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A new type of smectic A phase with long range modulation in the layers

G. Sigaud, F. Hardouin, M. F. Achard, A.M. Levelut

To cite this version:

G. Sigaud, F. Hardouin, M. F. Achard, A.M. Levelut. A new type of smectic A phase with long range modulation in the layers. Journal de Physique, 1981, 42 (1), pp.107-111.

�10.1051/jphys:01981004201010700�. �jpa-00208978�

A

new

type ^{of smectic A} phase ^with long range modulation ^{in the} layers

G.

Sigaud,

^F.

Hardouin,

^{M. F.} Achard and A. M. Levelut

(*)

Centre de Recherches Paul-Pascal, ^Domaine Universitaire, ^{33405 Talence} Cedex, France (Reçu ^le 16 juillet 1980, accepté ^{le 25} septembre 1980)

Résumé. ²⁰¹⁴ Nous avons révélé, ^{dans un} diagramme ^binaire isobare, l’existence d’une

phase smectique

^intermé-

diaire entre une

phase

smectique A monomoléculaire et une

phase

smectique bimoléculaire. Plusieurs

techniques

ont été mises en 0153uvre afin de prouver l’existence de cette nouvelle

phase.

^A

partir

^de l’analyse structurale par RX

une

description

^{de cette} phase ^{fluide est} proposée

qui

^nous suggère

l’appellation

^«

antiphase smectique

^{A »}

(SÃ).

Abstract. ²⁰¹⁴ In a

binary

^isobaric

diagram

there appears ^an intermediate smectic

phase

^{between a} monomolecular smectic A

phase

^{and a} bimolecular one. By ^means of several

techniques

evidence is

given

^{for this new}

mesophase

with a

liquid-like

short range

ordering.

^Some considerations about the

long

^{range structure} of this novel fluid

mesophase

^are put forward from X-ray

investigations

^{and lead us to name it as an} « SA

antiphase » (SÃ).

Classification

Physics ^Abstracts

61.30 ^- 64.70E

1. Introduction. ^- Two years ago, the existence of an

unexpected

transition between two

types

^of smectic A

phases

^was discovered

by

^{us in a}

binary

mixture constituted of a

cyanoderivative

(DB

^{5 for}

short)

^and

T.B.B.A.,

The methods

employed

^to detect this transition were

the D.S.C. or

magnetic susceptibility

measurements

[2].

But

optical microscopy

^{failed to} differentiate these

two

phases. Subsequent X-ray experiments [3]

^showed

that this

phase change corresponds

^to the transforma- tion of a

high temperature SA

with monomolecular

layer spacing

^{into a low}

temperature

^{one with bimo-} lecular

layer spacing (SA2).

We recall

briefly that,

in the nematic

phase,

^two

diffuse

spots

^are observable with commensurate wave vectors q2

= 2 qi.

^{The one}

corresponding

^to

d ⁼ 2

nlq2

¹

(d layer spacing, 1

^molecular

length)

transforms first into ^a

Bragg spot,

^{and the}

SA - Sp2

transition consists of the condensation of the second diffuse

spot

located at d ⁼ 2

nlql -

^{2 1.}

As a consequence of recent results

[4],

^{the mono-}

molecular-bimolecular

SA

transition is not the

only type

^of

SA-SA2 phase change.

^{One can}

suggest

^{a more}

general

définition for this transition to a bimolecular

SA2 . phase :

^that

is,

^a commensurate

locking

^{of twô}

wave vectors which could be commensurate or not, in the

higher

^{smectic A}

phase

and in the nematic one.

Moreover,

^some studies of various

cyanoderivatives

with three

phenyl rings

^{in the}

rigid

^{core have shown}

that different anomalies

of periodicity

^can be observed in their smectic A

phases [4, 5, 6]. Especially,

^the

X-ray

patterns ^{of some of them}

present

^a

Bragg spot corresponding

^to

layers

with d - 1 and a diffuse

scattering generally split

^{out of the Z axis}

(i.e.

