X-Ray study of the reentrant polymorphism N-SA-N-SA in a pure liquid crystal compound

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X-Ray study of the reentrant polymorphism N-SA-N-SA in a pure liquid crystal compound

F. Hardouin, A.M. Levelut

To cite this version:

F. Hardouin, A.M. Levelut. X-Ray study of the reentrant polymorphism N-SA-N-SA in a pure liquid crystal compound. Journal de Physique, 1980, 41 (1), pp.41-46. �10.1051/jphys:0198000410104100�.

�jpa-00209214�

X-Ray study ^{of the reentrant} polymorphism ^N-SA-N-SA

in a pure liquid crystal compound

F. Hardouin (*) ^{and A. M. Levelut}

Laboratoire de Physique ^{des Solides} (**), Université Paris-Sud, ^Bâtiment 510, 91405 Orsay, ^France

(Reçu ^le 19 juillet 1979, accepté ^{le 18} septembre 1979)

Résumé.

Nous avons effectué une étude photographique de diffraction des rayons ^X dans les phases ^méso- morphes ^du 4-n-octyloxy-benzoyloxy-4’-cyanostilbène. ^{En accord avec} de récentes études microscopiques ^et magnétiques, ^nous confirmons qu’en ^{abaissant la} température ^ces mésophases apparaissent respectivement ^comme

des phases nématique, smectique ^A, nématique réentrante et smectique A réentrante. L’évolution thermique ^de l’épaisseur des couches smectiques d implique ^{une structure} partiellement bicouche de la phase SA ^haute tempé-

rature tandis _{que la} phase SA ^basse température ^est monocouche. En termes d’ordre local, ^{ces deux} phases SA diffèrent également : ^les diffusions de la phase ^haute température ^sont identiques ^{à celles} habituellement décrites alors _que la phase ^basse température ^{révèle en} plus ^une surstructure bidimensionnelle compatible ^{avec la} symétrie

de la phase SA. ^La phase nématique réentrante semble être la conséquence ^{de la} compétition ^{entre l’ordre à courte}

distance de l’une et de l’autre des phases smectiques ^A.

Abstract.

X-ray diffraction patterns have been made on mesophases ^of 4-n-octyloxybenzoyloxy-4’-cyano-

stilbene. In agreement ^with microscopic ^and magnetic ^{studies we have} reported ^{the evidence of a} thermotropic

reentrant polymorphism nematic-smectic A-nematic-smectic A in this pure system. ^The evolution of the smectic

layer thickness d as a function of temperature requires ^{some kind of} bilayer ^{structure in the} higher temperature SA

whereas d corresponds ^{to the molecular} length ^{in the lower} temperature SA. ^{In terms} of local order the diffuse

scattering intensity which appears in the higher temperature SA ^{is the} usually ^{observed one,} but in addition we

have also seen an original two-dimensional superlattice consistent with smectic A symmetry in the lower tempe-

rature SA. ^{The reentrant nematic} phase ^{seems to be a} consequence of the competition between both short range smectic A orders.

Classification

Physics ^Abstracts

d1.30

^{64. 70E}

1. Introduction.

Some investigations [1, 2, 3, 4]

have shown for a few _years that the following sequence of phases : nematic-smectic A-nematic with decreasing

temperature may be obtained at atmospheric pressure

by mixing ^terminal polar ^{or non} polar liquid crystals

with certain _cyano derivatives. The nematic phase

at lower temperature than smectic A phase ^{is called}

reentrant nematic. The phenomenon ^{of reentrance}

has been also found at elevated pressure [5, 6] ^{in a}

pure cyanoalkoxy compound ^{or in} cyano binary

systems. ^In all mentioned _cases, these systems ^consist

of molecules which _possess two aromatic rings ^and generally supercooled ^{reentrant nematic} phase ^could

be observed. But recently, by the way of microscopic

observations then by ^the rotating magnetic ^field method, ^F. Hardouin et al. [7, 8] reported ^{the evidence}

of an enantiotropic ^{reentrant nematic} phase ^at atmospheric pressure in a pure compound ^{with three}

aromatic rings, ^the 4-n-octyloxybenzoyloxy-4’-cyano-

stilbene [9] (o T8 » ^for short) ^{with the} following

formula :

Another remarkable property ^{of the «} T8 » ^{is to}

exhibit one more smectic phase ^at still lower tempe-

rature than reentrant nematic; by ^{means of contact}

method this reentrant smectic was identified to a

(*) Permanent address : Centre de Recherche Paul-Pascal, 33405 Talence, ^France.

