Semiflexible liquid crystalline tetramers as models of structurally analogous copolymers

Article abstract of DOI:10.1021/ma0502304

The synthesis and characterization of a series of liquid crystal tetramers is reported. These consist of molecules containing four liquid crystal units connected via three flexible alkyl spacers. The length of the central spacer is held at six methylene u

Full text of DOI:10.1021/ma0502304

Macromolecules 2005, 38, 3307-3311
3307
Semiflexible Liquid Crystalline Tetramers as Models of Structurally
Analogous Copolymers
Peter A. Henderson and Corrie T. Imrie*
Department of Chemistry, School of Engineering and Physical Sciences, University of Aberdeen,
Meston Walk, Aberdeen AB24 3UE, United Kingdom
Received February 2, 2005
ABSTRACT: The synthesis and characterization of a series of liquid crystal tetramers is reported. These
consist of molecules containing four liquid crystal units connected via three flexible alkyl spacers. The
length of the central spacer is held at six methylene units, while that of the outer spacers is varied from
three to twelve units. All the tetramers exhibit an enantiotropic nematic phase. The transitional properties
exhibit a strong dependence on the length and parity of the outer flexible spacers. The properties of this
series are compared to those of the corresponding series containing a pentamethylene central spacer.
This change has a smaller than anticipated effect on the transitional properties and suggests that the
four mesogenic units in the tetramer are not strongly correlated. The behavior of the tetramers is shown
to strongly resemble that of a copolymer series containing differing lengths of spacers.
Introduction
to polymeric systems, here we report the synthesis and
characterization of a new series of liquid crystal tet-
ramers in which the length of the central spacer is held
at six methylene units while the lengths of the outer
spacers are varied from 3 to 12 methylene units:
Liquid crystal dimers consist of molecules containing
two mesogenic units linked by a flexible spacer1,2 and
exhibit properties quite different from those of conven-
tional low molar mass mesogens containing a single
semirigid core. In addition to being of considerable
interest in their own right, dimers have proved also to
be valuable model compounds for semiflexible main-
3
chain liquid crystal polymers. In particular, the tran-
The tetramers are referred to using the acronym
n-p6p-n in which n denotes the number of methylene
units in the outer flexible spacers. The properties of this
series will be compared to those of a corresponding one
but in which the central spacer contains an odd number
sitional behavior of dimers is strongly dependent on the
length and parity of the flexible spacer in a manner
strongly reminiscent of that seen for polymers. In other
respects, however, the dimers behave quite differently
from the polymers. For example, the tendency of a dimer
to exhibit smectic behavior decreases on increasing the
23
of methylene units, the n-p5p-n series. The extent
to which these materials can act as model compounds
to understand the behavior of liquid crystal copolymers
4
spacer length, while quite the opposite is true for
3
polymers, although it should be noted that dimers do
27
containing different lengths of spacers will be dis-
5
-7
exhibit alternating smectic phases
which were first
cussed for the first time.
8
,9
discovered for polymers.
The question now arises as to how the properties of
the dimers evolve into those of the polymers, and to
begin to answer this, a number of liquid crystal trimers
have been reported in which three liquid crystal groups
are interconnected via two flexible spacers.10-20 Again
the transitional behavior is strongly dependent on the
length and parity of the flexible spacers, resembling that
seen for polymers. It is not yet clear whether the smectic
properties of trimers are going to resemble those of
dimers or polymers because the number of reported
Experimental Section
The tetramers were prepared using the synthetic routes
shown in Scheme 1 and are identical to those used to prepare
the n-p5p-n tetrameric series.
2
3
1
,6-Bis(4-aminophenyl-4′-oxy)hexane, 2. The synthesis
of 1,6-bis(4-aminophenyl-4′-oxy)hexane is identical to that
23
described for 1,5-bis(4-aminophenyl-4′-oxy)pentane, and thus
only representative spectroscopic data for it and its intermedi-
ates are provided.
