-
54
Chapter 3
DENSITY STUDIES
3.1 INTRODUCTION
Since their first discovery back in 1888, interest in the
properties and
practical applications of liquid crystals has increased
dramatically [1, 2]. General
acceptance of liquid crystals as a distinct phase of matter was
slow, occurring
after some 30 years since they were first reported. The density
variation with
temperature is mostly used for the study of phase transitions in
liquid crystals.
From the density study as a function of temperature it is
possible to determine the
order of transitions.
Several workers in the field of liquid crystals used the
density
measurements to understand the nature of the phase transitions
and the pre-
transitional effects across different phase transformations. The
dilatometric
technique to obtain the density data with temperature on
different liquid crystals
is as old as more than hundred years as Schenk [3] carried out
density studies in
1898 and is still being used as one of the reliable
techniques.
Recently Datta Prasad et al [4] has reported density studies on
alkoxy
benzoic acids. They had discussed the I–N and N–SmC transition
in these
compounds. Also they [5] have carried out the density studies on
two benzylidine
aniline compounds viz., N-(p-n-decyloxy and undecyloxy
benzylidene)-p-
toluidines (10O.1 and 11O.1) and inferred that all the phase
transitions present in
these compounds are of first order nature. Fakruddin et al [6]
carried out the
density studies for N- (p-n-ethoxy and propoxy benzylidene)-p-n-
pentyloxy
aniline. From the density studies they observed that the nature
of I–N transition is
first order as expected. The variation of density with
temperature in a number of
higher homologues (phenyl benzylidene anilines) which exhibit a
variety of
liquid crystalline phases was carried out by Srinivasu et al[7].
Their study also
inferred the first order nature of different phase
transitions.
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55
Ajeetha et al [8, 9] reported density variation with temperature
in different
compounds viz., 5.O5, 5O.O5, 5.5, nO.O5 compounds and found that
the position
of oxygen in the LC moiety plays an important role in deciding
the phase variants
as well as the clearing temperatures. Padmaja et al [10]
reported the variation of
density with temperature in compounds which exhibit a direct
transition from
isotropic to smecticF phase and found that the nature of the
transition is first
order. The variation of the density and dynamic viscosity with
temperature for the
nematic and isotropic phases of 4-alkoxy-4'-cyanobiphenyl liquid
crystals was
reported by Burmistrov et al [11].
Gogoi et al [12] measured the variation of density with
temperature of
liquid crystal dimers 10.O12O.10 and CB.O10O.10 (CB –
cyanobiphenyl) and
reported the rare transition I-SmG in case of the compound
10.O12O.10 along
with the N-SmA transition in CB.O10O.10. They also reported the
nature of
different phase transitions in symmetric dimers like 6.O5O.6 and
6.O6O.6 [13].
The temperature variation of density, refractive index, magnetic
susceptibility
and X-ray intensity for a number of compounds was studied by
Paul et al [14,
15]. They calculated the molecular polarizabilities from
refractive indices using
Vuk’s formula and estimated the orientational order parameters
from
polarizability values.
Alapati and Saran[16] had reported a first order I-N and a weak
first
order N-SmA transition in the decyloxy and dodecyloxy homologues
of the 4-(-
4′-alkoxybenzyloxy) benzylidene-2''-methylanilines using
dilatometry, DSC and
ESR. Mallikarjuna Rao and Srinivasa Rao [17] measured the
temperature
variation of density and ultrasonic velocity in the compounds
6OCB and 7CB and
had inferred a first order IN transitions. Kiefer and Baur [18]
reported a second
order phase transition of SmA-SmG by observing the density
variation with
temperature for a number of compounds.
Lotke and Patil [19] studied the temperature variation of
density in the
compounds n-alkyl-4-4(4-n-Octyloxy benzyloxy) benzylidene
amino
benzoates and reported a first order I-N, N-SmA transition and a
second order
SmA-SmC transition. Rao et al [20] and Rao et al [21, 22]
reported density,
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56
ultrasonic velocity and dielectric permittivity results and
observed second order
N-SmA transition and nematic dimorphism in 4O.4.
Ratna et al [23] studied intra smecticA polymorphism using
dilatometry
and found that the density variations at these transitions are
extremely small.
