X-ray investigations and magnetic field effect on a nematic phase of disc-like molecules

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X-ray investigations and magnetic field effect on a nematic phase of disc-like molecules

A.M. Levelut, F. Hardouin, H. Gasparoux, C. Destrade, Nguyen Huu Tinh

To cite this version:

A.M. Levelut, F. Hardouin, H. Gasparoux, C. Destrade, Nguyen Huu Tinh. X-ray investigations and

magnetic field effect on a nematic phase of disc-like molecules. Journal de Physique, 1981, 42 (1),

pp.147-152. �10.1051/jphys:01981004201014700�. �jpa-00208983�

X-ray investigations ^and magnetic ^{field effect on a nematic phase}

of disc-like molecules

A. M. Levelut, ^{F. Hardouin*}

Laboratoire de Physique ^des Solides, Université de Paris-Sud, ⁹¹⁴⁰⁵ Orsay, ^France

H. Gasparoux, C. Destrade and Nguyen ^{Huu Tinh}

Centre de Recherche Paul-Pascal, Université de Bordeaux I, 33405 Talence, ^France

(Reçu ^{le 14} avril 1980, ^{révisé le Il} septembre, accepté ^{le 15} septembre 1980)

Résumé.

Les diagrammes de diffraction des rayons X d’échantillons

orientés de la mésophase ^{fluide de}

molécules

plateau, ^les hexaalkoxybenzoates ^de triphénylène, ^sont identiques ^{à la} figure de diffraction d’une

phase nématique ^{non orientée de molécules en} bâtonnets. De plus,

^{montrons le caractère uniaxe de}

^milieu anisotrope qui possède ^une anisotropie diamagnétique négative, ^car les molécules plates s’orientent parallèlement

les unes

autres sous l’action d’un champ magnétique ^{tournant. Enfin, les} diagrammes ^de diffraction d’échan- tillons orientés du dérivé hexahexyloxybenzoate ^montrent l’existence d’un ordre prétransitionnel analogue ^{à celui} qui ^{est observé dans la} phase nématique ^et

voisinage ^{de la} transition nématique ^{SmC. Ici, cet effet} prétransi-

tionnel est relié à l’existence d’une phase ^{de basse} température ^que

^{décrivons et} où les molécules sont empilées

en

colonnes, ^leur plan moyen étant oblique par rapport ^{à l’axe de la colonne.}

Abstract.

The X-ray diffraction patterns ^of powder samples ^{of the fluid} mesophase ^{of the} hexaalkoxybenzoates of triphenylene ^{which is}

disc-like molecule are similar to the diffraction patterns ^of

orientated nematic phase

of rod-like molecules. Furthermore,

give ^{evidence for}

^{uniaxial character of this} anisotropic medium with

negative diamagnetic anisotropy, ^{the flat molecules}

orientated parallel each other by

rotating magnetic ^field.

In addition, ^the X-ray patterns ^of orientated samples ^{of the} hexyloxy compound ^revealed pretransitional ^effects

in this nematic phase of disc-like molecules (ND). ^This short-range translational order, ^{sort of skewed} cybotactic

groups originates from the existence of

low temperature ^columnar phase ^with

original ^tilted packing ^here

described.

Classification

Physics ^Abstracts

61.30

64.70E

1. Introduction.

Recently ^{a new} mesogenic

series [1] of disc-like molecules has been discovered which exhibits a highly ^fluid mesophase, ^{the texture}

of which is similar to the classical nematic one’.

Moreover, ^{it can be} compared ^{to the textures} seen i

the carbonaceous mesophase ^{which is a} mesomorphi

state of a complex ^mixture containing polyaromatic

molecules with ^some aliphatic parts [2]. Therefore,

the new phase ^of disc-like molecules could be a pure component simple model for the carbonaceous _mesot-

phase.

We have performed X-ray ^and magnetic ^investi- gations ^{in order to obtain some} information on the structure and the symmetry properties ^{of this new}

fluid anisotropic phase.

