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Physica 139 & 140B (1986) 631-635 North-Holland, Amsterdam PHASE BEHAVIOUR UNDER PRESSURE OF TWO HEXASUBSTITUTED TRIPHENYLENES J.M. BUISINE Equipe de Dynamique des Cristaux Mol(culaires, Universit( des Sciences et Techniques de Lille, 59655 Villeneuve D'Ascq C(dex, France J. MALTHETE Laboratoire de Chimie des Interactions Mol(culaires, Coll~ge de France. 11 Place Marcelin Berthelot, 75231 Paris C(dex 05, France C. DESTRADE and Nguyen Huu TINH Centre de Recherche Paul Pascal, Domaine Universitaire, 33405 Talence C(dex, France The phase behaviour under pressure of two disc-like mesogens are studied with a metabolemeter. The investigated compounds are (-)-2,3,6,7,10,11-hexa-[S-(3-methyl)-n-nonanoyloxy] triphenylene and 2,3,6,7,10,11-hexa-(n- dodecanoyloxy) triphenylene. The weakly first order or practically second order mesomorphic transitions are detected. The pressure-temperature phase diagrams are established. The volume changes for the first order phase transitions are determined. 1. Introduction There are very few results relating to the volume changes at atmospheric pressure and pressure-temperature dependence of the phase transitions of disc-like mesogens [1]. A ther- mobarometric method has recently been used to study the phase behaviour under pressure of rod-like mesogens - pure [3, 4] or in binary mix- tures [5] -. We use it now to study the polymor- phism under pressure of two disc-like mesogens. Initial results relating to the pressure-tempera- ture dependence of first and practically second order phase transitions are given. 2. Principle and method The thermobarometric method and the prelimi- nary apparatus - metabolemeter - have been pre- viously described [2]. The principle consists in measuring the pressure of a small sample of mesogen enclosed in a metallic cell as a function of temperature. The simultaneous recording of both data gives the thermobarograms. The ex- perimental procedures and explanations relating to the interpretation and exploitation of ther- mobarograms are given elsewhere [3, 4]. 3. Substances The studied compounds were two 2,3,6,7, 10,11-hexa-substituted-triphenylenes: (-)-hexa- [S-(3-methyl)-n-nonanoyloxy] ((-)HAT-C10) and hexa-n-dodecanoyloxy (HAT-C12). (-)HAT-Clo HAT-C12 R R [[f,R * O R: n-C6H13-CH-CH2-C ~ I O- CH3 ~O R:n-CIIH23-C "" O - 0378-4363/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

Phase behaviour under pressure of two hexasubstituted triphenylenes

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Page 1: Phase behaviour under pressure of two hexasubstituted triphenylenes

Physica 139 & 140B (1986) 631-635 North-Holland, Amsterdam

PHASE BEHAVIOUR UNDER PRESSURE OF TWO HEXASUBSTITUTED TRIPHENYLENES

J.M. BUISINE Equipe de Dynamique des Cristaux Mol(culaires, Universit( des Sciences et Techniques de Lille, 59655 Villeneuve D'Ascq C(dex, France

J. MALTHETE Laboratoire de Chimie des Interactions Mol(culaires, Coll~ge de France. 11 Place Marcelin Berthelot, 75231 Paris C(dex 05, France

C. DESTRADE and Nguyen Huu TINH Centre de Recherche Paul Pascal, Domaine Universitaire, 33405 Talence C(dex, France

The phase behaviour under pressure of two disc-like mesogens are studied with a metabolemeter. The investigated compounds are (-)-2,3,6,7,10,11-hexa-[S-(3-methyl)-n-nonanoyloxy] triphenylene and 2,3,6,7,10,11-hexa-(n- dodecanoyloxy) triphenylene. The weakly first order or practically second order mesomorphic transitions are detected. The pressure-temperature phase diagrams are established. The volume changes for the first order phase transitions are determined.

1. Introduction

There are very few results relating to the volume changes at atmospheric pressure and pressure-temperature dependence of the phase transitions of disc-like mesogens [1]. A ther- mobarometric method has recently been used to study the phase behaviour under pressure of rod-like mesogens - pure [3, 4] or in binary mix- tures [5] - . We use it now to study the polymor- phism under pressure of two disc-like mesogens. Initial results relating to the pressure-tempera- ture dependence of first and practically second order phase transitions are given.

2. Principle and method

The thermobarometric method and the prelimi- nary apparatus - metabolemeter - have been pre- viously described [2]. The principle consists in measuring the pressure of a small sample of mesogen enclosed in a metallic cell as a function of temperature. The simultaneous recording of both data gives the thermobarograms. The ex- perimental procedures and explanations relating

to the interpretation and exploitation of ther- mobarograms are given elsewhere [3, 4].

