DOI : 10.17577/IJERTV4IS110136
- Open Access
- Total Downloads : 134
- Authors : S. Sreehari Sastry, L. Tanuj Kumar, D. M. Potukuchi, Ha Sie Tiong
- Paper ID : IJERTV4IS110136
- Volume & Issue : Volume 04, Issue 11 (November 2015)
- DOI : http://dx.doi.org/10.17577/IJERTV4IS110136
- Published (First Online): 06-11-2015
- ISSN (Online) : 2278-0181
- Publisher Name : IJERT
- License:
This work is licensed under a Creative Commons Attribution 4.0 International License
Cross-Over Temperature for Distinct Low-Frequency Modes in Ferro Electric Liquid Crystalline Phases
-
Sreehari Sastry, L Tanuj Kumar
Department of Physics, Acharya Nagarjuna University, Nagarjunanagar -522510, India.
D. M. Potukuchi
Department of physics,
Jawaharlal Nehru Technological University-Kakinada, Kakinada-533003, India.
Ha Sie Tiong
Department of Chemical Science,
Faculty of Science, Universiti Tunku Abdul Rahman, Jalan University, Bandar Barat, 31900
Kampar, Perak, Malaysia
Abstract Phase transitions and relaxation behaviour are investigated in the low f requency region (1Hz-1MHz) for two ferroelectric liquid crystal compounds viz., (S)-(-)-2- Methylbutyl4-(4-n-alkanoyloxybenzoyloxy) biphenyl-4- carboxylates (S-MB-nB-BC), for n=16 and 18, which exhibit enantiotropic Smectic A and Smectic C* phases. Phase transition temperatures are determined from LF dielectric method that are in agreement with the results obtained from concurrent microscopic textural (POM) and DSC studies, Dielectric relaxation behaviour is also investigated for SmecticA and SmecticC* phases. Dielectric dispersion has inferred the temperature variation for relaxation frequency, dielectric strength, loss maxima and degrees of freedom. Arrhenius shift has indicated the activation energies in higher range. Dielectric loss maxima in SmC* has exhibited a cross- over temperature that suggested the presence of two distinct modes of relaxation in SmC* phases. L F Relaxation in SmC* has behaved in a similar way of Curie -Weiss law in ferro electrics.
Keywords—Ferro electric liquid crystals; Smectic A & C* phases; dielectric parameters; cross over temperature; Goldstone mode.
-
INTRODUCTION
-
Thermotropic liquid crystal (LC) phase structures [1] such as Nematic and Smectics owe to their rich electro optic response [2,3]. Existence of ferroelectric (FE) response in tilted liquid crystals is reported [4,5] firstly by Meyer in 1975. LC molecules, those possessing chiral centre in their tilted phase structures like in SmC*, Sm I* and Sm F* phases are found[6,7] to exhibit ferroelectric response. LC SmC* phases have been attracted greater attention for studies due to their least viscous state and readily alignable stablized structure surface [8] geometry. Potential applications for [9] fast-switching, high-contrast, large viewing angle devices are noteworthy. The molecular director possesses helicoidal structures in the layered SmC*, and a constant tilt angle is maintained [10,11]. The coupling between applied electric field at given
molecular tilt and its electro-clinic response[12,13] in these ferroelectric liquid crystalline materials become interesting[7] on observing fast electro-optic response at sub microsecond. Investigations on low frequency (LF) dielectric response [14,15] in LCs will help to get important information that can be used in making of electro optic devices. Determining the properties like relaxation frequency (fR), strength , distribution parameter- and activation energy (Ea) in LCs corresponding to the LF relaxation behaviour would help to optimize their utility. Although LF dielectric response is initially reported [16] in nematics (N) with sluggish millisecond response, the investigations on FELC SmC* phases has resulted in useful information [17, 18] with regard to collective and independent molecular processes. The reported gold stone mode and soft mode, Curie Wiess behaviour and other orientational response modes in SmC* phase in low frequency (in few Hz to KHz) region have attracted the attention for studies on fundamental reorientation in the Quasi-2D crystalline soft condensed matter systems. The information for the reorientation response would be optimized during their utility in devices. Keeping in view the requirement of the ambient FELC SmC* phase structures, being exhibited [20] by LCs preferably for esteric moieties, structures, the effort is made have to bring out the dielectric dispersion properties in FELC materials S-MB-16B-BC and S-MB-18B-BC.
