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High resolution X-ray scattering study of the multiply reentrant polar mesogen DB9ONO2
E. Fontes, P.A. Heiney, J.L. Haseltine, A.B. Smith
To cite this version:
E. Fontes, P.A. Heiney, J.L. Haseltine, A.B. Smith. High resolution X-ray scattering study of the multiply reentrant polar mesogen DB9ONO2. Journal de Physique, 1986, 47 (9), pp.1533-1539.
�10.1051/jphys:019860047090153300�. �jpa-00210352�
High resolution X-ray scattering study of the multiply reentrant polar mesogen DB9ONO2
E. Fontes, P. A. Heiney, J. L. Haseltine (+) and A. B. Smith, III (+)
Department of Physics and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, PA 19104, U.S.A.
(+) Department of Chemistry and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, PA 19104, U.S.A.
(Reçu le 21 janvier 1986, accepté le 13 mai 1986)
Résumé. 2014 Les phases cristal liquide de la molécule polaire DB9ONO2 montrent des fluctuations selon deux échelles incommensurables et dépendantes de la température. Nous avons utilisé la diffusion des rayons X à haute résolution pour étudier la structure de ce matériau entre 80 °C et 230 °C à la pression atmosphérique. Nous con-
firmons la séquence de phases nématiques réentrantes multiples qui avait été annoncée précédemment. Les phases nématiques à basse température sont très ordonnées, avec des longueurs de corrélation qui dépassent 3 300 A
loin de la limite de phase. Nous trouvons également une phase Sc entre les phases basse température SA1 et Sc.
Abstract. 2014 Liquid crystal phases of the polar molecule DB9ONO2 display fluctuations on two incommensurate and temperature dependent length scales. We have used high resolution X-ray scattering to study the structure
of this material between 80 °C and 230 °C at atmospheric pressure. We confirm the previously reported phase
sequence of multiply reentrant nematic phases. The low temperature nematic phases are highly ordered, with
correlation lengths exceeding 3 300 A far from the phase boundary. We find evidence of an Sc phase between the low temperature SA1 and Sc phases.
Classification
Physics Abstracts
61.30 - 61.10 - 64.70
1. Introductioa
Recently, there has been considerable experimental
and theoretical interest in the structure and transi- tions of liquid crystals composed of polar molecules.
Many different thermodynamic phases are found in
these materials, and the structure and phase diagram
of a particular material can depend sensitively on the
details of the molecular structure. In addition to nor-
mal layered smectic SA phases, the polar nature of
the molecules may favour the formation of a dimer-like
layered structure labelled SAd. Ratna et al. [1] recently reported the observation of a smectic-A phase with
two collinear incommensurate density modulations,
which is presumably the result of a competition bet-
ween SA and SAd ordering. This type of incommen-
surability, in which both layer spacings play an equal role, is considerably different from that in systems such as charge density waves or adsorbates, in which
a structure with one wavelength is only weakly mo-
dulated by a structure with a different periodicity. The properties of such « soft » incommensurate systems
are not well known. Polar liquid crystals also com-
monly display a « reentrant » nematic phase appear-
ing at a lower temperature than a more highly ordered
smectic phase.
We present here structural studies of one such
polar molecule, DB90N02, which exhibits [2, 3] ten
known stable phases including a doubly reentrant
nematic phase. DB90NO2 (4-nonyloxyphenyl-4’-ni- trobepzoyloxybenzoate) consists of terminally nitro-
substituted triaromatic molecules
with a strong spontaneous electric moment. Pure, bulk samples of DB9ONO2 have been shown to exhibit the phase sequence at atmospheric pressure
on cooling :
Here the symbols I, N, and S refer to isotropic liquid, nematic, and smectic phases; SA phases have a density
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019860047090153300
1534
modulation parallel to the long molecular axis (direc- tor) and Sc phases have a density modulation at a
finite angle with respect to the director. Nematic reentrancy has been phenomenologically explained [4]
by a Landau theory incorporating an optimum den- sity for the smectic phase. Recent theories [5-10] have suggested that the unusual smectic polymorphism [11]
observed in DB90NO2 and similar compounds is the
result of frustration arising from two preferred length
scales. In this case, two lengths are simultaneously present at the molecular level : the steric length of the
molecule I, and a favoured dipole-dipole nearest neighbour pair length l’ (l l’ 2 1). In DB90N02,
the length l’ is found to vary significantly with tem- perature. As a result, quite different structures that
are very close in energy, including multiply reentrant
nematic phases, may develop.
