The temperature effect on dielectric parameters of bent-core liquid crystal (DHB)

Article abstract of DOI:10.1016/j.molliq.2017.04.114

In this study, we have been analyzed in detail the dielectric, ac conductivity properties and phase transition temperatures of the bent-core liquid crystal 3′-{4-[4-(3,7-Dimethyloctyloxy)benzoyloxy]benzoyloxy}-4-{4-[4-[11-(1,1,1,3,5,5,5-heptamethyltrisiloxan-3-yl)undec-1-yloxy]benzoyloxy]benzoyloxy}biphenyl (DHB) exhibiting a dark conglomerate mesophase (DC^[?] phase) by Impedance Spectroscopy measurements at different temperatures.

Full text of DOI:10.1016/j.molliq.2017.04.114

Accepted Manuscript
The temperature effect on dielectric parameters of bent-core liquid
crystal (DHB)
Nimet Yilmaz Canli, Hale Ocak, Alptekin Yildiz, Mustafa
Okutan, Belkız Bilgin Eran
PII:
S0167-7322(17)31204-7
doi: 10.1016/j.molliq.2017.04.114
MOLLIQ 7270
DOI:
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
18 March 2017
22 April 2017
24 April 2017
Please cite this article as: Nimet Yilmaz Canli, Hale Ocak, Alptekin Yildiz, Mustafa
Okutan, Belkız Bilgin Eran , The temperature effect on dielectric parameters of bent-
core liquid crystal (DHB). The address for the corresponding author was captured as
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ACCEPTED MANUSCRIPT
The Temperature Effect on Dielectric Parameters of Bent-
Core Liquid Crystal (DHB)
*,a
b
c
a
b
Nimet Yilmaz Canli , Hale Ocak , Alptekin Yildiz , Mustafa Okutan , Belkız Bilgin Eran
a
Yildiz Technical University, Department of Physics,,34220 Esenler, Istanbul, Turkey
b
Yildiz Technical University, Department of Chemistry, 34220 Esenler, Istanbul, Turkey
c
Istanbul TechnicalUniversity, Department of Physics Engineering,, 34469 Maslak, Istanbul, Turkey
*
Corresponding Author: Nimet Yilmaz Canli Email:niyilmaz@yahoo.com,
Phone: +902123834278, Fax: +902123834234
ABSTRACT
In this study, we have been analyzed in detail the dielectric, ac conductivity properties and
phase transition temperatures of the bent-core liquid crystal 3’-{4-[4-(3,7-
Dimethyloctyloxy)benzoyloxy]benzoyloxy}-4-{4-[4-[11-(1,1,1,3,5,5,5-heptamethyltrisiloxan-
3
-yl)undec-1-yloxy]benzoyloxy]benzoyloxy}biphenyl (DHB) exhibiting a dark conglomerate
[
*]
mesophase (DC phase) by impedance spectroscopy measurements at different temperatures.
Keywords: Bent-core Liquid Crystal, Impedance Spectroscopy (IS), Dark Conglomerate
Mesophase.
1
. INTRODUCTION
Liquid crystals, which are intermediate states between crystals and isotropic liquids, are one
of the most exciting research areas within materials science due to their technological
1
,2
applications in the last decades . Thermotropic liquid crystals are mainly formed by
3
calamitic liquid crystals or discotic liquid crystals . Calamitic liquid crystals showing the
4
nematic phase have been widely studied because of their application to flat panel displays . In
addition to these, the bent-core materials have received significant attention in the soft matter
area in the last two decades and become a new sub-field of liquid crystals due to their unique
mesophases and unusual physical properties which are not exhibited in more conventional
1
,2,3,4,5
mesogens.
. The most outstanding properties they present are polarity and chirality. A
1

