Document [original]

Smectic Ai

—

Smectic Ad Transition in Binary Mixtures

of Compounds with Strongly Polar Terminal Groups

Ch. Bahr*, G. Heppke*, and R. Shashidhar**

Z. Naturforsch. 40a, 1311-1315 (1985); received October 10, 1985

On the basis of high pressure studies we present here the evidence for the existence of a smectic

—

smectic Aj transition at high pressure in a binary liquid crystal mixture consisting of

terminally polar compounds. A feature of this observation is that both the constituent

compounds are three phenyl ring systems wherein the bridging dipoles are disposed additive

with respect to the polar end group.

1. Introduction

A smectic A liquid crystal can be described as an

orientationally ordered fluid with a one-dimen-

sional mass density wave along the optic axis [1, 2],

This mass density wave can be mathematically re-

presented by expanding the centre of mass density

Q(r) in a Fourier series. It has been pointed out by

Meyer and Lubensky [3] that the symmetry of the

SA phase does not necessarily correspond to the

onset of just one Fourier component and that the

development of the mass density has to be periodic

but may contain higher harmonics of The first

evidence of the coupling between the fundamental

and the second harmonic of the Fourier series has

been obtained by Sigaud et al. [4], While studying

the phase diagram of mixtures of strongly

polar 4-«-pentyl-phenyl-4'-cyanobenzoyloxybenzoate

(DB5CN) and the non-polar compound terephthal-

bis-4-rc-butyaniline (TBBA), they found evidence of

a phase transition between two forms of SA which

are optically identical. Subsequent X-ray studies

showed that one of the A phases was a monolayer

phase (A]) wherein the layer spacing (d) is approxi-

mately equal to the length of the molecule (/), while

the second A phase was found to be a bilayer phase

(A2) with d ^ 21. This discovery of Sigaud et al. [4]

has led to a lot of work on the smectic A phases ex-

hibited by polar systems. At least four different

* Technische Universität Berlin, Sekr. ER 11, D-1000

Berlin 12.

** Raman Research Institute, Bangalore 560 080, India.

Reprint requests to Prof. Dr. G. Heppke, Technische Uni-

versität Berlin, Sekr. ER 11, D-1000 Berlin 12.

forms of the A phase are known, viz., Ai, A2, Ad

and A [5, 6], Ad is a partially bilayer phase with a

layer spacing intermediate between / and 21. Ä is

the antiphase which has the same local order as

A2 but in addition has a modulation of the dipolar

heads within the layer. In addition to the Ai~A2

transition observed by Sigaud et al., four other

types of A—A transitions have already been ob-

served, viz. A]-Ä, Ä-A2, Ad~A] and Ad~A2

[6, 7].

Several important features are common to all the

systems which exhibit the different types of A—A

transitions discussed above:

(i) A—A transitions have so far been observed

only in three-phenyl ring systems possessing a

strongly polar cyano or nitro end group.

(ii) For a single component system to exhibit an

A—A transition, the disposition of the bridging

group dipole closest to the end chain should be such

that its longitudinal component opposes that of the

strongly polar end group.

(iii) In binary systems which show A-A transi-

tions, it has been observed that at least one con-

stituent compound should have a bridging dipole

that satisfies condition (ii).

Recently Shashidhar et al. [8, 9] reported mis-

cibility, high pressure and X-ray studies of binary

mixtures of 4-nitrophenyl-4'-(4"-hexyloxybenzoyl-

oxy)benzoate and 4-nonyloxybenzoyloxy-4'-cyano-

azobenzene. These studies showed clearly that for

certain concentrations these mixtures exhibit two

types of A-A transitions, Ai-Ad and Ad-Ad, the

latter having been observed for the first time. It was

also the only instance of an A-A transition being

observed in a binary system wherein both constituent

compounds have the longitudinal components of the

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1312 Ch. Bahr et al. • Smectic A, - Smectic Ad Transition in Binary Mixtures

linkage dipoles oriented additively with respect to

that of the end group dipole. We present here a

high pressure study of binary mixtures of 4-butyl-

oxybenzoyloxy-4'-nitroazobenzene (or 4 0BNAB)

and 4-nonyloxybenzoyloxy-4'-cyanoazobenzene (or

9 OBCAB), which again shows the existence of an

A| ~transition at high pressures.

2. Experimental

a) Compounds. 40BNAB has been newly synthe-

sized by us [10], whereas 9 OBCAB belongs to the

well known nOBCAB series [11], amongst the first

reentrant mesogens to be synthesized:

4-butyloxybenzoyloxy-4-nitroazobenzene (4 OBNAB)

K 150 SA, 160 N 284

4-nonyloxybenzoyloxy-4-cyanoazobenzene (9 OBCAB)

CgHig

K 93 (SA1 71) N 116 S. . 213 N 250 1

A1 rc AQ

b) X-ray studies. The X-ray experiments were per-

formed on magnetically oriented samples in sealed

Lindemann glass capillaries (0.5 mm diameter) [12].

Copper K^ radiation reflected from a bent quartz

monochromator (Carl Zeiss) was used in conjunc-

tion with a flat film for recording the diffraction

pattern. The temperature of the sample was main-

tained constant to within ±0.1 °C during any

exposure, a typical exposure time being about

20 minutes. The relative accuracy of the layer

spacing determination is about ± 0.01 nm or better.

c) High pressure studies. An optical high pressure

cell with sapphire window was used for the ex-

periments. This being a direct pressure transmitting

cell, the sample which was sandwiched between two

optically polished sapphire cylinders was isolated

from the pressure transmitting medium by an

elastomer material (fluran). The transition temper-

atures were detected by the optical transmission

technique. Experiments were always conducted

along isobars, i.e. the pressure was kept constant

and the temperature of the sample varied at a

controlled rate. The pressure was measured using a

Heise gauge to an accuracy of ± 1.5 bar, and the

temperature to an accuracy of ±0.1°C. Further

details of the pressure set up are given elsewhere

[13.14].