We have used X-ray scattering to probe the wave- length dependence of ordering in bulk samples of DB90NO2. We discuss in detail three regions of the phase diagram. Three N and two SAd phases are
interleaved between 200 °C and 121.5 OC, characte- rized by a sharp scattering peak at q 2 nll’ and
simultaneous diffuse fluctuation peaks at q z 2 rell.
The longitudinal correlation lengths in the lower temperature reentrant N phases are remarkably large.
In the SA1 -+ Sè phase transition we find scattering profiles which appear to be due to the coexistence of
SA1 and Sc phases with short range polarization modu-
lations. Lastly, we discuss briefly a third, incommen-
surate scattering peak which we have observed in the
SA2 phase.
2. Experimental method.
The DB90N02 sample was synthesized and purified using standard techniques, and kept in an inert atmosphere at all times when heated. The material
was loaded into a 2 mm-thick brass sample cell with
two 5 mm x 5 mm x 25 J.1 Be windows (Fig. 1). The sample cell sat inside a Be cylinder 12.7 mm in dia-.
meter and 25.4 mm high, which was fixed between the poles of a 3 kG permanent magnet Both the cylinder and the cell were temperature controlled,
with the cylinder 1-2 °C cooler than the inner sample
cell. Temperature reliability over the duration of data collection was ± 0.02 °C for the cylinder, and
± 0.002 °C for the sample cell. Both inner and outer heaters were balanced to give at most a 0.01 °C gra- dient across the sample. The temperature setpoints
were under computer control; this allowed slow,
careful adjustment to assure accurate temperature variation without overshoot. The sample cell remained in the magnetic field throughout the experiment. An
induced diamagnetic moment in the triaromatic
DB90NO2 molecule favours a bulk alignment of the
molecular long axis parallel to the magnetic field
direction. The homogeneity of the magnetic field was
better than 1 %.
Fig. 1. - Illustration of sample cell and scattering geome- try, showing the incident and final wavevectors k; and kc,
the reciprocal space basis (q jj, q,), and the magnetic field H.
The rectangular sample cell is surrounded by a Be cylinder.
A thermal insulating shroud (not shown) encloses the entire
assembly. Short lines inside the sample cell indicate sche-
matically the molecular orientation.
We used CuKa or MoKa radiation produced by rotating-anode X-ray generators together with a computer-controlled diBractometer to collect scat-
tering data from several samples. Low resolution data utilized a vertically bent graphite crystal monochro-
mator and Soller slit analyser with NaI scintillation detector. High resolution data were taken with a flat
germanium crystal monochromator and analyser.
The scattering geometry was as shown in figure 1;
the magnetic field rotated with the sample. We define q II as the component of the scattering wavevector parallel to the magnetic field and q.L as the component perpendicular to the magnetic field; the magnitude
of the scattering vector is given by q = (4n/ À) sin (0) =
2 nld The longitudinal correlation length is given by
Çll = 2/Aqll, where All is the measured peak full-
width at half-maximum (FWHM). Measurements above 150 °C were done in a low resolution configu-
ration characterized by Aq 11 = 0.0175 A-1 and eql =
0.004 A - ’; measurements in the vicinity of 115 °C
were done in a low resolution configuration characte-
rized by Aqjj = 0.0062 A-1 and eql = 0.003 A-1;
high resolution measurements were characterized by
Aqjj II = 0.0005 A-’ and Oql = 0.0002 A-’. (In most
cases, the actual width of profiles in the q.L direction
was limited by the >, 1 ° sample mosaic, rather than
by the instrumental optics). The diffraction lineshapes
were also influenced by the relatively poor out-of- plane instrumental resolution, which was kept broad
to maximize diffraction intensities; in some cases
this resulted in spurious scattering on the low q side
of a peak due to the orientational mosaic of the crystals.