They concluded that density variation at the transition SmA2 to
SmA1 is very
similar to that observed for a known second order N-SmA
transition. Orwall [24]
reported thermal pressure coefficients, specific volumes of
cyanobiphenyl
compounds which inferred larger volume jumps across I-N
transitions. Jaiswal
and Patel [25] had studied the temperature variation of density
and ultrasonic
velocity in the compound CBOOA and inferred that N-SmA
transition to be
weakly first order possibly due to bilayer formation resulting
in weaker smectic
ordering.
Obbie et al[26] and Kumar[27]
reported thermal conductivity
measurements across AC transition in 7O.4 of nO.m compounds, DSC
and X-ray
measurements in TBAA series and showed that the SmA-SmC
transition changes
from second order to first order as the alkyl chain length
increases, i.e. the
tricritical point occurs at TBAA5. However, Rao et al [28]
carried out the
systematic dilatometric studies on TBAA series and reported that
the SmA-SmC-
TCP in these homologous series exists at or around TB8A. Alapati
et al [29] also
reported the TCP of SmA-SmC phase transition in binary mixtures
of TBBA and
TBDA of different weight percent of some compositions.
Oweimreen et al [30] reported the density results in nematic and
isotropic
phases of p-pentyl-p'-cyanobiphenyl. Rao et al [31-38] reported
density (specific
volume), ultrasonic velocity, thermal microscopy and DSC studies
in different
liquid crystalline compounds including same nO.m compounds to
infer the order
of phase transitions. Shashidhara Prasad et al [39] reported the
density, heat of
transition and refractive index variation with temperature in
isotropic and re-
entrant isotropic phases and estimated the molecular
polarizabilities using the
Lippincott-δ-function method.
Pisipati, and his co-workers [4-10, 40-49] carried out extensive
studies
using thermal microscopy, DSC, density, refractive index,
magnetic resonance
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57
and X-ray diffraction techniques in nO.m compounds. Klopov et al
[50]
measured the temperature variation of density and viscosity in a
number of
compounds. Trostina et al [51] has measured the temperature
variation of density
for the compounds p-(hexyloxy) phenyl esters of p-alkyloxy
benzoic acids to
study the molar volume dependence on molecular structure.
Demus et al [52, 53] reported density and viscosity studies on
1, 4-bis
(4-n-hexylbenzyloxy)-2-substituted benzenes to study the effect
of varying lateral
branches of liquid crystal and their results confirmed the
dominant role played by
steric forces in enhancing the nematic thermal stability.
Further, they reported[54]
the density, heat of transition and refractive index results in
liquid crystalline
compounds of 5-n-hexyl-2-(4-n-alkoxyphenyl)pyrimidine homologous
series and
calculated the pressure dependence of I-N transition
temperature, order
parameter and the local field anisotropy. Demus et al [52,55-57]
also determined
the temperature variation of density in many compounds and
reported the first
order transitions across Isotropic to blue phase and cholesteric
to SmA phase in
cholesteryl myristate, and second order transitions across
cholesteric to blue
phase transition,
Kali et al [58] studied density and refractive index variation
with
temperature in the nematic phase of homologues of alkyl and
alkoxy phenyl
cyclohexane carboxylates and computed order parameters and
effective
polarizibilities using Vuks and Nuegebauer models. Hajdo et al
[59, 60] reported
density studies in different liquid crystals exhibiting smectic,
cholesteric and
nematic liquid crystalline phases. Bahadur et al [61-66] carried
outdensity and
ultrasonic velocity variation with temperature in number of nO.m
compounds and
found that the molar sound velocity and molar compressibility
are temperature
independent in the isotropic phase as expected. Lockart,
Gelerinter and Neubert
[67] determined the temperature variation of density for some
compounds and
reported a second order SmA-SmC transition.
Bouchet and Cladis [68] measured the variation of density as a
function
of temperature for the mixtures of cyano hexyloxy biphenyl and
cyano octyloxy
biphenyl compounds. They found that the variation of density
with temperature
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58
is not linear but exhibits an approximation to linearity at
lower temperature.
Leenhouts et al [69] carried out the density studies of
anisylidene-p-
aminophenylacetate in the isotropic and nematic phases and
inferred a first order
I - N transition. Richard Stimple et al [70] detected the
density and bulk
coefficient of volume expansion () of 4, 4′-
di-n-alkoxyazoxybenzenes and
found that nematic phase has positive , whereas isotropic phase
has negative
near nematic isotropic transition temperature.