Data for 1,6-Bis(4-acetamidophenyl-4′-oxy)hexane, 1.
1
3,14,16-19
smectogenic trimers is very small.
It should be
Yield: 83%. Mp: 203-204 °C. IR (KBr): ν (cm-1) 3307 (CONH);
noted that, like the dimers, trimers exhibit highly
unusual phase behavior including novel smectic phases.21
More recently we have reported the only examples of
complete homologous series of liquid crystal tetramers,
which consist of molecules containing four mesogenic
2
2943, 2867 (CH ); 1656 (CO), 1597, 1513 (Ar-H); 825 (para-
substituted aromatic). 1 was not soluble in common organic
1
solvents, and thus, a H NMR spectrum was not obtained.
Anal. Calcd: C, 68.7; H, 7.3; N, 7.3. Found: C, 68.9; H, 7.3;
N, 7.0.
units interconnected by three flexible spacers.22,23
Data for 1,6-Bis(4-aminophenyl-4′-oxy)hexane, 2. Yield:
A
1
9
6
4
2%. Mp: 147 °C. H NMR (CDCl
3
): δ (ppm) 6.7 (m, 4H, ArH),
CH2, J ) 6.6 Hz), 3.3 (s,
CH CH CH ), 1.5 (m, 4H,
,). IR (KBr): ν (cm ) 3395, 3316
); 824 (para-substituted aromatic).
The syntheses of the R-(4-methoxyazobenzene-4′-oxy)-ω-(4-
formylphenyloxy)alkanes 4 and the R-bromo-ω-(4-methoxy-
small number of tetramers have also been reported by
.6 (m, 4H, ArH), 3.9 (t, 4H, OCH
H, ArNH ), 1.7 (m, 4H, CH CH CH
CH CH CH CH CH
aromatic NH
2
24-26
other authors.
To continue our investigation of both
2
2
2
2
2
2
2
the novel mesomorphic phase behavior of oligomers and
how transitional behavior evolves from low molar mass
-1
CH
2
2
2
2
2
2
(
2
*
To whom correspondence should be addressed. Phone: (44)
23
1
224 272910. Fax: (44) 1224 272921. E-mail: c.t.imrie@abdn.ac.uk.
azobenzene-4′-oxy)alkanes 3 have been described previously.
1
0.1021/ma0502304 CCC: $30.25 © 2005 American Chemical Society
Published on Web 03/25/2005

3308 Henderson and Imrie
Macromolecules, Vol. 38, No. 8, 2005
Scheme 1. Synthesis of the Liquid Crystal Tetramers
Table 1. Transition Temperatures and Associated
n-p6p-n Series 5. The synthesis of the n-p6p-n series
23
Entropy Changes for the n-p6p-n Series
is identical to that of the n-p5p-n series described elsewhere
except that the n-p6p-n tetramers were purified by Soxhlet
extraction using toluene as the solvent. Due to their insolubil-
ity in an appropriate solvent, no NMR data were obtained for
the tetramers. Their IR spectra, however, were consistent with
the proposed structures. Specifically the bands associated with
the stretch and bend deformations of the N-H bond in the
amine 2 and that associated with the carbonyl band in the
aldehydes 4 are absent in the spectra of the products. A new
band associated with the imine stretch is seen at approxi-
mately 1600 cm . In addition, elemental analysis data are
consistent with the proposed structures, and representative
data are provided. Anal. Calcd for 3-p6p-3: C, 73.5; H, 6.2;
N, 8.0. Found: C, 73.7; H, 6.1; N, 7.7.