^the

perpendicular

^{to the}

layers

in the real

space)

^{the wave}

vector of which is not commensurate

along

^{Z with}

the one of the

long

range modulation

q2 =F 2

qiz,

But,

^{as the}

temperature decreases,

this diffuse

spot

never transforms into a

Bragg

reflection and a

locking

process is never noticed. We shall see that these events occur

by mixing

^{the DB 5 with a}

compound

^{of this}

last

class,

and in that _manner, one induces a transition to a new

exciting mesophase

^with

SA ordering.

2.

Expérimental

^{results. -}

First,

^we

present

^{in this} section some

physical

measurements

demonstrating

the existence of the new

phase. Then,

^{a structural}

analysis by X-ray

diffraction will be

reported.

(*) Laboratoire de Physique ^des Solides, ^Bât. 510, Université Paris-Sud, ⁹¹⁴⁰⁵ Orsay, ^France.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01981004201010700

108

We have studied mixtures of DB 5 with the

following compound :

1

This

compound presents

^a

N-SA

transition at

197°C,

but also a

SA-SB

^{one at 137 OC}

[7].

2. 1 DIFFERENTIAL SCANNING CALORIMETRY. - The thermal

analysis

^of

samples

^with

increasing

^molar

fraction of C 5 STILBENE in DB 5 was

performed.

In this _manner, the

binary diagram

^{of the}

figure

¹

has been obtained from the

temperatures correspond- ing

^to the various transition

enthalpies. Beyond

^a

Fig. ^{1. -} Binary ^isobaric diagram (1 atm.) between DB 5 (on left)

and C 5 STILBENE (on right).

molar fraction of about 40 moles

%

of C 5 STILBENE in DB

5,

^a

striking

feature is the appearance of ah intermediate

phase

^between

SA

^and

SA2 obviously

revealed

by

^{a new heat}

peak

^on the D.S.C.

recordings (Fig. 2).

^{One can notice the weakness of this new}

peak considering

^our

high sensitivity :

latent heat

10 mcal. g-l (see Fig. 2).

2.2 MAGNETIC SUSCEPTIBILITY MEASUREMENTS. -

The behaviour of the

previous

mixture is confirmed

by

the thermal evolution of the

magnetic suscepti- bility

measured in a direction

parallel

^{to the}

magnetic

field. The

figure

3 exhibits three

tiny

enhancements in

A x

^at the transition

temperatures,

all indicative of

a

slight improvement

of the orientational order

by decreasing temperature. Thus,

the molecules remain

Fig. ^{2. -} Cooling profile of the D.S.C. thermogram ^{of a 46} m% ^{C 5}

STILBENE in DB 5 mixture.

Fig. ^{3. - Thermal variation of the} diamagnetic anisotropy ^{for a}

46 m % C 5 STILBENE in DB 5 mixture.

on average

parallel

^{to the}

magnetic

field direction in the new

phase.

2.3 MICROSCOPIC OBSERVATIONS. ^-

Contrary

^to

the usual smectic-smectic transitions

(e.g. SA-SC, SA-SB...)

the observations

performed,

^{between a}

glass

slide and a cover

slip

^{in the case} of the DB 5-C 5 STILBENE

mixtures,

show that there is no textural difference under these conditions for the three smectic

phases (either

^with

homeotropic alignment

^{or with}

focal conic

textures).

But,

^{unlike the case of the}

SA-SA2

transition for which

nothing

^was

visible,

^the succession of textures

depicted

ⁱⁿ

figure

^{4 can} be observed on a free surface.

We

specify

that these

photographs

^were taken between crossed

polarizer

^and

analyser

^{on the free surface of}

a low thickness

droplet

^{which was} very

strongly

Fig. ^{4. - Thermal} evolution of the microscopic aspect ^{of a} droplet

free surface of a 46 m % C 5 STILBENE in DB 5 mixture ( x 300) : a) High temperature ^{smectic A at} 130 °C ; b) Intermediate phase ^at 122 °C ; c) ^{Transient texture} between the intermediate phase ^and SA2 ^at 118 °C ; d) SA2 phase ^{at 113 °C.}

illuminated. The bulk orientation is rather

good homeotropic,

^{as evidenced}

by

the small number of defects observable in the

high temperature SA phase (plate 4a).

