phase transition and structural studies of symmetric...

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.

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,

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

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

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.

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

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

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

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



)


10.8

34910

1.97

6831

1.40

4.63

17605

2.63



)


18.6

61856

2.86

9653

1.49



)


14.3

47199

9.79

32686

2.7

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

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

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.

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

phase transition and structural studies of symmetric...

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