ACCEPTED MANUSCRIPT
rich variety of LC phases of bent-core molecules arises due to their molecular shape, leading
6
,7
to polar order and supramolecular chirality . The polar order of bent molecules in smectic
layers can induce a chirality of the smectic layers although the individual molecules are
3
achiral.
The novel mesophases formed by bent-core mesogens are usually either smectic or columnar
but, many variants have also been reported. Among these, dark conglomerate (DC)
mesophase, which provides materials with very attractive functional properties such as ferro-
8
and antiferroelectric switching, has a great importance . The DC phases, usually formed on
cooling the isotropic liquid, have a characteristic texture, appearing completely dark or
9
,10,11,12,13,14,15,16,17
exhibiting very low birefringence under crossed polarizers
.On uncrossing
the polarizers, the chiral domains of opposite handedness appear bright and dark revealed in
the ground state. The brightness within these domains is interchanged on by decreasing the
1
,9-17
polarizers in opposite directions
.
To study phase transitions and physical properties of dark conglomerate (DC) phases the
following complementary methods have been employed: DSC calorimetry, polarizing
microscopy texture observation, dielectric spectroscopy, and reversal current method.
Although the dielectric method has been established for well over 100 years, it is still one of
the most important techniques for studies of the structure of the materials. The main goal of
dielectric studies has been focused on a relationship between dielectric constant and
molecular properties of liquid crystals. The anisotropic dielectric properties of liquid crystals
play an important role in determining the electro-optical response of liquid crystal devices.
The real and imaginary part of dielectric function, dielectric loss, conductivity, capacitance,
activation energies are crucial parameters for deciding the suitable material for a
technological applications. Here, we determined the dielectric parameters and phase transition
temperatures of a silylated achiral bent-core molecule exhibiting the dark conglomerate (DC)
phase via the dielectric investigations and conductivity measurements by using Impedance
spectroscopy (IS) as an alternative method to Differential Scanning Calorimetry (DSC),
which is one of the fundamental method in liquid crystal studies for determining transition
temperature.
2

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2
2
. EXPERIMENTAL
.1
Synthesis
and
Mesomorphic
Properties
of
3’-{4-[4-(3,7-
Dimethyloctyloxy)benzoyloxy]benzoyloxy}-4-{4-[4-[11-(1,1,1,3,5,5,5-heptamethyltrisiloxan-
-yl)undec-1-yloxy]benzoyloxy]benzoyloxy}biphenyl (DHB) Liquid Crystal
3
The
Dimethyloctyloxy)benzoyloxy]benzoyloxy}-4-{4-[4-[11-(1,1,1,3,5,5,5-heptamethyltrisiloxane-
-yl)undec-1-yloxy]benzoyloxy]benzoyloxy}biphenyl (DHB) has been carried out as reported
synthesis
of
bent-core
liquid
crystal
molecule,
3’-{4-[4-(3,7-
3
1
6
previously .
Firstly, the esterification of 4-[4-(3,7-Dimethyloctyloxy)benzoyloxy]benzoic acid with 4’-
1
8
1
2
benzyloxybiphenyl-3-ol
dimethylaminopyridine
Dimethyloctyloxy)benzoyloxy]benzoyloxy]biphenyl. The hydrogenolytic debenzylation (H2,
0% Pd/C in THF) of the benzylated intermediate, followed by DCC esterification with 4-[4-
by
using
dicyclohexylcarbodiimide
(DCC)
and
4-
(DMAP)
as
catalysts yields
4'-Benzyloxy-3-[4-(3,7-
1
1
9
(
10-Undecenyloxy)benzoyloxy]benzoic acid . In the final step, the hydrosilylation reaction of
the bent-core compound 3’-{4-[4-(3,7-Dimethyloctyloxy)benzoyloxy]benzoyloxy}-4-{4-[4-(10-
undecenyloxy)benzoyloxy]benzoyloxy}biphenyl and 1,1,1,3,5,5,5-heptamethyltrisiloxane by
using Karstedt’s catalyst (platinum-divinyltetramethyl-siloxane complex in xylene) yields the
1
6
target siloxane substituted compound DHB (Scheme 1).
3