2. Experimental ^{technics. - We} have studied two derivatives of the hexaalkoxybenzoates ^of tripheny-

lene (Fig. 1) ^with alkyl chains of six and eleven carbons.

Each compound ^{exhibits two} mesophases : ^the higb temperature phase ^{is fluid} (with ^similar textures) ^for

both compounds. ^{The low} temperature phase ^{is much}

more viscous. The texture of the C6 derivative is a

mosaic one [3] ^{which is observed} for the first time with disc-like compounds while the texture of the C11

derivative is similar to the texture of the D1 ^phases

of the alkanoates of triphenylene [4, 5].

X-ray investigations have been made on powder samples ^{with a Guinier camera} using ^the CuKà

radiation. At the same time, samples ^{held in a Lin-}

deman glass capillary ^have been ^{studied within a}

3 kG magnetic field and illuminated with a point- focusing X-ray ^beam CuK« perpendicular ^{to the} magnetic ^field.

*

Permanent address : Centre de Recherche Paul-Pascal, ^Univer- sité de Bordeaux 1, 33405 Talence, ^France.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01981004201014700

148

Fig. ^1.

Chemica.l formula of the hexaalkoxybenzoates ^{of tri-} phenylene.

To determine to what extent an extemal magnetie

field is apt ^{to induce a} single ^domain (in ^the bulk)

of the highly ^fluid mesophase, ^{we have also} performed magnetic measurements. First, by ^{means of the classi-}

cal Faraday ^method [6] (the magnetic ^field has a

vertical gradient) ^the magnetic susceptibilities arç measured. In addition, ^we have undertaken some

experiments ^{with a} homogeneous rotating magnetic

field of about 11 kG [7]. This latter technique ^{is necesr}

sary ^to define the sign ^{of the} diamagnetic anisotropy.

3. Results. - 3.1 X-RAY DIFFRACTION PATTERNS OF THE FLUID MESOPHASE. - X-ray diffraction pat-

terns of samples ^{held in a} capillary tube outside of the magnetic field in the fluid mesophase (Fig. 2) ^aré

very similar to the diffraction patterns ^{of the} isotrope liquid phase ^{of the} C11 derivative or other disc-like

mesogenic compounds (we ^{have not} attempted ^to

reach the isotropic phase ^{for the} C6 compound

T > 274°C). ^{We have two broad} diffraction rings,

Fig. ^2.

X-ray diffraction pattern of the fluid mesophase ^of

orientated sample (C6 derivative).

the inner ring being ^of high intensity compared ^{to the}

outer ring. ^{The outer} ring ^{lies at}

while the inner is related to the molecular size and

corresponds ^{to d}

^{20.5 A for the} C6 ^{and d}

26.5 À

for the Cl,. ^{This is} very similar to the diffraction by ^a

non orientated nematic phase. Nevertheless a striking

différence lies in the relative intensity ^{of the two} rings ^:

the inner is of higher intensity ^{in the case of this} flui4

mesophase ^{of disc-like} molecules while the outer is

more intense for nematics of rod-like molecules.

Moreover, ^{the 3 kG} magnetic ^{field has no effect on the}

fluid mesophase ^{of the} Cl 1 derivative.

Going down from the isotropic phase up ^to the columnar phase appears, ^we observed only ^a slight narrowing of the inner ring ^for X-ray patterns ^{in the} fluid phase ^{domain. In contrast, in the case of the} C6

derivative the magnetic field orientates the sample (Fig. 3). ^{We have to} notice that it is _necessary to heat at least 20 °C above the transition temperature ^from

Fig. ^3.

X-ray difïraction pattern of the fluid mesophase ^of

orientated sample ^of hexyloxybenzoate ^of triphenylene (the

indicates the direction of the director). a) ^{3 hours -} 220 °C ; b)

1 hour - 205 °C.

the columnar phase ^{in order to have a} good alignment

of the sample. Thus, ^the viscosity ^{of the} mesophase ^has likely ^{an influence} upon the sample alignment ^obtained by ^a rather weak magnetic ^field (- 3 kG).