Von Hecke et al [71] also studied the density variation with
temperature
for a number of compounds. Usha Deniz et al [72] calculated the
density and bulk
coefficient of volume expansion of HXBPA for three smectic
phases from the
layer thickness and in-plane internal separation from X-ray
diffraction. The
density is relatively constant with temperature except near the
isotropic to
smectic A transition where it decreases rapidly.
Adomenas and Grozhik [73] studied the temperature dependence
of
density for the methoxy to decyloxy homologes of
4-n-butyl-4'-n-alkoxybenzene
in the isotropic and nematic phases, and observed an average
increase in specific
volume for the addition of CH2 group. Armitage et al [74]
reported volumetric
studies on 1O.4 and PAA across I - N transition and interpreted
the results in the
light of Landau-de Gennes theory. Leadbetter et al [75] reported
variation of
density with temperature in p-n-octyloxy- p-cyanobiphenyl (8OCB)
and found
the first order nature for I - N and N - SmA transitions.
Nehring and Osman[76] reported density variation across nematic
to
smectic-B transition in
N-(4-n-alkylbenzylidene)-4'-n-alkylanilines with alkyl
chains varying from methyl to heptyl on left side and methyl to
hexyl on right
side. Blinov et al [77] had determined the density variation
with temperature of
PAA, MBBA and three homologues of p-alkyoxybenzylidene
p′-cynoaniline
series (alkoxy=ethoxy, butoxy and heptyloxy).
Chang [78] reported the critical phenomena in the nematic phase
of
MBBA employing precision volumetric measurements. Subsequently
Chang and
Gysbers [79] evaluated the volume-temperature relationships in
the nematic
phase of MBBA near IN transition by means of computer
minimization methods.
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59
Guillon et al [80-87] reported both density and X-ray studies in
compounds
exhibiting rich polymesomorphism. Padmini and her co-workers
[88-92]
computed molar sound velocity, adiabatic compressibility and
thermal expansion
coefficient to infer the nature of the phase transitions and the
pre-transitional
effects..
Of all the low molar mass liquid crystals discovered during the
1980s, one
class of compounds that attracted particular attention and which
still remains the
focus of much research are the so called liquid crystal
dimers.
Liquid crystal dimers contravened the accepted
structure-property
relationships for low molar mass mesogens by consisting of
molecules having a
highly flexible core rather than a semi-rigid central unit. In
this respect therefore,
these molecules actually represented an inversion of the
conventional molecular
design for low molar mass mesogens. The interest in these
materials stems not
only from their ability to act as model compounds for
semi-flexible main chain
liquid crystalline polymers but also from their properties which
are quite different
from those of conventional low molar mass mesogens[93]. In
particular, the
translational behaviour of dimers exhibits a dramatic dependence
of length and
parity of the flexible spacer in a manner strongly reminiscent
to that observed for
the polymeric systems.
Symmetric schiff base liquid crystal dimers which are formed by
linking
two mesogenic units through a flexible spacer (alkyl or alkoxy
chain) are an
interesting class of liquid crystals in that they exhibit quite
different and rich
smectic mesomorphism compared to that of their precursors viz.,
monomers
(nO.ms or nCBs) and are capable of acting as model compounds for
semi flexible
main chain liquid crystal polymers [93–100]. The liquid crystal
dimers are
classified into two catagories, viz., symmetric and non
symmetric. In case of
symmetric dimers the mesogenic groups are identical. On the
other hand, in non
symmetric dimers, the mesogenic groups are not identical. Some
important
aspects of these symmetric dimers, which attract special
interest, are that the
molecules with smaller spacer length promote smectogenic
behaviour whereas
the dimeric molecules with large spacer length promote nematic
behaviour, in
contrast to monomeric liquid crystals. Also, symmetric dimers
exhibit a large
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60
odd – even effect with number of carbon atoms in the spacer in
mesophase –
isotropic transition temperature as well as transition entropy.
