 ... (5)

where f!. IS = .f(S")p'1 cos "~/kTand f!,ls(S2 ) ~l2 s in 2/~kT, j(S2} and f!,(S") are functions of the SmA order parameter. In SmC phase

6~2 COSC ~ ( J . 2 1 f!. IS = ,) k T . I - "2S Il1 8

and

3 W SIIl - I-' . " ?~ n.( ) /;/.1 = "4 r H T . J cos 8 - 1

where 8 is the tilt angle of the SmC phase and r the intermolecular di stance .

Eqs ( I ) and (5) can be written in the foll ow ing fo rm B

£=A+T

.. . (6)

where va lues of A and B are different for different mesophases and different a li gnment s.

3 Results and Discussion Different mesophases and their transition tempera­

tures (0C) as determined by DSC of pure and y- irrad i­ated ChP and ChM, NOBA and the bicomponent eutectic mixture of ChP-NOBA are given below:

ChP (Pure) I I :

K(78.2) N* (89 .7) I (89.8) N* (74.() ) SmA (overn ight cooling) K

ChP (y-irradiated)11 : K(74 .2)N* (79.8)1 (80.2) N';' (58.6) SmA (ove rni ght

cooling) K ChM (Pure)1 2: K (69.3) SmA (77 .5) N*(82.9) I (83. 1) N* (77.6)

SmA (overni ght cooling) ChM (y-irrad.iated) 1'1 : K (67.0) N* (72.9) I (73 .2) N* (67.3) SmA (o verni ght

cooling) K NOS A (Purd 5

:

K (9 1.2)S mC( 115.8) N ( 14 1.8) 1 ( 14 1.9) ( 11 5.8)

SmC (90.5) K' (67.3) K Eutectic mi xture of ChP and NOBA (mole rati o 67 :

33)1.)

K (69 .9) SmA (80.7) SmA'!' (8 1.6) N* (93.8) (94.2) N* (83.8) SmA * (8 1.0) SmA (overni ght coo ling) K

where K. K' , Sm, N, N* and r represent crystal. mi xed crysta l, smectic, nemati c, chiral nematic (cholesteri c) and isotropic phases respecti vely.

1

SR IVASTAVA & DHAR: LIQUID CRYSTALLI NE PH ASES

Typical pl ots or the measured va lue of longitudinal

and transverse component of dielectric constant (c/ and

cr)with I /T(K- I)at IOOkHzareshowninFi g. 1 rorChP.

Fig. 2 shows vari ation of' dCI/dT and dc/dT with T (DC) .

c/ and Cr data show abrupt jump at transitions (see Fig.

I ), whereas dc//dT and dc/dT show peaks at transiti on tem peratl1l'es (see Fig. 2). Transition temperatures as

3.1 -

2.9

I w I

27 r

2 .5

obtained from the peaks dc/ dT and dc/dT are given in Table I alongwith transition temperature determ ined by DSC. As shown in Fig. I die lectric constan t of the

isotropic phase (c /) is same in planar and homeotropic

ali gnments (i.e. C/ == Cr == c,), but at the isotropic to

cholesteric transiti on temperature (T/N* ). C; jumps up­

ward whi le c/downward showing negat ive di elec tric

K

I .

2.3 L---=-2'""=. 7:----'!"'---:2:"-. 8;:-------'---:;2~. 9;---4--'3 -';:;.0----;3.'1'--- "{3.12 -----:3t. 3~--' 1000/ T (KI)

Fi)!. I - Var iati on o f Inngitudin ,ll ~nd transverse co mponents of dielectri c constnnt (E/ nnc! Eri for pure ChP at 100 kH z in the coolin)!

cyc le wi t h I ()()OIT ( K- t): (I) represents E/ and (+ ) represents Er. Verti cal hal'S represent uncertaint y in the measurement or E/ and Er anc! vert i­

cal arrows represent transition temperature obtained rrom DSC

~--------~,------------------~ 0 .7

0.6

0 .5

0 . 4

f- 03, -f--+-

~ 02r

'OJ 0.1~

O.O~ ~

K

- O .l~L I 25 35 45 ---5"--5

I SmA' SmA

L L-++-+-JI j>-!--I -!-I k }tH+ll-t--+ I t 11ft

I I !

