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Coexistence of a 2D discommensuration wall network and of a hexatic molecular ordering in a liquid
crystalline phase
A.M. Levelut, Nguyen Huu Tinh
To cite this version:
A.M. Levelut, Nguyen Huu Tinh. Coexistence of a 2D discommensuration wall network and of a
hexatic molecular ordering in a liquid crystalline phase. Journal de Physique, 1987, 48 (5), pp.847-
853. �10.1051/jphys:01987004805084700�. �jpa-00210504�
Coexistence of a 2D discommensuration wall network and of a hexatic molecular ordering in a liquid crystalline phase
A. M. Levelut (*) and Nguyen Huu Tinh (**)
(*) Laboratoire de Physique des Solides, Associé au CNRS, Université Paris-Sud, 91405 Orsay Cedex, France (**) Centre de Recherches Paul Pascal, Domaine Universitaire, 33405 Talence Cedex, France
(Recu le 20 octobre 1986, accepté le 20 janvier 1987
Résumé. 2014 Une nouvelle série de mésogènes polaires présente à la fois des phases smectiques B et des phases
en rubans Sc. Nous avons pu mettre en évidence une phase hexatique B incommensurable dans laquelle une
modulation de l’orientation des dipôles de grande période se superpose à l’ordre hexagonal de chaque couche smectique. Nous comparons les caractères structuraux de cette phase à ceux des phases hexatiques et Sc.
Abstract.
2014A new series of polar mesogens exhibits both a smectic B phase and a ribbon phase Sc. In this series, an incommensurate hexatic B phase has also been found. In this phase, a long wavelength
modulation of the dipolar orientation is superimposed over the in-layer 2D hexagonal array. The structural features of this new phase are compared to those of the normal hexatic B and Sc phases.
Classification
Physics Abstracts
61.30E
-64.70M
Introduction.
During the last decade the polymorphism of « polar
mesogens » has been extensively studied [1].
By «polar mesogens », we mean rod-like molecules with one nitrile or nitro end group since these groups induce a large longitudinal dipole. The
richness of their polymorphism is due to their ability
to form several smectic A phases of different layer
thicknesses. Two characteristic periods can coexist
in the same compound and the competition between
these two periods is at the origin of SmA-SmA transitions or of reentrant nematic or smectic phases.
Moreover, the coupling of the two characteristic
lengths can induce the formation of a 2D periodic phase made of ribbons. The interfaces between
adjacent ribbons are of two kinds : one of the two is
made of the methyl terminal end groups of the molecule and the second one is between two zones
of reverse orientation of the electric dipole.
The 2D lattice can be centered rectangular (SÃ)’ rectangular [2] or oblique (S:;). In all these phases a 2D liquid order of the molecular centers of
mass is superimposed over the 2D long period periodic order. In fact, very few polar compounds
exhibit a layer structure with a regular array of molecules inside the layers. Except for a modulated
SmE phase [3], the ordered phase of polar molecules
are of the 5mB type with a thickness ot the layer
comparable to the molecular length. We present here a new series of polar molecules with the general
chemical formula :
In this series, we have seen a direct transition SmA-SmB for 3 -- n : 8 and a new sequence
SA SC SB for n = 9, 10, 11. Therefore, we have undertaken an X-ray structural study of the SmB
phases and of the SC SB transition.
1. Experimental conditions.
1.1 SYNTHESIS.
-The studied polar compounds
were prepared according to the following scheme :
4-Bromethylbenzonitrile 2 and 4-hydroxybenzal- dehyde 3 were obtained from Fluka Company.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01987004805084700
848
-
4-(4’cyanobenzyloxy) benzaldehyde 4 : a solu-
tion of 2 (7.28 g ; 0.04 mol), 3 (4.88 g ; 0.04 mol),
and anhydrous K2CO3 (28 g ; 0.2 mol) in 200 ml acetone was refluxed for 2 h. The solution was
filtered off, the solid was washed with hot acetone and the solvent was evaporated. The recrystallization
of the residue from ethanol resulted in 4 (6.8 g ; 80 %), m.p. : 114 °C.
-
4-n-alcoxyanilines 5 : These compounds were prepared according to the literature (4) by heating a
solution containing potassium hydroxyde, p-hydrox- yacetanilide and alkyl bromide following the hydro- lyse with concentrated potassium hydroxide solution.
-
4-(4’-cyanobenzyloxy) benzylidene-4"-n-alko-
xy aniline 1 : a solution of 5.5 mmol of aniline 5 and 5 mmol of aldehyde 4 in 15 ml of absolute ethanol with one drop of acetic acid was refluxed for
2 h. After cooling the crystals obtained were recrys- tallized from absolute ethanol until the transition temperatures were constant. Yield : 50-76 %.