(**) Laboratoire associe

C.N.R.S.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198000410104100

42

smectic A phase. Therefore this substance allows the authors [7, 8] ^{to introduce a new} original thermotropic liquid crystal tetramorphism ^{with 3} N-SA transitions :

A thermodynamical study ^{of the} 4-n-alkoxy- benzoyloxy-4’-cyano-stilbene ^{series is} published ^else-

where [10]. Briefly it reveals that the nonyloxy ^deri-

vative gives ^a metastable reentrant nematic and below it a metastable smectic A phase; ^the decyloxy ^deri-

vative still exhibits reentrant behaviour. Replacing

stilbene by ^tolan linkage, analogous properties ^have

been found [11] ^{and we} report ^elsewhere [12] ^new microscopic observations and X-ray investigations ^of

the reentrant phenomenon ⁱⁿ previously synthesized 4-cyanobiphenyl-4’-n-alkoxybenzoate ^esters [13, 14].

Concerning ^{the reentrant nematic} phase ^these

results support the theoretical arguments [15, 16]

which predict ^{that the} necessary conditions for the

occurrence of reentrant phase ^{are not} particularly

unusual. Whenever it arises the observation at

atmospheric pressure of the reentrant nematic depends

upon the maximum _pressure (Pm) ^{at which the} (higher temperature) ^{smectic A} phase ^{exists. We}

note that for ^« T8 » Pm + ^{0.5 kbar} [17].

It is interesting ^to carry ^out X-ray experiments

on such a system (« T8 ») ^{in order to} know the main structural characteristics of both nematic and both smectic phases.

2. Results.

The X-ray diffraction technique using

oriented samples provides ^us informations on the effects due to thermal agitation ^and local order.

Nematic single ^{domains are obtained} by orientating

the substance in a magnetic ^field (0.3 T). ^CuKa (1.54 A)

radiation monochromated by ^a double bent pyrolitic graphite crystal and collimated to a small beam of 0.5 mm is diffracted by ^the sample. ^{It is contained}

in a Lindemann glass capillary ^{of 1.5 mm diameter,}

the temperature of which is constant within 0.5 OC.

The diffracted X-rays ^{are collected on a flat} photo- graphic ^{film and} optical density ^{are measured} by

Le Service de Microdensitométrie du CNRS, Orsay.

Vacuum inside the permanent magnet-sample-film

system is made in order to avoid the X-ray absorption by ^{air. To ensure a} good orientation in the two smectic A modifications the patterns ^are taken after the sample ^is aligned ^{in the nematic} phase. ^On average, the director is approximately perpendicular ^{to the}

incident beam and parallel ^{to the film. In this} position

we see two distinct regions ^on X-ray patterns :

-

at small diffraction angles ^the X-ray patterns of nematic and smectic phases differ in their aspect.

X-ray patterns of smectic phase present Bragg

diffraction spots resulting ^{from the smectic} layers periodicity (figs. ^{lb, d). A weak} intensity ^scattered

and located around these Bragg spots is visible.

As we shall ^see afterwards these diffuse spots ^are

different in the two smectic A phases. ^In higher

temperature ^{nematic or reentrant nematic} patterns (figs. la, c) instead of the 0 0 1 Bragg diffraction spots ^observed for smectic A the X-ray reflections become slightly ^more diffuse and nothing ^{is visible}

at the 0 0 2 location. Although ^the longitudinal long range order is destroyed, ^the high intensity ^and

the sharpness ^{of the diffuse} scatterings indicate that the size of the cybotactic groups, in particular ^{in the}

whole _range of temperature ^{of the reentrant nematic} phase, is rather large;

-

both nematic and both smectic A phases give

at large diffraction angles ^two diffuse and broad

scatterings positionned ^{on one of the} equatorial

line (fig. 1). They ^{are due to the} interaction between

neighbouring molecules and point ^out the lack of

periodic order in the directions perpendicular ^to

the long molecular axis. The position of these dif- fraction maxima enable us to measure the _average lateral distance between adjacent ^molecules [18]

(2 d sin 6 1.117 À) : ^: 5.3 A ^at 263 °C ; 5.1 A at

150 °C ; ^5.0 A at 115 °C ; ^4.85 A at 75 °C. We note that these values are quite consistent with most results reported ^{so far} [18, 19].

Since the evolution of the lateral molecular packing

appears ^{not to} be affected by ^reentrant phase ^tran- sitions, ^we shall focus on the behaviour of the mole- cular longitudinal ^order along ^the mesomorphic temperature range. After considering the average

longitudinal ^{order we} shall discuss on fluctuations in smectic phases ^and finally ^we shall take ^account of local order in nematic phases.