65 75 8 5 T(C)

95 105

Fi)!. 2 - V ,lr iat inn o f cI E//liT and li lr/liT with T (i n "C) for ChP i ll the coo ling cycle. Curve I represenb liE//cIT and curve 2 represents ,lnL!

cIf/Crr Curve I has hecn shifted hy O.J. Vert ical arrows represent transi tion temperatures obtained from DSC

lNDIAN J PURE APPL PI-IYS. VOL 37. DECEMBER 19()l)

anisotropy ( 1-.£ = £r£, = -ve l in N* phase. At TIIII ,,, . d£/dT

pcak is posirive whereas d£/dT-peak is negat ive. Below

FI,\ ' as temperature decreases, 11-.£1 increases. At N* to

SIllA transition temperature (7;v , .. d, £1 and £, both show

upward jump and d£/ dTand d£/dT peaks are negati ve.

At the crystalli zati on temperatures ofthe sample, £1 and

E, both JUIllP downward and d£/dT and d£/dT show posit ive peaks. Tem perature of the crystallizati on in

pure ;Ind y-irrad iated ChP and ChM could not be deter­mined by DSC because the process tak es very long time.

Inthc isotropic phases of all the samples £ versus l iT pi nts are straight lines (as is obtained fo r the non­assoc iated isotropic l iquicl~) with correlat ion coeffic ient hetter than 0.99. Correlat ion coeffic ient of I corresponds tn perfect march of the data on the strai ght line. First

cil:ri vali ves OfE with respect to IITohtained from Eq. (6) !,!i v ' the va lue of R in d ifferent mesophases. Var i­

ali on uf H ( = d£/d( 117') wi th T s how~ peaks at the Ir;l1lsi tion remper(ltures (st'e Fig. :1 for ChP), but va lues of n h;I\'c been found to he alillos t temperature inde­pcnlicill within a dielect rica ll y stabili zed l11esophase (execI'I in NOBA). Corre lat ion factors g ls ,lIld g ,.\' of Eqs (4 ) (In ci C'i) secm to be weak ly temperature dependent. Due tn very weak temper;ilurc dependence of B within

a ll1 e~()phase . £ versus J / 1' piots represents a straight line IC\Cepi in NOBA ) with inrerce pt A ane! slope B (sec Eq.

h). LC(lst-squares fil (LSF) nf £ versus ilTplot in c1iller­cnl Il1csnphases ~i ves d i frerenl va lucs of I he slone R. Thc

'I 2S - 2

: : 2

val ues of B fo r d ifferenr ll1esophases of different sam­pl es in different alignments are given in Tab le 2. The vaiue of B depends upon number density (N), order parameter (5) and dipolar correlation coeffi cient (g) or different mesophases, hence B can be re lated with Gi bb~ free energy (G) . Since N. Sand g have different values for different lllesophases.J·I.ll.J, B should have diffe rent

va lues in dilTerent mesophases, see Table 2. Although

pl o t ~ or d£/clT (Fig. 2) and d£/d( I IT) (Fig. :1) wi t h T shmv peaks at the transiti on temperature, the variati on of

d£/d ( l iT) vers u:, T is more sensiti ve than d£/d( I/T) ver­sus 1'. Hence it is preferable to determine the tran siti on

temperature from d£ld ( I IT) versus T pl ots, see Fi g. :1 .

Value of B in the isotropic phLl.~e (B = N/1'( fl "] £nk) of pure ChP (569 K) is larger than that of ptlre ChM (4H I K). Molecule of ChM is longer than that oj' ChP ami the refore, number density (N) for ChP shou ld he larger than that or ChM and lhal may be the reason I'm the larger va lue of B of pure ChP than that or ChM. Va lue

of R for the isotropic phase of y-irradiated ChM is 5XO K wh ich is larger than the va lue of B for pure ChM (48 1 K) and suggest the poss ibility of increase in d ipole

moment ( fl) of ChM on irradiati on. The values of B for the isotropic phases of the pure and irradiated ChP are almost same within the uncertainty limits (569 and 5() I

K), suggesting that dipole moment ( )1. ) or ChP rCl1la in:-­ullchall ,!!ecl on irrad iation . Infrared (I R) spec lra \)1' i rra­dialed ChM show the runlure or ~ s ler lin kin g O-C hOlld

~

+ 0

o f' t-·+I----+-

~ -4 ~ !

K \ i Sm A SmA I N' 1

! I 'C "­' 1-

: - 6 r I i

I ' !