The best orientation effects are obtained after a

rotation of 7c/2 ^{of the} capillary tube around its axis which is perpendicular ^{to the} magnetic field. The same process of orientation has been successful for the nematic lyotropic mesophase ^of type ^II [8] ^{which is} supposed ^{to have identical} symmetry ^and magnetic properties ^as the disc-like nematic phase (see 3.4).

Further rotation of the sample ^{does not} induce 4 change in the diffraction pattern. ^We develop further

the effect of a rotating magnetic ^{field on this} phase

(see 3.4). 1

By changing ^the angle between the X-ray ^{beam and}

the magnetic ^{field we} have established that the diffrac..

tion pattern ^{of the} C6 ^fluid phase (Fig. 3) ^{has an infmite}

rotational symmetry ^axis parallel ^to the axis of the

capillary ^{tube. One can see two} broad discs at

while the inner ring ^is split into four broad spots.

The structure of the inner ring is similar to the dif- fraction patterns ^{of a nematic} phase ^of elongated

molecules with skewed cybotactic groups [9] ^howevet

the angular extension of the four spots ^{on the innef} ring ^is greater in the disc-like compound (these diffuse spots ^are poorly ^{visible on the} over-exposed patterns

of the figure 3). ^The presence of such spots ^{led us} tô study the columnar phase ^{in order to check} if they are

indicative of a pretransitional local order. Neverthe- less, ^the two outer discs lying ^{Y g} at u - ^d 1 A -1 ^{. are}

characteristic of a preferred orientation of the molecu.

lar lateral substituents, ^these elongated substituents

lying ^{in the} plane ^{of the} applied magnetic ^field (plane perpendicular ^to the axis of the capillary tube).

3.2 X-RAY DIFFRACTION PATTERNS OF THE COLUM- NAR PHASES.

Non orientated sample of the columnar

phase has been obtained from the Cl 1 derivative.

The powder pattern ^{of this} mesophase ^is similar

to the powder pattern ^{of the} Dl phase ^{of alkanoates}

derivatives of triphenylene [10]. Thereforé, ^this phase

is likely constituted by columns of molecules in a

rectangular ^two dimensional lattice (space ^group Pgg)

the lattice parameters ^{are a}

^51.8 Á, ^b

^{32.6 À} (table I) ^{and the} specific ^area AIM

^{2.58 x 10’ cm2} g- 1 (A ^{is the area of a} column and M the molecular mass)

and this specific ^area is similar to that of the dode-

canate of triphenylene ^{in the} Dl phase [10]. ^We recall

that in this phase ^{the molecules are} stacked in column$

with their plane perpendicular ^to the column axis,

the columns forming ^a herring-bone pseudo-hexagonal

array.

In the case of the C6 derivative the orientation obtained in the fluid phase largely remains in thé

Table I.

Reticular distances observed on powder

patterns of the columnar phases.

columnar phase by ^slow cooling. Although ^thèse samples ^{are not at all} single domains, there is gene-

rally ^one principal ^{domain in the} X-ray ^beam. Indeed, rotating ^the sample, ^the magnetic field of the X-ray

device is unable to reorient the molecules in this _very viscous phase. Then it is possible ^to obtain various

X-ray patterns corresponding ^to successive rotations of the capillary tube around its axis and describing

the successive orientations of a same principal ^domain.

One example ^{of such an} X-ray pattern ^is given figure ^4a

for which we can analyse qualitatively ^{the structure.}

Fig. ^4a.

X-ray diffraction pattern of the tilted columnar phase ^of

the hexyloxybenzoate ^of triphenylene. The intense 110 reflections

are

pointed ^out by

^{The other} spots

^{at least}

^{order of} magnitude less intense and originate from other domains.

Two sets of two broad reflections are observed at

The first one indicates a liquid-like order of the substituents and the second, ^{which is} only visible for the columnar phase, is characteristic of a linear

stacking order of the cores of the molecules.