We present the data in the form of high resolution q II scans and also as intensity contours from low reso-
lution diffraction grids, or meshes, in the q I, - q I
plane. Although the two sets of data were taken on
different samples of DB90N02, each series was
collected during a monotonic decrease in temperature of the same sample. This method was followed because it was found that cycling through a transition tempe-
rature produced the same phase, but could grow a
crystal with different mosaic orientation.
3. Results and discussion.
Table I summarizes our diffraction data, which were
taken in order of decreasing temperature. Since it was found that samples of DB9ONO2 degraded at high temperatures over the time necessary to collect high
resolution data, phases found above 150 OC were
only studied with low resolution. The high tempera-
ture I and N phases show diffraction maxima at q z 2 x/40 A. In comparison, the S 1 phase condenses
with a layer spacing of d = 30.76 Å. The stereomodel
prediction [2] for a fully extended monomolecular
length gives 1 = 32.5 A. These data reveal, therefore,
that as in the lower temperature phases, the I and N phases show a strong disposition to form stable
dipole-dipole dimer pairs with length l’ z 1.29L.
The N -+ SAd phase transition at 195 °C proceeds
as commonly found in nematic to smectic A phase
transitions : the longitudinal peak scattering inten- sity grows and sharpens, signaling the onset of a long
range density modulation characteristic of an SA phase. The molecular axis is aligned by the magnetic
field in the N phase, yielding a mosaic of 10° FWHM just above 195 OC; the mosaic evolves to z 1.2°
FWHM in the SAd phase. The finite mosaic in the
SAd phase is most likely due to inhomogeneity in the
direction of the magnetic field (on the order of ± 0.60)
and possibly also to wall pinning effects.
At temperatures below 145 °C the material was
found to be free from degradation for as long as two weeks, allowing detailed lineshape studies with high
resolution (Fig. 2). Pure samples of DB90NO2 show
the following properties above 121.5 °C : 1) three N
and two SAd phases are interleaved, 2) there is a strong
Table I. - Summary of measured DB90N02 diffraction peak positions and widths as a.function of temperature.
Measurements were made in either low or high resolution configurations, as discussed in text. q is the compo-
nent of the scattering vector parallel to the molecular director, and ql measures scattering perpendicular to the
director. Aq is the peak ,full-width at half-maximum. The SA2 and SC2 phases had a large orientational mosaic, and in these cases q 11 = q is the magnitude of the scattering vector.
1536
Fig. 2. - High resolution scattering profiles along the
molecular director (qlj) for the phases Ni,re (139.8 °C), SAd,re (133.8 °C), N,,,.(123.6 °C), and SAI (121.5 oC). Plots
at 117.7 °C and 115.7 °C show the angular or powder
average of low resolution data in the corresponding dif-
fraction mesh of figure 3. The noise is primarily a numerical
artifact due to the averaging procedure rather than being
statistical in nature. The intensity scales on these two plots
are the same, but not in agreement with the other four
high-resolution profiles.
temperature dependence of the dipole-dipole pair length 1’, 3) as the temperature decreases there is an
evolution of the molecular correlation length ç II in the three N phases from 30 A to 1 000 A to >, 3 300 A.
(The lowest temperature nematic phase, N2,re, was primarily distinguished from the SAd phase via optical microscopy, although we also observed changes in
the X-ray peak intensity near the phase boundaries.)
Our measurements show that the dimer length I’ varies continuously from z 1.2 1 to z 1.5 l over tempera-
tures from 227°C to 121.5°C. Dielectric studies [3]
have shown that the frequency of relaxation and the dielectric constants 8 jj and 81 also vary smoothly over
this same temperature range, except for a small jump
at 195 OC and an abrupt evolution of the N2,re phase.
We expect, then, that at the reentrant phase boundaries
there should be only small differences in the local molecular environments and hence small differences in the free energy densities of the adjoining phases. In
addition to the smectic phases immediately above and
below the reentrant nematic phases, it is known [11] ]
that the of a small amount of DBloON02 will restore
the smectic phase. Previous observations [12] have
shown that a smectic phase which is nearby along a
concentration axis can induce strong layer fluctuations in the nematic phase. Thus, the dramatic growth of
all II in the reentrant nematic phases can be explained by the presence of nearby smectic phases along both
the temperature and concentration axes. In the Nre phases, the viscosity appears to quite high, and the samples have a clumpy texture somewhat different from the critical opalescence seen in ordinary nematic
fluids.
