Synthesis, characterization, and dilatometric studies on N-(p-n-alkoxybenzylidene)-p-n-pentyloxyanilines compounds

Article abstract of DOI:10.1080/15421400500447108

Synthesis, textural characterization, differential scanning calorimetry, and dilatometry studies are carried out on a new homologous series of N-(p-n-alkoxy benzylidene)-p-n-pentyloxy anilines (nO.O5) where n=1-18. The lower homologues of this series (n=1-5) and a higher homologue (n=16) exhibit monotropic Liquid Crystal (LC) phase. The other members of the series show enantiotropic LC phases, while 18O.O5 is a nonliquid crystal. The dilatometric studies on 7O.O5, 10O.O5, 13O.O5 and 16O.O5 reveal the first-order nature of Isotropic to Nematic Phase (IN), Isotropic to Smectic A Phase (IA), Nematic to Smectic C Phase (NC), Smectic A to Smectic B Phase (AB), Smectic C to Smectic B Phase (CB), and Smectic A to Krystal Phase (AK) transitions. The pretransition effects are estimated from dilatometry results. The results are compared with the available data on nO.m compounds.

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Synthesis, Characterization,
and Dilatometric Studies on N-
(p-n-Alkoxybenzylidene)-p-n-
pentyloxyanilines Compounds
N. Ajeetha a , V. G. K. M. Pisipati a , M. Ramakrishna
Nanchara Rao b & P. V. Datta Prasad c
a Centre for Liquid Crystal Research and Education,
Faculty of Physical Sciences, Acharya Nagarjuna
University , Nagarjunanagar, India
b Department of Physics , A. J. Kalasala,
Machilipatnam, India
c Department of Physics , Hindu College ,
Machilipatnam, India
Published online: 31 Jan 2007.
To cite this article: N. Ajeetha , V. G. K. M. Pisipati , M. Ramakrishna Nanchara Rao
& P. V. Datta Prasad (2006) Synthesis, Characterization, and Dilatometric Studies on
N-(p-n-Alkoxybenzylidene)-p-n-pentyloxyanilines Compounds, Molecular Crystals and
Liquid Crystals, 457:1, 3-25, DOI: 10.1080/15421400500447108
To link to this article: http://dx.doi.org/10.1080/15421400500447108
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Mol. Cryst. Liq. Cryst., Vol. 457, pp. 3–25, 2006
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400500447108
Synthesis, Characterization, and Dilatometric Studies
on N-(p-n-Alkoxybenzylidene)-p-n-pentyloxyanilines
Compounds
N. Ajeetha
V. G. K. M. Pisipati
Centre for Liquid Crystal Research and Education, Faculty of Physical
Sciences, Acharya Nagarjuna University, Nagarjunanagar, India
M. Ramakrishna Nanchara Rao
Department of Physics, A. J. Kalasala, Machilipatnam, India
P. V. Datta Prasad
Department of Physics, Hindu College, Machilipatnam, India
Synthesis, textural characterization, differential scanning calorimetry, and dila-
tometry studies are carried out on a new homologous series of N-(p-n-alkoxy
benzylidene)-p-n-pentyloxy anilines (nO.O5) where n ¼ 1–18. The lower homolo-
gues of this series (n ¼ 1–5) and a higher homologue (n ¼ 16) exhibit monotropic
Liquid Crystal (LC) phase. The other members of the series show enantiotropic
LC phases, while 18O.O5 is a nonliquid crystal. The dilatometric studies on
7O.O5, 10O.O5, 13O.O5 and 16O.O5 reveal the first-order nature of Isotropic to
Nematic Phase (IN), Isotropic to Smectic A Phase (IA), Nematic to Smectic C Phase
(NC), Smectic A to Smectic B Phase (AB), Smectic C to Smectic B Phase (CB), and
Smectic A to Krystal Phase (AK) transitions. The pretransition effects are esti-
mated from dilatometry results. The results are compared with the available data
on nO.m compounds.
Keywords: dilatometry; liquid crystals; pretransitional effects
INTRODUCTION
The benzylidene aniline Schiff base liquid-crystalline (LC) compounds,
N-(p-n-alkoxybenzylidene)-p-n-alkylanilines, popularly known as
nO.ms, exhibit rich but subtle polymesomorphism [1,2]. It is very
Address correspondence to V. G. K. M. Pisipati, Centre for Liquid Crystal Research and
Education, Faculty of Physical Sciences, Acharya Nagarjuna University, Nagarjunanagar
522 510, A.P., India. E-mail: venkata_pisipati@hotmail.com
3