ACCEPTED MANUSCRIPT
O
OH
HO
O
+
BnO
O
O
1
. DCC, DMAP, dry CH Cl r.t.
2 2,
2
. H , Pd/C, THF, 40 °C, 18 h
2
O
O
O
HO
O
O
O
O
OH DCC, DMAP, dry CH Cl r.t.
2 2,
O
O
9
O
O
O
O
O
O
O
O
O
9O
Si
Si
O
O
H
Karstedt's catalyst, toluene, r.t., 48 h
Si
O
O
O
O
O
O
O
O
O
Si
Si
O
O
DHB
Si
O
1
1
16
Scheme 1. Synthesis of the siloxane substituted compound DHB .
4

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1
13
29
Compound DHB was characterized H-, C-NMR and Si-NMR (Varian Unity 500 and
Varian Unity 400 spectrometers, in CDCl solutions, with tetramethylsilane as internal
3
standard). Microanalysis was performed using a Leco CHNS-932 elemental analyzer.
16 1
Compound DHB : H-NMR: δ (ppm) = 8.29 (d; J ≈ 8.7 Hz; 2 Ar-H), 8.28 (d; J ≈ 8.7 Hz; 2
Ar-H), 8.15 (d; J ≈ 8.9 Hz; 2 Ar-H), 8.14 (d; J ≈ 8.9 Hz; 2 Ar-H), 7.65 (d; J ≈ 8.5 Hz; 2 Ar-H),
7
.51-7.50 (m; 2 Ar-H), 7.45 (broad s; 1 Ar-H), 7.37 (d; J ≈ 8.7 Hz; 2 Ar-H), 7.36 (d; J ≈ 8.7
Hz; 2 Ar-H), 7.30 (d; J ≈ 8.5 Hz; 2 Ar-H), 7.23-7.20 (m, 1 Ar-H), 6.98 (d; J ≈ 8.9 Hz; 2 Ar-
H), 6.97 (d; J ≈ 8.9 Hz; 2 Ar-H), 4.11-4.08 (m; 2H OCH ), 4.04 (t; J ≈ 6.6 Hz; 2H, OCH ),
2
2
1
.85-1.79 (m; 3H, CH , CH), 1.69-1.45, 1.35-1.15 (2m; 25H, CH, 12 CH ), 0.95 (d; J ≈ 6.4
2 2
Hz; 3H, CH ), 0.87 (d; J ≈ 6.4 Hz; 6H, 2 CH ), 0.44-0.42 (m; 2H, SiCH ), 0.07 [s; 18H, 2 Si-
3
3
2
13
(
CH ) ], -0.02 (s; 3H, Si-CH ). C-NMR: δ (ppm) = 164.46, 164.44, 164.32, 164.30 (CO),
3 3 3
1
1
63.82, 163.79, 155.42, 151.32, 150.63, 142.06, 138.01, 126.84, 126.80, 120.96 (Ar-C),
32.40, 131.82, 129.84, 128.31, 124.67, 122.10, 122.05, 120.60, 120.43, 114.42, 114.41 (Ar-
CH), 68.38, 66.72 (OCH ), 39.20 (CH ), 37.23, 35.97 (CH), 33.20, 29.81, 29.58, 29.54, 29.34,
2
2
2
0
9.07, 27.94, 25.96, 24.62, 23.04 (CH ), 22.67, 22.57, 19.62 (CH ), 17.60 (SiCH ), 1.83, -
2 3 2
2
9
.28 (CH ). Si-NMR: δ (ppm) = 6.83, -21.17. C H O Si (1181.69 g/mol); Anal. Calc.:
3
68 88 12
3
C, 69.11; H, 7.50. Found: C, 69.18; H, 7.43%.
Transition temperatures were measured using a Mettler FP-82 HT hot stage and control unit
in conjuction with a Leica polarizing microscope. The associated enthalpies were obtained
from DSC-thermograms which were recorded on a Perkin-Elmer DSC-7, heating and cooling
-1
rate: 10 °C min .
Spectroscopic data and mesomorphic properties for the target material DHB had already been
1
6
reported previously . The phase transition temperatures and molecular structure of DHB
have been shown in Figure 1.
5