" i ~ -10~ ___ ~ ______ ~ _________ ~ ______ ~ _________ ~ ______ -L _________ ~ ______ ~

2') 35 45 55 65 T tel

7 5 85 9 5 10 5

l'i ).! . . ) - V ~ lr i ati ()fl (I I' dflldT;lIld clc/dT wilh T( in ~c) l'or Ch P int ilc coo ling cyc lc. Curvc I rcprcsents df/hlT and cllrvc::: rq )I'c sCllis ,Ind

de ;,,1 7. Cur\L' ::' hils heen shiltcd hy - I .. ). VCrlic;iI arrows rcprcscnllransil ion Icmpcraturcs ohlaincci I'rom DSC

~ ---

SR IVASTAVA & DHAR : LIQUID CRYSTALLINE PHASES

Tahle I - Mcsophase transit ion tcmperaturc of differcnt materials as dctermi ned fro m dielectr ic data (presen t work ) and from DSC

Materi :ti

ChP (Purc)

ChM (Purc i

ChP ('I-irradiated)

CI1M ('I-irradiated )

NOBA

Eutect ic Mi xturc (ChP-NOBA)

Phase transition

I-N ':'

N *-SmA

SmA-S mA'

SIllA'-K

I-N '"

N ':'- SmA

SmA-SmA'

SmA'-K

1- N olo

N ':'-SmA

SmA-K

I-N ':'

N "' -SmA

SIll A-K

I-N

N-SmC

SmC-K'

K'- K

I -N ':'

N * -SmA ':'

SIl1A * -SmA

SmA-K

Transi tion tcmpcralUrc Reference

T hi s work DSC

89.5 ± 0.4 89.8 II

74.6 ± 0.2 74.0

6X. 1 ± 0.:1 (,X .2

(,() ± I

SO.X ± O.~ 83.1 12

76.<) ± 0.7 77.6

69.4 ± 0.2

55 ± ~

76.4 ± 2.0 XO.2 II

(,0.7 ± O.9 5X.6

55 ± I

729± I .X 73.2 12

(,7.0 ± O.X ('7 .~

52 ± I

141.2 ± 0.4 141 .9 1.5

114.7 ± O.4 115.X

93.3 ± 1.7 90.5

77 ± X 67.3

94.6 ± 2.0 94.2

XX.4 ± 2.0 X3 .X

XO,X ± 02 X 1.0

52 ± I

T,lh le 2 - Va lucs of IJ I. in K ) for diffcrclllll1csophases obtaincd fromlcast square fit 01' £ versus liT plot,

Vlatcri :11 Isotropi c (I ) Cholestcric (N"' ) Smeetic A ISmA)

Planar Homeotropic Planar Homeotropic

ChP )6<) ± XO 1397 ± 233 252 ± 13 1229 ± 172

(Pure) 1969 ± 141 1232 ± 18

ChM 481 ± )(, 13 12± 162

(Pllre) IR52± :10 13() ±4

ChP 591 ± lOS 1134 ± 148 81 ± 10 (y- irr,lliialcli )

ChM 580 ± 11 2 776 ± 50 42<) ± )O IOJX ± 249 72) ± II X ('I-irrad iated )

Eutecti c Mix lUre 642 ± 64 826 ± 71 6<)0 ± 62 435 ±)6 rChP-NOB A)

l l 70 ± I (,X "227 ± 72

I tbta for SmA' phase , -d,lta for Sm A'" phasc

INDI A .I P RE APPL PI-IYS. VOL :'7. DECEMBER Il)l)l)

or ChM on irradiation, whi ch seems responsible for the change in dipole moment of C hM on irradiation I ) .

in N* phase o r pure ChP, B in the homeorropic

alignrnent (B = Nit; ~t ~ g'NI 1-( 1- 3cos ~~)S I/3£ok) is 252

K and in the planar ali gnment (fJ = Nlry\t1grNI I + (1 -

3c() s ~ BJ .()12I/3 £nk) is 1397 K . ubstituting the va lue or

Nlry\t ~/3k rrom the isotropic phase (assumin g th at it is

almoslthe sa me for I and N':' phase) and taki ng l6 ~ = 6 ()c,

and S = O.S. g IN and g,N are estimat ed to be 0.39 and 2.S6. Estimated values or gIN and g,N suggest that the dipo le corre lat ion in homeotropic al ignment is Llnt iparall e l and in pla nar alignment it is paralle l. For ChM. die lec trica ll y stabi li 7.ed N'!' phase could not be studied due to small

temperatu re ran gc ( - 2- 3°C).