4
N. Ajeetha et al.
interesting to note that the position of the oxygen atom on either sides
of the rigid core (nO.m and n.Om) plays a major role in the occurrence
of the phase variant and the clearing temperature of the mesogenic
compound [3,4]. The recent investigations on similar Schiff bases with
n.Om and nO.Om chains revealed that the clearing temperatures are
elevated to above 100ꢀC whereas nO.m compounds exhibit a clearing
temperature below 100ꢀC [5].
A physical system at a given temperature is completely described by
the equilibrium value of the relevant order parameter and the fluctu-
ation effects. During phase transition, the free energy of the system
remains continuous, and the thermodynamic quantities such as vol-
ume, entropy, and heat capacity undergo discontinuous changes.
The dilatometric investigation of thermotropic liquid crystals, to
obtain relations between the molecular structure and mesomorphic
properties, have been performed using the specific volume (v ¼ 1=q)
and molar volume (Mv ¼ molecular weight=density (q)), which are clo-
sely related to density. The thermal expansion coefficient (a ¼ (1=Mv)
dMv=dT) of the material can also be obtained from dilatometry. Dilato-
metric studies are long known to determine the order of the phase
transitions and the associated pretransitional behavior at the tran-
sition involving isotropic liquid to the first mesomorphic phase, which
infers the molecular interaction and contribution for the growth and
stability of the LC phase [6,7].
As a part of our ongoing systematic studies on benzylidene aniline
LC compounds, the present article reports the synthesis, characteriza-
tion, and dilatometric studies on N-(p-n-alkoxybenzylidene)-p-n-pen-
tyloxyanilines, nO.O5 compounds. The phase variants, transition
temperatures, associated enthalpy values, and nature of the transi-
tions involving isotropic (I), nematic (N), smectic A (SA), smectic C
(SC), smectic B (SB), and smectic G (SG) phases are presented. The
pressure dependence of the transition temperatures is estimated from
measured volume jumps and the enthalpy data. The results are com-
pared with the data available on nO.m compounds.
EXPERIMENTAL
The compounds are prepared by condensation of the corresponding p-
n-alkoxybenzaldehyde (0.1 mol) from n ¼ 1–16 with pentyloxyaniline
(0.1 mol) by refluxing the reactants in absolute ethanol in the presence
of a few drops of glacial acetic acid. After refluxing the mixture for 4 to
5 h, the solvent is removed by distillation, and the compound is further
purified by recrystallization in cold ethanol. The molecular formula for
nO.O5 compounds is given in Fig. 1. The textural identification and

