High pressure studies on reentrant nematogens

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High pressure studies on reentrant nematogens

A.N. Kalkura, R. Shashidhar, N. Subramanya Raj Urs

To cite this version:

A.N. Kalkura, R. Shashidhar, N. Subramanya Raj Urs. High pressure studies on reentrant nematogens.

Journal de Physique, 1983, 44 (1), pp.51-55. �10.1051/jphys:0198300440105100�. �jpa-00209573�

High ^{pressure studies on reentrant} nematogens

A. N. Kalkura, ^R. Shashidhar and N. Subramanya Raj ^Urs

Raman Research Institute, Bangalore ⁵⁶⁰ 080, ^India (Rep ^{le 12} juillet 1982, accepté ^{le 8} septembre 1982)

Résumé. 2014 Nous étudions sous pression ^huit composés purs qui présentent ^{chacun une} phase nématique ^réentrante

à la pression atmosphérique. ^{Dans tous les} cas, la frontière smectique A-nématique ^{a une forme} elliptique. ^{A haute} pression, pour deux des substances, la transition smectique A-nématique réentrante change de caractère (de ^mono- tropique ^à énantiotropique). ^La pression ^maximale de stabilité de la phase smectique ^{A est} liée à l’étendue de la

phase nématique ^à pression atmosphérique.

Abstract

We report here pressure studies ^on eight pure compounds all of which exhibit the reentrant nematic

phase ^at atmospheric pressure. In all ^cases the smectic A-nematic phase boundary ^{has an} elliptic shape. ^{For two}

of the substances, the smectic A-reentrant nematic transition changes its character from monotropic ^{to enantio-} tropic ^at high pressures. The maximum pressure of stability of the smectic A phase ^{is seen to} be related to the range of the nematic phase ^at atmospheric pressure.

Classification

Physics ^Abstracts

61. 30E

1. Introduction

Reentrant nematic behaviour

was first observed in mixtures at atmospheric pres-

sure [1] and later in _pure compounds ^at high ^pres-

sure [2, 3]. Subsequently this behaviour ^was also observed in _pure compounds ^at atmospheric pres-

sure [4, 5] and this has led to a variety ^of experimental

studies. We report here the pressure studies ^on eight

pure substances all of which exhibit the ^reentrant nematic phase ^at atmospheric pressure.

2. The compounds. - ^{We have} studied the eleventh and twelfth members of 4 different homologous ^series.

All these compounds, which show either ^a monotropic

or enantiotropic ^{reentrant nematic} phase, ^{have been} synthesized ^{in our} chemistry laboratory (see ^refe-

rences [61 ^and [7] ^{for details} regarding the methods of synthesis). ^{The molecular} structures of these

compounds ^are given ^{in table I,} while table II gives

their transition temperatures ^{and the heats} of transi- tions as determined by Perkin-EImer DSC-2 diffe- rential scanning calorimeter. All these compounds

have three phenyl rings, ^{a lateral} methyl ^or methoxy

substituent in the middle phenyl ring ^{and a} cyano end group. Two of the compounds, viz., ^{11 CPMCB}

and 11 CPMaMCB show enantiotropic ^{smectic A-}

reentrant nematic (A-RN) transitions while in all the other ^cases the reentrant nematic phase appears

as a monotropic phase. The heat associated with

the A-RN transition has been given only ^{for 4 com-} pounds, since for the other 4 substances the heats

are too small to be determined with sufficient accu-

racy.

3. ExperimentaL

^The phase transitions were

detected by ^the optical transmission technique. ^An optical high pressure cell equipped ^with sapphire

windows ^was used for the experiments. ^The liquid crystalline sample ^was sandwiched between two

optically polished sapphire cylinders and enclosed in a

fluron tube. The transmitted light intensity ^{was moni-}

tored by ^a photo cell. The experiments ^were always

conducted along isobars, i.e., ^to determine the tran- sition temperature ^at any pressure, the pressure ^was

kept ^{constant and the} temperature ^{varied at a rate} of 1-2 OC/min. ^The temperature ^at which there was an

abrupt change in the transmitted light intensity ^was

taken as the transition temperature. ^{There was howe-}

ver a difficulty in that all the compounds ^{have a natu-}

ral tendency ^for homeotropic alignment which makes the detection of the transition by ^this technique ^rather

difficult To avoid such an alignment ^{SiO was} deposit-

ed at an oblique angle ^on the surfaces of the sapphire cylinders sandwiching ^the sample. ^{Pressures were}

maintained and measured to an accuracy of ± ^{10 bars}

while the temperatures could be read to better than

± ^{0.025 .} Further details of the experimental ^set

up ^are described elsewhere [8].

