Existence of a third crystalline phase in TBBA

Article abstract of DOI:10.1080/744818943

Positron annihilation lineshape (S-parameter) measurements and small angle X-ray scattering (SAXS) using synchrotron X-rays on liquid crystalline terepthal-bis-butylaniline (TBBA) are reported at cryogenic temperatures. The crystalline ? crystalline transition at 233 K deviates from the first order nature, as evidenced by the observation of a pronounced hysteresis in the layer spacing and the S-Parameter during the heating and cooling cycles. Presence of a possible third crystalline phase at a still lower temperature of 180K is identified from X-ray diffraction patterns in corroboration with the slope change in layer spacing and electron density variations reflecting on the line-shape parameter.

Full text of DOI:10.1080/744818943

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EXISTENCE OF A THIRD
CRYSTALLINE PHASE IN TBBA
Alok D. Bokare a & Archita Patnaik a
a Department of Chemistry, Indian Institute of
Technology, Madras, Chennai-600 036, India
Published online: 02 Feb 2011.
To cite this article: Alok D. Bokare & Archita Patnaik (2003) EXISTENCE OF A THIRD
CRYSTALLINE PHASE IN TBBA, Molecular Crystals and Liquid Crystals, 403:1, 23-31
To link to this article: http://dx.doi.org/10.1080/744818943
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Mol. Cryst. Liq. Cryst., Vol. 403, pp. 23–31, 2003
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421400390234039
EXISTENCE OF A THIRD CRYSTALLINE PHASE IN
TBBA
Alok D. Bokare and Archita Patnaik*
Department of Chemistry, Indian Institute of Technology
Madras, Chennai-600 036, India
Positron annihilation lineshape (S-parameter) measurements and small angle
X-ray scattering (SAXS) using synchrotron X-rays on liquid crystalline ter-
epthal-bis-butylaniline (TBBA) are reported at cryogenic temperatures. The
crystalline $ crystalline transition at 231 K deviates from the first order nat-
ure, as evidenced by the observation of a pronounced hysteresis in the layer
spacing and the S-Parameter during the heating and cooling cycles. Presence of
a possible third crystalline phase at a still lower temperature of 180 K is
identified from X-ray diffraction patterns in corroboration with the slope
change in layer spacing and electron density variations reflecting on the line-
shape parameter.
Keywords: liquid crystals; positron annihilation; layer spacing
INTRODUCTION
Among the numerous thermotropic mesogens known, terepthal-bis-(4-n-
butyl aniline), commonly known as TBBA, occupies a special position by
virtue of its polymorphism. It possesses 4 well known mesophases: smectic
H (SH), smectic C (SC), smectic A (SA), and the nematic (N) phase, in
addition to the isotropic phase. Calorimetric studies have shown that TBBA
also adopts monotropically 2 additional smectic phases between the
smectic H and the crystalline phase on cooling. Moreover, the monoclinic
crystalline phase transforms into a triclinic crystalline phase at cryogenic
Received 30 January 2003; accepted 23 April 2003.
This work has been supported by the Department of Science and Technology (DST), Gov-
ernment of India under grant No. SP/S1/H-33/95. Alok D. Bokare thanks the DST, Govt. of India
for a Research Fellowship. The authors would especially like to acknowledge Dr. Heinz
Amenitsch, Synchrotrone Trieste, Italy for helping with the SAXS data analysis and Mr. G.L.N.
Reddy, MSD, IGCAR, India for the low temperature XRD measurements.
*Corresponding author. E-mail: archita59@yahoo.com
23