n
TCrN/°C
TNI/°C
∆SCrN/R
∆SNI/R
3
213
251
215
240
206
229
197
222
187
216
248
291
246
274
234
253
236
243
213
231
23.1
28.6
25.6
38.8
32.6
41.2
30.6
41.7
34.3
46.0
2.15
4.35
2.52
4.53
2.92
5.26
3.37
4.89
3.83
5.32
4
5
6
7
8
9
10
11
12
-
1
for the n-p6p-n series is shown in Figure 1. Both the
melting and clearing temperatures exhibit a pronounced
odd-even effect as the outer spacer lengths are in-
creased in which the even spacers exhibit the higher
values. The magnitude of the alternation in the melting
points is largely unchanged on increasing the spacer
lengths, and similar behavior has been observed for
Characterization and Thermal Analysis. The tetramers
and their intermediates were characterized using a combina-
1
tion of H NMR spectroscopy (where suitable), using a Bruker
AC-F 250 MHz spectrometer, FTIR spectroscopy using an ATI
Mattson Genesis Series FTIR spectrometer, and elemental
analysis carried out by Butterworth Laboratories. The thermal
behavior of the materials was investigated by differential
scanning calorimetry (DSC) using a Mettler Toledo DSC 820
differential scanning calorimeter equipped with a TS0801RO
sample robot and calibrated using indium and zinc standards.
The heating profile in all cases was heat, cool, and reheat at
liquid crystal tetramers,2
2,23
trimers,
16,17
and dimers.
1,3
This is thought to indicate either that the change in the
conformation statistical weights of the spacer on melting
into a nematic phase is small for even-membered
spacers but large for odd-membered spacers or that this
alternation reflects the difficulty that the bent confor-
mations of the odd-membered compounds experience in
packing efficiently into a crystal lattice compared with
the more elongated all even-membered tetramers. The
magnitude of the odd-even effect seen for the nematic-
-1
1
0° min with a 3 min isotherm between heating and cooling.
All samples were heated from 0 °C to 20-40 °C above their
clearing temperatures. Thermal data were normally extracted
from the second heating trace. Phase characterization was
performed using polarizing light microscopy using an Olympus
BH2 polarizing light microscope equipped with a Linkam TMS
92 hot stage.
Results and Discussion
The transition temperatures and associated entropy
changes for the n-p6p-n series are given in Table 1.
The data listed were extracted from the reheat DSC
traces, and several members of the series showed
complex melt/crystallization behavior before the forma-
tion of the nematic phase. All the members of the series
exhibit an enantiotropic nematic phase. The nematic
phase was identified on the basis of the observation of
characteristic schlieren textures containing both types
of point singularity and which flashed when subjected
to mechanical stress.
Figure 1. Dependence of the transition temperature on n,
the number of methylene units in the two outer spacers, for
the n-p6p-n tetramer series: Cr-N (b), N-I (O).
The dependence of the transition temperatures on the
number of methylene units in the outer flexible spacers

Macromolecules, Vol. 38, No. 8, 2005
Semiflexible Liquid Crystalline Tetramers 3309
Figure 2. Dependence of the entropy change associated with
the nematic-isotropic transition for the n-p6p-n (O) and
n-p5p-n (4) series on the number of methylene units in the
outer alkyl spacers.
Figure 3. Comparison of the melting (filled symbols) and
nematic-isotropic (open symbols) transition temperatures of
the n-p6p-n (b, O) and n-p5p-n (2, 4) series.
orientational order ensures that for even-membered
dimers many of the bent conformers are converted to a
linear form, enhancing the orientational order of the
nematic phase. This increases ∆SNI/R relative to what
would be expected for a monomer. For an odd-membered
dimer, however, the difference in free energy between
the bent and linear conformers is too large for the
orientational order of the nematic phase to convert bent
into linear conformers. Thus, odd-membered dimers
exhibit smaller values of ∆SNI/R than even-membered
dimers. For tetramers we now have three spacers to
consider, one of which is always even for this particular
series. It would seem reasonable to assume that this
model would also successfully predict the transitional
properties of the tetramers. The odd-even effect shown
by the nematic-isotropic transition temperatures at-
tenuates on increasing n (see Figure 1), whereas that
shown by ∆SNI/R does not (see Figure 2). This is also
archetypal behavior for dimers and presumably reflects
that on increasing the spacer length the greater number
of available conformations means that the average
shapes of the molecules, and hence the interaction
strength parameters between them, become more simi-
lar. Thus, the alternation in TNI attenuates, but there
will still be a marked difference in the conformational
distributions of the spacers, giving rise to the odd-even
effect seen for ∆SNI/R.
isotropic transition temperatures is attenuated on in-
creasing n, with the temperatures for the even members
falling more rapidly than those of the odd members.