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O
O
O
O
O
O
O
O
O
Si
Si
O
O
DHB
Si
O
1
1
-1
Figure 1. Phase transition temperatures T (°C) and enthalpies ΔH (kJmol ) of DHB
Compound
T/°C [ΔH kJ/mol]
16
[*]
DHB
Heating: DCFE 150 [18.2] Iso
[
*]
[*]
Cooling: Iso 146 [17.3] DCFE 70 DC(g)
−
1
[*]
(
Perkin-Elmer DSC-7; heating/cooling rates 10 K min and abbreviations: DC
= dark
FE
*]
[
conglomerate phase showing ferroelectric switching under electric field, DC(g) = non-
switching glassy state of dark conglomerate mesophase; Iso= isotropic phase).
The siloxane substituted compound DHB containing a racemic branched chain exhibits dark
conglomerate mesophase which is composed of chiral domains with opposite chirality. As
seen in Fig. 2, the dark and bright domains which are typical for dark conglomerate phases
have been observed by rotating the polarizer with a small angle (±4°) in the opposite
direction.
a)
b)
[
*]
Figure 2. Polarised light optical photomicrographs of DCFE mesophase at 120 °C of DHB
between uncrossed polarizers (±4°).
6

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2
.2 Preparation of Liquid Crystal Cell
Empty cell was prepared by indium-tin-oxide (ITO) coated glass plates purchased by Instec
Colorada Inc. The thickness of the empty cell was set at d =9 ± 0.2μm by Mylar spacer. ITO
cell was filled by capillary action with DHB.
3
. Results and Discussions
Dielectric measurements of DHB Liquid crystal were done using dielectric spectrometer
based on HP 4192A Impedance Analyzer controlled by PC using a program written on
Labview. Substances were put by means of capillary action into 9 μm indium-tin-oxide (ITO)
coated glass plates purchased by Instec Colorada Inc. The dielectric spectra were acquired in
0
the frequency range from 5Hz to 13 MHz at various temperatures varying from 25 C to
0
1
80 C. The variations of the real part of the dielectric constant with angular frequency at these
temperatures have been given in Figure 3. As is seen from Figure 3 real part of the dielectric
o
constant sharply decreased after phase transition at 75 C.
Figure 3. The real component of dielectric constant of DHB at different temperatures.
7

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In order to define the effect of temperature on dielectric relaxation type of DHB, the
dispersion curves for each temperature have been fitted by Origin Lab 8.5. The equation of
fitting function has been given in ref Eq. (1)


1

1





1
 () sin 
2

'()  
 (low freq. high freq.
)
(1)



high freq.
1
1
2(1 )
1 2() sin   ()
2
where  and  are absorption coefficient and relaxation time. As is known, absorption
coefficient parameter, α takes values between zero and one (0 < α ≤1). When α equals to zero,
it corresponds to Debye type relaxation. The non-Debye type occurs when the value of
2
0,21
absorption coefficient varies between; 0<<1 region
8