2.1 AVERAGE LONGITUDINAL ORDER IN BOTH SMEC- TIC A PHASES.

First, ^we confirm that the two smectic phases ^are certainly ^{smectic A} phases ^because

the director (in ^the bulk) ^is parallel ^{to the} magnetic

field direction and ^we always ^see Bragg reflections

positionned along ^{this direction} (figs. ^{lb, d), thus,}

smectic layers ^{are normal to} the director. With long

exposure time ^we reveal two orders of layer reflections, the ratio of 12 (0 0 2) ^over h (0 0 1) intensities can be

roughly estimate, despite the fact that ^we have not

performed rocking ^curves (the sample ^{is fixed but}

the magnetic ^{field makes a 85°} angle ^{with the} X-rays

beam and the 0 0 1 and 0 0 2 reflections are more or

less simultaneously ^{on the Ewald} sphere ^{on one side}

of the pattern). ^{The ratio} 12/h decreases in the higher temperature SA ^{from 145 °C} (= ^{2 x} 10-3) ^{to 195 °C}

(£r 10-3); ^{in the lower} temperature SA ^{we find the}

same order of magnitude ^{as at 145 °C.}

The Bragg reflections spots ^{at small} angles ^cor- respond with the thickness of the smectic layers d (calculated ^from Bragg’s law). ^{Shown in} figure ^{2 is}

the thermal dependence ^{of d.} Comparing ^the present result with earlier ones [2, 3] obtained from mixtures

we corroborate that the layer thickness exhibits

no pretransitional ^{effects when we} approach ^the

nematic reentrant phase. ^{However we note that in}

Fig. ^1.

Intensity contour map of X-ray patterns of the different phases (exposure ^{time : 6} hours) : a) ^{Nematic T}

250 °C ; b) ^{Smectic A}

T

185 °C ; c) Reentrant nematic T

120 °C ; d) ^{Reentrant smectic A T}

78 °C. The 0 0 1 Bragg reflections on the layers ^{in the}

tic A phases

over-exposed ^{and the contour lines} give ^the shape ^{of the} surrounding ^diffuse scatterings. ^{The arrows} point the 0 0 2 reflec- tions. These sharp reflections

characteristic of

smectic order.

Fig. ^2.

^Smectic layer thickness d

function of temperature.

our system ^the layer spacing decreases with decreasing temperature. ^This change ^{in the} higher temperature smectic A phase represents ^{about a 3} % ^variation.

On the other hand, ^no apparent change ^{in d with} temperature ^is found in the whole low temperature smectic A range and the corresponding value 31.1 A

is significantly lower than the value extrapolated ^from higher temperature SA. Finally, calculating ^the length

of the molecule in its most extended conformation L

31.4 Å (Dreiding Stereomodels), ^{it is clear that}

the ratio d/L varies from 1.24 at 250 OC to 1.11 at 140 OC. Thus, ^as for certain _cyano derivatives [20, 21, 22] ^{some kind of} bilayer ^{smectic A} packing is, required

to explain ^{that the} layer spacing ^is larger ^{than mole-}

cular length ⁱⁿ higher temperature SA phase. ^As strongly suggested [5, 23, 24, 25] ^we suppose ^a bimole- cular head-to-tail arrangement. ^More precisely, ^{if we}

refer to the A. J. Leadbetter et al’s works [25] ^on

cyano derivatives with two aromatic rings, ^{we can}

show that at 250 OC « T8 » ^{is on the line} indicating

44

an approximately linear relation between d and

(a ^{+ 2} b) ; a ^{is the molecular core} length ^{and b is}

the length ^{of the tail} (see fig. 2, ^ref. [25]). ^{This is}

consistent with a structure in which the molecular

cores overlap. The effects of decreasing temperature in the high temperature SA would tend to increase the

overlapping ^{of such dimers,} and/or ^the penetration

of the molecules in each smectic layer ^{into the two} adjacent layers and/or ^{the tilt} angle (if ^{we consider as}

certain authors [26, 27, 28] ^{a tilt} distribution of long

axes orientations in smectic A). On the other hand, in the entire low temperature SA region ^{the value} d/L - ^0.99 corresponds ^{to a} monolayer ^{smectic A.}

Besides, the diffuse scattering intensity ^around Bragg spots ^{is also} clearly different in each smectic A phase.