However, the
alteration in temperatures is attenuated as the spacer grows in
length. In contrast,
the alteration in the entropy is essentially unattenuated; at
least for spacers
containing upto twelve carbon atoms [101]. In addition, the
entropy change at
nematic – isotropic transition for dimers with odd spacers is
comparable to that of
monomers while for even spacer dimers the transitional entropy
is typically three
times larger. The behaviour of the transitional entropy suggests
that the
orientational order for even spacer dimers should be
significantly greater than
that for odd spacer dimers.
The results of the study of nature of different phase
transitions in
symmetric Schiff base liquid crystal dimers by measuring the
variation of density
as a function of temperature in the compounds of the homologous
series α,ω–
bis(4-n-alkylaniline benzylidene-4’-oxy) alkane (hereafter
referred to as
m.OnO.m) are presented in this chapter. The compounds studied
are 6.O12O.6,
7.O6O.7, 7.O12O.7 and 8.O12O.8. . The compounds 6.O12O.6 and
7.O12O.7
exhibit crystal ‒ nematic (Cr ‒ N) and nematic‒ isotropic (N ‒
I) phase
transitions. The compound 7.O6O.7 exhibits crystal ‒ smecticF
(Cr ‒ SmF),
smecticF ‒ smecticA (SmF ‒ SmA) and smecticA ‒ isotropic (SmA ‒
I) phase
transitions while the compound 8.O12O.8 exhibits crystal ‒
smecticG (Cr ‒
SmG) and smecticG ‒ isotropic (SmG ‒ I) phase transitions.
3.2 Result and Discussion:
A general molecular structure of the compounds is shown
below:
CH=N H2m+1Cm N=CH O(CH2)nO CmH2m+1
6.O12O.6 : m=6 and n=12
7.O6O.7 : m=7 and n=6
7.O12O.7 : m=7 and n=12
8.O12O.8 : m=8 and n=12
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61
The phase sequence and transition temperatures exhibited by the
dimers studied
is given below:
It may be noted that as there could be difference in transition
temperatures
in heating and cooling cycles, only temperatures during heating
cycle are shown.
DSC scans of the compounds 6.O12O.6, 7.O6O.7, 7.O12O.7 and
8.O12O.8 are
shown in Figure 3.1(a, b, c, and d) respectively.
The variation of density as a function of temperature and the
variation of
estimated thermal expansion coefficient (α = dlnV/dT, where V is
the molar
volume and T is temperature) with temperature for the compounds
6.O12O.6,
7.O6O.7, 7.O12O.7 and 8.O12O.8 are shown in Figures 3.2[(a),
(b), (c) and (d)]
and 3.3[(a), (b), (c) and (d)], respectively. In all the
compounds, the density
increases as the temperature decreases, except in the vicinity
of the phase
transitions. The molar volume of 6.O12O.6 at (TNI + 5) o
C is found to be 778.61
x 10-6
m3mol
-1; for 7.O6O.7 at (TAI +5)
oC it is found to be 719.37 x 10
-6 m
3mol
-1.
This was reported to be 686.80 x 10-6
m3mol
-1 for 6.O6O.6 [12] and a comparison
of this value with those obtained for 6.O12O.6 and 7.O6O.7
infers an increment
N 6.O12O.6 : Cr I
129.1oC 132.2
oC
I SmA
115.8oC
7.O6O.7 : Cr SmF
144.1oC
182.5oC
127oC
7.O12O.7 : Cr N I
133oC
8.O12O.8 : Cr SmG I
129oC 124
oC
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62
of 15.03 x 10-6
m3mol
-1 per methylen unit in 6.O12O.6 and 16.29 x 10
-6 m
3mol
-1 in
7.O6O.7. The compound 6.O12O.6 exhibits an N - I phase
transition and the
thermal range of nematic phase is 3.1o
C. While the compound 7.O6O.7 exhibits
I-SmA and SmA – SmF phase transitions. Similarly the molar
volume of
7.O12O.7 at (TNI + 5)o
C is found to be 808.90 x 10-6
m3mol
-1 and that for
8.O12O.8 at (TSmGI +5)oC is found to be 812.9x 10
-6 m
3mol
-1. Here also an
increment of 15.15x10-6
m3mol
-1 per methylene unit is observed for 7.O12O.7 and
15.33x10-6
m3mol
-1 for 8.O12O.8 as compared to the compound 7.O6O.7 [100].
Various phases and phase transition temperatures, transition
entropy, enthalpy
and density jumps of the compounds are shown in the table 3.1.