-
MATERIALS AND EXPERIMENTS
-
Materials
The compounds (S)-(-)-2-Methyl butyl4-(4-n- alkanoyloxy benzoyloxy) biphenyl-4-carboxylates for n= 16 and 18) mesogens [22] were synthesized.
The molecular structure for the chosen FELC compounds (S)-(-)-2-Methylbutyl 4-(4-n-alkanoyloxybenzoyloxy) biphenyl-4-carboxylates (where n= 16 and 18) in conjunction of chiral centre (*) presented in Template -1.
Template 1: Molecular structure of S-MB-nB-BC, where n=16,18
-
Measurements
Measurements for textures and phase transition temperatures [1] were carried out by using polarizing optical microscope (POM), coupled with hot stage of Meopta DRU 3 model (Meopta Global Manufacturers, Hauppauge, NY, USA) and Canon EOS Digital REBEL XS/ EOS1000D to record textural images at given POM crossed polar configuration. The FELC compounds [22] were filled through capillary action method in LC1 ITO coated liquid crystal cells of 6 m space that are received from Instec, (USA). The temperature and frequency variation of dielectric response were at a low frequency range for 1Hz to 1 MHz measured by LCR Meter(Model PSM1700 , Newtons4thLtd., Loughborough.UK). The sample was initially heated to isotropic state and kept it until thermal equilibrium attained. Through capacitance and loss factor, the dielectric response was measured against an input 1Vp-p oscillating signal. The accuracy for dielectric constant and loss values is 1% and 2%, respectively. The accuracy for temperature variation is ± 0.1oC. Phase transition temperatures were determined from the studies to temperature variations on POM textures, capacitance C() and loss factor Tan(). The off centred dielectric dispersion behaviour [23, 24] was investigated based on temperature and frequency variations of capacitance and loss factor.
-
Computational Details
The dielectric dispersion was measured for variation in capacitance C (T) and loss factor Tan (T) at specified different temperatures for different LC phases during cooling scan. The observed variation of C() and Tan() is presented in figure-1 and figure-2. In the wake of the temperature invariant capacitance being exhibited by the empty cell (~38.99pF) for the frequency range 1Hz 1 MHz, the relative permittivity of 100 KHz (or r) is estimated by
*() = () – j () ——————————— (1)
r ()= () = C()/(38.99——————————- (2)
where C () is the observed capacitance of the LC cell at a specific temperature in any LC phase corresponding to the frequency , = 2f of the input ac signal.
The dielectric loss () is estimated by
() = r() Tan() ———————————- (3)
where Tan is the observed loss factor exhibited by the LC phase structure at a specified temperature corresponding to
Figure-1: Variation of capacitance and loss factor of empty cell with temperature
Empty cell
54
52
Capacitance (pF)
50
48
46
44
42
40
38
0 1 2 3 4 5 6
Log (freq)
Figure-2: Variation of capacitance for empty cell with frequency
the frequency of input ac signal. The dielectric dispersion is given [14-15] by
() = + {() (1 + [j]1)} ———————- (4)
where, = [o ] is the dielectric strength, estimated by extrapolating on to the r axis through Cole Cole plots
, the relaxation time given by 1/fR, where fR corresponding to thefrequency at which exhibits maximum value
for 2f where f is the frequency applied ac E field, for the distribution parameter reflecting upon the degrees of freedom exhibited by the phase in any LC phase structure.