24
A. D. Bokare and A. Patnaik
FIGURE 1 Phase transition scheme for TBBA.
temperature [2]. The phase transition scheme for TBBA is shown in
Figure 1 with the transition temperatures in Kelvin.
Positron annihilation lifetime spectroscopy (PALS) utilizes positron and
its bound state, positronium, as atomic probes for studying electronic
structure, phase transitions, and relaxation processes in condensed and
liquid media using their lifetimes and the line width/line shape parameters
[3] from the annihilation radiation energy distribution spectra. In this
context, details of the phase transition behavior in a variety of matrices
such as polymers [4], micellar media [5], and superconductors [6] have been
successfully undertaken. Amongst the very few studies on liquid crystals
[7,8], the sensitivity of this technique in discovering new phases has been
highlighted on account of its ability to monitor small changes in the elec-
tron densities occurring across a phase transition.
In this article, we report the existence of a new crystalline phase tran-
sition at 180 K using low temperature Positron Annihilation Lineshape
(PAL) measurements combined with Small angle X-ray scattering (SAXS)
studies using the high brilliance and high resolution of a synchroton beam.
We also provide evidence for the deviation of the solid-solid transition at
231 K from obeying a first order behavior.
EXPERIMENTAL DETAILS
TBBA was prepared by refluxing 4-n-butylaniline and terepthaldehyde in
absolute ethanol for 5–6 h according to the procedure of Taylor et al. [9].
The product after isolation was recrystallized several times from ethanol
until the transitions temperatures remained constant. These were deter-
mined from changes in the characteristic textures of the mesophases
observed under a polarizing microscope (Olympus) fitted with a hot stage.
The heat of mesomorphic transition was calculated from the thermographs
obtained on a Netzsch DSC-204 calibrated with indium. Details of the
Positron Annihilation Lineshape measurements and the SAXS studies have
been described earlier [10]. Low temperature X-ray diffraction studies were
carried out on a Siemens D500 diffractometer operating at 45 KV power

Existence of a Third Crystalline Phase in TBBA
25
using Cu-Ka as the incident X-ray beam. The temperature was maintained
with an Oxford Continuous Flow Cryostat with an accuracy of Æ 0.5 K. The
(111) diffracted peak of Silicon NBS 640a was used as an internal standard.
RESULTS AND DISCUSSION
Figure 2 shows the time-resolved evolution of the layer spacing in the small
angle region when the sample was cooled down from the solid crystalline
phase to 123 K and heated back to room temperature. In order to avoid the
exposure of the sample for a long period to the incoming X-ray beam, time
slicing during the SAXS measurements was performed. Series of scattering
data with high time resolution (5 ms) and angular resolution (50) were
acquired. This proves beneficial since the signal-to-noise and signal-to-
background ratio is increased, thereby assisting in the observation of also
the weak scattering reflections. The time resolved data was converted into
a 2-D plot (Figures 3(a) and 3(b)) for the variation of the layer spacing and
scattered intensity, obtained from the main scattering peak, with tem-
perature. At room temperature TBBA crystallizes in the monoclinic system
FIGURE 2 Time resolved plot of the variation of layer spacing in the temperature
range 298 K–123 K–298 K (cooling and heating) for pristine TBBA. (See COLOR
PLATE I)

26
A. D. Bokare and A. Patnaik
FIGURE 3 (a) Temperature dependence of the layer spacing for the heating and
cooling cycles. (b) Temperature dependence of the scattered intensity from the
main Bragg peak during the heating and cooling cycle.

Existence of a Third Crystalline Phase in TBBA
27
and a collective motion of the molecules involves a correlation length of
about 7 molecules [10]. This monoclinic structure undergoes a structural
change to a triclinic crystalline phase reported by Doucet et al. at 231 K [2]
and is also evident in this study from the changes in the layer spacing
(Figure 3(a)) and the scattered intensity (Figure 3(b)) near 231 K, both
during the cooling and heating cycles. There is a sharp drop at around
231 K, indicating a predominantly first order transition. Also, the trans-
formation shows an evident hysteresis; in Figure 3(a) the transition tem-
peratures we obtain from the cooling and heating cycles are 234 K and
227 K. The hysteresis is also confirmed in the Positron Annihilation Line-
shape measurements (Figure 4) where the variation of the lineshape
parameter, defined as a ratio of the integrated counts in the central portion
of the Doppler shift ꢀ0.7 to 0.7 keV to the total counts in the energy range
ꢀ7 to 7 keV (peak counts corresponding to zero energy), is plotted with
temperature. The transition temperatures obtained thus from the cooling
and heating cycles differ by ꢁ7 K. The presence of a pronounced hysteresis
in both of the studies then excludes the transition we observe to be a real
first-order transition.
An interesting observation can be made from the analysis of Figure 3(a).
A significant change in the slope of the layer spacing profile is seen at the
FIGURE 4 Hysteresis phenomenon seen in the temperature dependence of the
lineshape parameter (S) for the heating and cooling cycles.