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198300440105100

52

Table I.

The compounds and their molecular struc- ture,-

IV 4-cyanophenyl-3’-methoxy-4’44"-n-unde- cyloxy ^or dodecyloxy-a-methyl ^cinna- moyloxy) ^benzoate (11 ^CPMeOaMCB

or 12 CPMeOaMCB).

4. Results and discussion.

The P-T diagrams ^of

the compounds ^{studied are} given ⁱⁿ figures ^{1-8. The} melting ^{as well as the} nematic-isotropic (N-I) phase

boundaries ^are straight lines while the smectic A- nematic (A-N) phase boundary ^is elliptic ⁱⁿ shape ⁱⁿ

all cases. Pm, the maximum _pressure of existence of the smectic A phase, ^are listed in table III.

4.1 MOLECULAR STRUCTURE AND Pm. - In ^the

case of compounds of series I (nCPMBB), ^{both the} bridging groups ^are carbonyloxy moeities and there is only ^{one lateral} group, viz., ^the methyl group attach- ed to the middle phenyl ring. ^The stability ^{of the A} phase ^{for 11} CPMBB is rather low (Pm

^{0.55 kbar,} Fig. 1). ^{The addition} of another CH2 ^{end group}

enhances P. considerably, Pm for 12 CPMBB being

2.50 kbar (Fig. 2). ^It appears therefore that P. ^is

very sensitive to changes ^{in molecular} length. ^The

same behaviour is seen for all the series as seen in table III.

Table II.

Transition temperatures ^{and heats} of

transition of ^the compounds.

( ) ^Denotes monotropic transition.

(*) The transition is _very weak, ^{no accurate} estimation of eH can

be made.

K-crystal, ^A-smectic A, N-nematic, RN-reentrant nematic.

Table III.

Pm of ^{the reentrant} nematogens.

Fig. ^{1. - P-T} diagram of 11 CPMBB. The dotted line indicates that the transition is monotropic.

Fig. ^2.

^P-T diagram of 12 CPMBB.

Fig. ^3.

^P-T diagram ^{of 11 CPMCB.}

The only difference between the compounds ^of

series II (nCPMCB) and those of series I is that the former _possess an additional ethylene (CH=CH) linkage ^{in one of the} bridging groups. This ^{seems to} decrease the Pm values which are 0.29 kbar and 1.05 kbar for the eleventh and twelfth homologues respectively. ^{It must} also be mentioned that the A-RN

Fig. ^4.

^P-T diagram of 12 CPMCB.

Fig. ^5.

^P-T diagram of 11 CPMaMCB.

Fig. ^6.

^P-T diagram of 12 CPMaMCB.

Fig. ^7.

^P-T diagram of 11 CPMeOaMCB.

54

Fig. ^8.

^P-T diagram of 12 CPMeOaMCB.

transition in 12 CPMCB is monotropic ^at atmospheric

pressure but becomes enantiotropic ^for pressures

higher than 0.44 kbar. This is the first observation of such a change of character involving ^{a reentrant}

nematic phase, although ^a similar behaviour has been observed before for the nematic (or cholesteric) phase [9, 10].

In the case of compounds of series III

(nCPMaMCB), ^the a-hydrogen ^{has been} replaced by

a methyl group. This does not seem to affect the Pm

values _very much. Also, ^{as in the case of 12} CPMCB,

the twelfth homologue of this series shows a mono-

tropic-enantiotropic change ^{in nature of the A-RN}

transition at a pressure of 0.57 kbar. The compounds

of series IV (nCPMeOaMCB) ^{have a lateral} methoxy

group attached to the middle phenyl ring ^{instead of}

a methyl group of the compounds ^of series III. This appears ^to increase the stability of the smectic A phase considerably ^{as seen} by ^the Pm values, viz., ^{1.14 kbar} for 11 CPMeaMCB and 3.45 kbar for 12 CPMeOaMCB which are much higher ^{than the} Pm ^values (0.19 ^{kbar and 1.35} kbar) ^{of the} correspond- ing members of series III.