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6
7
Table 1. Dielectric parameters calculated by high frequency fitting (10 -10 Hz) dispersion
curves for different temperatures.
2
Temp
α
τ
Δε
Adj. R
Value
0.9997
Value Value StdErr
Value
StdErr
Value
StdErr
2
6
6
7
7
8
8
9
9
5
0
5
0
5
0
5
0
5
0.0729 0.0044 5.08E-08
0.0968 0.0089 5.07E-08
0.1308 0.0170 4.80E-08
0.0994 0.0053 2.28E-08
0.0861 0.0053 2.50E-08
0.0966 0.0057 2.48E-08
0.0956 0.0111 2.47E-08
0.0967 0.0104 2.38E-08
0.0981 0.0101 2.30E-08
5.40E-10 13.8045 0.0869
1.17E-09 14.2638 0.1882
2.41E-09 14.6098 0.4007
0.9988
0.9958
0.9999
0.9999
0.9998
0.9998
0.9998
0.9998
0.9999
0.9998
0.9998
0.9998
0.9998
0.9999
0.9999
0.9996
0.9997
0.9998
0.9998
0.9998
0.9998
0.9997
4.09E-10
4.02E-10
4.39E-10
5.96E-10
5.68E-10
5.58E-10
5.53E-10
6.56E-10
7.0764 0.0931
7.5315 0.0899
7.9184 0.1022
8.1514 0.1688
8.2881 0.1679
8.3362 0.1698
8.7558 0.1827
9.4303 0.2388
1
1
1
1
1
1
1
1
1
1
1
1
1
1
00 0.1137 0.0096 2.14E-08
05 0.1322 0.0109 2.01E-08
10 0.1520 0.0112 1.90E-08
15 0.2006 0.0149 1.66E-08
20 0.2490 0.0164 1.31E-08
30 0.2952 0.0113 1.20E-08
40 0.2288 0.0075 1.63E-08
50 0.0555 0.0058 2.39E-08
55 0.0610 0.0046 2.28E-08
60 0.0564 0.0040 2.11E-08
65 0.0482 0.0037 2.06E-08
70 0.0443 0.0038 1.99E-08
75 0.0361 0.0036 1.93E-08
80 0.0455 0.0050 1.80E-08
7.04E-10 10.1296 0.2789
1.02E-09 11.7080 0.4853
1.15E-09 13.5159 0.7282
8.22E-10 16.2816 0.6221
4.98E-10 13.9998 0.2787
4.90E-10 10.0853 0.1549
3.88E-10 10.6637 0.1360
3.33E-10 10.9162 0.1334
2.99E-10 11.1340 0.1285
2.99E-10 11.5257 0.1402
2.82E-10 11.8721 0.1447
4.02E-10 12.8322 0.2393
9

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Figure 4. The effect of temperature on absorption coefficient and dielectric strength,  for
s
the compound DHB.
Dielectric relaxation parameters, ,  and dielectric strength,  have been given in Table 1.
s
The effect of temperature on absorption coefficient and dielectric strength,  have also been
s
given in Figure 4. The dielectric strength,  , has also been calculated by taking the
s
difference between the ′low freq. and the ′high freq. values. It has been clearly observed that
o
dielectric strength sharply decreasing behavior at phase transition temperature 70 C and then
o
increased to 130 C (See Figure 4 and Table 2). The decrease in the value of  can be
s
interpreted as the fact that the molecular alignment is getting easier as the temperature
increases from this temperature region. Since absorption coefficient values for all
temperatures are very close to zero, the relaxation type of DHB molecule has been considered
as nearly-Debye.
Dielectric relaxation parameters, ,  and dielectric strength,  have been also calculated by
s
double fitting and these data have given in Table 2. The real component of dielectric constant
o
of DHB at double fitting example for 95 C has also given in Figure 5.
1
0

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o
Table 2. Dielectric parameters calculated by double fitting dispersion curves for 85 C -115
o
C.
2
Temp
#
α
τ
Δε
Adj. R
Value Value
1
Value
0.0518
0.0956
0.0701
0.0967
0.0759
0.0981
0.0939
0.1137
0.1011
0.1322
0.1014
0.1520
0.1076
0.2006
StdErr
0.0398
0.0111
0.0311
0.0104
0.0304
0.0101
0.0327
0.0096
0.0361
0.0109
0.0404
0.0112
0.0507
0.0149
Value
StdErr
Value
1.8395
8.1514
2.4183
8.2881
2.5931
8.3362
2.6204
8.7558
2.6771
9.4303
2.5552
StdErr
0.0904
0.1688
0.0809
0.1679
0.0769
0.1698
0.0750
0.1827
0.0824
0.2388
0.0850
Value
0.9781
0.9998
0.9851
0.9998
0.9848
0.9998
0.9806
0.9999
0.9755
0.9998
0.9684
0.9998
0.9519
0.9998
5.22E-06
2.47E-08
3.59E-06
2.38E-08
2.77E-06
2.30E-08
1.99E-06
2.14E-08
1.71E-06
2.01E-08
1.49E-06
1.90E-08
1.47E-06
1.66E-08
4.46E-07
5.96E-10
2.32E-07
5.68E-10
1.69E-07
5.58E-10
1.29E-07
5.53E-10
1.21E-07
6.56E-10
1.14E-07
7.04E-10
1.42E-07
1.02E-09
8
9
9
5
0
5
2
1
2
1
2
1
2
1
2
1
2
1
2
1
1
1
1
00
05
10
15
10.1296 0.2789
2.3988 0.1072
11.7080 0.4853
Figure 5. The real component of dielectric constant of DHB at double fitting example for 95
o
C.
1
1