This is clearly understood if one considers the diffraction pattern ^{of a} liquid-like ^{line of} points : this would be constituted of two diffuse planes perpen- dicular to the column axis ; the distance between these

two planes ^{is related to the mean} distance between

the points ^{on the line. Now, if we} have disc-like

molecules instead of the points, ^the intensity ^{in the}

150

diffuse plane is modulated by the molecular structure factor which in this case has a cylindrical symmetry.

Consequently ^the intensity ^scattered by ^{one column}

is localized into two diffuse discs, ^{the axis} joining thé

centres of the discs being perpendicular ^{to the mole-}

cular plane. ^The distance between the two discs gives

the mean distance between two molecules in a column.

At last, me ^{diameter of the diffuse} disc is related to the size of the disc-like scattering entity and in this manner we can see in our case that the central part ^of the molecules alone contributes to such kind of broad

reflections. In addition, the location of the 1 A- 4.5 spots ^indicates that the lateral groups are more or less

in the same plane ^as the molecular cores. Near the

centre of the pattern (Fig. 4a) ^{two intense} équivalent points are seen on reciprocal ^rows going through ^thç origin. ^These reflections originate from the two dimensional lattice formed by ^the arrangement ^{of the} columns. The line joining ^{these two} spots gives ^thé

direction of a plane perpendicular ^{to the column axig.}

Therefore, remembering ^{what was described for thé}

diffuse discs, ^{we see on} figure ^{4 that the} plane ^{of thë}

Fig. 4b.

Projections ^{of the} reciprocal space

the (001) ^and (110) planes : ^{1 :} equatorial ^section ofthe Ewald sphère ; ^{2 :} section of the Ewald sphere ^{at the level} 1 o A-1; 3 : localization of the scattered intensity by the molecular

molecules is not perpendicular ^{to the} column axis and

we conclude for the first time in favour of a tilted

columnar phase. Unfortunately ^{we are unable} tg deduce the reciprocal ^{lattice of column} arrays from

such samples ^since they ^{are too} badly ^oriented for

this purpose. Powder patterns ^{enable us to} assign a

centred rectangular ^{lattice for the column} array ; the lattice parameters are, in a plane perpendicular

to thé column axis (Fig. 5) a

^30.7 À, ^b

^{28.4 À} (table I).

The symmetry properties ^{of this} phase ^{can be}

described by ^{the three} dimensional monoclinic space group Cm ^or C2/m (depending ^{on the molecular}

conformation and on an eventual orientational disor-

der) ^{in which we} suppress the periodic translational order along ^{the c} direction. We have also to point ^out

that as there is no periodicity along ^{c, the column}

Fig. ^5.

^Schematic representation of the lattice of the tilted columnar phase ^{of the} hexyloxybenzoate of triphenylene.

lattice is described in a plane perpendicular ^{to the}

column axis that is to say perpendicular ^{to the c}

direction. We assume also that the shorter axis b is

perpendicular ^to the mirror plane because this

assumption ^{ensures a better} packing of the columns.

Thus it is not the conventional description ^{for a}

monoclinic cell.

_

Figure 4b shows the projection ^{of the} reciprocal

space ^on the a* b* plane and its intersection with the Ewald sphere ^{when the} X-ray ^{beam is} parallel ^{to this} plane ^and perpendicular ^to the 110 ) ^direction.

The X-ray pattern ^of figure ^4a corresponds ^{to an}

orientation which differs from the scheme 4b by ^a

rotation ( N 30°) ^{of the} sample ^around the 110 >

row. From the figure 4a, ^{one can estimate the} angle ^ac

between the core plane and the 100 direction : this

angle ^a is of the order of 550 (Fig. 5).

The specific ^{area of a molecule can be deduced from}

the area per column :

since there are two columns _per rectangular ^lattice.