23
Similar behavior was seen for the n-p5p-n series and
other tetramers.2
2
The dependence of the entropy change associated with
the nematic-isotropic transition on the number of
methylene units in the outer spacers is shown in Figure
2
. A pronounced odd-even effect is observed in which
the values for the even members are typically 1.5-2
times larger than those of the odd members. The general
trend is for the clearing entropy to increase upon
1
-3
increasing n, as is seen in dimers, trimers,
and
2
8
polymeric systems, although there is a less regular
dependence on the value of ∆SNI/R for both tetramer
series when n is even. Unlike polymers, the entropy
values are not scaled for the number of mesogenic units
in the molecule, and it is not entirely clear to what
extent this should be carried out for low oligomeric
materials. Further speculation should await simulation
studies on this class of oligomers.
We have seen in Figures 1 and 2 that both the
nematic-isotropic transition temperatures and associ-
ated entropy changes of the n-p6p-n series depend
critically on the length and parity of the spacer in a
manner strongly resembling that seen for the transi-
1,3
16,17
tional properties of dimers, trimers,
and semiflex-
2
8
Comparison of the n-p5p-n and n-p6p-n Tet-
rameric Series. Figure 3 compares the melting and
nematic-isotropic transition temperatures of the
n-p5p-n and n-p6p-n series. The melting points of
the n-p6p-n series are higher than those of the
corresponding members of the n-p5p-n series with the
exception of the butyl homologue for which the melting
points are essentially the same. For the remaining
homologues, the difference in melting points, however,
is small and on average is just 7 °C. Similar behavior
is seen for the nematic-isotropic transition tempera-
tures (see Figure 3), and the n-p6p-n series exhibits
higher values than the corresponding member of the
n-p5p-n series. The differences are, however, rather
small for the lower homologues but tend to increase on
increasing the length of the outer flexible spacers. The
TNI for 4-p6p-4 is lower than expected, and this may
be attributed to the onset of decomposition at these high
temperatures. The increasing difference between the
nematic-isotropic transition temperatures of these two
series on increasing the length of the spacers reflects
that the odd-even effect observed for TNI for the
n-p5p-n series is attenuated more rapidly than that
seen for the n-p6p-n series.
ible main-chain liquid crystal polymers. In dimers
such behavior is most often attributed to the dependence
of the molecular shape on the parity of the spacer
considered in the all-trans conformation. Thus, in an
even-membered dimer the mesogenic units are anti-
parallel, whereas in an odd-membered dimer they are
inclined at some angle with respect to each other. This
more linear structure for even-membered dimers is then
considered to be more compatible with the molecular
organization found in the nematic phase than that of
the bent odd-membered dimers, and it is this greater
compatibility which results in, for example, the higher
values of ∆SNI/R found for the even members. This
interpretation, however, completely neglects the flex-
ibility of the spacer, and a more realistic interpretation
of the dependence of the transitional properties on the
parity of the spacer must certainly include a wide range
of conformations and not just the all-trans conformation.
3
Such a model has been developed and shows that in
the isotropic phase approximately half the conformers
of an even-membered dimer are essentially linear
compared to only 10% of the conformers of an odd-
membered dimer. At the transition to the nematic phase
the synergy that exists between conformational and

3310 Henderson and Imrie
Macromolecules, Vol. 38, No. 8, 2005
A comparison of the entropy changes associated with
the nematic-isotropic transition for the n-p5p-n and
n-p6p-n series is shown in Figure 2. The n-p6p-n
series exhibits higher values of ∆SNI/R than the corre-
sponding member of the n-p5p-n series. The values
of ∆SNI/R for the two series are considerably larger than
those exhibited by conventional low molar mass me-
sogens and fit the emerging trend that ∆SNI/R increases
on passing from the monomer to the dimer, trimer, and
16,22
tetramer.
