Indian Journal of Pme & A[J[J li cd Ph ys i c~ Vol. 40, December 2002, P[J. 90 1-907

Optical and thermodynamic studies of binary mixtures of nematic liquid crystals from

homologous members of alkyloxybenzoic acid Ravindra Dhar' & R S Pandey

Ph ysics Department , Ewi ng Chri stian Co llege, All ahabad 2 1 I om and

V K Agrawa l

Ph ysics De[Jartmei1l, All ahahad Uni versity, All ahabad 2 11 002

Received 20 March 2002: revised 17 Ju ne 2002: acce[Jted 6 August 2002

Transition rem[Jerature (Tp) , transiti on ent halpy (/:;.H) and transition entropy (/:;.S) rur the hi nary mixtures or heptyloxybenzo ie acid (HOB A) and decy loxyhenzoic acid (DORA) in dilTcrent mole rat ios have been determ ined u ~ ing dirrerential scann ing calorimeter (DSC). The phase diagrams 1'01' the heating and coo ling cyc les hJve been drawn whi ch show th e eutectic co mpositi on or DOB A- HOBA hinary system in the rat io or 48:52 mol e [Jer cent. A wide range smect ic C (S mC) phase (range - 40 ec) rol lowed hy the nematic (N) phase (range -34 ec) has been ohserved around the eutectic co mposition or DOBA-HOB A hi nary system. SmC-N and I- I (isotropic) transi ti un temperatures (Tc_:.: and TN_I) have been round \() vary, almust linearl y, with the mu le rraction or components , indicat ing the system as an ideal mi xllIre . For the eutecti c cumposi tion or the HOBA-DORA system, it has been [Jossihl e to bring down SmC phase to nO ec wit h en hanced temperature range. whereas no member or the alk yloxybenzoic ac id seri es has SmC phase helow 92 °C.

1 Introduction

One of the main requirements for the application of liquid crystalline material s in display devices is the ava il ab ility of wide range of required mesophase at room temperature'. Most of the single component liquid crysta l materi als do not fulfill this simple bas ic requirement. More often, co mpos iti on of su itable binary, ternary or multi-component mi xtures, fulfill this, as well as other requirements for the application in display dev ices l-). Besides technica l importance, the study of liquid crystal mixtures is ga ining interest due to occurrence of induced, rc-cntrant , twist grain boundary (TGB) phases and so me other criti ca l phenomenolyl-7. There are a few binary mi xtures, which conform to simple eutec ti c behaviourx.'J, whereas the others do not"'-11. Tn many cases it is not always possible to bring down the temperature range of mesophase concerned without affecting the properties of the constituent pure components ').

Members of the alkyloxybenzoi c acid series lJ

have nematic (N) and smectic C (S mC) mesophases. but they lie above 92 DC, which make them unusable for any device application despite the fact th at, nematic phase is the most widely used in twisted nematic (TN) and super-twisted nematics (STN) type liquid crystal displ ays (LCD) whereas, helica l SmC (S mC*) phase is primaril y required for ferroeleclricity in liquid crys tals_ The authors have observed TGB A and SmC* phases In nonyloxybcnzoic acid (NOBA ) by introduci ng helical twisting due to choles teryl estel's "'." . As SIl1C phase of NOBA li es above 93 DC, therefore, SmC* phase has also been observed above 93 DC in the binary mi xtures of NOBA with cholesteryl pelargonate l1 and with choles teryl caprylate'5 In the present work , thc authors have tried to bring down the temperature range of the ex istence of SmC mesophase by preparing the binary mi xtures of heptyloxybenzoic acid (HOB A) and decyloxybenzoic acid (DOB A) that belong to alkyloxybenzoic ac id series.

902 INDI AN J PURE & APPL PHYS , VOL 40, DECEMBER 2002

2 Experimental Details

Pure grade sampl es of hepty loxybenzoic ac id (HOB A) and decy loxy benzoic ac id (DOBA) have been procured from Institute o f Phys ics, Academy of Sciences of Ukraine, Ki ev (Ukrai ne) and they have been used without any furthe r purification . Seven binary mixtures of different mo le rati os of HOBA and DOBA have bcen prepared by weighing out pure samp lcs using e lectro-balance of Calm

(mode l C-33) having an accuracy of I ~lg . Mo le per cent rati os in diffe rent binary mi xtures of DOBA and HOB A are respectively , 85.6: 14.4 (M I)' 72.7:27.3 (M1)' 65.9:34.1 (M ,), 49.8:50.2 (M~), 40.2:59.8 (M,), 72.0 :28.0 (Mr.), 12.9:87. 1 (M 7)' Before taking the measurements, these mi xtures have been homogeni zed by adequate stirring in the ir isotropic liquid phase.