3. Results and Discussions

The variation of the layer spacing (d) with tem-

perature is given in Figs. 1 and 2 for 4 OBNAB and

9 OBCAB, respectively. For 4 OBNAB it is seen

that d has a constant value of 2.58 nm throughout

the A phase. The length of the 4 OBNAB molecule

measured in its most extended configuration using a

Dreiding model is 2.6 nm. Thus the A phase of this

compound can be characterized as Ai. The A

Fig. 1. Variation of the layer spacing (d) with temperature

in the smectic A phase of 4 OBNAB.

Fig. 2. Variation of the layer spacing (d) with temperature

in the monolayer smectic A (A,), reentrant nematic (Nre)

and partially bilayer smectic A (Ad) phases of 9 OBCAB.

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Ch. Bahr et al. • Smectic A, - Smectic Ad Transition in Binary Mixtures 1313

phases in 9 OBCAB have already been established

as A, and Ad. The temperature variation of d in this

compound (Fig. 3) shows that d is significantly

temperature dependent in the Ad phase while it is

temperature independent in the Ai phase [11].

The temperature-concentration diagram of binary

mixtures of 4 OBNAB and 9 OBCAB is given in

Figure 3. It is seen that addition of 4 OBNAB

quickly destabilises the reentrant nematic phase

which gets completely bounded in the t - x plane.

For A' = 0.8 (x being the mole fraction of 9 OBCAB

in the mixture) there is no reentrant nematic phase

and the three A phases (viz., the Ad and A! phases

of 9 OBCAB and the A, phase of 4 OBNAB) appear

to be isomorphous. It may be recalled that our

earlier pressure studies on 6 ONNPBB-9 OBCAB

mixtures [8] clearly demonstrated that the signature

of the A-A transitions can be seen dramatically on

the A-N phase boundary in the p—t plane. We

have undertaken a pressure study of the A—N

phase transitions in 4 OBNAB. 9 OBCAB and their

binary mixtures.

The p — t diagram of 4 OBNAB is given in Fig. 4

while that of 9 OBCAB is given in Figure 5. In the

case of 4 OBNAB the Ai-N phase boundary is

straight throughout, the phase getting bounded only

because of the crystallization line intersecting the

A,-N line. The p - t diagram of 9 OBCAB (Fig. 5)

shows the marked difference in the pressure be-

haviour of Ai and Ad phases. The Ad-N boundary

has the expected elliptic shape, characteristic of the

phase diagram of reentrant mesogens. On the other

hand, the Ai-Nre boundary is a straight line

throughout.

It is also easy to see by extrapolation to negative

pressures that the A|-Nre and the Nre-Ad phase

line would intersect at a pressure of about - 1.7 kbar

(Figure 5). This would lead to a direct Ad-A]

transition at this pressure. With decrease of x the

pressure at which the Aj-Ad transition occurs is

expected to shift to higher values and for x = 0.89

(the concentration at which the reentrant phase

coexists at 1 bar) this transition would occur at

atmospheric pressure. Extending the concentration

to even lower values, the x = 0.80 mixture shows

that the presence of this transition manifests itself

dramatically on the A-N phase boundary (Fig. 6)

as an abrupt change in the slope at 0.3 kbar. An

exactly similar behaviour is seen in the p—t dia-

gram of the x = 0.76 mixture also (Figure 7). Thus,

290

°C

-I

270

250 -

230 -

•

210

190 -

t 170 -y7 (Ad)

150

130

110

90 - (A,)

i i , i

(A,)

0.0 ,2

4OBNAB

X ,6 5 1.0

9 OBCAB

Fig. 3. Temperature-concentration diagram of binary mix-

tures of 4 OBNAB and 9 OBCAB. X is the mole fraction of

9 OBCAB in the mixture.

Fig. 4. Pressure-temperature diagram of 4 OBNAB.

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1314 Ch. Bahr et al. • Smectic A, - Smectic Ad Transition in Binary Mixtures

Fig. 5. Pressure-temperature diagram of 9 OBCAB. Fig. 7. Pressure-temperature diagram of the .x = 0.76 mix-

ture.

although the A)-Ad transition is itself not ob-

servable at high pressure by the optical transmission

method, its signature on the A-N phase boundary

is clearly seen at high pressure. It should also be

pointed out that the point at which the A]-Ad line

meets the A-N phase boundary should be a bi-

critical point. This is because by symmetry arguments

[15] the Ai~Ad transition should be at least weakly

first order while the A]-N and Ad-N transitions

are expected to be second order because of the low

rAN/rNI values [16]. In fact, the p-t diagram for the

x = 0.80 and 0.76 mixtures resemble the topology of

a phase diagram exhibiting known bicritical points

[17], Further studies are in progress to probe the

A,-Ad transition at atmospheric pressure as well as

at high pressures using X-ray diffraction experiments.

Acknowledgements

150 170 190 °C 210

The financial support of the Gesellschaft von

5re 6' Pressure"temperature dia§ram of the * = a8° mix" Freunden der Technischen Universität Berlin is

acknowledged.

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1315 Ch. Bahr et al. • Smectic A, - Smectic Ad Transition in Binary Mixtures

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