The transition
temperatures and enthalpy values obtained in this study were in
good agreement
to those reported in literature [93].
Table 3.1 Various phases, transition entropy, enthalpy and
density jumps of
liquid crystal dimers studied.
Comp-
unds
Parameters Cr-N
Cr-SmF
-SmG Cr
SmF-
SmA
N-I SmA-I SmG-I
6.O12O.6 Entropy (∆S/R)
Enthalpy (∆H/J mol-1
)
Density Jump (∆ρ/ρ%)
16.7
55829
1.94
6535
2.09
7.O6O.7 Entropy (∆S/R)
Enthalpy (∆H/J mol-1
)
Density Jump (∆ρ/ρ%)
10.8
34910
1.97
6831
1.40
4.63
17605
2.63
7.O12O.7 Entropy (∆S/R)
Enthalpy (∆H/J mol-1
)
Density Jump (∆ρ/ρ%)
18.6
61856
2.86
9653
1.49
8.O12O.8 Entropy (∆S/R)
Enthalpy (∆H/J mol-1
)
Density Jump (∆ρ/ρ%)
14.3
47199
9.79
32686
2.7
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63
An estimate of the pressure dependence of transition
temperatures can be
obtained by using Clausius – Clapeyron equation
H
VT
dP
dT
Where T ‒ the transition temperature Δ V ‒ Molar volume change
associated with the transition
Δ H ‒ the heat of transition.
3.2.1 Isotropic – Nematic transition in 6.O12O.6 and
7.O12O.7:
The continuous rotational symmetry of the isotropic phase is
broken at the I-N
transition. The IN transition, which involves a change of
disordered isotropic
phase into a long range orientationally ordered nematic phase is
generally a first
order transition. A density jump (Δρ/ρ%) of 2.09% in 6.O12O.6
and 1.49% in
7.O12O.7 along with a peak in thermal expansion coefficient of
441 x 10-4
oC
-1
in 6.O12O.6 and 31 x 10-4
oC
-1 in 7.O12O.7 confirms the strong first order
nature of the transition in both of these compounds. Further,
the I-N transition is
accompanied by a small biphasic region of 0.8oC in 6.O12O.6 and
0.6
oC in
7.O12O.7 which is a signature of a first order transition.
However, maximum
density jump observed was completed within 0.3oC in both of the
compounds.
The observed density jump in both 6.O12O.6 and 7.O12O.7 is large
compared to
those of I-N transitions in nO.m compounds (monomers) which is
of the order of
0.30 to 0.40. This must be due to the large number of methylene
units present in
the alkyl chains of the spacer as well as in the terminal alkyl
chains of the dimer
molecules which are expected to contribute to the large entropy
change at the
transition. It may be noted that the observed entropy change
(∆S/R) at this
transition is 1.94 in 6.O12O.6 and 2.86 in 7.O12O.7. In
compounds exhibiting
two phase transitions separated by a narrow temperature range,
large density
jumps are observed in some compounds at the phase transition on
the higher
temperature side [46]. In the present case, the compound
6.O12O.6 has a nematic
phase range of 3.1oC while the compound 7.O12O.7 has a nematic
phase range of
6oC. This must be the reasons for 6.O12O.6 exhibiting higher
density jump than
7.O12O.7 although the entropy changes observed are just reverse
at I-N transition
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64
of these two compounds. A comparison of density jumps and the
estimated
values of pressure dependence of I-N transition temperature of
the compounds
6.O12O.6 and 7.O6O.7 is presented in table 3.2 along with the
monomers. This
comparison shows that the estimated values of pressure
dependence of I–N
transition of 98.70 K/kbar for 6.O12O.6 is rather large compared
with those of
7.O12O.7 (which is 49.8 K/kbar) and CB.O10O.10 (which is 26.83
K/kbar).
However, the value of 49.8 K/kbar observed for 7.O12O.7 is
broadly comparable
to those for monomers of nO.m and TBnA series.