The area of a molecule is taken in a plane parallel ^to

the molecular core Amal

435.9 Â2 cos 55° ^{; and from the}

mass of one molecule 2.94 x 10-21 _g we deduce a

specific ^{area of 2.58 x 10+’ cm2} g-1, ^{therefore we}

obtain the same value for the C11 ^and C6 compounds.

3. 3 PRETRANSITIONAL LOCAL ORDER IN THE FLUID MESOPHASE.

From the structure of this phase ^we

are now able to understand the diffraction pattern ^of the C6 ^nematic phase. ^{The four diffuse} spots ^{in the} inner ring ^{are at the same distance as the 110} reflections of the columnar phase. In the nematic phase ^small

domains with a columnar order are present : ^{as the} domains are small we only observe the first order of reflection and moreover it is broad. In fact, ^{the four} spots correspond ^to the intersection of two circles

with the photographic plate ; these circles are perpen- dicular to the capillary ^{axis and we can assert that}

all the discs ^are parallel, the order of the centres of

mass is liquid-like ^whereas the column directions of the different domains are on a cone of 420 of half appex.

The axis perpendicular ^{to the molecular} plane ^is perpendicular ^{to the} plane ^defmed by ^the magnetic

field and the rotation of the capillary ^{and we can}

measure the apparent diameter of the molecule looking

at the first intensity maxima in the direction of the

magnetic ^field (c.-- 23 Â).

3.4 MAGNETIC MEASUREMENTS.

The magnetic experiments ^were performed ^{on different} samples ^of

the same compounds using ^{two different} apparatus.

The Faraday ^{device is} equipped ^{with a} magnet provid- ing ^fields up ^to 15 kG. Moreover the rotating magnet gives ^{a maximum field of 11 kG.}

First, ^{in order to check the influence of the} magnetic

field _upon the degree ^of orientation of the nematic

phase ^we have studied the diamagnetic susceptibility

of these compounds. The thermal evolutions of this

parameter x (measured ^{in the direction of the} magnetic field, during ^slow cooling ^{from the} isotropic phase)

corroborate the existence of a spontaneous orientating

effect of the applied ^{field at the} isotropic-fluid ^meso, phase transition for both C6 ^and C11 compounds (i.e. ^a sharp decrease of the absolute value of the

diamagnetic susceptibility, figure 6). ^{In addition,}

we have to note that, contrarily ^to the columnar phases of triphenylene derivatives investigated ^elsewhere [ 11],

this effect as a function of the magnetic field value is saturated for H >, ^{5 kG.}

Furthermore, ^the study ^{of the} behaviour of a sample hung by ^a quartz ^{wire in a} homogeneous rotating magnetic field of about 11 kG [7] ^{shows that the fluid} mesophase ^is isotropic ⁱⁿ the rotation plane ^{of the} magnetic ^{field : no} magnetic torque ^{occurs and the} sample remains in its initial position. Thus, ^a single

domain of this fluid mesophase is obtained with a

rotating magnetic ^{field :} this medium is magnetically

uniaxial with the preferred ^axis perpendicular ^{to the} magnetic ^{field rotation} plane. ^{For an uniaxial} system, the magnetic anisotropy ^{is defmed} by

where X~ Il is connected to the magnetic susceptibility

measured in a direction parallel ^to the main axis and

Fig. ^6.

Discontinuity ^{of the} magnetic susceptibility ^{at the iso-} tropic-fluid mesophase transition.

xi ^to the susceptibility ^measured perpendicularly ^to

this axis.

In the case of this fluid mesophase xl is the diama-

gnetic susceptibility ^{in the} magnetic ^{field direction} thus | X~ | 1 > 1 Xl. 1 (x ~ and X, 0) ^{and the diama-} gnetic anisotropy ^{in this} mesophase ^{is hence} negative :.

We can note the strong tendency ^{for this} mesophase

to give extinction between crossed polarizers : ^this

also suggested ^an optically ^uniaxial mesophase.