-
-
RESULTS AND DISCUSSION
-
Textures and Phase Transition Temperatures:
Enantiotropic phases and transitions temperatures exhibited by the liquid crystalline SmA and SmC* phases are initially determined by POM. The POM textures shown by (S)-(-)-2-Methylbutyl4-(4-n-alkanoyloxybenzoyloxy) biphenyl-4-carboxylates(S-MB-nB-BC for n = 16 and 18) for SmA and SmC* phases are shaped in focal conic fan (plate-1) and arced focal conic (plate-2) respectively. The
Plate 1: Focal conic fan texture of SmA phase for S-MB-18B-BC at 380.5 K
transition temperatures TIA and TAC* are in agreement with POM and DSC [21] data reports. The observed phase transition temperatures determined by POM (in heating and cooling cycles) are presented in Table-1. The thermal span for heating scan periods of LC phases has differed slightly from those observed for cooling scan. However, the hierarchy in occurrence has remained invariant.
Table1: Phase transition temperatures Tc by POM, DSC, LF Dielectric methods for S-MB-nB-BC for n= 16 and 18
n=
Method
Data of Phases, Transition
Temperatures (Tc in K)
Ref
16
POM
H
Cr337SmC*385.4 SmA 426.2 Iso
pres ent
C
Cr1325.4Cr2333.7SmC*
375.4SmA425.8Iso
POM
/DSC
H
Cr337SmC*385.4 SmA 426.2 Iso
[21] C
Cr1325.4Cr2333.7SmC*
375.4SmA425.8Iso
LF
Dielectric
C
Cr1325.4Cr2337.0SmC*
379.5SmA424.8Iso
pres ent
18
POM
H
Cr346.3SmC*385 SmA
425 Iso
pres ent
C
Cr1335.9Cr2341.5SmC*
377.5SmA424.8Iso
POM
/DSC
H
Cr346.3SmC*385 SmA
425 Iso
[21] C
Cr1335.9Cr2341.5SmC*
377.5SmA424.8Iso
LF
Dielectric
C
Cr1335.9Cr2341.5SmC*
379.0SmA422.7Iso
pres ent
H: HEATING, C: COOLING
-
Phase Transitions by Temperature variation of Dielectric constant r(T) and Loss Factor Tan(T) :
The S-MB-nB-BC cell is connected to the LCR meter which is operated at fixed 100 kHz frequency and triggered by 1Vp-p oscillating signal. The observed temperature variations in dielectric constant r(T) and loss factor Tan(T) exhibited for cooling run of FELC samples are presented in figure-3 and figure-4 for n=16 and 18, respectively.
160
140
Dielectric constant (')
120
100
80
60
40
20
0
Crystal
SmC*
SmA
S-MB-16B-BC
Iso
0.8
0.7
0.6
Tan
0.5
0.4
0.3
0.2
0.1
Plate 2: Arced focal conic texture of SmC* phase for S-MB-16B-BC at
350.5 K
320 340 360 380 400 420 440
Temperature(K)
Figure 3 Temperature variations of dielectric constant and loss factor Tan
() for S-MB-16B-BC
160
140
Dielectric constant (')
120
100
80
60
40
20
Crystal
SmC*
SmA
S-MB-18B-BC
Iso
1.2
1.0
0.8
0.6
0.4
0.2
0.3
Cr
0.2
0.1
d/dT
0.0
-0.1
SmC*
Tan
341.5 K
S-MB-18B-BC
SmA
Iso
422.7 K
0
320 340 360 380 400 420 440
Temperature(K)
0.0
-0.2
-0.3
379.0 K
Figure – 4 Temperature variations of dielectric constant and loss factor Tan ()for S-MB-18B-BC.