Synthesis, Characterization, and Dilatometric Studies
5
FIGURE 1 General molecular structure of nO.O5 compounds.
the phase transition temperatures are determined using a polarizing
microscope (Olympus BX 50) supplemented by an optical display
(DP–10) at a scan rate of 0.1ꢀC per min. A Perkin-Elmer DSC-7 instru-
ment is used to determine the transition temperatures and the asso-
ciated enthalpy values.
RESULTS AND DISCUSSION
Characterization of Liquid-Crystalline Phases of N-(p-n-alkoxy
benzylidene)-p-n-pentyloxy anilines (nO.O5)
Although the optical textural observations reveal that these compounds
exhibit traditional textures, they are found to exhibit interesting phase
variants compared to the compounds of N-(p-n-alkoxybenzylidene)-
p-n-pentylanilines (nO.5) [8]. The textural observations of this series
of compounds exhibit two monovariant, one divariant, one trivariant,
and one quadravariant phase-sequenced systems. The compounds
1O.O5 to 6O.O5 exhibit one type of monovariant phase sequence,
whereas the higher homologue 16O.O5 exhibits a qualitative type of
monovariant sequence. The middle members of the series exhibit rich
polymesomorphism, quadra- (7O.O5) and tri- (8O.O.5 to 10O.O5) vari-
ant phase sequences. However, the higher homologous series are found
to exhibit divariant (11O.O5 to 14O.O5) phase sequences.
On cooling from the I phase, these series of compounds with n ¼ 1–7
are found to grow a N phase, exhibiting threaded marble texture at
95.2, 119.3, 100.5, 110.9, 109.7, 110.5, and 106.5ꢀC respectively. The
texture observed in these compounds is similar to those in the nO.5 ser-
ies of nO.m compounds that exhibit an N phase [9]. On further cooling,
these compounds with n ¼ 1–6 are found to condense into solid (crystal-
line) phase at 83.5, 88.5, 96.2, 109.2, 109.1, and 102.5ꢀC respectively.
The compounds with n ¼ 4 and 5 show very narrow nematic thermal
ranges of 1.7 and 0.6ꢀC. Although our earlier report [3] on 5O.O5 pre-
sented it as SG, our present meticulous and systematic studies on the
compound confirmed that the phase is nematic. The error in our earlier
report may be attributed to the extremely small thermal range
involved with the LC phase. Because, the transition from isotropic to
LC phase occurs around ꢁ109ꢀC in the remaining lower homologues

6
N. Ajeetha et al.
(with n ¼ 1–6) that exhibited the nematic phase, it may be readily
accepted that the LC phase in 5O.O5 is nematic conclusively.
However, on further cooling, the compound 7O.O5 was found to
exhibit an altogether different texture (from usual marble thereaded
N texture) consisting of schlieren brushes characteristic of SC phase
occurrence at 95.2ꢀC. This compound is unique in this series as it exhi-
bits a different phase variant than the other compounds. On further
cooling of the sample, these schlieren brush texture is found to trans-
form smooth glossy focal conic texture at 89.5ꢀC, characterizing the
phase as SB. Further lowering of temperature leads to a change of
focal conic texture into striped broken focal conic fan texture at
86.2ꢀC. A similar trend of phase variance is observed in the case of
7O.5, characterizing the phase as SG [8]. The overall textural observa-
tions in this compound, 7O.O5, reveals that it is a case of Nematic,
Smectic C, Smectic B, Smectic G (NCBG) phase variant.
The remaining compounds in the nO.O5 series with n ¼ 8–11, 13–
16 show a SA phase in the form of batonnets at 104.6, 105.3, 106.2,
89.1, 104.5, 99.5, and 91.3ꢀC on cooling these samples from isotropic
melt. These batonnets that grow in the I phase coalesce to form a clear
focal conic fan texture with the further decrease of temperature. This
SA phase is also found to exhibit pseudo-I appearance in the homeotro-
pic aligned region. This observation further confirms the orthogonal
structural arrangement of the smectic phase. These observations are
similar to those observed in the case of 8O.5 and the lower homologues
of 13O.m, 14O.m, and 16O.m compounds [10]. These observations in
the wake of reports on 8O.5, 13O.m, 14O.m, and 16O.m compounds
taken together confirm that the phase occurs in n ¼ 8–11, 13, and
16 of nO.O5 series and is SA. In the case of compound 16O.O5, the cool-
ing of I liquid results in the formation of a solid (crystal) phase at
90.2ꢀC. However, in the case of the rest of the compounds, further low-
ering of temperature results in the appearance of transient transition
bars followed by the appearance of smooth focal conic fans with glossy
surfaces at 95.3, 100.3, 104.3, 87.2, 89.4, 91.3, and 88.2ꢀC for com-
pounds 8O.O5, 9O.O5, 10O.O5, 11O.O5, 13O.O5, 14O.O5, and
15O.O5 respectively. The appearance of transient transition bars
and the formation of smooth focal conic fan texture with reduced dis-
continuities at boundaries implies that this transformation is from SA
to SB. However, this occurrence of SASB transitions falls as a common
observation because many Schiff-based nO.m compounds show SA and
SB phases [11].
On further cooling of compounds with n ¼ 11, 13, 14, and 15, the SB
focal conic fan texture is found to change to a solid (crystal) sandy
appearance at 73.4, 76.2, 78.4, and 81.3ꢀC. The other compounds with