On

decreasing

^the

temperature,

^{when one reaches} the intermediate

phase, specific

^defects

looking

^like

iso-level lines arise

(plate 4b). Cooling

^down

further,

a transient texture appears ^{at a}

temperature

^corres-

ponding

^to the third latent heat

peak (plate 4c). Then,

this texture vanishes

together

^{with the}

line-defects, restoring

^a

picture

^{similar to the one of the}

high temperature SA phase (plate 4d).

Increasing

^{now the}

temperature

^{from the}

SA2 phase,

^{one can}

successively

^observe

again

^{the tran-}

sient texture, the iso-level line defects in the ^new

phase

which are erased at the

temperature

of transition to the

high temperature SA phase.

The

study

of these defects is not

yet completed,

^but

should be of a

great

interest in the

understanding

^of

the intermediate

phase.

Thus,

^we have confirmed

by optical microscopy

the

general

form of the transition lines of the

binary diagram (Fig. 1).

^In

addition,

the observation of smectic films

(suspended

^{in a}

hole) suggested

^{that the}

viscosity

of the intermediate

phase

^was

larger

^than

the one of

high

^{and low}

temperature SA phases.

^This

has led us to

perform viscosity

measurements.

2. 4 VISCOSITY. - The thermal evolution

of’

the

apparent viscosity (measured by using

^{a ROTOVISCO}

HAAKE with a

plane-cone system)

^is

given

ⁱⁿ

figure 5

for a 42 moles

%

of C 5 STILBENE in DB 5. A

change

in the

slope

^{indicates the}

N-SA

transition.

Then,

^ah

important pretransitional

effect takes

place

^{in the}

high temperature SA phase, leading

^{to a rather}

high

viscosity

in the intermediate

phase (with

^{some tur-}

bulences). Finally

^the

viscosity

^decreases

sharply

when the

SA2 phase

is reached.

Thus,

these results

support

^the

qualitative microscopic observations,

^and

we shall see further here how such a behaviour can

be

explained

^with

respect

^{to the structure of the new}

phase.

Additional information is

given by

^{the varia-}

Fig. ^{5. -} Thermal evolution of the apparent viscosity ^{for a 42 m} %

C 5 STILBENE in DB 5 mixture.

110

Fig. ^{6. -} Thermal evolution of the apparent viscosity ^{for a 18 m} %

T.B.B.A. in DB 5.

tion of the

apparent viscosity through

^the

SA-SA2 transition,

^{such as the ones induced}

by mixing

^the

DB 5 with T.B.B.A.

(Fig. 6).

^{One can observe a}

slight

variation of the

slope

^{at the}

SA-SA2 change,

^{but the}

viscosity

^{in no} way

presents

^the

pretransitional

effects or the

discontinuity

observed in

figure

^{5 when}

the intermediate

phase

^exists.

2.5 X-RAY DIFFRACTION INVESTIGATIONS. ^- The X- ray

patterns

of these different

phases

^{are obtained}

from

samples

^{oriented in} the nematic

phase by

^a

magnetic

field. The

X-ray

^{beam is}

perpendicular

^to

the

direction of the field

(thus perpendicular

^{to the}

director of the

mesophases

^as

just

^{seen in}

2.2).

^The

mixture considered is

composed

of 46 moles _per cent of C 5 STILBENE in DB 5 : the intermediate

phase

is stable between 127 °C and 118 °C

(Fig. 1).

In the

high temperature SA phase (Fig. 7a)

^{two mass}

density

modulations _appear at small

angles

^with

their wave vectors, ^{such that}

q2 1= 2 ql(q2lql - 1.8)

as

yet

observed and elsewhere described for the pure C 5 STILBENE

compound [4, 7].

The diffuse

spot with qi

^{wave vector}

(2 n/qi -

⁴⁷

À)

^{is due to a}

highly damped

modulation. The other modulation connected

to the

layers gives

^{rise to one intense} first order

Bragg

reflection 001 indicative of a

layer spacing

^{d close to}

the average molecular

length 1 (d

^{= 2}

nlq2 ’" 1

^{= 26.8}

Â).