The r in cooling run has exhibited peaks, where as Tan
(T) displayed dips in the vicinity of phase transition. The increase of r(T) with the decrease of temperature has indicated the increasing dipole correlation due to that LC phase structures grew with higher order. As the reason of change in r(T)and Tan(T ) is apparently marginal, the derivative curve is drawn for the observed temperature variations of r(T) in figure-5 and figure-6 for n= 16 and 18, respectively. The data for transition temperatures determined from LF dielectric and microscopic observation (Table-1) are in agreement with the reported [22] data.
0.4 S-MB-16B-BC
320 340 360 380 400 420 440
Temperature(K)
Figure – 6 Temperature variation of differential dielectric constant (T) for S-MB-18B-BC
The AC* transition noticed here which was not reported
[22] for any enthalpy in DSC studies, so that it is considered as a second order transition. However, an anomalous behavior is observed to r(T)and dr(T)/dt across AC* transition. In view of these, the LF dielectric method is a capable method to detect second [25] order transitions also. -
Dielectric Dispersions:
The LF dielectric dispersion i.e., frequency variation of capacitance C () , loss factor Tan() are recorded at
0.3
Cr
0.2
d/dT
0.1
0.0
-0.1
-0.2
337.0 K
SmC*
SmA
379.5 K
Iso
424.8 K
different temperatures for SmA and SmC* phases exhibited by FELCs. The dielectric constant (i.e., r = ) at different frequencies is estimated using equation-2. From the data for capacitance variation () of the FELC compounds are presented in figures -7, -8, -9 and-10 at specified temperatures for SmA and SmC* phases. Both compounds have shown decreasing trend of with increasing frequency.
379 K
389
408
418
160
-0.3
320 340 360 380 400 420 440
Temperature (K)
Figure 5 Temperature variation of differential dielectric constant (T) for S-MB-16B-BC
140
Dielectric constant (')
120
100
80
60
SmA in S-MB-16B-BC
40
20
0
0 1 2 3 4 5 6
Log (freq)
Figure 7 Frequency variation of dielectric constant () in SmA of S- MB-16B-B
160
140
Dielectric constant (')
120
100
80
60
40
20
0
SmC* in S-MB-16B-BC
345 K
353
363
373
The steep fall in is noticed in lower frequency region, and it is marginal at higher frequencies. Steep fall of at lower frequency (Few kHz) range would indicate the predominant response of LC molecules during the dipolar orientational process. It is also noticed that dielectric constant is increased with increasing temperatures in LC phases. At higher frequency, the values of dielectric constant are lower, that might reflect the lesser contributions of LC molecules dipole moment to the orientational mechanism. The higher value of r at lower frequencies is attributed to the response of large polarization in LC compound.
From the observed data of capacitance C and loss factor
0 1 2 3 4 5 6
Log (freq)
Figure 8 Frequency variation of dielectric constant () in SmC* of S- MB-16B-BC
160
Tan the dielectric loss () exhibited by the FELC in the SmA and SmC* LC phases are estimated by the equation-3 in view of the response of empty cell. The loss spectrum
() exhibited by FELC in their SmA and SmC* phases at specified temperatures is presented in figures -1, -12 and -13, -14 for n=16 and 18, respectively.