Table 3.2 Density jumps and the estimated values of pressure
dependence of
transition temperature at I – N transition of some liquid
crystal
compounds
3.2.2 Isotropic - Smectic A Transition in 7.O6O.7
Isotropic – smectic A (I – SmA) transition is a manifestation of
the
simultaneous growth of orientational as well as translational
ordering which is
accompanied by the breakdown of infinite rotational symmetry of
the isotropic
Name of the compound Density jump
at I – N
transition
(Δρ/ρ%)
Estimated
value of dT/dP
(K/kbar)
Reference
6.O12O.6
2.09 98.70 Present work
7.O12O.7 1.49 49.8 Present work
CB.O10 O.10 1.04 26.83 12
TB5A 0.356 56.05 102
TB7A 0.351 60.07 102
4O.8 0.31 23.3 40
5O.2 0.13 21.4 40
5O.5 0.34 36.5 40
5O.6 0.30 33.00 40
5O.7 0.33 29.20 40
5O.8 0.25 26.30 40
6O.2 0.24 26.40 40
60.8 0.43 37.00 103
70.4 0.28 22.50 40
70.5 0.34 28.40 40
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65
phase. I - SmA phase transition in 7.O6O.7 is accompanied by a
large density
jump (Δρ/ρ%) of 2.63% and a peak in thermal expansion
coefficient of 523 x 10-4
oC
-1 which confirm this transition to be a first order transition.
Besides, the higher
slope of the density value in the smecticA phase compared to the
isotropic phase
indicates the dense packing and higher structural ordering in
smecticA phase than
in the isotropic phase. Further co-existence of isotropic and
SmA phase is
observed for a temperature range of 0.7oC, but a large part of
density jump is
completed within 0.4oC. It may be noted that the density jump at
the I – SmA
transition is much larger than that observed for the transition
in monomers such
as nO.m and TBnA compounds. But if this density jump observed
for I – SmA
transition is compared with that observed for I – N transition
in 6.O12O.6 and
with the entropy value of 4.63 at I – SmA transition of 7.O6O.7
with that of 1.49
at I – N transition in 6.O12O.6, the density jump observed at I
– SmA transition
in 7.O6O.7 is not as much as it should be. This is presumably
due to the large
SmA range of 38.4oC observed for 7.O6O.7. Analogous results were
also
reported for monomers as well as at N – SmA transition in
monomers. The
estimated pressure dependence of transition temperature, using
Clausius –
Clapeyron equation, was found to be 47.8 K/kbar. A comparison of
the density
jumps, heat of transition and dT/dP values observed for this
transition in some
other compounds of the dimers as well as monomers is shown in
Table 3.3. From
the comparison it is observed that the density jump (Δρ/ρ %) for
the compound
7.O6O.7 at I – SmA transition is broadly comparable to the
compound
10.O10O.10. It is also observed that the pressure dependence of
transition
temperature (dT/dP) at I – SmA transition for the compound
7.O6O.7 is the
highest in case of symmetric dimers.
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66
Table 3.3 Density jump (Δρ/ρ%), transition enthalpy
(ΔH/Jmol-1
) and
pressure dependence of transition temperature(dT/dP K/kbar)
at
I – SmA transition of few mesogenic dimers and monomers.
Name of the compound Δρ/ρ% ΔH/Jmol-1
dT/dP
K/kbar
Reference
7.O6O.7 2.63
17605
47.80
Present work
7.O4O.7 1.57 17666 29.08 104
7.O5O.7 0.95 9047 30.07 104
6.O5O.6 0.78 7511 26.89 12
6.O6O.6 1.80 14342 39.60 12
10.O10O.10 2.26 20515 39.71 105
TB10A 1.82 7080 55.50 106
Di-n-hexadecyl 4,4'-azoxy
Cinnamate
0.04 4764 26.70 107
Di-n-undecyl 4,4'-azoxy-ɑ - Methyl Cinnamate
1.21 8565 18.90 107
n-amyl-4(4-n-
dodecyloxybenzylidene amino)
Cinnamate
1.28 8414 33.70 107
N-(4-n-heptyloxybenzylidene)
4'-n-octylaniline
1.04 5870 27.30 108
N-(4-n-octyloxybenzylidene)
4'-n-butylaniline
1.11 5680 26.50 109
Terephthalylidene-bis-p-n-
decylaniline
1.82 7080 72.20 45
Terephthalylidene-bis-p-n-
octylaniline
0.96 5670 42.0 110
3.2.3 Isotropic – SmecticG transition in 8.O12O.8:
The isotropic to Smectic G phase transition is a very rare kind
of phase
transition. The large density jump and the peak in thermal
expansion coefficient
confirm the first order nature of transition of Isotropic –
SmecticG (I – SmG)
transition. The density jump and the peak in the thermal
expansion coefficient are
found to be 2.7% and 368 x 10-4 o
C-1
respectively. Further co-existence of
isotropic and smecticG phase is observed for a temperature range
of 0.8oC, but a
large part of density jump is completed within 0.3oC. It is
observed that the
density jump is much less than what would have been expected for
a transition of
entropy change (∆S/R) of 9.79. However, this density jump is
found to be higher
-
67
compared to the smecticA to isotropic, smecticC to isotropic as
well as smecticG
to isotropic transitions reported for the compounds 10.O10O.10,
10.O4O.10 and
10.O12O.10 [12,105].The density jump is also higher than that of
isotropic
nematic transition of the compounds 6.O12O.6 and 7.O12O.7. It
may be due to
the dimensionality and crystal structure order change is
different at the isotropic
to smecticG transition. It is due to the fact that the isotropic
phase is completely
disordered state where the molecules are randomly oriented but
the SmecticG
phase is a phase of three dimensional structures with
considerable disorder, i.e.,
the layer distribution is not sharp as in the case of crystals.