3. Substances

The studied compounds were two 2,3,6,7, 10,11-hexa-substituted-triphenylenes: (-)-hexa- [S-(3-methyl)-n-nonanoyloxy] ((-)HAT-C10) and hexa-n-dodecanoyloxy (HAT-C12).

( - ) H A T - C l o

HAT-C12

R

R

[[f ,R

* O R: n-C6H13-CH-CH2-C ~

I O - CH3

~ O R : n - C I I H 2 3 - C

"" O -

0378-4363/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

Page 2: Phase behaviour under pressure of two hexasubstituted triphenylenes

632 J.M. Buisine et al. / Phase behaviour under pressure of disc-like mesogens

Table I Literature data for the temperature T(°C) and enthalpy changes AH (kcal mol -~) at the transitions, experimental data for the equilibrium curves (dP/d T)e (bar K - ~ ) of the P - T diagrams and ratio a /X (bar K ~ ) of the expansion coefficient a and isothermal compressibility X and calculated data for the volumes changes at the transitions for the volume changes at the transit ions for (-)-HAT-C10 and HAT-CI2

( - ) -HAT-C j0

K D 2 D l I Ref.

T - 59 - 64.5 - 97.5 - [6] AH 5.9 0.016 1.06 [6] (dP/dT)¢ 29.6 21 26.1 AV 25.09 0.094 4.58 a~ X 16.6 7.1 5.5 3.5

HAT-C12 K D o Dhd Drd I

T - 80 - 93 - 111 - 122.3 - [10] AH 14.2 --0 0.035 0.5.77 [10] (dP/dT)e 26.4 20 14.2 17.6 A V 63.69 0.268 3.46 a~ X 10.5 5.6 6.5 4.7

In (-)HAT-C~0 synthesized by Destrade et al. [6], two columnar mesophases D 1 and D 2 were identified [7], the structure of which has been studied in detail [8]. The phase sequence under atmospheric pressure is

K 59 D 2 64.5 D 1 97.5I.

The enthalpy changes have been measured [6], see table I.

In HAT-C~2, synthesized by Destrade et al, [9], three mesophases were identified [10]. The two mesophases stable at higher temperature are col- umnar phases respectively hexagonal Dhd and rectangular Drd [11]. The phase sequence under atmospheric pressure is:

K 80 D O 93 D h d 111 Drd 122.31.

The enthalpy changes are known [10], see table I. For both compounds no data rel~iting to the

volume changes at atmospheric pressure and pressure-temperature dependence of the trans- formations are known. The quantity of tested matter we used with the rnetabolemeter was 1.2 mg, that gives the optimal pressure changes at the transition [3].

4. Results

In figs. 1 and 2 are given examples of ther- mobarograms obtained on heating (a) and on cooling (b) respectively for (-)HAT-C~0 and HAT-C12. They exhibit at the melting and clear- ing transitions of each compound very clear pressure changes (characteristic of first order phase transitions [2]). But such pressure changes are not detectable for the other transformations; only weak changes of slope relative to [4] the different values of the thermal expansions and isothermal compressibilities appear on both sides of the transitions. In figs. lb and 2b can be seen the supercooling of the D 2 and D o phases, respec- tively. For HAT-C~2 the D 0 - - D h d , D h d - - D r d and Drd-I transitions are more easily detectable on thermobarograms obtained on cooling than on heating (as can be seen for both D0-Dhd and D h d - - D r o transitions in figs. 2a and b).

For samples larger than 1.2mg, the pressure change (AP)T at a first order phase transition is independent of the quantity of tested matter [1]. Moreover, (AP)T is a decreasing function of the molar volume of the studied compound [3]. The lower amount of data for (AP)r detectable with the metabolemeter is at 10bar [12]. Then, for

Page 3: Phase behaviour under pressure of two hexasubstituted triphenylenes

J.M. Buisine et al. / Phase behaviour under pressure o f disc-like mesogens 633

60C

20G

k p (ha,s) a

K+D2

/ /

i / 6O

T(°C)

60C

20C

L p (bars)

a

!/7 / /

8 0 120 160

Dr d

T(°C)

600

400

200

L p

(ba rs) ] ,~

b ~ . 1 D 1

~ 7

D z

40 80 120

T~C)

Fig. 1. Experimental thermobarograms obtained on heating (a) and on cooling (b) relating to the different transitions for (-)-HAT-C,,.

900

600

300

o

k p (bar,) J

Drd • ] b

r(°c) 50 110 170

Fig. 2. Experimental thermobarograms obtained on heating (a) and on cooling (b) relating to the different transitions for HAT-C~2.