140
Dielectric constant (')
120
100
80
60
40
20
0
SmA in S-MB-18B-BC
70
383 K
393 60
403
413 50
Dielectric loss ('')
40
30
20
10
0
SmA in S-MB-16B-BC
383 K
389
408
418
0 1 2 3 4 5 6
Log (freq)
Figure 9 Frequency variation of dielectric constant () in SmA of S- MB-18B-BC
160
0 1 2 3 4 5 6
Log(freq)
Figure 11 Frequency variation of dielectric loss () in SmA of S- MB-16B-BC
140
Dielectric constant (')
120
100
80
60
40
20
0
SmC* in S-MB-18B-BC
357 K
361
368 60
377
50
Dielectric loss('')
40
30
20
341 K
345
353
361
363
368
373
0 1 2 3 4 5 6
Log (freq)
Figure 10 Frequency variation of dielectric constant () in SmC* of S-MB-18B-BC
10
0 SmC* in S-MB-16B-BC
0 1 2 3 4 5 6
Log (freq)
Figure 12 Frequency variation of dielectric loss () in SmC* of S- MB-16B-BC
60 SmA in S-MB-18B-BC
50
Dielectric loss ('')
40
383 K
393
403
413
423
2.0
1.8
1.6
Sm A
1.25 eV
30
20
10
0
0 1 2 3 4 5 6
Log (freq)
1.4
Log f
R
1.2
1.0
0.8
0.6
0.4
S-MB-16B-BC
0.97 eV
H.T.SmC*
L.T SmC*
0.64 eV
Figure 13 Frequency variation of dielectric loss () in SmA of S-
MB-18B-BC
-
Activation Energy:
Variation of fR(T) and () max in SmA and SmC* phases exhibited by FELCs is presented in Table-2. The data for fR have shown decrease trend with the decreasing temperature
2.4 2.5 2.6 2.7 2.8 2.9
1000/T
Figure-15 Reduced Temperature plots in different phases of S-MB-16B- BC
22..55
Sm A
in both SmA and SmC* phases. This trend is attributed to increasing viscosity on cooling LC phase structure. The value of fR in good agreement with the reported values for other FELC compounds [17-19, 25]. From data of fR(T), the Arrhenius plots are drawn (figure -15 and -16 ) for SmA and SmC* phases.
22..00
Log f
R
11..55
11..00
00..55
1.32 eV
S-MB-18B-BC
H.T.SmC*
1.01 eV
L.T SmC*
0.73 eV
40
35
Dielectric loss ('')
30
25
357 K
359
361
368
373
375
377
20
15
10
5
0 SmC* in S-MB-18B-BC
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Log (freq)
00..00
2.3 2.4 2.5 2.6 2.7 2.8
1000/T
Figure-16 Reduced Temperature plots in different phases of S-MB-18B- BC
The estimated activation energy Ea (Table-2) for LC SmA and SmC* phases are recorded relatively with lower values that would reflect lower potential barrier for the process to re-orient the tilted SmC* phases, than the orthogonal SmA phase structures.
Table 2: Data of dielectric parameters in SmA and SmC* phases of S-MB
nB-BC for n= 16and18
n
Phase
Tempe
-rature (K)
Relaxat ion frequen cy
(fR)
(Hz)
Dielectri c strength
Max. loss ()max
Distribut ion Paramete r
16
418
2.080
113.05
61.96
0.174
SmA
[1.25eV]
408
1.959
113.60
52.04
0.226
399
1.714
115.239
50.63
0.261
389
1.591
116.86
47.08
0.296
379
1.469
117.95
44.25
0.349
373
1.340
140.36
56.24
0.191
SmC*(H T)
[0.97eV]]
368
1.220
144.65
55.34
0.209
363
1.102
152.53
54.62
0.244
361
0.979
144.30
52.28
0.139
SmC*(L T)
[0.64353
0.734
147.95
55.62
0.157
Figure 14 Frequency variation of dielectric loss () in SmC* of S- MB-18B-BC
18
eV]
345
0.612
150.29
57.29
0.191
341
0.489
152.38
59.01
0.209
423
2.326
120.03
49.39
0.209
SmA
[1.32eV]
413
2.200
121.33
47.48
0.226
403
2.080
121.98
46.21
0.244
393
1.836
123.28
44.29
0.261
383
1.591
124.59
39.82
0.279
377
1.224
152.38
47.29
0.08
SmC*(H
T)
[1.01eV]375
1.102
154.47
46.28
0.122
373
0.979
157.26
43.89
0.157
368
0.857
160.73
42.67
0.174
361
0.612
162.82
41.89
0.244
SmC*(L T)
[0.73eV]
359
0.489
165.6
44.86
0.261
357
0.244
168.99
45.85
0.296
6 5 4 3 2 1 0
60 60
SmC* in S-MB-16B-BC
341 K
50 345 50
Dielectric loss( '')
40
30
20
L.T.Mode
10
0
353
361
363 K
368
373
H.T.Mode
40
30
20
10
0
0 1 2 3 4 5 6
Log(freq)
Figure 18 Cross over temperature modes in SmC* phase for S-MB-16B- BC
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
40
35
Dielectric loss ('')
30
25 L.T.Mode
40
H.T.Mode 35
30
25
An overview of data of Ea in SmA and SmC* phases has revealed the higher magnitudes than the reported [26] values. The higher value of Ea is attributed to the additional contribution of transverse dipole moment (t) to the tilted chiral structure.