7

8
N. Ajeetha et al.
n ¼ 8, 9, and 10, on further cooling of SB phase, results in the forma-
tion of striped broken focal conic fan texture (from smooth focal conic
SB texture) at 60.0, 85.6, and 87.4ꢀC respectively. These observations
are similar to those observed in the case of 8O.5 and other nO.m com-
pounds, indicating the growth of SG phase. Later, further cooling of
the sample results in the transformation of SG phase to solid (crystal)
phase at 73.6, 75.2, and 71.2ꢀC in the nO.O5 compounds with n ¼ 8, 9,
and 10 respectively.
These results imply that these homologous compounds (nO.O5)
exhibit five types of variants; N, NCBG (Nematic, Smectic C, Smectic
B, Smectic G), A, AB, ABG. This observation stands contrast to those
N, NABG, NB, NACFG, NACBG, ABG, and AF phase variants
exhibited by nO.5 compounds. This clearly indicates that the introduc-
tion of an electronegative oxygen atom on the aniline side of the LC
molecule quenches some of the mesomorphic phases and at the same
time elevates the clearing temperatures. The present textural obser-
vations concur with the results obtained in the case of nO.m com-
pounds. The phase variants exhibited by the nO.O5 series and the
reported nO.5 series are presented in Table 1. The phase diagram of
nO.O5 compounds is given in Fig. 2.
Table 2 provides information regarding the transition tempera-
tures from Differential Scanning Calorimetry (DSC) and thermal
microscopy and the corresponding enthalpy valuesfrom DSC atdifferent
FIGURE 2 Phase diagram of nO.O5 compounds.

9

10

11

12

Synthesis, Characterization, and Dilatometric Studies
13
FIGURE 3 DSC heating and cooling thermogram of 3O.O5 compound.
phase transitions for nO.O5 compounds. The DSC thermograms of
3O.O5, 7O.O5, 10O.O5, 13O.O5, and 16O.O5 are presented in Figs. 3–7.
Dilatometric Studies
The transition temperatures from dilatometric studies are found to
agree with those obtained earlier from thermal microscopy and DSC
measurements (Table 2). The slight temperature differences recorded
from different techniques may be due the different scan rates. The
phase-transition studies by density measurements are carried out
across IN, AK, NC, IA, AB, and CB phase transitions in four members
of nO.O5 series: 7O.O5 (NCBG), 10O.O5 (ABG), 13O.O5 (AB), and
16O.O5 (A) and on 5.5 Nematic, Smectic G (NG) and 5.O5 (NG).
The estimated molar volume (molecular weight=density) in iso-
tropic phase per an increment of methylene unit estimated at
T(IN=IA) þ 5 is in the range of 15–16.5 ꢀ 10ꢁ6 cmꢂmolꢁ1 and agrees with
the body of the data available on nO.m compounds and the values
reported for normal liquids [7,12]. The temperature variation of den-
sity q(T) and thermal expansion coefficient a(T) for the compounds
(n ¼ 7, 10, 13, and 16) studied are shown in Figures 8–11. The trends

14
N. Ajeetha et al.
FIGURE 4 DSC heating and cooling thermograms of 7O.O5 compound at two
different scan rates.
FIGURE 5 DSC heating and cooling thermogram of 10O.O5 compound.

Synthesis, Characterization, and Dilatometric Studies
15
FIGURE 6 DSC heating and cooling thermograms of 13O.O5 compound.
FIGURE 7 DSC heating and cooling thermograms of 16O.O5 compound.