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The variations of the imaginary part of the dielectric constant with angular frequency at
0
0
various temperatures varying from 25 C to 180 C have been given in Figure 6.
Figure 6. The imaginary component of dielectric constant of DHB at different temperatures.
The imaginary component of dielectric constant has a peak that corresponds to dielectric
relaxation (Fig.6). It has shown that while the relaxation frequencies shift to higher
frequencies as the temperature increases, the peak intensities decrease. Moreover the decrease
0
in relaxation frequency above 150 C may be associated with the phase change of the material
from dark conglomerate phase to isotropic phase. Dielectric relaxation mechanism of DHB
has been analyzed by Cole-Cole curves given in Figure 7a and 7b. As shown in Figure 7a,
DHB exhibit nearly semicircle and straight line; but as the temperature increases the shape of
the semi-circles deforms. The variation of temperature on Cole-Cole characteristics also
manifests itself as a noticeable decrease in the diameter of the semi-circles. According to
publications in literature, vertical lines appear in the low frequency region of Cole-Cole plot
2
2,23,24,25,26,27,28
are accepted as a super-capacitive behavior
. As is seen from the Cole-Cole
o
curve at 60 C (Figure 7a), it also indicates the system may behave as a super- capacitor.
1
2

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(
a)
1
3

ACCEPTED MANUSCRIPT
(b)
Figure 7. Cole- Cole Plots of DHB (a) and (b) at different temperatures.
Figure 8. The temperature dependence of a real part of the dielectric constant of DHB at
different frequencies.
The temperature dependence of a real part of the dielectric constant of DHB at different
frequencies have also shown in Fig.8. The real part of dielectric constant of DHB sharply
decreased at phase transition temperature in all frequencies.
ı
ıı
The three dimension plots of the complex dielectric permittivity real ε and imaginary part ε
versus angular frequency and temperature for DHB molecule have also drawn in Figure 9a
and 9b. The phase transition temperatures for DHB also can be determined from these graphs.
1
4

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(a)
(b)
ı
Figure 9. The typical 3D plot of the complex dielectric permittivity real ε (a) and imaginary
ıı
part ε (b) versus angular frequency and temperature for DHB liquid crystal.
1
5

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3
.2. Analysis of Temperature Effect on Frequency Dependent Conductivity
To analyze the effect on temperature on electrical properties of DHB, alternative current (ac)
conductivity,  , has been calculated by Eq. (2).
ac
ac= "
(2)
0
where  and " represent angular frequency, dielectric constant of free space, and the
0
2
9
s
imaginary part of dielectric constant, respectively . Conductivity power law obeys  

s
30
relation,
is the dc component and
= is the ac component . According to Jonscher,

ac
the value of frequency exponent “s” was restricted to be between zero and one. While s≈0
3
1
state corresponds to dc conductivity , 0<s<0.7, 0.7≤s<1, s=1 and s>1 conditions represent
Correlated Barrier Hoping (CBH) Mechanism, Quantum Mechanical Tunneling (QMT)
model, Nearly Constant Loss (NCL) and Super Linear Power Law (SLPL),
3
2,33,34,35,36
respectively
. The frequency exponent values for each temperature have been
calculated at the low, intermediate and high frequency regions by the slope of ln =f (ln)
ac
curves given in Figure 10. Frequency exponent values, s and related conductivity mechanisms
0
0
0
o
0
o
o
at 25-55 C, 60-65 C, 70-80 C, 85-95 C, 100-115 C, 120-125 C, 150-180 C temperatures
have also been given in Table 3.
1
6