357 K
20 359
361
15
10
5
0
SmC* in S-MB-18B-BC
368 K 20
373
375 15
377
10
5
0
-
Cross-Over of temperture for LF modes in SmC* phase:
The temperature variation of loss maximum () max in SmC* phase presented in figure-12 and-14 has indicated that it is accompanied by a reversal of trend (figure -17)
max(T) at a particular temperature.
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Log (freq)
Figure19 Cross over temperature modes in SmC* phase for S-MB-18B- BC
The mode cross- over temperature in SmC* lower for lower end-chain compound FELC i.e., for n=16. The observation of TCO also has implied a lower energy configuration during the reorientation process corresponding to distinct modes
60
58
56
54
52
max
('') 50
48
46
44
42
40
L.T mode of SmC* in S-MB-16B-BC H.T
L.T mode of SmC* in S-MB-18B-BC H.T
340 345 350 355 360 365 370 375 380
Temperature(K)
-
Cole Cole plots:
The dispersions of, () and () are seemed to be asymmetric about , where in () has shown maximum in both SmA and SmC* phases. The fall of with frequency is not symmetrical about the ()max. Hence, such off-centered dielectric dispersion [24] exhibited by the SmA and SmC* phases. In order to analyze the observed low frequency (LF) dielectric response and also the distinct (time scale wise) mode of relaxation behavior, the data for
and r , LF dielectric dispersion to all LC phases are received by computation using equation-4. The derived dielectric dispersions are presented as Cole-Cole plots in figure -20 (a) to -20 (f).
Figure 17 Variation of Dielectric maxima with temperature for S-MB-
16B-BC and S-MB-18B-BC
For a decreasing temperature ()max the loss decreased initially, but started to increase from a characteristic temperature TCO., Since the () max reflected the loss of energy corresponding to a dielectric medium, the reversal trend is related to the contribution made from a different and distinct modes in SmC*phases. As a result,a possible distinction should be present to differentiate the high temperature (HT) mode (figure -18 and -19) from low temperature mode (LT) in the SmC* phase.
The data for LF dielectric parameters corresponding to the distinct modes, such as relaxation frequency fR, loss maximum max, dielectric strength , distribution parameter- and activation energy Ea are estimated from Cole-Cole plots drawn for S-MB-nB-BC for n=16 and -18 and presented in Table-2. In order to determine , o and
from Cole-Cole plots the value of is extrapolated on to the r axis. The extrapolated point towards the lower frequency side is read as o and for higher frequency side as . An overview of the Cole-Cole plots for LF
relaxations has shown a greater temperature r shift in the o end for all phases. However its shift towards high frequency end, remains constant almost. The observed large temperature o shift informed the relative dielectric susceptibility of FELC in the LF (KHz) region.
Figure 20 Cole- Cole plots for SmA and SmC* phases in S-MB-16B-BC and S-MB-18B-BC
The trend for -parameters (Table-2) of SmA and SmC* phases as shown the increasing degree of freedom for decreasing temperatures. Increase in -parameter values are due to increasing restrictions on rotational freedom for longitudinal dipole moment (l).