The decreasing trend
of the density jump with the increase of the flexible spacer
length irrespective to
type of transition is in agreement with that reported [102] in
the case of TBNA
and 12O.m series, where this type of behaviour was reported for
increasing
terminal chain length (the spacer length). Moreover, at this
transition the infinite
rotational symmetry of isotropic phase is broken with the growth
of three-
dimensional positional correlation of bond orientational order
[108,109]. The
calculated pressure dependence of transition temperature is
found to be 117. 70
K/kbar which is larger than that at any transition in these
dimers reported so far.
The density jump (Δρ/ρ%) and pressure dependence of transition
temperature for
the compound 10.O12O.10 exhibiting I – SmG transition is 1.57%
and 18.25
K/kbar respectively [12].
3.2.4 Smectic A – smectic F transition in 7.O6O.7
The smectic A – Smectic F transition is inferred by a large jump
in density
and a peak in thermal expansion coefficient at the transition.
The observed
density jump of 1.40% and a large peak in thermal expansion
coefficient of 488 x
10-4
oC
-1 confirms the SmA – SmF transition to be a first order
transition. Further
co-existence of SmA and SmF phase is observed for a temperature
range of
0.9oC, but a large part of density jump is completed within
0.3
oC. SmecticA
phase is a disordered phase which is characterized by a
one-dimensional density
wave along layer normal and the molecules are orthogonal to the
layer planes
with no correlations among the centre of mass positions of the
molecules.
Whereas the SmF phase is an ordered phase in which the molecules
are packed in
layers with a pseudo hexagonal arrangement with a 2-dimensional
order of the
-
68
positional order and long axis is tilted with respect to the
layer planes (i. e., with
uncorrelated layers but long range bond orientational order). As
a result SmA –
SmF transition is expected to be first order transition.
However, this transition is
observed rarely and hence attracts much attention for phase
transition studies.
The other liquid crystal compounds (dimers) on which density
studies were
reported so far across SmA – SmF transition are 6.O6O.6,
7.O4O.7, 7.O5O.7 and
10.O10 O.10 in which the density jump is higher for the compound
10.O10O.10.
[104]. The estimated pressure dependence of transition
temperature is found to
be 57.4 K/kbar for 7.O6O.7 which is found to be highest reported
so far at this
transition to the best of our knowledge [13]. A comparison of
the density jumps
and dT/dP values observed for SmA-SmF transition in the above
mentioned
dimers is shown in Table 3.4.
Table 3.4 Density jump (Δρ/ρ%), and pressure dependence of
transition
temperature(dT/dP K/kbar) at SmA-SmF transition of few
mesogenic
dimers.
The salient features observed from the density studies are
The I – N transition in 6.O12O.6 and 7.O12O.7 is found to be
first
order as expected.
The I - SmA, SmA – SmF transitions in 7.O6O.7 and I – SmG
transition in 8.O12O.8 are also first order.