Decreasing

^the

temperature

in this smectic A

phase,

we observe that the diffuse

spot (ql) slightly splits

into two diffuse

spots

^{located out of}

the 001 )

^row

(Z axis)

^{with a}

large angular spread.

^{Such a behaviour}

was

reported already

in the smectic A

phase

^{of various}

compounds,

among which is pure C 5 STILBENE

[4, 5, 6].

Now, going

down in the

temperature

range ^cor-

responding

^{to the new smectic}

phase

the

X-ray patterns

exhibit the

following striking

features

(Fig. 7b) :

-

First,

^{the two diffuse}

spots

^located

off the 001 )

row seem now condensed. We

point

^{out that the}

Fig. ^{7. -} X-ray diffraction photographs ^{for a 46 m} % ^{C 5 STIL-}

BENE in DB 5 mixture : a) High temperature SA phase ^{at 140} °C ; b) Intermediate phase ^at 121 °C ; c) SA2 phase ^{at 112 °C.}

001

Bragg spot

^remain

unchanged

^{with d = 2}

nlq2 -

¹

and this is

incompatible

^{with a}

long

range

Sc

^arran-

gement.

-

Second,

in the limit of our

experimental

^accu-

racy, the two wave vectors ql and _q2 ^can be considered

commensurate

along

^{Z = in}

fact,

^{the wave vector} qiz has shifted in order that

q2/qiz

^{= 2}

(q2

^{did not}

move).

-

Third,

the diffuse broad

scattering

^at

large angles

^{confirms that the mass centres of the molecules}

are still

randomly

distributed within the

layers :

^the

short _range

ordering

^is

liquid-like

^{in the}

layers.

In other

words,

^a

long

range monomolecular modu- lation

along

^Z remains in this

phase.

In

addition,

^the ql ^{wave vector} loses its unidi- mensional form and

long

range fluctuations with a

large wavelength (2 n/Qlx

> 130

Á)

^take

place

^{in the}

layers. ^However,

the fluid local order is

kept

^{in this}

intermediate

phase.

Cooling

down towards the low

temperature SA

phase,

the lateral modulation mode

collapses (i.e.

^the

corresponding wavelength

^{tends to}

infinity)

^and

fmally

^{a collinear} commensurate

locking

^{of the two}

wavelengths

^{’on the Z axis occurs at T =118}

OC, indicating

^{that the}

phase changes

^{into a} smectic A2 : i.e. one bimolecular modulation with d ⁼ 2

nlqi -

^{2l 1}

and _q2 ⁼ _{2 ql}

(Fig. 7c).

3. Discussion. - This novel _sequence of structural

changes

has led J. Prost to propose ^an

explanation

within his recent

theory

of the nematic and smectic A

phases

constituted of

polar

^molecules

[8, 9]. According

to it such a frustrated

system prefers

^to

keep

^one

underlying

modulation in the

longitudinal

^direction

(director)

^{and to}

develop

^a

long

range

periodic

^struc-

ture in the

layers,

rather than to

accept

^{two incom-}

mensurate

density

^{wave vectors collinear to the}

director.

Nevertheless,

^{from an}

experimental point

of view the structure

resulting

^{from the transverse}

modulation cannot be

yet fully

^described

by X-ray investigations

^alone.

First,

^{we cannot} decide between a one or a two

dimensional modulation

lying

^{in the}

layers :

^our

X-ray pattern

^{has a}

cylindrical symmetry

^{around the}

magnetic

^{field direction in such a manner that the}

four spots ^at ± qix,

± q, are

ⁱⁿ fact the intersection of two

rings

with the Ewald

sphere.

^{Moreover no}

harmonics _{in qix} are seen and thus we cannot

assign

a lattice for the

in-layer

modulation.

Second,

^two models for the modulation can be considered : a pure

dipolar

modulation in which

thç

centre of mass of the molecules remains on

planes

while the number of _up or down molecules in a

layer

varies

periodically

^{in the x} direction. Correlations between

layers

^are of antiferroelectric

type.