1.2 PHASE CHARACTERIZATION. - The characteri- zation was performed through optical texture obser-
vations and miscibility method. Some compounds
were studied by X-ray diffraction. The transition temperatures and type of mesophases are given in
table 1. The first two compounds (n = 1, 2) only
exhibit the nematic (N) phase with schlieren or
marbled textures. From the propyloxy to octyloxy in
addition to the nematic phase, the compounds display two smectic phases : SA and SB. On cooling
from the nematic phase, the SA phase appears with focal conic fan or homeotropic textures. Further cooling produces a transition to the SB phase, the
transition is marked by transition bars which are
usually observed at a smectic A to crystal B phase change. The bars cross the fans at the start of the transition, widen, meet and disappear.
In the nonyloxy derivative, below the N and
SA phases, one has three smectic phases Sc, SC, SB. Schlieren and fan shape textures characterize the Sc phase. The St is also seen for n = 10, 11, 12 and presents the typical developable domain texture [5] figure 1. For n > 13, the Sc disappears and a Sc phase is seen with probably a partially bilayered organization.
1.3 X-RAY DIFFRACTION.
-Our first experiments
on a magnetically aligned sample in the nematic
phase confirm the existence of a crystalline SmB phase for compound n = 5, 6, 8. For the n = 9 and 10 derivatives, it appears that a two dimensional hexatic structure can take place below the St phase.
In order to characterize the molecular ordering with
better accuracy, we have performed X-ray scattering experiments on aligned samples using the synchrot-
ron radiation facility at LURE.
The sample is held in a Lindemann glass tube of
1.5 mm of diameter and aligned in a magnetic field
of 0.3 T. The capillary tube is horizontal. The
magnetic field is vertical and the sample holder can
rotate around the vertical axis which is perpendicular
Table I.
-Transition temperatures (OC) of compounds 1.
The meanings of the signs used in this table are :
K crystalline phase
N nematic phase
S smectic phases, A, C... : SA, Sc
I isotropic phase
.
the phase exists
-
the phase does not exist
( ) monotropic phase.
Fig. 1.
-Optical textures of the decyloxyderivative. a) Smt; b) SmB.
to the incident X-ray beam. The diffracted X-rays
are collected on a photographic plate (the film-to- sample distance is 164 mm). The wavelength is
1.68 A. Within these conditions, the resolution
Aq in a horizontal plane is of 5 x 10- 3 Å - 1 and in the vertical one of 3 x 10- 3 Å - 1 but we have to note that for large diffraction angles (= 20°) the size of the sample can limit the resolution since the diameter of the tube is equal to 10- 2 times the film-to-sample
distance.
2. Experimental results.
Three compounds of different chain lengths have
been studied. For the shortest chain n = 8, we have
a monolayer SA1 phase and at low temperature a crystalline SB phase of nearly the same layer thick-
ness. The structure is a hexagonal compact (HCP)
one but incommensurate diffuse scattering is seen along the axis parallel to the director corresponding
to a period of about 1.8 molecular lengths. Such a
diffraction pattern has been already seen [6] and we
will focus our discussion on the other two com-
pounds :
For n = 9, we have a monolayer smectic A phase.
In the Sc phase, one has simultaneously several
orientations for the 2D lattice with a global uniaxial symmetry. Going down into the ordered smectic
phase, the director is still aligned along the magnetic field, and one obtains a fiber pattern of a SB phase
with a layer thickness of one molecular length ;
diffuse spots are seen at the positions of the Bragg peaks of the Sc phase (Fig. 2).
Fig. 2.
-X-ray diffraction pattern of the SmB phase of
the nonyloxyderivative T == 118.2 °C. The magnetic field
is parallel to the vertical direction.
At large angles two reflections characteristic of an
in-plane hexagonal array (100 and 110) are seen.
They are both elongated in a direction parallel to the
director and sharp in a perpendicular direction.
Microdensitometer traces of the 100 reflection for different temperatures are shown in figure 3. Very
close to the SC phase, no Bragg spot can be seen except those corresponding to the reflection on the
layer planes. The diffraction pattern is thus charac- teristic of a stack of uncoupled 2D crystalline layers.