16
N. Ajeetha et al.
FIGURE 8 Temperature variation of density q(T) and the thermal expansion
coefficient a(T) of 7O.O5 compound.
FIGURE 9 Temperature variation of density q(T) and the thermal expansion
coefficient a(T) of 10O.O5 compound.

Synthesis, Characterization, and Dilatometric Studies
17
FIGURE 10 Temperature variation of density q(T) and the thermal expan-
sion coefficient a(T) of 13O.O5 compound.
of slopes in all LC phases are found to increase as one goes down from
the isotropic to higher ordered LC phases. However, the values are
relatively lower than those observed for nO.m compounds in any LC
FIGURE 11 Temperature variation of density q(T) and the thermal expan-
sion coefficient a(T) of 16O.O5 compound.

18
N. Ajeetha et al.
TABLE 3 Slopes of Different Phases (the Magnitude Is Increasing from the
Isotropic to Higher Ordered Liquid-Crystalline Phases)
Phase
Nematic=
Compound variant Isotropic Smectic A Smectic C Smectic B Smectic G Crystal Ref.
7O.O5
NCBG
ABG
AB
5.7
4.2
4.5
4.8
6.4
18.6a
5.8
36.0a
7.2
8.3
6.0
7.5
PW
PW
PW
PW
10O.O5
13O.O5
16O.O5
7.2
A
9.5
aThe thermal range of smectic A in these compounds is short, and hence the slope
values are not reliable.
PW, Personal Work.
phase [7]. The slope values obtained for the compounds 7O.O5,
10O.O5, 13O.O5, and 16O.O5 are given in Table 3.
Isotropic to Nematic Transition (IN) in 7O.O5
The density of this compound is found to increase with the decrease of
temperature at I, N, SC, and SB phases in compound 7O.O5 except in
the vicinity of phase transitions. The vertical distance between the
density values (q1 and q2) obtained by extrapolating the observed liner
dependence to transition temperatures from either sides of the tran-
sition, when normalized over the average value of these two density
values (i.e., [(q1 ꢂ q2)=fðq1 þ q2Þ=2gꢃ is taken as the density jump
(Dq=q) across the transition.
The observed density jump ꢀof 0.28% and the thermal expansion coef-
ficient maxima of 23.5 ꢄ 10ꢂ4 Cꢂ1(Fig. 8) indicate the IN transition to
be first order. The density jump and thermal expansion coefficient
maxima of these compounds along with the values for the some nO.m
compounds with equal chain lengths are given in Table 4. The
TABLE 4 Density Jump and Thermal Expansion Coefficient Maxima of
7O.O5 Compound along with the Values of the Compounds with Equal Chain
Lengths of Isotropic–Nematic Transition
ꢀ
Compound
Phase variant
% of (Dq=q)
a ꢄ 10ꢂ4 Cꢂ1
Ref.
7O.O5
7O.5
5O.7
6O.6
4O.8
NCBG
NACBG
NACBG
NABG
0.28
0.34
0.33
0.30
0.31
23.5
103.6
110
109
95
PW
[15]
[18]
[19]
[20]
Note. The density jump is smaller compared to 7O.5 and 5O.7.