Thermodynamic study of these binary mixtures has been carri ed out on a diffe renti a l scannin o b ca lorimete r of Perkin-Elmer (mode l DSC-7), which has a built-in software, PEAK, to determine the

15

II. 25

~

:::: E 7.5

~ 0 -' LL

>--

DHAR el a/.:BINARY MIXTURES OF NEMATIC LIQUID CRYSTALS 903

transition enthalpies (t1H) have been determined with the maximum uncertainty of 5 % even where peak areas have been resolved manuall y.

i- I

[ 1 ~ s[

SrrC-N i

u ssl

Q.

f-75:

I K -SmC I 1

sst I!I

K K' -0

J~ sc an

."li --- - .--1\-*---"" j-N

~ N-SmC

1 1

6 e 9 ~ SmC - K'

G-_ - .Q K' -;( & - - - ~

10 11 14 r ate (C/m ln)~

I

I 16

Fig. 2 - V.lria tion of peak tran sition temperature (T I' in ec)

wi th scanning rate (in eC/min ) for different transitions of the mixture M " (HOI3A 72.0 mole per celli ). Lines having posi ti ve slopes represent transi tions temperatures of the heating cycles whereas the li nes with negative slopes represent transiti on temperatures of the cooli ng cyc les

Different mesophases have been identified by using a polarizing mi croscope of Cens ico model Izumi-7626. The temperature of the sampl es under the cross po larizers has been controll ed with the help of a hot stage of lnstec with temperature acc uracy ±0.003 DC. The textures have been photographed with the photographic camera of Yashica mode l FX-D quartz with the magnificat ion of 100.

3 Results and Discussion

Peak transiti on temperatures (7"1') and enth alpy (t1H ) of pure as we ll as binary mixtures have been determined by the acquired DSC data at different sca nning rates viz. 5.0, 7.5, 10.0, 12.5 and 15.0 °C/min. A typical DSC thermogram at the scanning rate of 10 DC/min for heat ing and coo ling cyc les both is shown in Fig. I for mixture M.I . It has been observed that, peak transiti on temperatures of the mixtures also depend linearly on the scanning rates

like pure samples I7- l ') A typi cal plot of Tp (in DC) with the scannin g rate (in DC/min) is shown in Fig. 2 for MI' Fig. 2 shows that , the transition temperatures recorded in the heating cycles are hi gher than those recorded in the corresponding coo ling cyc les. In the heating cycles , Tp increases, linearl y, with the scanning rate (positive slope) whereas, in the cooling cycles , Tp decreases, linearl y (negative slope) with the scanning rate. Difference between transiti on temperatures recorded in heat ing and coo ling cycles decreases with decrease in scanning rate and becomes almost zero (for a enanti otropic transiti on) at the extrapol ated scan nin g rate of 0 DC/min (see Fig. 2). The ex trapo lated transition temperatures at the scanning rate of DC/min have been taken as true transiti on temperatures and have been determined for all the mixtures as we ll as for the pure components . Ext rapo lated transiti on temperatures obtained from DSC almost agree with the transition temperatures obtained from texture stud y except at higher temperatures where, temperatures obtained from texture study have been found to be sli ghtl y hi gher (- I DC) .

U

a. E 0 +'

(

0

+'

150 ---------__

130

~I

110

I

901

Sm-C

70 + + K+S'nC

_ ._-- --,

+ Kf-S'nC +

~J

( ro

: :~~~, _~K-~~~, -~-~~-~ o ~ w ~ ~

mole pe rcent H0 3r.~

Fig. :1 - Vari at ion of ex trapolat eci transition temperatures at the scanni ng rate of 0 °C/Illin obtaineci from OSC ( in ec) with mole per cent of HORA for the heat ing cycle of OOI3 A- HOI3 A binary system

Phase diagrams as shown in Figs 3 and 4 have been drawn uSing extrapo lated transiti on temperatures for the heating and coo ling cyc les of DOBA-HOBA binary system. Figs 3 and 4 show

904 INDI AN J PURE & APPL PHYS, VOL 40. DECEMBER 2002

simple eutec tic phase diagramsN') with the eutectic temperature (7J = 57 DC and eutectic compos iti on ."i2±2 mole per cent of HOBA. Phase diagrams of heating and coo ling cycles are almost similar, ensurin g repeatab le mesophase transiti ons, whi ch are not often observed in li quid crys ta l mi xtures. In DOBA. range of SmC phase en DC) is larger th an the range of N phase ( 19 DC) whereas in HOB A, range or N phase (44 DC) is larger than the range of SmC phase (."i DC) . While approaching towards the eutec ti c compos iti on, mi xed phase (c rysta l+SmC) to SmC transiti on temperature (TK.(f' .c, ) decreases and show minimum at eutect ic co mpos iti on leav ing wide range SmC phase. The range of SmC i:, very large e -40 DC) for eutec ti c compos ition and that too at considera bly lower temperature in compari son to pure components. N to I transiti on temperature ( T~ ,) increases linea rl y, with the mole perce ntaae of HOBA whereas, SmC to N transiti on t empe~lture (Tc.;) decreases with mole per centage of HOB A and hence. ran ge of N phase is maximum in HOB A. Eutec ti c compos iti on has also been estimated by empirica l relati onshi p2":

\ = (To -T.) " T., + T2 - 27~.