-
Curie Weiss behavior in FELC in SmC* phase:
The data for temperature variation (Table-2) of LF frequency dielectric strength pertaining to the HT LF mode in the SmC* phase are fitted to the equation given by
1/ (T) ——————————————- (5)
The data for fitted (Figure 20 and 21) is shown
with the solid line. The estimated exponential value () is 1.0846±0.001 for n=16, and for n=18 it is1.04±0.001. The value – exponent is in agreement with reported [27] the expected Curie wises behavior for FE SmC* phase.
0.0072
0.0071
0.0070
1/
0.0069
0.0068
0.0067
0.0066
T
A-C*
H.T. mode of SmC* in S-MB-16B-BC
0.0065
154
152
150
148
146
144
-2 0 2 4 6 8 10 12 14
142
140
H.T. mode of SmC* in S-MB-16B-BC
355 360 365 370 375 380
Temperature(K)
0.00660
0.00655
0.00650
1/
0.00645
0.00640
0.00635
T
A-C*
-
mode of SmC* in S-MB-18B-BC
wishes to thank Universiti Tunku Abdul Rahman for a grant from the UTAR Research Fund (6200H10).
REFERENCES
-
G.W.Gray and J.W.Goodby In Smectic Liquid Crystals Textures and Structures, Leonad Hill, London, (1984).
-
J.W.Goodby, R.Blinc, N.A.Clark, S.T.Lagerwall, S.A.Osipov, S.A.Pikin, T.Y.Yushino and B.Zeks, Ferroelectric Liquid Crystals, Principles, Properties and Applications, Philadelphia, Gordon and Breach, (1991).
-
-
R.B.Meyer, L.Liebert, L.Strzelecki and P.Keller, Ferro electric
158
157
156
155
154
153
152
-2 -1 0 1 2 3 4 5
T
A-C*
H.T mode of SmC* in S-MB-18B-BC
365 370 375 380
Temperature(K)
Liquid Crystals, J. Phys. Lett.1975;36: 69-71.
-
R.B.Meyer, Ferroelectric Liquid Crystals: A Review, Mol. Cryst. Liq. Cryst., 40 (1977) p. 33-48.
-
S.Singh, A.S.Parmar and A.Singh, Phase transition in ferro electric liquid crystals, Phase trans., 81 (2008) p. 815-855.
-
H.Takezoe and Y.Takanishi, In Antiferroelectric and ferroelectric phases, in Chirality Liquid Crystals, New York, (2001).
-
H.R.Brand, P.E.Cladis and P.L.Finn, Helielectric-Ferroelectr Transition Mediated by a Tilt Suppressing Intermediate Phae in Liqui Crystals, Phys. Rev., A31 (1985) p. 361-365.
-
S.T.Lagerwall, N.A.Clark, J.Dijon, and J.F.Clerc, Ferro electric liquid crystals development of devices, Ferroelectrics, 94 (1989) p. 3-62.
-
A.Beresnev, L.M.Blinov, M.A.Osipov and S.A.Pikin, Ferroelectric Liquid Crystals , Mol. Cryst. Liq. Cryst., 158 A (1988) p. 1-150.
-
P.G.de Gennes, In The Physics of Liquid Crystals Clarendon Press, Oxford (1974).
-
S.Garoff and R.B.Meyer, Electroclinic Effect at the A-C Phase Change in a Chiral Smectic Liquid Crystal, Phys. Rev. Lett., 38
Figure 21 Curie wises behavior exhibited by dielectric increment relevant to H.T mode of SmC* phase for S-MB-16B-BC and S-MB-18B- BC
-
-
CONCLUSIONS
The following conclusion are drawn from the present LF dielectric study
-
LF Dielectric studies can be confidently used to determine the phase transitions temperature in FELCs for involving second order transitions.
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The temperature variation in fR (shifting to the lower frequency side) has inferred high activation energy in SmA and SmC* phases that suggested for greater strength of the potential barrier.
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The Dielectric loss spectrum accompanied by a cross- over temperature for SmC* phase would helps to resolve distinct modes.