Page 4: Phase behaviour under pressure of two hexasubstituted triphenylenes

634 J.M. Buisine et al. / Phase behaviour under pressure o f disc-like mesogens

rod-like mesogens, a pressure change (zlP)r is detectable with the metabolemeter when the en- thalpy changes are bigger than 0.015 kcal mol -~ [3]. For disc-like mesogens, the molar mass is generally three times larger than for rod-like mesogens; so, to induce a pressure change the transformations of disc-like mesogens have to present an enthalpy change greater than 0.05 kcal mol ~. Then at the D2-D 1 transition of (-)HAT-C~0 and at the D0--Dhd, Dhd--Drd transi- tion of HAT-C12 no pressure increment can be detected by our barometric measurements.

For each compound, the pressure-temperature phase diagrams, deduced from thermobarograms are given in figs. 3 and 4. For the D0-Dhd, Dhd--Drd and Drd-I transitions of HAT-C~2, the equilibrium curves are obtained from ther- mobarograms on cooling. Because of a possible supercooling of the sample that can be observed in different temperature ranges and/or errors in the experiments [4] a spread exists of the data that gives less and more imprecise data on the temper- ature of the transition at atmospheric pressure

60

400

200

L p

(b ~)

I

6O

T(°C)

80 100 120

Fig. 3. Pressure-temperature phase diagram for (-)-HAT- Clo.

,00 i

400 t K 200

Fig. 4. Pressure-temperature phase diagram for HAT-C12.

(respectively 90, 109 and 119°C). The values of the slopes (dP/dT)e of the equilibrium curves taken under atmospheric pressure of the P-T diagrams are reported in table I. For all the studied transformations, without the D0-Dhd transition of HAT-C~2, the volume changes can be calculated from the Clapeyron's relation (dP/ dT)e = AH/T AV [13] on using the (dP/dT)e and AH data; they are reported in table I. The ratio a/X of the thermal expansion a and the isother- mal compressibility X, that is the slope out of the transitions of the thermobarograms [2], are also reported in table I.

5. Conclusion

The metabolemeter previously used to detect phase changes under pressure on rod-like mesogens has been used to study first order, weakly first order and practically second order phase transformations in two disc-like mesogens. On using about 1.2mg samples, the transitions between both columnar mesophases of (-)-HAT- Cl0 and between the D 0 mesophase, hexagonal

Page 5: Phase behaviour under pressure of two hexasubstituted triphenylenes

J.M. Buisine et al. / Phase behaviour under pressure of disc-like mesogens 635

and r ec t angu la r c o l u m n a r m e s o p h a s e s o f H A T - C~2 have been d e t e c t e d u n d e r p ressure . F o r bo th c o m p o u n d s , the p r e s s u r e - t e m p e r a t u r e phase d i a g r a m s are given. F o r the t r ans i t ions with en- tha lpy changes , the vo lume changes are calcu-

la ted .

References

[1] S. Chandrasekhar, B.K. Sadashiva, K.A. Suresh, N.V. Madhusudana, J. Kumar, R. Shashidhar and G. Venka- tesh, J. de Phys. 40 C3 (1979) 120; A. Beguin, J. Billard, F. Bonamy, J.M. Buisine, P. Cuvelier and P. Le Barny, Mol. Cryst. Liq. Cryst. 115 (1984) 297.

[2] J.M. Buisine, B. Soulestin and J. Billard, Mol. Cryst. Liq. Cryst. 91 (1983) 115.

[3] J.M. Buisine, B. Soulestin and J. Billard, Mol. Cryst. Liq. Cryst. 93 (1983) 397.

[4] J.M. Buisine, Mol. Cryst. Liq. Cryst. 109 (1984) 143. [5] J.M. Buisine and J. Billard, Mol. Cryst. Liq. Cryst., to

appear. [6] C. Destrade, Nguyen Huu Tinh, J. Malthete and J.

Jacques, Phys. Lett. 79A (1980) 189. [7] J. Malthete, J. Jacques, Nguyen Huu Tinh and C.

Destrade, Nature 298 (1982) 46. [8] A.M. Levelut, P. Oswald, A. Ghanem and J. Malthete,

J. de Phys. 45 (1984) 745. [9] C. Destrade, M.C. Mondon and J. Malthete, J. de Phys.

40-C3 (1979) 17. [10] C. Destrade, M.C. Mondon-Bernaud and Nguyen Huu

Tinh, Mol. Cryst. Liq. Cryst. Lett. 49 (1979) 169. [11] C. Destrade, M.C. Bernaud, M. Gasparoux, A.M.

Levelut and Nguyen Huu Tinh, Proc. Int. Conf. on Liq. Cryst. Bangalore (Heydon, London, 1980) p. 29.

[12] J.M. Buisine, Th~se, Lille (1984). [13] E. Clapeyron, J. Ec. Polytech. 14 (1834) 153.