2.2 FLUCTUATIONS IN SMECTIC A PHASES.

In the

high temperature range of the smectic A phase (fig. I b),

the Bragg reflections characteristic of the smectic

periodic ^{order are} surrounded by ^{a diffuse} spot;

this spot ^{forms a disc} lying ^{on the} (0 01) plane (parallel

to the smectic layers) ^{the center} of the disc is the 0 0 1 reflection. This diffuse scattering intensity ^which

is usually ^{observed on oriented} SA patterns originates

in the undulation modes of the layers [29]. ^{Let us}

recall that, ^{in a} periodic system, fluctuations of the

periodic ^{order of wave vector} q ^scatter the X-rays

on a point Q ^{of the} reciprocal space (in ² nlÀ units) :

where R is a reciprocal point corresponding ^{to a} Bragg reflection. As the scattered intensity ^is pro-

portional ^{to the square} amplitude ^{of the} fluctuations, the large amplitude fluctuations are seen first. If we

take into account the selection rules on the pola-

rization of the fluctuations, ^{we can associate the} observed diffuse spots ^to the existence of low frequency

and large amplitude undulation modes of the smectic

layers.

In the low temperature supercooled range of the smectic A phase (fig. 1d) ^{the scattered} X-rays intensity coming ^{from the} undulation modes are still visible but we also see scattered X-rays ^{localised in the} reciprocal space ^{on a cone} of large angle, ^the pitch

of which is on a 0 0 1 or 0 0 2 point ^{and its axis is}

the [0 0 1] ] reciprocal ^{row. Near the} recrystallization

temperature ^{the scattered} intensity ^{is localised on}

spots. ^{On over} exposed patterns (fig. 3), ^{we can see}

two spots ^{out of the} [0 0 1] ] row ^{and below the 0 0 2}

reflections. Similar spots ^{are seen, on less} exposed films, ^{under the 0 0 1} reflections ^at the same distance from the [0 01] ^{axis and at the same} distance from the

corresponding Bragg spots. ^{On the} figure ^{3 a} spot ^is also seen at half-distance between the 0 0 2 and 0 0 3

points. ^The dissymmetry ^{of the} pattern along ^the

I direction is due ^to the geometrical ^conditions (the magnetic ^{field is at 850 of the} X-rays beam).

These spots ^are indicative of a two dimensional

superlattice periodicity. ^A modulation of the position

of the center of mass takes place and this modulation has a period ^of roughly ^four layers (120 A) ^{in the}

director direction (OZ) ^{and of} 70 A in the perpen- dicular direction (OX). ^The displacements ^{of the}

molecules occur evidently ^{in a direction} parallel ^to

the director. A model in which the displacement ^{u of}

a molecule out of the mean position along ^{the z}

direction is sinusoidal can be proposed :

Fig. ^3.

a) X-ray pattern of the lower temperature SA phase (T

^{60 °C} exposure time 20 h) ; b) Intensity ^contour map of the small

angle part ^{of this} X-ray pattern. ^A circular Aluminium filter is put

^{the center of the} pattern ^{in order to} clear the external part. The super- lattice reflections

indicated by ^{arrows. As}

^{matter of fact, the} separation between the two superlattice spots under the 0 0 2 reflections is

more

evident

the original pattern.

with

Such a model is consistent with the observed

X-ray pattern (fig. 3) ^{with a maximum} displacement

uo - 2 ^to 3 A. This model describes sort of peristaltic

modes of the layers, ^{at low} temperature (fig. 4)

Fig. ^4.

^Schematic representation ^{of the structure of the}

tic A phase ^{at low} temperature. ^{Each arrow is}

molecule with the nitrile head.

one of this mode becomes predominant (if ^very

low frequency), ^{while at} higher temperature ^different modes with different _q vectors are present leading

to the conic diffusions (fig. ld). ^{Moreover two remarks}

can be made :

1) ^the X-ray pattern ^{at low} temperature ^{has a} revolution symmetry around the [0 0 1] ^{row in such}

a way that the spots ^{are in fact flat} rings ^{in the} recipro-

cal space (excepted, ^{of course, for the} spots lying ^on

the [001] axis). ^{We can} explain ^this symmetry ^with

two models. In the first one the peristaltic ^{modes are} locally anisotropic ^{in the} layers plane (due ^{to defects}

for example) but all the orientations of projection

of the wave vector of these modes are present ^{in the}

sample. ^{In a} second model we can imagine ^{that the} bumps ^{and the} depressions of smectic layers ^are

rather circular with regular ^{size and form a} liquid

array in the smectic layers (the surface of a smectic

layer looks like sand-hills in a desert) ;

2) ^{a second remark concerns the absence of} superlattice reflections above the 0 01 and 0 0 2 reflec- tions on the layers. Such reflections would be in a

region ^of very low diffracted intensity observed in all the patterns ^{of this} compound ^and probably corresponding ^{to a zero of the molecular structure}

factor.