ACCEPTED MANUSCRIPT
(a)
(
b)
Figure 10. The ln versus ln graphics of DHB (a) and (b) at various temperatures.
ac
1
7

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Table 3. Frequency exponent values, s and related conductivity mechanisms at 25-55 C, 60-
0
0
o
0
o
o
6
5 C, 70-80 C, 85-95 C, 100-115 C, 120-125 C, 150-180 C temperatures.
Temperature
middle freq.
low high
low freq.
high freq.
o
(
C)
2
6
7
8
5-55
0-65
0-80
5-95
SLPL
DC Cond.
QMT
QMT
CBH
SLPL
SLPL
SLPL
CBH
CBH
SLPL
SLPL
SLPL
SLPL
NCL
SLPL
SLPL
QMT
SLPL
1
1
1
00-115
20-125
50-180
DC Cond.
DC Cond.
SLPL
SLPL
As is seen from Table 3, different conductivity mechanisms have been observed at different
temperatures. According to high frequency s parameter values before phase transition DHB
o
exhibited CBH mechanism after phase transition at 70 C DHB exhibited stable conduction
o
mechanism as SLPL. In the isoptropic phase at 150 C conductivity mechanism changed as
NCL.
3
.3. Arrhenius Plot Analysis
The temperature dependence of conductivity is given by Eq. (3)
= exp(E /k T)

(3)
0
a
B
where  is a pre-exponential factor, E is the activation energy, k is the Boltzmann constant
0
a
B
(
ev/K), and T is the temperature (K). In Figure 11, the variation of ln with (1000/T) has
ac
been given some selected angular frequencies. Arrhenius plots show a sudden jump of
conductivity around the phase transition temperature. According to Table 4, the activation
energies have been sharply changed in phase transition regions too. The decrease in activation
energy with increasing frequency can be useful for the low energy liquid crystal device
applications.
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Figure 11. The temperature dependence of ln versus 1000/T of DHB for =5.3k rad/s, 11
ac
k rad/s, 23k rad/s, 47k rad/s.
Table 4. Angular frequency exponent values, temperature regions and activation energies at
o
o
o
o
o
2
5-69 C, 70-84 C, 85-115 C, 116-149 C, 150-180 C.


25-69
(eV)
70-84
(eV)
85-115
(eV)
116-149
(eV)
150-180
(eV)
(
rad/s)
1
.7516
.5037
.3180
.1338
0.5841
0.5655
0.6460
0.8083
0.3604
0.2711
0.1710
0.0468
0.7282
0.5889
0.4231
0.2508
0.0213
0.0328
0.0443
0.0612
5
.3k
1
1
1
1
1k
3k
7k
2
4
1
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4
. Conclusions
In this study, we have been analyzed in detail the dielectric, ac conductivity properties and
phase transition temperatures of the bent-core liquid crystal DHB exhibiting a dark
[
*]
o
conglomerate mesophase (DC phase, between 70-150 C) by impedance spectroscopy (IS)
measurements at different temperatures.
According all IS results, dielectric parameters (dielectric relaxation parameters, ,  and
dielectric strength,  ), conductivity mechanisms, activation energies have been sharply
s
changed in phase transition regions. From this point of view, real phase transition
temperatures could be verified with Impedance Spectroscopy measurements.
The decrease in activation energy with increasing frequency has also observed and this data
can be useful liquid crystal device applications.
Acknowledgements
This research has been supported by Yıldız Technical University Scientific Research Projects
Coordination Department with the Project Number 2014-01-GEP05.
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References:
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Highlights
1
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3
- Dielectric parameters of the bent-core liquid crystal have been researched.
- Activation energies for some selected angular frequencies have been calculated.
- The conductivity mechanism has been changed with increasing temperatures.
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