As we decrease the temperature, Bragg peaks corre- sponding to an HCP structure appear superimposed
to a more diffuse scattering. The correlations be-
tween layers increase as we decrease the tempera-
ture. (We must notice that a disordered stack of
crystalline hexagonal layers with the molecules lo- cated randomly at position A, B, C of the hexagonal
network will induce a sharp Bragg peak at
110 position, which was never seen here). It seems
that increasing the chain length in our series favours both the fQrmation of SC and the formation of
uncoupled hexagonal 2D crystalline layers. This
trend was confirmed by our results on the next homologous n = 10.
The smectic A phase has a layer thickness equal to
=1.6 times the molecular length. In the Sc phase (Fig. 4a), we obtain several large domains of the ribbon lattice. Going into the ordered phase, the
diffraction pattern is that of a polydomain sample (Fig. 4b).
A comparison between the pattern of figure 4b,
patterns of the same sample but with another
850
Fig. 3.
-Densitometric scans of the reflection upon the
hexagonal lattice planes q, is parallel to the molecular director and the intensity is measured in arbitrary unit.
orientation and powder patterns show that a large
scale 2D lattice similar to that of the SC one is superimposed over a hexagonal ordering of parallel
molecules. Table II gives the lattice spacings of these
two lattices. A further analysis of figure 4b allows a
better description of the structure of this phase.
Besides an interrupted ring lying at a lattice spacing
of 4.35 A, one sees two. distinct rectilinear bars lying
Table II.
-Lattice spacing for the two 2D lattices of
the SmB phase of the decyloxyderivative T = 112.5°.
Fig. 4.
-X-ray diffraction patterns of two smectic phases
of the decyloxyderivative. The magnetic field is parallel to
the vertical axis. a) Sc: phase, T =126 °C ; b) SB phase,
T = 112.5 °C.
at the same distance from the origin. At small angle,
two lines of Bragg spots parallel to each of these bars
can be identified. These Bragg spots all correspond
to reflections on 11 planes of the ribbon oblique
lattice. The other reciprocal points of this oblique
network are not seen in figure 4b, but diffuse
scattering intensity at nearly the same location as the
01 spot can be observed. It is possible, from some symmetry arguments, to extract from the pattern 4b
two sections of the reciprocal space in which one finds a bar (A) at large angle, two or three aligned Bragg spots at small angle (B) and some diffuse
spots in the same zone (C). The two sections of the
reciprocal space in figure 5 correspond to two diffe-
rent single crystals and therefore can bring com- plementary information. Let us initially neglect the
diffuse spots C. Our pattern is similar to that of
figure 2, the layer spacing corresponding to the
molecular length. In scans along and perpendicular
to the bars lying at ql. = 4.;; A (Fig. 6), the inten-
sity dependence is characteristic of a stacked hexatic
Fig. 5. - Schematic drawing of the pattern of figure 4b.
The axes I and II are respectively parallel to the director of two single crystals. The open spots come from crystal I and
the hatched one from crystal II. A are the diffraction bars from the 2D hexatic lattice. B the Bragg peaks of the oblique lattice and C diffuse spots. The central part B and C is not at the same scale as the outer part A.
Fig. 6.
-Scan of the reflection coming from the hexatic lattice of crystal I of the decyloxyderivative. The dashed
line is a Lorentzian curve corresponding to a correlation
length of 300 A.
phase [7]. A ql scan shows clearly that we have no positional correlation of the hexagonal order be-
tween adjacent layers. The maximum of intensity along the bar lies in the equatorial plane. Therefore
within our accuracy, the director of the molecule is
perpendicular to the layer plane for the two sections
of the reciprocal space. Nevertheless, a slight differ-
ence appears on q, scans between the two sections.
In both cases we have a Lorentzian shape but the
width is slightly larger for crystal II than for crystal I (Figs. 5, 6). The corresponding correlation lengths
are respectively 260 A and 300 A at T = 112.5 °C
(which is fairly large compared to usual hexatic B systems (8)).
’Going down in temperature, the width decreases and the shape of the line is dominated by the size of
the single crystal. Therefore, we estimate that this size reaches at least 0.1 mm. A second important
characteristic of the hexagonal lattice is its azimuthal disorientation.
We have not performed a systematic exploration
of reciprocal space, but we can notice that a small rotation ( 3°) of the sample around a vertical axis
(at 20° from the director) does not change the intensity of the bar. Nevertheless, with a rotation of
20°, the bar disappears and no other hexagonal
reflections are seen. We can thus estimate that the disorientations do not exceed ten degrees.
As we have seen above, the peaks characteristic of
a long period 2D ordering are not seen in figure 4b.