Synthesis, Characterization, and Dilatometric Studies
19
TABLE 5 Density Jump and Thermal Expansion Coefficient Values for
Nematic–Smectic C Transition obtained for Compound 7O.O5 vs. NOBA
Compound
Phase variant
% of (Dq=q)
a ꢄ 10ꢂ4 ꢀCꢂ1
Ref.
7O.O5
NOBA
NCBG
NC
0.28
0.23
23.5
50
PW
[13]
magnitude of density jump in all compounds is almost found to be same
with total flexible identical chain length and exhibit IN transition.
Nematic to Smectic C Transition (NSC) in 7O.O5
A transition from an orientational ordered N phase to SC phase posses-
sing additional layered structure with a tilted order is expected to be
the first order. The observed density jump (Dq=q ¼ 0.45) and the ther-
ꢀ
mal expansion coefficient maxima (a ¼ 27.5 ꢄ 10ꢂ4 Cꢂ1) confirm the
NSC transition as first order. The data on nO.m compounds reveal that
no compound exhibits NSC transition [1]. The values obtained for this
compound are slightly lower to those obtained for NOBA [13] and are
given in Table 5.
Isotropic to Smectic A Transition (ISA) in 10O.O5, 13O.O5,
and 16O.O5
The ISA transition in 10O.O5, 13O.O5, and 16O.O5 is accompanied by
a large density jump of 0.71%, 0.71%, and 2.10% and thermal expan-
ꢀ
ꢀ
sion coefficient maxima of 35.4 ꢄ 10ꢂ4 Cꢂ1, 52.3 ꢄ 10ꢂ4 Cꢂ1, and
ꢀ
180.0 ꢄ 10ꢂ4 Cꢂ1 respectively indicate the transition is of first-order
nature. The density jumps and thermal expansion coefficient maxima
of the present nO.O5 compounds along with the other in nO.m com-
pounds that exhibit ISA transition are presented in Table 6.
It is noticed that the density jumps obtained in the compounds
10O.O5 and 13O.O5 are slightly less than those found in the case of
nO.m compounds, whereas the value of Dq in the compound 16O.O5
is found to be highest reported for benzylidene anilines (nO.m) com-
pounds (exhibiting a single phase variant, SA) [14]. The dilatometric
studies across ISA transition are accompanied by large thermal span
of fluctuation-dominated nonlinear regions (FDNLR) to an extent of
ꢁ2.0ꢀC in these three compounds. The growth of SA phase is observed
as a pyknometer as the translucent SA phase grows at the bottom of
the bulb, while transparent isotropic liquid floats over it. The SA phase
with a clear separation boundary is found to spread gradually from the

20
N. Ajeetha et al.
TABLE 6 Density Jumps and Thermal Expansion Coefficient Maxima of
These Compounds along with the Other Compounds in Benzylidene Aniline
Series that Exhibit Isotropic–Smectic A (ISA) Transition
Compound
Phase variant
% of (Dq=q)
a ꢄ 10ꢂ4 ꢀCꢂ1
Ref.
10O.O5
13O.O5
16O.O5
7O.8
8O.7
9O.6
ABG
AB
A
0.71
0.71
2.10
1.04
1.32
1.51
0.93
0.90
1.50
0.99
0.90
35.4
52.3
PW
PW
PW
[21]
[22]
[22]
[23]
[15]
[12]
[24]
[7]
180.0
225.0
146.0
440.0
133.7
239.0
629.0
124.0
123.0
ACBG
ACBG
ACFG
A
ABF
ACFG
ABG
ABG
140.1
8O.10
10O.8
7O.14
9O.12
isotropic liquid all through the pyknometer with the decrease of tem-
perature. The formation of SA embryos at the bottom of the bulb and
their consequential growth clearly indicates the nucleation type of
growth at ISA phase transition. The visual observation of translucent
SA phase, which is clearly distinct from milky or cloud ¼ like appear-
ance in the entire bulb of nematic phase, suggests the simultaneous
onset of orientational and translational orders rather than preferen-
tial growth of one over the other.
Smectic A to Smectic B (SASB) Transition in 10O.O5
and 13O.O5
The SASB transition is associated with the development of long-range
three-dimensional order and molecular correlation of hexagonal in-
plane stacking with its layered structure. The observed density jumps
(Dq=q) of 0.25 and 1.18 and thermal expansion coefficient maxima
ꢀ
ꢀ
(amax.) of 23.5 ꢄ 10ꢂ4 Cꢂ1 and 54.8 ꢄ 10ꢂ4 Cꢂ1 for the 10O.O5 and
13O.O5 respectively indicate the first-order nature of transition. The
large variation of density jump values in the case of these two com-
pounds is not an uncommon trend as observed in the case some
nO.m compounds (8O.m series) [15]. The density jumps and thermal
expansion coefficient maxima for these compounds along with the
other compounds are presented in Table 7. However, the sticky nature
of substance (to the inner walls of the pyknometer) did not permit
further investigation of q(T) deep into the SB phase as well as the SBSG
phase transition in the 10O.O5 compound because the compound
develops breaks in the pyknometer columns.