^Another model in which the molecular centres of mass are

located ^on undulated surfaces would

imply

reflections

at

0,

qi ; qlx, 2 q l ... etc., which ^{are in} fact not observed.

The first model fits well with our

X-ray

observations since we

expect only

reflections for

0,

2 nq 1 and

(2 m

⁺

1)

q 1 x,

(2 n

⁺

1 ) ql .

^{This is the}

reciprocal

space characteristic of an

antiphase [10]

^{and such an}

antiphase

is encountered in the ^case of

quenched

ordered

alloys

^{such as AuCu or}

AuCu3.

^{In our case}

the

density

^of up and down

dipoles

^can vary either

Fig. ^{8. -} Tentative two-dimensional model for the fluid antiphase (SÂ).

abruptly

^or

slowly,

^as

represented

^on

figure

^{8. The}

first case can be described

by

^a

periodic

^{structure of}

domain walls or solitons

[11].

In the second case a

sinusoidal variation of the

dipolar density

^{in the}

x direction would _suppress the

high

q 1 x orders of reflection. An intermediate model in which small

displacements

^{of the centre of mass are}

coupled

^to

the

dipolar

modulation cannot be

completely excluded,

but such a model seems

unlikely.

Nevertheless,

^we propose ^to index this new

phase SÂ (SA antiphase)

because the

long

range order remains of smectic A

type,

and because of the existence of a

modulation in the

layers (a

tentative two-dimensional model of this sort of fluid

antiphase,

^is

depicted

ⁱⁿ

figure 8).

^{One can} better understand now the

larger viscosity

^{of the}

SÂ phase

^{as a result of} its bi-or tridi- mensional

(i.e. crystalline)

character.

Finally,

^{one can}

suggest

^{that the} iso-level lines observed

by microscopy

would be the consequence of the

in-layer

structures,

just

^above

mentioned,

^{and due to} connections bet-

ween monodomains with different orientations of the

wave vector modulation.

Acknowledgments.

^{- We are indebted to M. Jous-}

sot-Dubien and Dr.

Nguyen

Huu Tinh for the _syn- thesis of the

compounds.

References

[1] SIGAUD, G., HARDOUIN, F., ACHARD, ^M. F., GASPAROUX, H., J. Physique Colloq. ⁴⁰ (1979) ^{C 3-356.}

[2] SIGAUD, G., HARDOUIN, F., ACHARD, ^M. F., Phys. ^{Lett. 72A} (1979) ^24.

[3] HARDOUIN, F., LEVELUT, ^A. M., BENATTAR, ^J. J., SIGAUD, G., Solid State Commun. 33 (1980) ^337.

[4] ^{HARDOUIN, F.,} LEVELUT, ^A. M., SIGAUD, G., To be published

and presented ^to the 8th International Liquid Crystal Confe-

rence, Kyoto, ^1980.

[5] HARDOUIN, F., LEVELUT, ^A. M., ^J. Physique ⁴¹ (1980) ^41.

[6] BROWNSEY, ^G. J., LEADBETTER, ^A. J., Phys. Rev. Lett. 44 (1980) 1608.

[7] NGUYEN HUU TINH, HARDOUIN, F., SIGAUD, G., Mol. Cryst.

Liq. Cryst. ^{Lett. 56} (1980) ^189.

[8] PROST, J., Proceedings of ^the Liquid Crystals of ^{one- and}

two-dimensional order conference, Garmisch-Parten- kirchen, ^1980.

[9] PROST, J., 8th International Liquid Crystal Conference, Kyoto, 1980.

[10] ^SATO, H., TOTH, R. S., Metallurgical Society Conferences ²⁹ (1963). Alloying behavior and effect ⁱⁿ concentrated solid solutions, ^{T. B.} Massaki, ^Ed. (Gordon ^and Breach),

p. 295-419.

[11] See, e.g. Solitons and Condensed Matter Physics ^edited by

A. R. Bishop ^{and T. Schneider} (Springer-Verlag, Berlin)

1978.