Name of the
compound
Density jump
at SmA-SmF
transition
(Δρ/ρ%)
Estimated
value of dT/dP
(K/kbar)
Reference
7.O6O.7 1.40 57.4 Present Work
6.O6O.6 0.61 53.59 12
7.O4O.7 0.55 37.25 104
7.O5O.7 0.62 16.82 104
10.O10 O.10 1.80 45.76 105
-
69
The density jumps in case of these dimers are large compared
to
their precursor nO.m compounds.
The pressure dependence of transition temperature at I – N
transition in 6.O12O.6 and I – SmA transition in 7.O6O.7 is
the
highest, reported so far at these transitions.
The pressure dependence of transition temperature at I – SmA
transition in 7.O6O.7 is broadly comparable to those for
monomers of nO.m and TBnA series.
The pressure dependence of transition temperature at I – SmG
transition in 8.O12O.8 is larger than that at any transition in
these
dimers reported so far.
-
70
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115 120 125 130 135 140
-8
-7
-6
-5
-4
-3
-2
-1
0
He
at F
low
[a
.u.]
Temperature /OC
Cr N I
Figure 3.1(a) DSC scan of the compound 6.O12O.6.
-
77
80 100 120 140 160 180 200
-5
-4
-3
-2
-1
0
He
at F
low
[a
.u.]
Temperature / oC
Cr ISmF SmA
Figure 3.1(b) DSC scan of the compound 7.O6O.7.
-
78
100 110 120 130 140 150
-10
-8
-6
-4
-2
0
He
at F
low
[a
.u.]
Temperature/ OC
Cr N I
Figure 3.1(c) DSC scan of the compound 7.O12O.7
-
79
110 115 120 125 130 135
-9
-8
-7
-6
-5
-4
-3
-2
-1
He
at F
low
[a
.u.]
Temperature/ oC
Cr SmG I
Figure 3.1(d) DSC scan of the compound 8.O12O.8.
-
80
128 130 132 134 136 138
0.93
0.94
0.95
0.96
0.97
0.98
0.99
1.00
I
De
nsity (
X 1
03 k
g m
-3)
Temperature/ OC
N
Figure 3.2 (a) Variation of density as a function of temperature
in
Isotropic and Nematic phases of 6.O12O.6
-
81
130 140 150 160 170 180 190
0.91
0.92
0.93
0.94
0.95
0.96
0.97
0.98
0.99
1.00
1.01
1.02
De
nsity (
x 1
03 k
g m
-3)
Temperature/ oC
SA
SF
I
Figure 3.2 (b) Variation of density as a function of temperature
in
isotropic, smectic A and smectic F phases of 7.O6O.7
-
82
128 130 132 134 136 138
0.93
0.94
0.95
0.96
0.97
0.98
0.99
De
nsity (
x 1
03 k
g m
-3)
Temperature / oC
IN
Figure 3.2 (c) Variation of density as a function of temperature
in
isotropic and nematic phases of 7.O12O.7
-
83
124 126 128 130 132 134 136
0.92
0.94
0.96
0.98
1.00
1.02
De
nsity (
x1
03kg
m-3)
Temperature/oC
ISmG
Figure 3.2(d) Variation of density as a function of temperature
in
isotropic and smectic G phases of 8.O12O.8
-
84
128 130 132 134 136 138
0.00
0.01
0.02
0.03
0.04
0.05
Temperature/ oC
Th
erm
al e
xp
an
sio
n c
o-e
ffic
ien
t
IN
Figure 3.3(a) Variation of thermal expansion coefficient for I –
N transition of
6.O12O.6
-
85
130 140 150 160 170 180 190
0.00
0.01
0.02
0.03
0.04
0.05
0.06
Th
erm
al E
xp
an
sio
n C
oe
ffic
ien
t
Temperature / oC
SmF SmA I
Figure 3.3(b) Variation of thermal expansion coefficient at I -
SmA
and SmA – SmF transitions of 7.O6O.7
-
86
128 130 132 134 136 138
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
Th
erm
al E
xp
an
sio
n C
oe
ffic
ien
t
Temperature / oC
IN
Figure 3.3(c) Variation of thermal expansion coefficient at I -
N transition of
7.O12O.7
-
87
126 128 130 132 134 136
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
Th
erm
al E
xp
an
sio
n C
oe
ffic
ien
t
Temperature/ oC
SmG I
Figure 3.3(d) Variation of thermal expansion coefficient at SmG
- I
transition of 8.O12O.8