V. A. E. Shaikh et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 10 (2007) 1979
showed birefringence at about 130 ꢂC (Fig. 4b), which contin-
ued to increase until the sample became completely birefrin-
gent (Figs. 4c and 4d). The birefringence started flowing and
disappearing (isotropization) from 170 ꢂC onwards. The sam-
ple turned totally isotropic at about 180 ꢂC. It was held at
this temperature for about five minutes and then cooled. Slight
birefringent regions were seen upon cooling from 125 ꢂC
(Fig. 4e), and it increased slightly on further cooling and re-
mained until room temperature (Figs. 4f and 4g). The same
sample was reheated (2nd heating) at the same rate of 10 ꢂC
per minute. The birefringence that formed on cooling started
to disappear from 135 ꢂC onwards, and the sample became iso-
tropic (Fig. 4h). The possibility of a homeotropic phase forma-
tion was ruled out, because birefringence could not be seen
even after the sample was deformed by slight moving and
pressing the cover glass, while heating as well as cooling. It
was found to melt at the same temperature as above.
In an another attempt to characterize the CDCh under OPM,
the sample was heated directly to 145 ꢂC at a programmed
heating rate of 10 ꢂC per minute. It was held at this tempera-
ture for 10 min and cooled to room temperature. The birefrin-
gence developed at 145 ꢂC was retained at room temperature,
indicating formation of ‘‘liquid-crystal glass’’ (Fig. 4i). Clearly
identifiable textures were not seen in any of the case under
OPM.
Differential scanning calorimetric (DSC) (Fig. 5) was con-
ducted to establish the liquid-crystalline behavior of the
CDCh. The heating and cooling rate was programmed at 10 ꢂC
per minute. Here, the sample was heated from ꢃ50 ꢂC instead
of room temperature, as the later was featureless during first
heating, cooling and second heating run. The DSC study was
in reasonable agreement with that of OPM study. However,
the structure of BCD plays a role in the final properties of
the product. It was anticipated that, due to oligomeric (small)
structure of ꢀ-cyclodextrin, CDCh would show clear thermal
transitions in contrast to the corresponding polysaccharide
derivatives,11–13 which is due to their very high molecular
weight, polydispersity, non-solubility of the backbone, non-
melting character, non-uniform substitution of the mesogens,
etc. However, no clear transitions were observed in the DSC
thermograms. This is further evidenced by the fact that these
polysaccharide derivatives also did not have any clearly iden-
tifiable textures under OPM. In the present investigation, even
though the backbone utilized was oligomeric, the textures were
not clearly identifiable. Only birefringence could be seen under
OPM. In addition, the melting character of the product was
unique. The sample, when heated in a capillary, first under-
went a volume contraction at about 130 ꢂC and then it stuck
to the sidewalls of the capillary as if showing the signs of melt-
ing at about 150–160 ꢂC. On further heating the sample spread
on the sidewalls of the capillary at about 170 ꢂC and then
underwent volume expansion above 175 ꢂC. However, when
observed under OPM, it was found that the sample starts ex-
hibiting birefringence at about 130 ꢂC that increased until it
started melting above 170 ꢂC. The melt was very viscous and
sticky in nature, suggesting that the product is not of the typi-
cal low molecular weight material. The overall ‘‘abnormal’’
melting behavior of the CDCh can be attributed to the basic
non-melting and cyclic structure of the parent BCD. Also,
the higher DS (2.00 in the present work) of the bulky mole-
cule–cholesterol molecule that causes ‘‘overcrowding’’ on the
BCD makes the CDCh molecule ‘‘stodgy.’’ The first heating
DSC thermogram showed an exotherm in the range of 162–
180 ꢂC corresponding to the organized phase formation as seen
under OPM. It continued melting and decomposing from about
200 ꢂC onwards. At this temperature, the sample turned dark
brown in color as observed under OPM. In addition, the sam-
ple was investigated for decomposition by IR spectroscopy
after first heating. The peak due to hydroxy groups (free) dis-
appeared, whereas other peaks remained intact, indicating par-
tial decomposition. The cooling thermogram was featureless
whereas the second heating showed a glass to liquid-crystal
transition at about 120–125 ꢂC. This is in contrast to the OPM
observations, in which no birefringence was seen during the
second heating. Slight birefringence was seen on cooling
around 125 ꢂC, which remained at room temperature, and on
reheating, it disappeared at about 135 ꢂC. No endotherm was
seen during the second heating, most probably due to the fact
that the sample did not crystallize on cooling. This is further
supported by the observations made under OPM, during which
crystallization was not observed on cooling. It can be interpret-
ed that the bulky and complex nature of the molecule hinders
the crystallization process. Also, the partial decomposition as
studied by IR supports the non-reproducibility of LC phase
formation on cooling and second heating.
It is very interesting to note here that the temperature of
transition, i.e., crystal to liquid crystal and liquid crystal to iso-
tropization, are comparable to that of the mesogen (ChMS)
transitions. As anticipated, the mesogenic character overrides
the oligomeric character. That is, the transition temperatures
are more or less similar to those of the mesogen (Tm ¼ 180
ꢂC) than the oligomer (decomposes without melting). This
can be attributed to the ‘‘excess’’ of ChMS (DS 2.00) on the
oligomer, which causes the properties of ChMS to be more
prominent and the properties of BCD to fade away. However,
it is important to note that, in the case of ChMS, the mesogenic
character was observed while cooling25 whereas CDCh ex-
hibits mesogenic character during the first heating.
2.0
Cooling
1.5
1.0
0.5
0.0
second heating
-0.5
first heating
80
-1.0
20
Exo Up
40
60
100
120
140
160
180
200
Author Shaikh V.A.E. thanks the Department of Science
and Technology, India, for financial assistance through grant
under the research project SR/FTP/CS-28/2004.
Universal V4.1D TA Instruments
Temperature (°C)
Fig. 5. DSC thermograms of CDCh.