E vers us I IT pl ots or pure C hP and ChM show two dilleren t va lu cs o r n with in the SmA phases suggesting somc molec ular rearran gc ment wit hin the SmA phase

,Ind transrorJ1lin g it to a new SmA' mesophase as te m­perali lre goes down . The change of slopes occ urs at

6:->.4°C in ChP and 69.2DC in ChM. This tran siti on has not bcc n detec ted in ChM by DSC, whereas on careful exa mination of DSC thermogram of C hP, a weak peak

(L'l H- 53 .J imole) has bee n observed at 68.2 DC (Refs 1, 17), whi ch remains und~ec tcd on usual DSC thermo­

gra m. At t hi s t ran si ti on fi rst deri vat i ve or £, and £, seems continlloll s (see F igs:2 and 3) suggcs ting it to be a second orde r phasc transiti on. Arrheniu s pl ots or re la xati on

timcs (1) also show the cx istencc of thi s tran siti on in ChP

3.2r-I

3J lI ! i ll ~~111 111 111 11'111

l f 1 t I II f II \,111 ~ ij I i I I \ \1

and ChM gIving two different activation energIes in 11.1 7 I ~. I I) SmA phase . Ratna el al. and Slgaud el 0 /. ha ve

also reported such a trans iti on in SmA phase of 4-

cynophenyl -3'meth yl-4' (4- n-undecy lbenzoy loxy) ben­zoa te by Arrheniu s pl ot.

In the case of NOSA, plots of the £, and 1::, wilh l i T

are non linear as show n in Fig. 4 and the va lu e~ or /J a.,

obtained by differentiatin g £ / and £, with respect to l i T

can not be tak en to be constant in a me:-.ophase . £, and E, with l i T show abrupt jumps at the transi tion tempera­

tures (see Fig. 4) and variation s of dE/d ('ilT) and d£/ d ( lIT) with T show peaks (see Fig. 5) at the transi ti on te mperature. 1n fact the theory discussed th rough ECJs I to 5 may not be app licable 1'0 1' NOSA, and the dipole

mOlllent (p) see m to chan ge with temperature s u ~gcsl ­

in g the rormati on or dimcrs1(J.2 1. In NOS A th ree typIC' '' or

I " the ll10 lecul ar c lu ste rs have been sugges ted ' ; Il1 UnlllllCr with s in gle molecules, cyc li c dilllers or two Il1lliecul es with two hydrogen bond s bet ween COO H groups uf NOSA and open dimers of two molecu les with onl' broken hyd rogen bonel. For a cyc li c d illl T nct ciipo iL'

moment (p ) becomes almost ze ro~l due to antiparallcl combination of molecul es . However for an open d llller there is poss ibility of increase in tran sverse componellt

of dipole moment (p,) and decrease inlongituciinal com­

ponent of dipol e Illoment ( P I). In the isotrop ic ph,lse or o BA, popula l: ion of mono mers see ll1s to be III a x i IllUIll .

In N nhase iu st below the isotToni c to nell1 ,lti c tran siti on

3 0 , 2 1

I II I II I 29 r N Sm C I I I k' K

,+ 1 11 1 111 1 1

I I 2 .7~ I II I I j 2 6 LI--_2,.LiL,,------1-----;;'2't-,,.6:----L---;2-::-.na- - - -'---t --3!,1 ----'-- 3.2

10 OO/T (K- l)

h t'. -J. - V:lr i: llioll or IOllgillidinal anti l ransvcrsc cOlllponenl s 01 dieleclric conSI;lIlI (EI and E,) for . 081\ ;11 I ()() kHl ill 1i1c coo ling l' \C!c

", ilh I(jOWl'( ill K I ): (I) n.: prc>e nl> r, and (+ ) rcprc,cnl> r,. K an d K ' dala h;l ve hecn shined hy 0.:1 . V crti c; i1 hal'S reprc,c llI IIll l'C r ' :ll llI\ ill

Ihe J11l'aS lln: lllCIl I o r f l ;lI ll l [, ;lIlti ve rti c il ;IITOWS rcprcscnllr:lIlsilio IIICJ11pCr;lIl1 rc llhlilillCd frolll DSC