It has been argued that for oligomers these
values should be scaled according to the number of
mesogenic groups, and indeed such a scaling is routinely
performed when data are analyzed for semiflexible
main-chain liquid crystal polymers. Thus, the scaled
values of ∆SNI/R for 6-p6p-6 (i.e., [∆SNI/R]/4 ) 1.13)
and for 5-p5p-5 (0.46) are comparable in magnitude
to those of trimers containing two hexamethylene or
Figure 4. Schematic representations of odd- and even-
membered tetramers and the polymers they resemble.
1
6,29
pentamethylene spacers, respectively.
It is interesting to note that both the n-p5p-n and
n-p6p-n series are exclusively nematogenic and in-
creasing the spacer length has not resulted in smectic
phase formation. This is similar to the behavior ob-
served for liquid crystal trimers16 and symmetric dimers4
but quite unlike that of semiflexible main-chain poly-
mers for which increasing the spacer length enhances
smectic behavior.30,31 Although we are at an early stage
in the development of the empirical rules relating
molecular structure to properties in both trimers and
tetramers, these data do suggest that the driving force
for smectic phase formation must differ between liquid
crystal oligomers and polymers. Presumably for the
polymers the driving force must be an entropic one to
disentangle the polymer chains. The situation is some-
what different for other liquid crystal tetramers for
which smectic phase stability does not simply increase
or decrease on increasing the length of the outer spacers,
but for these tetramers the driving force for smectic
phase formation was believed to be a specific interaction
between the unlike mesogenic groups in the tetramer.22
For the n-p5p-n and n-p6p-n series such specific
interactions do not exist.
Figure 5. Dependence of the clearing temperature on the
27
relative fraction of odd-membered spacers in ([) copolymers
and (b) tetramers.
6-p6p-6, may be considered as model compounds for
homopolymers, while those having differing spacers,
5-p6p-5 and 6-p5p-6, may serve as model compounds
for 2:1 copolymers (see Figure 4).
Percec and co-workers27 have reported the thermal
behavior of a series of copolymers containing odd- and
even-membered spacers differing by only one methylene
unit
We have seen that for a given value of n passing from
an odd- to an even-membered central spacer only
slightly increases the nematic-isotropic transition tem-
perature and entropy but that this difference increases
on increasing n. This is rather surprising behavior as
it might have been expected, and particularly for small
values of n, that varying the central spacer would give
the dramatic alternation in transitional properties seen
on varying the spacer length in liquid crystal dimers.
The physical significance of this observation is not clear,
and the generality of this behavior must be established
for a greater number of compounds.
and expressed the dependence of the clearing tem-
perature in terms of the relative fraction of odd-
membered spacers, given by B/(A + B). Although the
mesogenic units in the tetramers and copolymers are
not identical, we can assume that the interaction
strength parameters between the mesogenic units within
each system will be similar, the similarity in transi-
tional behaviors serving to highlight the possible gen-
erality of the result. The clearing temperatures do not
show a linear dependence on the relative amount of the
odd-membered spacers (see Figure 5). Instead, the
clearing temperature is rather insensitive to increasing
the amounts of the odd-membered spacer up to ca. 0.6,
at which point the clearing temperature falls. Further
increases in the amount of odd-membered spacers have
essentially no effect on the clearing temperature. The
authors did not comment on this rather unexpected
result.