4.2 RELATION ^BETWEEN Pm AND THE NEMATIC RANGE.

It has been pointed ^out by ^{Cladis et al.} [3, 11] ^{that for the} substances studied by ^them Pm ^exhi-

bits a monotonic dependence ^{on the McMillan num-}

ber [12]. ^{For the} compounds ^studied by ^{us, we have} attempted ^{to relate} P. ^{with the range of the nematic} phase (R)

TNI - TAN, at ^{1 bar.} Figure ^{9 shows a} plot ^of log Pro ^{versus R for all the 8} compounds. ^The

data points ^{fall on a} straight ^{line so that} they ^{can be} represented by ^the expression

The constants Po and m evaluated from a least _squares

Fig. ^9.

^Plot of maximum pressure of smectic stability (Pm) ^versus the range of the nematic phase (TN, - TAN)

at 1 bar. The data points 1, 2, 3, ^{etc. on} line A are for three

phenyl ring compounds ^{listed in table III. The data} points ^on

line B are for two phenyl ring compounds, viz., ^{V-8 OCB} [3],

V-CBOOA [3], ·-8 OCB/6 OCB mixtures [13, ^{14], A-} CBOOA/HBAB ^mixtures [3], D-CBNA/CBHA ^mixtures [3], 0-8 OCB/40.8 ^mixtures [15].

fit of the data to a straight ^{line are} 3 769 bars and

0.071/°C. This result implies that with increasing

nematic _range, i.e., ^with decreasing ^smectic stability, Pm ^{should decrease.}

Since P-T data are known for several other reen- trant nematogens, viz., ^two pure compounds (CBOOA

and 8 OCB) [3] ^and many binary ^mixtures

⁸ OCB/

6 OCB [13,14], CBOOA/HBAB [3], CBNA/CBHA [3]

and 8 OCB/40.8 [15], ^it is worthwhile to see if such

a relation is valid in these cases also. The plot ^of Pm

versus (TNI - T AN) 1 bar for these substances is also shown in figure 9. These data again ^{fall on a} straight

line (line B) ^{with a} different slope. ^{The constants for}

this line are Po

^{14 OSO bars and m}

O.l40/oC.

It is interesting ^{to note} that all the substances whose

Pm ^{data fall on} line B have two phenyl rings ^{while all}

the compounds whose data fall on line A have three benzene rings. We therefore tentatively ^conclude

that for a reentrant nematogen P. ^is uniquely ^related

to the nematic _range and hence the stability ^{of the}

nematic ordering, ^{the constants of} proportionality being dependent ^on the molecular structure. Also,

the constant Po ^for the lines A and B indicate that the

stability of the smectic A phase ⁱⁿ compounds ^with

two phenyl rings ^is higher ^{than the smectic} stability

of the three phenyl ring compounds. ^{Hence the for-}

mation of the reentrant nematic phase ^{is more favour-}

ed in compounds with three benzene rings.

The shape ^{of the A-N} phase boundary ^{for 8 OCB}

has been analysed by ^Clark [16] ^and independently by Klug ^and Whalley [17] who have also evaluated the various thermodynamic parameters determining

the phase boundary. Also, ^{Pershan and Prost} [18]

have shown that the smectic A - reentrant nematic

phase transition follows from the Landau theory ^{if one}

assumes the existence of an optimum density ^for

smectic ordering. ^Recent high resolution X-ray expe- riments of Kortan et al. [19] strongly support ^this concept ^{of «} optimum density ^{». We} have ascertained that the fit of an ellipse ^to the data for all the com-

pounds ^studied by ^{us is} very good. ^Detailed analysis

of the A-N phase boundaries of the various substances will be published ^elsewhere.

Acknowledgments.

The authors are highly ^indebt-

ed to Professor S. Chandrasekhar for his keen interest

throughout this work and for _many illuminating

discussions. Thanks are also due to Dr. B. R. Ratna for some useful discussions and to Mr. K. Vasudevan for his help ^{in the} design ^and fabrication of the optical high pressure cell.

References

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