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@PHDTHESIS{Dey:452033,
author = {Dey, Kartick Ch.},
othercontributors = {MANDAL, PRADIP KUMAR},
title = {{INVESTIGATION} {ON} {SOME} {ANTIFERROELECTRIC} {LIQUID}
{CRYSTALS} {AND} {THEIR} {MIXTURES}},
school = {University of North Bengal},
type = {Dissertation},
reportid = {PUBDB-2020-04474},
pages = {222},
year = {2020},
note = {Dissertation, University of North Bengal, 2020},
abstract = {Liquid crystals are fascinating self–assembled soft
materials in which the molecules are orientationally ordered
and partially positionally ordered. Liquid crystals easily
response to small perturbations like the electric field,
magnetic field, surface effect, etc. for which they find
applications in diversified fields along with display
technology.At present high definition displays (AMLCDs) with
a viewing angle of ~178 degrees are available in the market.
Twisted nematic liquid crystals are generally used in such
displays. The high value of response time (~ms) governing
the frame rate, ghost effect and contrast ratio is still a
matter for further improvement. As a result, ferroelectric
liquid crystal (FLC) and antiferroelectric liquid crystal
(AFLC) draw a special interest from the last decades of the
20th century due to their low response time (µs). But
manifold problems were faced in the development of displays
based on FLCs. Some of them are small cell spacing (1–2
µm) to unwind the helix, the problem of mechanical shock
due to unstable molecular anchoring at the surface, contrast
ratio, etc. The AFLCs are promising materials to solve the
issues because of their fascinating properties – tristable
switching behavior, micro-second order response time,
intrinsic analog gray-scale capability, hemispherical
viewing angle (in-plane switching geometry) high contrast
ratio and no-ghost effect, etc. AFLCs are also interesting
for the basic studies in soft condensed matter field as
these materials show various sub-phases with distinct
macroscopic properties. So enough scope is here for the
improvement of the display industry in the coming years with
fascinating properties of antiferroelectric liquid
crystals.Physical properties of six antiferroelectric liquid
crystals and their three mixtures have been studied in this
dissertation using different experimental techniques viz.
polarizing optical microscopy, differential scanning
calorimetry, synchrotron X-ray diffraction, dielectric
spectroscopy, electro-optic study. The first four compounds
(DM0, DM1, DM2, and DM3) are biphenyl benzoate core-based,
in which the number and position of lateral fluorination in
the core differ. The first compound DM0 is a core protonated
compound but in DM1 one fluorine atom is introduced at ortho
position of the benzoate core, while it is introduced at
position meta in DM2, but in DM3 fluorine atoms are
introduced both at ortho and meta positions of the benzoate
core structure. In chapter 3, the effect of lateral
fluorination on the mesogenic behaviour of these four
compounds is discussed. All the four compounds exhibit
Cr–SmCA*–SmC*–SmA*–Iso phase sequence but in
different temperature ranges. Due to lateral fluorination,
the clearing point decreases in all the fluorinated
compounds but the melting point and the stability of SmCA*
phase decrease or increase depending upon the position of
lateral fluorination. Layer spacings increase in DM1 and DM2
whereas it is found to decrease in DM3. All the compounds
show a tricritical nature of SmC*-SmA* transition and
orthoconic nature of tilt. They also exhibit very large
dielectric increments in SmC* phase but in singly
fluorinated DM1 and DM2 its value is less and in doubly
fluorinated DM3 it is more than the core protonated
compound. Four relaxation modes (PL, PH, GM, and SM) are
observed and their critical frequencies are found to
decrease in all fluorinated compounds. Domain mode is also
observed. Fluorination results in the slower response under
a square pulse, of the order of a few hundred
microseconds.Properties of a binary eutectic
antiferroelectric mixture, formed mixing 50 $wt.\%$ of two
pure compounds DM0 and DM3 are discussed in chapter 4. The
behaviour of the mixture is not always found to be
proportional to the concentration of the protonated and the
fluorinated components although it shows the same phase
sequence as in the pure compounds but with an increased
range of SmCA* phase. In this eutectic mixture also four
collective relaxation modes (PL, PH, GM, and SM) are
observed and their critical frequencies are found to lie in
between the pure compounds. However, PL and SM critical
frequencies are not proportional to the concentration of the
pure compounds but for PH and GM they are almost
proportional. The critical field for the suppression of the
PL and Goldstone modes is found to increase significantly.
The GM mode dielectric increment decreased substantially.
SmC*–SmA* transition temperature (Tc) is found to increase
linearly with a bias field and the phenomenon has been
explained using the Landau model. Spontaneous polarization
is found to be in between those of the pure compounds.
Switching time also exhibits similar behaviour and observed
to be about a few hundred microseconds. Motivated by the
formulation of the binary eutectic mixture, a room
temperature multi-component high tilt antiferroelectric
mixture is prepared by mixing DM0, DM1, DM2, and DM3 in
equal $wt.\%$ and is discussed in chapter 5. The range of
SmCA* phase is increased significantly and extended far
below room temperature (~ -8oC to 65 oC), keeping the phase
sequence the same as the pure compounds. Some properties of
the mixture are found to be equal to the average of the pure
compounds. The mixture is of orthoconic nature and SmC*-SmA*
transition shows tricritical nature like the pure compounds.
Correlation lengths across and within the layers are found
to increase from paraelectric to ferroelectric to
antiferroelectric phases. Four collective modes viz.PL, PH,
GM and SM and the domain mode are observed. $f_SM$ and
$〖∆ε〗_SM$ were found to follow the Curie–Weiss law.