Synthesis, Characterization, and Dilatometric Studies
21
TABLE 7 Density Jumps and Thermal Expansion Coefficient Maxima
obtained for Smectic A–Smectic B Transition along with the Comparision
of nO.m Compounds
Compound
Phase variant
% of (Dq=q)
a ꢄ 10ꢂ4 ꢀCꢂ1
Ref.
10O.O5
13O.O5
7O.10
7O.14
8O.4
8O.5
8O.8
8O.9
8O.10
8O.14
9O.12
9O.14
9O.16
ABG
AB
AB
0.25
1.18
1.37
0.99
0.40
0.54
0.24
0.60
1.41
1.10
0.68
1.60
0.74
23.5
54.8
357
124.0
85.0
92.0
PW
PW
[11]
[24]
[25]
[11]
[21]
[11]
[15]
[24]
[7]
ABG
ABG
ABG
ABG
ABG
ABF
ABG
ABG
ABG
ABG
71.0
60.0
376.0
154.0
107.0
156.0
112.0
[24]
[7]
Smectic C to Smectic B (SCSB) Transition in 7O.O5
The SC phase is the tilted analogue of SA phase. The smectic layers
possess a liquid-like unstructured arrangement of a lath-like mole-
cules that are tilted with respect to the layer planes at angles, which
vary from compound to compound. Further, for a given compound, the
tilt angle may either vary or stay relatively constant with temperature
over the thermal range of SC phase. The SB phase that forms from the
SC phase on cooling regains the orthogonal alignment of the molecules
in addition to the long-range order within the smectic layer planes.
The SB phase possesses long-range three-dimensional (3D) order and
positional correlations of the hexagonal in-plane packing of the mole-
cules as well as the layer stacking. Hence, the transition from SC to SB
phase is expected to be first order. The observed large density jump
(Dq=q) of 1.76% and the thermal coefficient maxima amax. of
157.2 ꢄ 10ꢂ4 ꢀCꢂ1 (Table 8) confirm the order of the transition to be first
order in the compound 7O.O5. The density variation across the SCSB
transition does not, however, correspond to a step function. Instead,
it exhibits an almost linear behavior with a finite slope. The values
of density jumps across this transition are generally found to be less
than at the SASB transition. As the structural environment on the
higher temperature side SASB transition is much less ordered (in com-
parison to that across SCSB transition), a lower jump in the latter case
is justifiable. For reference, the observed density jumps in other nO.m
compounds, which exhibit the same transition, are given in Table 8.

22
N. Ajeetha et al.
TABLE 8 Observed Density Jumps and Thermal Expansion Coefficients
for Smectic C–smectic B Transition of 7O.O5 Compound along with the nO.m
Compounds that Exhibit the Same Transition
ꢀ
Compound
Phase variant
% of (Dq=q)
a ꢄ 10ꢂ4 Cꢂ1
Ref.
7O.O5
7O.5
5O.7
4O.7
7O.7
7O.8
8O.6
8O.7
10O.9
NCBG
NACBG
NACBG
NACB
NCB
ACBG
ACBG
ACBG
ACBG
0.28
0.76
0.50
0.41
0.93
0.84
0.79
0.79
1.27
23.5
163.0
108.0
267.0
249.0
92.0
101.0
107.0
381.0
PW
[10]
[18]
[26]
[19]
[21]
[11]
[11]
[12]
Smectic A to Crystalline Solid (SAK) in 16O.O5
This transition involving a change from long-range orientational order
with layered arrangement to a perfectly three-dimensional crystal is
expected to first order. However, the dilatometric investigation, as
in the case of 16O.O5, showed the transition temperature involved is
high compared to those compounds that exhibit this transition in
nO.m compounds. Further, the thermal range of SA phase in 16O.O5
is found to be as low as 1.9ꢀC. The observed density jump (Dq=q) of
ꢀ
3.0% and the thermal coefficient maxima amax. of 330.8 ꢄ 10ꢂ4 Cꢂ1 at
this transition confirm the order of transition as first order in the com-
pound 16O.O5. Further, the observed largest density jump across the
SASK transition implies the lower phase as nearer to the perfect solid
phase. For comparison, the transition temperature involved and the
thermal range of SA phase exhibited by different compounds in
nO.m series are presented in Table 9.
Pretransitional Effects Across Isotropic to First LC Phase
The observed nonlinear density variation on the isotropic side (fluctu-
ation dominated nonlinear regions, FDNLR) represents the varied vol-
ume swept by the molecules (with characteristic hindered rotational
degrees of freedom). As such these fluctuations are accompanied by
the density variation, which grows during the condensation of the
low-temperature LC phase. In the vicinity of the phase transform-
ation, the volume occupied by the molecules with a specified potential
energy is influenced by the growth of low-temperature mesophase
with a specific molecular structure. Further, the FDNLR on both sides