28
A. D. Bokare and A. Patnaik
lower end of the temperature scale at around 180 K, associated with both
the heating and the cooling profiles. A more accurate picture of the slope
change is presented in Figure 5, where the main scattered reflection at
different temperatures is plotted as a function of the layer spacing.
A change in the peak centroid at 180 K can be easily identified. This finding
is further corroborated by a small but significant change in the positron
annihilation lineshape parameter at the same temperature (Figure 6). Our
previous studies [10] had shown similar changes in the lineshape parameter
arising from order parameter fluctuations in the smectic C and H phases of
TBBA, which caused electron density fluctuations and were mapped by
the S-parameter. However, the changes observed in Figure 6 cannot
be assigned to any molecular fluctuations since all molecular motions are
effectively reduced at such low temperatures. Under these circumstances,
a small structure change occurring in the molecule appears as the only
possible reason for the changes in the peak position and the lineshape
parameter.
FIGURE 5 Measured X-ray scattered intensity versus layer spacing for the tem-
perature range 190 K to 169 K. A shift in the peak centroid from 180 K onwards is
clearly evident. The pronounced shift in the peak position at the 231 K transition is
shown for comparison.

Existence of a Third Crystalline Phase in TBBA
29
FIGURE 6 S-parameter profile as a function of temperature near 180 K.
In order to examine and confirm this possibility, X-ray diffraction using
a cryogenic setup was carried out. Figure 7 shows the variation in the
diffraction patterns as a function of decreasing temperature from 290–
160 K. The solid-solid phase transition at 231 K is clearly characterized by
1. Appearance of a new diffraction peak at 2y ¼ 16.58ꢂ (marked by *).
2. Complete disappearance of the (312) reflection at 2y ¼ 21ꢂ.
ꢀ
3. Decrease in the intensity of the (112) reflection and a corresponding
increase in the intensity of the shoulder peak adjoining the reflection.
4. Complete reversal of the doublet peak intensity ratio containing the
(004) reflection.
5. Increase in the separation of the (204) and the (911) reflection by 0.35ꢂ
accompanied by a decrease in their intensities.
6. An overall observable centroid shift for all the major peaks.
Similarly, Figure 7 shows evidence of yet another transition occurring at
a lower temperature of 180 K. The new reflection observed in the second
crystalline phase (existing below 230 K) splits up into a doublet at 180 K
and finally splits up completely to form 2 distinct peaks at a lower tem-
perature. The other interesting feature to develop at 180 K is the increase
in the intensity ratio of the doublet at 2y ¼ 22.74 and 23.15ꢂ from 1:1.2 at
220 K to 1:1.8, and which further increases to 1:3.7 at 160 K. This is also

30
A. D. Bokare and A. Patnaik
FIGURE 7 X-ray diffraction peaks in TBBA as a function of decreasing
temperature.
accompanied by a shift in the peak centroids of all the other peaks and
a decrease in the intensity of the (204) and (911) reflections. Thus, a new
phase transition at 180 K to a third crystalline phase in TBBA is evident
from the diffraction experiments and excellently corroborates the predic-
tion of the Position Annihilation and SAXS measurements.
CONCLUSION
Crystalline polymorphism was studied in TBBA using Positron Annihilation
Lineshape measurements along with the high brilliance and high resolution
of 8 keV X-rays from a synchrotron radiation source with time slicing. We
found that the crystalline phase transformation at 231 K exhibits a pro-
nounced hysteresis behavior leading to a deviation from being an ideal first
order transition. Another structural change occurring at a still lower tem-
perature of 180 K, identified through the observed slope change in the layer
spacing and change in the lineshape parameter, is further confirmed
through appearance of new features in the X-ray diffraction patterns.
A complete indexing and characterization of the new phase in under
progress.

Existence of a Third Crystalline Phase in TBBA
31
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