Full miscibility of disk- and rod-shaped mesogens in the nematic phase

Article abstract of DOI:10.1021/ja037075y

Mesogens containing rod-shaped moieties as well as one disk-shaped group exhibit nematic phase behavior and are entirely miscible with rod- and disk-shaped mesogens in the nematic phase. In the phase diagram, an extra nematic phase appears at nearly equim

Full text of DOI:10.1021/ja037075y

Published on Web 08/19/2003
Full Miscibility of Disk- and Rod-Shaped Mesogens in the Nematic Phase
Paul H. J. Kouwer and Georg H. Mehl*
Department of Chemistry, UniVersity of Hull, Cottingham Road, Hull, HU6 7RX, U.K.
Received July 4, 2003; E-mail: g.h.mehl@hull.ac.uk
Scheme 1. Synthesis of the Disk-Rod Mesogens
Rod-shaped (prolate) and disk-shaped (oblate) liquid crystals can
form a uniaxial (Nu) mesophase, characterized by long-range
orientational order in one direction. Although the nematic phases
for both type of molecules are fundamentally identical, mixing
between disks and rods in the thermotropic nematic phase has so
far never been observed. Those mixtures are of particular interest,
as already 33 years ago the existence of the nematic biaxial (NB)
phase1 has been predicted at certain ratios;2 see Figure 1. The NB
phase is characterized by orientational order in two directions.
containing moiety 3 was prepared by etherification of 4-hydroxy-
4′-cyanobiphenyl and 1,10-dibromodecane under Williamson con-
ditions. Subsequent etherification of the product with ethyl 3,4-
dihydroxybenzoate yielded 3. Deprotection of 3 and esterification
with 2 afforded the linked disk-rod mesogen 1.13 The phase
behavior of the pure components is summarized in Table 1. All
materials exhibit a nematic mesophase, although the mesophase of
3 is monotropic.
Figure 1. Schematic phase diagram containing the NB phase. For board-
shaped molecules, x scales with the ratio b/a; for mixtures of disks and
rods, x represents the fraction of disks. The exact shape of the phase
transitions between the NB and the Nu phase depends on the model used.
Experimentally, the nematic biaxial phase has been accomplished
for ternary lyotropic mixtures,3 and also results for side-chain
polymers indicate some phase biaxiallity.4 Efforts to realize the
NB phase in thermotropic systems include two main routes of
investigation. One approach combines the rod and disk shape of
conventional mesogens to a more boardlike shape.5 An alternative
approach is based on mixtures of prolate and oblate mesogens6-9
requiring the miscibility of both species over the entire phase
diagram. To avoid phase separation between the two entities,
molecules combining disk- and rod-shaped moieties have been
prepared and studied;10 however, no continuous rod-disk phase
diagrams have been constructed. Mixtures of oblate and prolate
mesogens have been subject to intensive computer simulations.
Common for all models is the occurrence of the NB phase around
the minimum of the isotropization temperature in the disk-rod
phase diagram (see Figure 1). Furthermore, the various models
predict a strong decrease of the latent heat of the Nu to I transition
upon approaching the tricritical point.
Encouraged by promising initial results for related materials,11
we synthesized the novel mesogen 1 (see Scheme 1) bearing both
calamitic and discotic mesogens. Compound 1 mixes without a
miscibility gap with 3, and the transitions of intermediate mixtures
show a nearly linear dependence of the clearing temperature with
the mol fraction of 1. In addition, 1 mixes with the discotic mesogen
2 over the entire phase diagram. The characteristics of the phase
diagram of 1 and 2 are presented, and the results of the miscibility
studies are correlated with theoretical and simulation studies.
The synthesis of the investigated products is outlined in Scheme
1. The preparation of 2 has been published before.12 The rod-
Table 1. Phase Behavior of Mesogens 1-3
material
phase behaviora
1
GN
Cr
Cr
31
137 (37)
118 (104)
N
N
N
77 (1.0)
246 (0.2)
106 (5.1)]
Ib
I
I
2
3c
a Transition temperatures in °C (latent heat in kJ mol-1), GN ) glassy,
nematic phase frozen in, N ) nematic, I ) isotropic, Cr ) crystalline.
b Compound 1 shows a single melting transition (Tm ) 113 °C, ∆H ) 3.4
kJ mol-1) during the first heating run that does not return in subsequent
runs. c Monotropic phase transition.
The mixing behavior was investigated by studying contact
samples with optical microscopy and by preparation of discrete
mixtures, which were studied by both OPM and DSC. The resulting
binary phase diagrams are shown in Figure 2a and b. Contact studies
of 1 and 2 and 1 and 3 showed a complete miscibility over the
entire composition range.14 Attempts to detect nonmiscible regions
have failed. A nematic phase, characterized by a typical schlieren
or marbled texture, is observed over the full width of the phase
diagram of 1 and 2.13
Destabilization due to the mixing process causes a minimum in
the clearing temperature (∼55 °C) at φ2 ≈ 0.56. The latent heat of
the clearing transition decreases strongly, going from high fractions
of 1 to the center of the phase diagram (Figure 2c). A minimum is
observed in the region of the lowest transition temperatures. Indeed,
these mixtures have isotropization enthalpies below 0.1 J g-1
,
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10.1021/ja037075y CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Mixtures of disk fractions 0.54 e φ2 e 0.60 show a second
transition in the DSC traces between the glass transition and the
clearing temperature; see II and III in Figure 2d. The small transition
is not associated with a change of the optical texture, even after
prolonged annealing. Preliminary X-ray studies show only diffuse
halos in the wide and the small angle region, indicating the absence
of any positional order and, hence, excluding the assignment to
smectic or columnar phase structures.13
As no demixing could be observed experimentally, the assign-
ment of the extra NX phase to a NB phase correlates the achieve-
ments of theory and the experiment best. Demixing would have
pointed toward a phase separation process into two uniaxial phases,
predicted by theoretical models8,15 to occur at temperatures below
the NB phase or instead of the NB phase.
In conclusion, these results indicate that it is possible to
synthesize molecules which are completely miscible in the nematic
phase with rod-shaped and disk-shaped molecules. To the best of
our knowledge, we presented the first experimentally obtained phase
diagram of thermotropic discotic and calamitic mesogens. The
construction of such a phase diagram is a major step forward in
the design and development of future materials exhibiting a nematic
biaxial phase. As theory for the NB phase predicts, the mixtures
exhibit a minimum in the transition enthalpies when approaching
the minimum nematic to isotropic transition. Furthermore, a
subsequent nematic-nematic transition can be observed, in line
with theoretical predictions, thus requiring further investigations
(e.g., solid state NMR on systematically deuterated samples) to
confirm phase biaxiallity.
Acknowledgment. We thank Prof. D. Photinos, Patras, Greece
for helpful discussions, and the Ramsay Memorial Fund and the
EU (contract HPRN-CT2000-00016) for financial support.
Supporting Information Available: Experimental procedures,
phase diagrams, OPM and XRD data of mixtures (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
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Figure 2. (a) Phase diagrams of 1 and 3. (b) Phase diagrams of 1 and 2.
Experimental data (symbols) are obtained from DSC studies, apart from
the mixtures showing the Nu+I biphasic area (data from OPM). The lines
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The dotted line represents the maximum resolution of the DSC for
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0.57; and (IV) 0.37:0.63. The arrows indicate the maxima of the phase
transitions.
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