Nevertheless, by rotation of the sample we are able
to obtain four small angle reflections. An oblique
lattice can be defined. The (11) planes correspond to
the smectic layer planes. The other three reflections
are the 01, 02,11 reflections (1). By analogy with the Sn-i,C phase, we can assume that this second network which lies in a plane perpendicular to the layers originates in the periodic modulation of the electric
longitudinal dipole, figure 7. Apparently, this modu-
lation does not couple strongly with the hexagonal
molecular ordering. We have no direct information upon the orientation of the ribbons with respect to the hexagonal lattice. Nevertheless, the existence of diffuse spots C can bring some evidence about the symmetry of the reciprocal space. The diffuse spots C are nearly in symmetrical positions with respect to
a 110 plane of the hexagonal reciprocal lattice for
crystal I ; but for crystal II a large disymmetry is
seen around a similar plane of the hexagonal lattice.
Since the length of the wave vectors for the diffuse
spots is that of the 01 reflection of the oblique
network we can assume that diffuse scattered inten-
sity is localized around this point and thus the symmetry of these diffuse spots in three dimensions
(1) These reflections come from small domains and cannot be assigned to one of the two single crystals
described below.
852
Fig. 7.
-Schematic representation of the modulated structure of the hexatic B phase of the decyloxyderivative.
The exact position of the molecule in each layer is not represented here, and the symbolic arrow is put in order to show the dipolar modulation which is superimposed over
the hexatic molecular ordering.
corresponds to the symmetry of the 2D oblique
lattice. Therefore, the reciprocal plane of this 2D
lattice is nearly perpendicular to the section of
crystal I, and makes a small angle with the section of
crystal II, figure 5b. Figure 8 illustrates this point,
and gives a schematic drawing of the section of the diffuse spot consistent with figures 4 and 5.
Moreover, the existence of such a scattered intensity
is indicative of some dipolar disordering which
occurs inside the ribbons. The same kind of disorder but with at least a six-fold symmetry exist in crystal B phases of shorter homologous n = 8 and 9, and is at
the origin of the diffuse spots seen in figure 2.
Moreover, the energy scattered out of the Bragg peaks is localized near the intense peak 01 of the oblique lattice. In other words, fluctuations of
wavelength, equal to the thickness layer in the SAd phase, are predominant.
Since the relative orientation of the two networks is known, we can determine the ratio between the two periods along their common row, which is [10]
for the hexagonal lattice and [11] for the oblique
one. This ratio is equal to 21.1 while the in-phase planes for the polarization modulation are tilted with respect to the normal to the layer planes at an angle
of 11°2. Finally we consider the effect of the modulation of the polarization upon the stacked hexatic molecular ordering.
The superposition of a polarization modulation to the hexatic order improves the short range molecular
positional order. The correlation length for this
order is 300 A in a direction perpendicular to the
modulation (crystal I). At an angle of vl3 of this
Fig. 8.
-Projection of the central upper part of the reciprocal space in modulated hexatic B phase. The arrows point in the direction of the 6 equivalent (100) axes of the
hexatic reciprocal lattice, the section of crystals I and II corresponding to figure 4b and 5 are shown and double
arrows point the reflections of the hexatic lattice seen on
figure 4b for each crystal (taking into account the Ewald sphere curvature). The solid line is a contour line of the diffuse spot which surrounds the 10 reflections of the
oblique lattice. This spot is a portion of a spherical surface going through the 10 points rather than a planar section of
the reciprocal space.
direction, the correlation length is slightly less (260 A for Crystal II). These lengths have been
measured half a degree below the Sè - SB transition
and they increase as the temperature decreases.
3. Conclusion.
The X-ray study of a new series of polar mesogens has added insight regarding the influence of the
polar ordering upon the molecular ordering.
The long range unidimensional periodic modu-
lation of polarization, which is present in a number of fluid mesophases of polar molecules disappears in crystalline smectic B phases, but can coexist with a
stacked hexatic ordering. The unidirectional charac- ter of the modulation is disturbed when the hexatic
ordering occurs since short range correlations of
nearly the same wavelength can exist in other
directions. Compared to non-polar molecules, the hexatic short-range positional ordering is improved
and a slight anisotropy of the correlation length is
induced by the existence of the modulation of
polarization. Let us remark that the Sc and this new
« incommensurate » hexatic phase have the same symmetry and that the existence of a real phase
transition is questionable, as it is for the Sc S, problem. The imbrication of two 2D lattices could
imply specific viscoelastic properties. An exper-
imental study of these properties must be done on
large single domains, and we have to improve our
technique of orientation.
Acknowledgments.
’