Tetramers as Model Compounds for Copoly-
mers. As we have noted already it might have been
expected, a priori, that varying the central spacer in a
tetramer would give rise to an alternation in the
clearing temperature and entropy at least of a magni-
tude similar to that observed on varying the spacer in
2,3
a liquid crystal dimer. This appears not to be the case,
however, and only a rather small difference in clearing
temperature is observed (see Figure 3). We will now
consider whether this surprising behavior is consistent
with that seen for semiflexible main-chain liquid crystal
polymers; i.e., to what extent can the tetramers be used
as model compounds for the polymers? Specifically, the
tetramers having three identical spacers, 5-p5p-5 and
The dependence of the clearing temperatures of the
tetramers on the relative fraction of odd-membered

Macromolecules, Vol. 38, No. 8, 2005
Semiflexible Liquid Crystalline Tetramers 3311
range of the n-pxp-n family of tetramers and of
analogous copolymers should now be investigated to test
the generality of these observations and to enhance our
understanding of liquid crystalline phase formation and
behavior in polymeric and oligomeric systems.
References and Notes
(
1) Imrie, C. T.; Henderson, P. A. Curr. Opin. Colloid Interface
Sci. 2002, 7, 298.
(
2) Imrie, C. T. Struct. Bonding (Berlin) 1999, 95, 149.
3) Imrie, C. T.; Luckhurst, G. R. In Handbook of Liquid Crystals;
Demus, D., Goodby, J. W., Gray, G. W., Spiess, H. W., Vill,
V., Eds.; Wiley-VCH: Weinheim, Germany, 1998; Vol. 2B, p
801.
(
Figure 6. Dependence of the clearing entropy on the relative
fraction of odd-membered spacers in the ([) copolymers and
(
4) Date, R. W.; Imrie, C. T.; Luckhurst, G. R.; Seddon, J. M.
Liq. Cryst. 1992, 12, 203.
(
b) tetramers.
(
5) Watanabe, J.; Komura, H.; Niori, T. Liq. Cryst. 1993, 13, 455.
6) Watanabe, J.; Niori, T.; Choi, S.-W.; Takanishi, Y.; Takezoe,
H. Jpn. J. Appl. Phys. 1998, 37, L401.
(
spacers is also shown in Figure 5. As seen for the
polymers, the clearing temperature does not show a
linear dependence on the relative fraction of odd-
membered spacers. Instead and as we have seen al-
ready, 5-p6p-5 and 5-p5p-5 exhibit rather similar
clearing temperatures as do 6-p6p-6 and 6-p5p-6,
with the latter showing the higher clearing tempera-
tures. Thus, the tetramers are showing behavior very
similar to that of the polymers. This strongly suggests
that orientationally the four mesogenic units are not
strongly correlated and thus varying the length of the
central spacer does not give rise to the expected pro-
nounced odd-even effect in the clearing temperature.
(
7) Watanabe, J.; Izumi, T.; Niori, T.; Zennyoji, M.; Takanishi,
Y.; Takezoe, H. Mol. Cryst. Liq. Cryst. 2000, 346, 77.
(8) Watanabe, J.; Hayashi, M. Macromolecules 1988, 21, 278.
(
9) Watanabe, J.; Hayashi, M. Macromolecules 1989, 22, 4083.
(
(
(
10) Keller, P. Mol. Cryst. Liq. Cryst. 1985, 123, 247.
11) Furuya, H.; Asahi, K.; Abe, A. Polym. J. 1986, 18, 779.
12) Attard, G. S.; Imrie, C. T. Liq. Cryst. 1989, 6, 387.
(13) Centore, R.; Roviello, A.; Sirigu, A. Mol. Cryst. Liq. Cryst.
1990, 182B, 233.
(
14) Ikeda, T.; Miyamoto, T.; Kurihara, S.; Tsukada, M.; Tazuke,
S. Mol. Cryst. Liq. Cryst. 1990, 182B, 357.
15) Tsvetkov, N. V.; Zuev, V. V.; Tsvetkov, V. N. Liq. Cryst. 1997,
22, 245.
(
(16) Imrie, C. T.; Luckhurst, G. R. J. Mater. Chem. 1998, 8, 1339.