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For decreased temperatures, value increased that has reflected upon the relatively more dipole moment in the LC phase.
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The increase dielectric strength () in SmA and SmC* phases for the decreased temperature, would suggest
that r is more susceptible to lower frequency eld.
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ACKNOWLEDGEMENTS
The authors gratefully acknowledge University Grants Commission Departmental Research Scheme at Level III program No. F.530/1/DRS/2009 (SAP-1), dated 9 February 2009, Departmental Special Assistance at Level I program No. F.530/1/DSA- 1/2015 (SAP-1), dated 12 May 2015,
and Department of Science and Technology-Fund for Improving Science and Technology program No.DST/FIST/ PSI002/2011 dated 20-12-2011, New
Delhi, to the Departent of Physics, Acharya Nagarjuna University for providing financial assistance. S.T. Ha
(1977) p. 848-851.
-
S.Garoff, R.B.Meyer, Electroclinic Effect at the A-C Phase Change in a Chiral Smectic Liquid Crystal, Phys. Rev. A., 19 (1979) p. 338- 347.
-
N.E.Hills, W.E.Wanghan, A.H.Price and M.Davies, In Dielectric Properties and Molecular Behaviour, Van Nostrand (Ed.), New York. (1969).
-
A.K. Jonscher In Dielectric Relaxations in Solids, Chelsea Dielectrics Press, London, (1983).
-
H.Kresse, J.K.Moscicki, In Advances in Liquid Crystals Research and Applications, Pergamon Press, Oxford, Budapest, (1980).
-
F.Gouda, K.Skarp and S.T.Lagerwall, Dielectric studies of the soft mode and Goldstone mode in ferroelectric liquid crystals, Ferroelectrics, 113 (1991) p. 165-206.
-
K.K.Raina, A.K.Gatahania and B.Singh, Observation of relaxation modes in a room temperature ferroelectric liquid crystal mixture, J. Phys. 52 (1999) p. 443 -451.
-
Ch.Bahr, G.Heppke, N.K.Sharma, Dielectric Studies of The Smectic-C* – Smectic-A Transition of A Ferroelectric Liquid Crystal With high Spontaneous Polarization Ferroelectrics, 76 (1987) p. 151-157.
-
G.W.Gray, In Molecular Structure and the Properties of Liquid Crystals , Academic Press: New York, (1962).
-
P.A.Kumar and V.G.K.M.Pisipati, Ambient Monocomponent Ferroelectric Liquid Crystals with a Wide Smectic-C* Range (< 20 to
65 °C) , Adv. Mater. 2 (2000) p.1617-1619.
-
Sie-Tiong Ha; Guan-Yeow Yeap , Peng-Lim Boey, Synthesis and smectogenic A and C* properties of (S)-(-)-2-Methylbutyl 4'-(4''-n alkanoyloxybenzoyloxy) biphenyl-4- Carboxylates, Int. J. Phy .Sci
, 5 (2010) p.182-191.
-
R.H.Cole and K.S.Cole, Dispersion and Absorption in Dielectrics – I Alternating Current Characteristics, J. Chem. Phys. 9 (1941) p. 341-352.
-
R.H.Cole and D.W.Davidson, High Frequency Dispersion in n- Propanol, J.Chem. Phys. 20 (1952) p.1389-1391.
-
K.Skarp, I.Dahl, S.T.Lagerwall, and B.Stebler, Ferroelectric Liquid Crystals Mol. Cryst. Liq. Cryst. 114 (1984) p.283-291.
-
G.P.Rani, D.M.Potukuchi, N.V.S.Rao, and V.G.K.M.Pisipati, Multiple Relaxation Phenomena in Low-frequency Dielectric investigations of Smectic Polymorphism, Solid State Commun, 88 (1994) p.795-801.
-
C.Kittel , Introduction to Solid State Physics, Wiley Eastern Private Limited, New Delhi (1974).