Tc is found to increase with bias which was explained using
the Landau model. The mixture also exhibits sub-millisecond
switching, near the onset of SmCA* phase. The key feature
behind the compatibility of LCs and CNTs is the highly
anisotropic nature of both. In the recent past, few groups
studied the CNT composite of ferroelectric liquid crystals
(FLCs). But studies on nanocomposites of antiferroelectric
liquid crystals are very less. In chapter 6 we have made a
comparative study of the dynamic behaviour and electro-optic
response of the pure AFLC (DM1) and its MWCNT composite
(0.12 $wt.\%).$ The phase boundaries are lowered and the
stability of the SmA* and the SmC* phases are found to
increase but that of SmCA* phase is decreased. Distinct
textural changes are observed in different phases in the
nanocomposite. Pitch of the helicoidal structure is found to
increase with decreasing temperature in the pure compound in
both the tilted phases, the opposite trend is observed in
the composite in the SmC* phase. The absolute values and the
ranges of the anti-phase antiferroelectric mode critical
frequency $(f_PH)$ and the absorption strength $(ε_PH^'')$
are found to decrease in the doped system. In the composite,
the dielectric increment $(〖∆ε〗_GM)$ of GM decreases
and the critical frequency (fGM) increases and has been
explained using generalized Landau theory. The critical
field for suppression of the Goldstone mode is increased by
2 times in the composite, signifying the helical structure
in the nanocomposite is more stable than in the pure
compound. A significant reduction of spontaneous
polarization and switching time is observed in the
composite. A lower value of conductivity in the composite
signifies trapping of impurity ions by the CNTs.To probe the
structure-property relationship further, we have
investigated the effect of fluorination in the achiral chain
in chapter 7. For this, we have selected two partially
fluorinated terphenyl based pure antiferroelectric liquid
crystals 4F6T and 6F6T. While the first compound has 4
fluorinated carbon atoms and 6 oligomethelene spacers, the
second one has 6 fluorinated carbon atoms and the same 6
oligomethelene spacers in the achiral chain. Another
compound 5F6T, having 5 fluorinated carbon atoms and 6
oligomethelene spacers in the achiral chain, belonging to
the above homologous series, was studied and published by
our group before, its results are compared with the present
compounds for the sake of better understanding of
structure-property relations. The molecules of this series
structurally differ from the DM series, discussed in Chapter
3, by the number of fluorinated carbon atoms at achiral
chain, instead of alkoxy group in the chain carbonyl group
is attached here, the core is doubly fluorinated terphenyl
based instead of biphenyl benzoate in DM series. Here we
discuss the change of properties of these three compounds
due to the major change with respect to the biphenyl
benzoate core-based series as well as the change of
properties of compounds within the series due to the change
of the number of fluorinated carbon atom at ahiral chain.
Although all of them exhibited Cr-SmCA*- SmC*-SmA*-Iso phase
sequence, the melting point is observed to decrease with
increasing achiral chain length but the clearing point shows
the opposite trend, range of SmCA* and SmC* phases showed
odd-even effect like the molecular dipole moments and
spontaneous polarization. Layer spacings, average
intermolecular distances and correlation lengths across the
smectic planes are also observed to increase with achiral
chain length. Like DM series, SmC*-SmA* transition shows
tricritical nature. Dielectric increments are found to
decrease with increased achiral chain. Both soft mode and
Goldstone mode critical frequencies are found to decrease
with decreasing temperature in the lower derivative but
opposite behaviour is observed in the higher derivative.
Both the compounds, however, show Curie–Weiss behaviour in
soft mode near $T_c.$ Goldstone mode critical frequency is
much higher in 6F6T. Both PL and PH modes are observed in
SmCA* phase in 6F6T but in 4F6T only PH is observed and in
5F6T both modes are absent. Optical tilts exhibited
orthoconic nature of the SmCA* phase in both the compounds.
However, X-ray tilt is much less and the discrepancy has
been explained. Both the compounds show sub-millisecond
order switching time which is also found to increase with
achiral chain length. Antiferroelectric-ferroelectric
transition temperature $(T_AF)$ is observed to decrease with
increasing ac field, effect is more in the higher derivative
compared to the lower one. There is an indication of the
presence of a sub-phase (SmC$_a$*) between SmA* and SmC*
phases in 6F6T. These two compounds are expected to be
suitable for the preparation of mixtures suitable for
display and non-display applications because of their
orthoconic nature, sub-millisecond switching time and
moderate spontaneous polarization. Thus the number of
fluorinated carbon in the achiral part is found to influence
the mesogenic properties of various liquid crystalline
systems. Conclusions of all the experimental results have
been summarized in Chapter 8.},
cin = {DOOR ; HAS-User},
cid = {I:(DE-H253)HAS-User-20120731},
pnm = {6G3 - PETRA III (POF3-622) / FS-Proposal: I-20170021
(I-20170021) / FS-Proposal: I-20110546 (I-20110546) /
INDIA-DESY - INDIA-DESY Collaboration
$(2020_Join2-INDIA-DESY)$},
pid = {G:(DE-HGF)POF3-6G3 / G:(DE-H253)I-20170021 /
G:(DE-H253)I-20110546 / $G:(DE-HGF)2020_Join2-INDIA-DESY$},
experiment = {EXP:(DE-H253)P-P07-20150101},
typ = {PUB:(DE-HGF)11},
doi = {10.3204/PUBDB-2020-04474},
url = {https://bib-pubdb1.desy.de/record/452033},
}
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