17) Luckhurst, G. R. Macromol. Symp. 1995, 96, 1.
(18) Chen, B.-Q.; Kameyama, A.; Nishikubo, T. Macromolecules
(
A small alternation in clearing temperatures has also
been reported for an incomplete series of tetramers.24
1999, 32, 6485.
It appears, therefore, that the two outer spacers play a
larger role in the odd-even effect seen in the clearing
temperatures than does the central spacer. This sug-
gests that the length and parity of the spacer having
the major compositional fraction determines the transi-
tion temperatures and that the orientational motions
of the four mesogenic units are not strongly correlated
along the length of the tetramer.
(
(
(
19) Nishiyama, I.; Yamamoto, J.; Goodby, J. W.; Yokoyama, H.
J. Mater. Chem. 2003, 13, 2429.
20) Abe, A.; Hiejima, T.; Takeda, T.; Nakufuku, C. Polymer 2003,
44, 3117.
21) Imrie, C. T.; Henderson, P. A.; Seddon, J. M. J. Mater. Chem.,
2004, 14, 2486.
(22) Imrie, C. T.; Stewart, D.; Remy, C.; Christie, D. W.; Hamley,
I. W.; Harding, R. J. Mater. Chem. 1999, 9, 2321.
(23) Henderson, P. A.; Inkster, R. T.; Seddon, J. M.; Imrie, C. T.
J. Mater. Chem. 2001, 11, 2722.
(24) Griffin, A. C.; Sullivan, S. L.; Hughes, W. E. Liq. Cryst. 1989,
The dependence of the clearing entropies for the
tetramers on the relative fraction of odd-membered
spacers is shown in Figure 6. We now have an apparent
linear dependence of the clearing entropy on the relative
fraction of the odd-membered spacer. The corresponding
data for the polymers are also shown in Figure 6,
expressed as the entropy change per repeat unit. The
copolymers also appear to show a linear dependence of
the clearing entropy on composition. The value for the
all-even-membered n ) 6 homopolymer is not included
as the melting and clearing transition peaks could not
be resolved in the DSC trace and the authors also noted
that the rather low molecular weight of the even-
membered polymer may mean that the estimated clear-
ing entropy lay in the molecular weight dependent
4, 677.
(25) Yelamaggad, C. V.; Anitha Nagamani, S.; Hiremath, U. S.;
Shankar Rao, D. S.; Krishna Prasad, S. Liq. Cryst. 2002, 29,
231.
(
26) Sasanuma, Y.; Ono, T.; Kuroda, Y.; Miyazaki, E.; Hikino, K.;
Arou, J.; Nakata, K.; Inaba, H.; Tozaki, K.; Hayashi, H.;
Yamaguchi, K. J. Phys. Chem. B 2004, 108, 13163.
27) Percec, V.; Asami, K.; Tomazos, D.; Feijoo, J. L.; Ungar, G.;
Keller, A. Mol. Cryst. Liq. Cryst. 1991, 205, 67.
(
(
28) Chiellini, E.; Laus, M. In Handbook of Liquid Crystals;
Demus, D., Goodby, J. W., Gray, G. W., Spiess, H. W., Vill,
V., Eds.; Wiley-VCH: Weinheim, Germany, 1998; Vol. 3, p
26.
(
29) Henderson, P. A.; Cook, A. G.; Imrie, C. T. Liq. Cryst. 2004,
31, 1427.
(
30) Ober, C. K.; Jin, J.-I.; Lenz, R. W. In Advances in Polymer
Science; Gordon, M., Ed.; Springer-Verlag: Berlin, 1984; Vol.
59, p 103.
31) Finkelmann, H. In Thermotropic Liquid Crystals; Gray, G.
W., Ed.; Wiley: Chichester, U.K., 1987; Chapter 6.
2
7
regime. The molecular significance of the dependence
of the clearing entropy on spacer lengths for both the
tetramer and polymer is unclear, although it appears
that both types of spacer contribute equally. A wider
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