... ( I )

where T, and T] are melting points of lower and hi gher melt ing components (HOB A and DOBA res pecti ve ly), T.. is eutec ti c temperature and x, is the eutec ti c mole fracti on of lower meltin u component (HOBA). Eq. ( I) gives eutec ti c comp~si ti o n at 51 mole per cent of HOBA.

Schroder-Van Laar eguationX '" has also been app li ed to find the eutec ti c composit ion of DOBA-HOBA binary system:

. - M-f ., (I I 1 . In .\ . - -- --- - In f · l R T. T . I

.,

DHAR et al.:B IN ARY MIXTURES OF NEMATIC LIQUID CRYSTALS 905

1 5B~

IJ -+ U

liB Q.

E

+' 90 (

Sm-C 0

+' 10t Kt SmC

w " Kl-SmC

(

ro 5d l

+' I

-+--------''!-L--,----".---- -- I I

K

lB I 0 13 10 60 BO 100

mo le perce nt HOBP

Fig. 4 - Variation of extrapolated tran sition temperatures at the scanning rate 01'0 DC/min obtained from DSC (i n DC) wi th mole per cent of HOBA for the cool ing cycle of DOB A- 1-I0BA bi nary system

Enthalpy (/)'H ) and entropy (/).S) of SmC-N and N- J transiti ons are governed by the following mix ing ru l e"' ~·L

... (5)

where XI, /).H I , /).S I are mole fracti on, enthalpy and entropy for HOB A and X 2, /)'H~ . /).S2 are the respec ti ve parameters for DOBA. The best-fit va lues of the mi xing constants All and As for SmC to N transiti on have been found to be -(O.2S±O. 13) and -(0 .24±O. 14) , respecti ve ly, whereas for N to J transiti on these va lues are (0. I S±O. 12) and (O.2 1±O.09) respec ti ve ly. Plots of f,.H for SmC-N and N-J transiti ons are given in Fig. 5 with the best-fit va lues of All for the ex perimental data and wi th ;\ 11=0 (for hypotheti cal non interac ting mi xtures) . Simil ar plots of /).S for SmC- and -I transiti ons arc given in Fig. 6 with the best-fit va lues of As for the ex perimental data and with A,,=O (fo r hypothetical non-interact ing mi xtures). Negative va lues of All and As for SmC-N transition suggests that the interacti on of HOB A and DOBA molec ules

of slightl y different length s is des tabi lizing SmC phase. This also accounts for the non-linearity in the plot of T eN in small range (see Figs 3 and 4). Pos itive values of All and As for N-J transiti on suggest stabili zati on of N phase in these mi xtures. The members of alkyloxybenzoic acid are known to show dimerizationI.1.2.' . The numbers of dimers are expected to be larges t in SmC phase'.' . In the mixture of HOBA and DOBA, fol lowing types of clu sters are poss ible:

I ) Monomer of HOBA; 2) Monomer of DOBA ; 3) Dimer of two HOBA molecules, 4) Dimer of two DOBA molecules; and 5) Dimer of one DOBA and another HOBA molecu les.

3.5

l.O

1,5 ~

0 E 1,0 \ f-]

:{

v I I

1. 51

906 INDIAN J PURE & APPL PHYS. VOL 40. DECEMB ER 2002

DOBA has onl y 1 more alkyl chains in compari son to HOBA. therefore, destabili za ti on effec ts in DOBA-HOBA binary systems are sma ll in compari son to systems where, molecul ar lengths are wide ly d ilTerent U

8.Jr--

r

I 7.0 [

l 6.0 , ~

0 S.C · E

>-, .:(

v ;.C -

U1

DHAR et al. :BINARY MIXTURES OF NEMATIC LIQUID CRYSTALS 907

Acknowledgement

The authors thank Prof M V Kurik , In stitute of Physics, Ukraine Academy of Sciences, Kiev (Ukraine), for providing liquid crys tal material s. Their sincere thanks are due to Prof S L Srivastava, Coordinator, K Banelj ee Centre for Atmospheric and Ocean Studies, Allahabad University, Allahabad , for hi s help and suggestions at various levels of the work. One of the authors (RD) spec ially thanks the University Grants Commission, New Delhi , for financial ass istance under a maj or research project.

References

Lilillid crvslals: Apl,/icalioll alld II SI'S , (Ed ) 8 Fl ahadur (World Scicnlific), 19