Moreover, the upper X-ray study ^has pointed ^{out that}

the aromatic cores of the molecules are parallel ^{to the}

rotation plane ^{of the} magnetic ^field. Thus the pre- ferred axis (i.e. ^the director) ^is perpendicular ^{to the}

molecular core in good agreement with the disc-like molecule symmetry and with the chemical structure of the molecule (Fig. 1) (the diamagnetism essentially depends ^on the aromatic part and thus the largest magnetic susceptibility ^{in absolute value is} likely per-

pendicular ^{to the} polycondensed rings).

In order to determine the magnetic anisotropy ^of

this phase, ^{we have to} notice first that it is not sure to obtain a single ^{domain in a static} magnetic ^field.

Indeed, if A x ^is negative, the director ^can be orientated not only ^{in one} direction but can point everywhere ⁱⁿ

the plane perpendicular ^{to the} magnetic ^field (as ^{it is}

the ^case for the nematic of rod-like molecules of the

alkyl bicyclohexyl carbonitriles [12] and for the

lyotropic mesophase ^of type ^II [8]).

152

Nevertheless, in the limit of our experimental

accuracy, the value of the susceptibility along ^the magnetic ^field direction is unchanged ^after turning

the sample ^{around a} vertical axis in order to improve alignment. Therefore, ^the diamagnetic susceptibility

measured in the direction of the static magnetic ^field by ^the Faraday ^{method seems to} correspond ^{to the} susceptibility perpendicular ^{to the director} (i.e. /J.

If we assume, like for rod-like nematogenic ^com- pounds, ^{that the} average magnetic susceptibility ^does

not depend ^{on the} temperature ^{and on the state of the} material we can write :

and then :

Thus, ^{as for a classical nematic} [6], ^the magnetic susceptibility measurement in the direction of the held, respectively ^{in the} isotropic phase and in the fluid

mesophase, ^{lead to} determine the magnetic anisotropy

of this latter phase. ^In fact, very small amounts of

ferromagnetic impurities prevent ^from obtaining ^thç

intrinsic thermal evolution of the disc-like compound anisotropy although they ^{do not} disturb the discon-

tinuity ^at the transition. Thus near the isotropic phase

we find :

We can explain the different values of OxM ^between

the C6 ^{and the} C11 compounds - ^first considering

that larger the aromatic ^over aliphatic parts ^ratio is, larger the molecular anisotropy ^is

secondly, may be the degree of orientational order at the isotropic phase - ^fluid mesophase transition has to be taken in account. This difference in the magnetic anisotropies

associated to a higher viscosity ^{of the} Cl, ^could explain ^{that we cannot} get orientated X-ray patterns (under ^{3 kG} magnetic field) with this compound contrarily ^to what is observed for the C6 compound.

4. Conclusion. - To conclude, ^our X-ray ^results

agree with the first microscopic observations of the fluid mesophase of disc-like molecules. Indeed, ^this mesophase ^{can be} considered as a nematic phase ^of

disc-like molecules (ND ^for short) because there is no

translational order and the flat molecules are more or

less parallel each other (from ^the X-ray patterns ^{it is} difficult to precise ^{the molecular} fluctuations).

In addition, ^we give evidence for an uniaxial cha- racter of this medium with a negative diamagnetic anisotropy, this fluid mesophase ^is spontaneously

orientated in the bulk by ^a rotating magnetic ^field.

As for rod-like mesogenic compounds, ^the X-ray patterns of orientated samples have revealed _pre- transitional effects in the ND phase. Also, ^{as for series}

of rod-like materials, ^{the low} temperature ^ordered phase ^{which succeeds to the nematic} phase ^{can exhibit}

different structures according ^{to the} length ^{of the}

substituents. Thus, ^the possibilities ^of investigations supplied by the nematic phase ^of disc-like molecules

seem to be as large ^{as the} expected ^{ones from the}

classical nematic phase ^a long ^time ago.

References

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72A (1979) ^251.

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International Conference (1979), ^{to be} published.

[4] DESTRADE, C., MONDON, ^M. C., MALTHETE, J., ^J. Physique Colloq. ⁴⁰ (1979) ^C3-17.

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