These surprising peristaltic undulations seem to be related to the monolayer ^structure of these nitrile molecules because the conic diffuse scattering appears in SA phase ^{of lower} homologues ^{of «} T 8 » ^{with a} layer thickness of one molecule too [10], ^{and in the}

lower temperature range of the reentrant nematic

phase ^{of «} T8 ^».

2.3 LOCAL ORDER IN NEMATIC AND REENTRANT NEMATIC PHASES.

The longitudinal local order in the reentrant nematic depends ^on the relative proximity respectively ^{of the} higher ^{and lower} temperature SA phases. ^{In the} high temperature ^{side of the reentrant} nematic domain the diffuse spots ^{at small} angles correspond ^{to the} cybotactic groups of bilayer

smectic A. In the middle of the temperature range of the reentrant nematic phase ^{these diffuse} spots visible at small angles ^{seems to} split ^{into two maxima}

of intensity corresponding ^{to two different} scattering

vectors (in amplitude) ^{in such a} way that bilayer ^and monolayer fluctuations are present simultaneously.

The correlation lengths associated to these two

types ^of fluctuations are relatively short in this temperature domain and it is not reliable to give ^the apparent ^{thickness of the} layers (fig. 2). Upon cooling

down the sample progressively ^{in the reentrant}

nematic phase conic diffuse scatterings appear ^on the X-ray patterns ^{and in the same time} only monolayer

fluctuations remain. Thus, ^{in terms} of local order

we have in fact to distinguish ^{the lower} temperature smectic A-reentrant nematic transition from the reentrant nematic-higher temperature SA transition, this latter is similar to the higher temperature SA-

nematic transition. At last, contrarily ^{to D. Guil-}

lon et al’s finding [2] ^{on a} monotropic ^reentrant

nematic system ^{we have not} observed in «T 8 »

extra Bragg spots ^{in the reentrant nematic} phase.

3. Conclusion.

The X-ray study ^{of the} o T8 » compound ^is quite coherent with the previous ^obser-

vations made on this compound [7, 8]. Moreover,

we have observed an unusual temperature dependence

of the layer thickness in the SA phase, ^{since the} layer

thickness decreases from 1.25 molecular length ^at

250 °C to 1 molecular length below 96 °C. This variation does not seem to be related to the existence of a reentrant nematic phase : ^{in reentrant mixture} [2]

the smectic layer thickness increases slightly ^with decreasing temperature ^{and is constant} in another

binary system [3] ^{or in a} pure ^reentrant compound [12].

46

Nevertheless in the T8 » ^the temperature dependence

of the smectic layer spacing ^{is more} likely ^{related to}

the existence of the reentrant smectic A phase. ^In

this phase ^{we have observed} peristaltic modes of the smectic layers. ^A condensation ^at a well defined value of the wave vector takes place ^{in the} supercooled

smectic A phase. Up ^{to now, we have} not accurate measurements of structure factor and we are unable to

precise ^the respective role of the aliphatic parts ^and

the rigid ^core parts ^{in such a} molecular arrangement.

Despite ^this fact, ^{we can make a} tentative model of the low temperature ^state (fig. 4) : ^{in a} layer ^{we should}

have alternatively bilayer clusters beside monolayer

ones. A splay ^{of the} molecules in the monolayer

clusters due to large dipolar ^{effect insure a mean} layer thickness of one molecule while the _super-

position ^{of the two kinds of clusters} along ^{a direc-}

tion (OZ) perpendicular ^{to the} layers preserves ^the

mean smectic periodicity. Competition ^between elastic, electrostatic (and wall) energies ^{can stabilize}

modes with a given ^{wave vector.}

Finally, ^{the reentrant nematic} phase ^{of «} T8 » ^{is a}

consequence of the competition between local smec-

tic A orders which are different at low and at high temperature. ^We have observed a similar competition

in the reentrant nematic phase of another _pure system [12] but in this last case between a smectic A and a smectic C.

Acknowledgments.

^{We are} grateful ^{to Prof.}

M. Lambert and Drs. J. Prost and G. Sigaud ^for

valuable discussions. We thank M. Joussot-Dubien and Dr. H. T. Nguyen ^{for the} synthesis ^{of the «} Tg » compound.

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