The appearance of the SA1 phase is signaled by the growth and sharpening at 121.5°C of a scattering peak at q 11 = 2 x/30.76 A = 0.2042 Å - 1. The q scans
in figure 2 are plotted using a logarithmic intensity
axis to make visible the thermal evolution of this
peak. Although reduced in intensity by a factor of 1 000, the peak is clearly visible far from the transition.
This scan sequence shows that the system displays at
all temperatures fluctuations into two incommensu- rate lengths; in general at most one has condensed into a long range layer structure and other other
persists as a thermal fluctuation. Detailed fits to these
lineshapes are in progress.
We illustrate the SAl -+ Sc phase transition via contour plots of two-dimensional diffraction data shown in figure 3 and cuts through these data in the
Fig. 3. - Equal intensity contours of low resolution data taken in the q jj
- ql scattering plane, at the temperatures shown (OC). Contours are drawn at values 4 642, 2 154, 1 000, 464, 215,100, 46, 21, and 10 counts/2 seconds. Repre-
sentative peak intensities are shown in figure 5. The straight
lines on the 119.7 °C plot indicate the limits of data collec- tion due to sample cell occlusion of the X-ray beam.
ql direction shown in figure 5. Previous work [2, 3]
established 119 °C to 100 °C as the temperature
range of the Sc phase. Within that region, we have
found an evolution between two different scattering profiles : a vanishing SAt phase accompanied by a developing Sc phase with SA-like polarization modu- lation, and below 116 °C resolution limited peaks typical of the Sc phase. This region of the phase dia-
gram was studied carefully in five different samples,
Fig. 4. - Top : Real space model (Ref. [6]) of the Sc phase,
decorated with a polarization modulation unit cell. Arrows indicate the average polar orientations of the molecules,
and are only shown at maxima of the polarization modula-
tion. An equal density of « up » and « down » molecules is found halfway between adjacent maxima. Solid lines
indicate the average positions of the molecules, and show
the locations of constant phase in the density modulation (with normal Zp). Broken lines, with normal 1,1, indicate
lines of constant phase in the polarization modulation.
H is the direction of the long molecular axis and the ma-
gnetic field The unit cell vectors a and b and the reciprocal
vectors a and b are defined and indexed as shown.
Bottom : Equal intensity diffraction contours (as described
in Fig. 3) in the Sc phase at 105.7 °C. Reciprocal space vectors are indicated as discussed above. The mosaic average of the crystal about the magnetic field duplicates the three primary peaks.
Fig. 5. - X-ray intensity as a function of ql, at constant
qll, extracted from the mesh scans in figure 3. The corres- ponding temperatures (OC) and qll (A-1) values are given.
Note the difference in vertical scale between the low q i
(left) and high (right) plots.
with reproducible results. The samples were checked periodically for degradation by verifying several
transition temperatures; the SAZ phase, for example,
is extremely sensitive to sample purity [3]. Tempera-
tures were changed slowly « 1 OC/5 min) to guaran- tee thermal equilibrium, and no evidence for meta-
stability or annealing was seen over periods as long
as 72 hours, although the mosaicity did increase somewhat with decreasing temperature and increasing
tilt angle.
The Sg phase may be described [6] as an energeti- cally driven commensurate locking, in two dimen- sions, of the incommensurate density and polarization
modulation lengths, resulting in a tilted layer struc-
ture with a skewed polarization modulation. The real space structure (Fig. 4a) is decorated with unit cell basis vectors to display a general polarization lattice (two non-collinear density waves) superimposed on
an Sc density modulation. Generally, in an Sc phase
the tilt angle between the molecular director and the
density wave may vary with temperature. Accordingly,
a general description of the real space structure of an
Së phase (which treats the tilt angle of the density and polarization modulations equally) must include the
following parameters : the length of the molecule l,