Synthesis, Characterization, and Dilatometric Studies
23
TABLE 9 Transition Temperature Involved and the Thermal Range of SA
for Different Compounds in nO.m Series
Compound
Transition temperature (ꢀC)
Thermal range of SA phase (ꢀC)
Ref.
16O.O5
2O.16
13O.1
14O.1
91.0
70.1
76.3
76.5
1.9
17.9
15.3
7.9
PW
[27]
[28]
[16]
of the interface depends on the LC phase (on the low-temperature side
of the transition). The growth of a LC phase through the underlying
density fluctuation from isotropic liquid is estimated by an exponent
a by fitting the observed density data [in FDNLR for T > TC] to the
following relation
eff:Þ
jqI ꢂ qcjajT1 ꢂ Tcjð1ꢂa
;
ð1Þ
where qi is the observed density at temperature of interest Ti and qc is
the observed density at isotropic to LC transition (i.e., TI ꢂ N=A). As DT
¼ jTi ꢂ Tcj reflects the thermal range of pretransition of fluctuation in
the FDNLR, they imply the longevity of the fluctuations. However, aeff
reflects the strength of the fluctuations.
The observed temperature variation data of density q(T) and tran-
sition temperatures Tc are fitted to the relation (1), and the resulting
aeff values and the data of thermal span of FDNLR are listed in
Table 10 along with the data of some nO.m compounds. The goodness
TABLE 10 Resulting aeff Values along with the Data of Thermal Span of
FDNLR along with the Data on Some nO.m compounds
Compound
Transition
aeff
FDNLR (ꢀC)
Ref.
7O.O5
7O.5
6O.6
4O.8
10O.O5
9O.6
8O.7
7O.8
13O.O5
10O.8
16O.O5
7O.14
IN
IN
IN
IN
IA
IA
IA
IA
IA
IA
IA
IA
0.20
0.57
0.26
0.79
0.26
0.62
0.18
0.45
0.38
0.63
0.35
0.62
2.0
0.4
0.4
0.4
2.5
1.3
1.4
0.5
1.9
1.2
2.2
1.4
PW
[15]
[19]
[20]
PW
[22]
[15]
[21]
PW
[12]
PW
[24]

24
N. Ajeetha et al.
of the fit is demonstrated through the p values (>0.996) corresponding
to the v2 test in the present compounds.
CONCLUSION
It may, therefore, be concluded from this discussion that the place-
ment of an oxygen atom plays an important role in forecasting the
polymorphism, and the alkyl chain length further tunes the occur-
rence of phase variant in benzylidene aniline compounds. Further,
the oxygen atom on both sides of the rigid core moiety has no major
effect on the order of transition in comparison with nO.m compounds.
ACKNOWLEDGMENT
The financial support of the Department of Science and Technology
(Grant No. SP=S2=M-34=2000), New Delhi, India, is gratefully
acknowledged.
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