1

-

SR IV AST A V A & DHAR: LIQUID CRYSTALLINE PH ASES II!) 7

temperature (TIN), a rapid combination of two monomers

into a cyclic dimer decreases the effective value of j.1

appearin g in Eqs ( I ) and (2), thereby decreasi ng both £,

and £, and the average va lue of the dielectric constant in

the nemati c phase ((£NL ,· = ( 1/3)(£,+ 2£,)) becomes less

than f l . The magnitudes of jump of £, and £, at TIN depend L1 pon the number of mon omers converted into the dimers whi Ie goi ng from I to N phase. In the nemati c phase also, as the temperature dec reases, population of monomers further decreases and formation of open dimers takes place. At N-S mC transiti on temperature (TNc) due to

sudden increase in the populati on of open dimers, £,

shows upward jump and £, downward jump. Within N and SmC phases there is continu ous change in the popu­lat ion of monomers and dimers with temperature, due to

whi ch the effec ti ve va lu e of the dipole moment ( j.1) does not remai n constant. Despite the non- appl icabi I ity of the Ma ier and Meier theory for NOSA, die lectric data are explainable and the mesophase transition tempera­

ture of NOSA determin ed by d£, Id( 117) and d£,Id( l in peaks agree well within the uncertainty limit with those obtai ned from DSC. Hi gher va lue of the crystallization temperature obtained by di elec tric ex periment (see Ta­ble I) ma y be due to the formati on of nucleation centres at a hi gher temperature in the sample kept open in the dielec tri c ce ll . Sample in DSC is co mpl etely sea led, and ill thi s ce ll Iluclea ti on occLirs at lower temoerature.

+0 - 2

Sicomponent mixtures of ChP and NOSA (having

NOSA concentrations 18. 18, 25.35, 33.33 and 40.68

mole percent) from SmA* phase ' . Detecti on of N*­

SmA * transiti on on DSC thermogram is very di ffi cult

as it has a very low transiti on enthalpy (f..H- 55 J/mole)

and that too is observed as a shoulder in N*-S mA

thermogram l. At the N*-S mA * transiti on £, and £, show

abrupt jumps and is therefore easi Iy detectable (see Fig.

6 for the eutectic compos iti on i.e. 33.33 mole percent of

NOSA). N*-SmA transition is c lea rl y apparent by the

pl ot of £, and £, with I IT and d£,Id( I IT) with T for other

compositi ons also. Summari zing the above we find that even those tran -

siti ons whi ch are not eas il y detectable by DSC polariz­

ing mi croscope, th e di e lect ri c co nstant derivati ve

tec hnique makes them detectable . Superi ority of the

tec hnique may be lying in the fact that whil e me~ut ri ll g

£, and £, ali gned samples are taken whereas in DSC

measurement sampl es are unali gned. Al so in DSC. tem­

perature var iation is dynamic whereas in the present

tec hnique temperature variat ion is quasi-s tati c due to

which system attain s thermodynami c equilibrium . Thi s

is manifested also in the fact that the width of the

tran sit ions of t l and E. are narrower than those of DSc:.

2

;;- -3 TI --­"" D

I-V III II~H /l1111+-tt~ 1 H+t--t

" - 4 -CD

- 5 -

K K , ~ SmC N

- 6 ;~ ______ ~, .------~~j--___ --~~I------~~I~--~~~I ~~ 5 0 70 90 110 130 150

Fig. ) -V ~lr i ati ()n o l dE,fd( l fT) and dE,Id( l f T) with T( in DC) lor NOI3A in !he cooling cycle. Curve I repreSeI1lSciE{fLi ( l fT) :lIl d curve 2

represents and de,ftl ( 1!7 } Curve I has heen shifted hy -2.5. Verti cal arrows represellt!ransi ti on temperatures ohtained lrolll DSC

8l)8 INDIAN J PURE APPL PHYS, VOL 37, DECEMBER 1991)

o

2: .!::::.. -2

~ w u -3 " ro

- 4

- 5 30

I

I ~ I

K

40 50 60

2

1

SmA

70 r ( C)

r (w,ll ~ I

~~~ I

It N* 1

80 90 100 11 0

Fig. 6 - Variation or uEJ/d( liT) anc! dEl d( I IT) with T ( in °C) 1'0 1' eutectic mixturc or ChP and NOBA in the coo ling cyclc. Clirve I

rcrrcsents dE/ld( I IT ) and curvc 2 rcrrescnts and dEt/d( I IT). Curve I has been shi !'ted by - I . Vert ical arrows rerrcsent transi ti on temrcratures obtained rrom DSC

Acknowledgement Authors thank Univers ity Gra nts Commiss ion, New

Delhi for the financial support in the form SAP and COSIST.

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2

4

5

7

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