Thermotropic liquid crystalline polyesters derived from 2-chloro hydroquinone

Article abstract of DOI:10.1007/s12039-017-1342-y

Abstract: Synthesis of thermotropic liquid crystalline polyesters derived from bis[4-hydroxy benzoyloxy]-2-chloro-1,4-benzene (BHBOCB) and aliphatic dicarboxylic acid chlorides by interfacial polycondensation methodology is presented. Synthesised polyesters consist of bis[4-hydroxy benzoyloxy]-2-chloro-1,4-benzene as a mesogen and aliphatic diacid chloride as flexible spacer. The length of oligomethylene units in the polymer was varied from the trimethylene to the dodecamethylene groups. Synthesized polyesters were characterized by differential scanning calorimetry and optical microscopy. The transition temperatures and thermodynamic properties were studied for all these polymers. These polyesters exhibited thermotropic liquid crystalline behavior and showed nematic texture except decamethylene spacer. Decamethylene spacer based polyester showed marble texture of smectic C. Mesophase stability of these polyesters was higher than 123 ^°C (except first heating cycle of PE-1). Graphical Abstract:: SYNOPSIS The present study deals with the synthesis of thermotropic liquid crystalline polyesters derived from bis[4-hydroxy benzoyloxy]-2-chloro-1,4-benzene (BHBOCB) and aliphatic dicarboxylic acid chlorides by interfacial polycondensation methodology. [Figure not available: see fulltext.].

Full text of DOI:10.1007/s12039-017-1342-y

J. Chem. Sci. Vol. 129, No. 9, September 2017, pp. 1461–1468.
© Indian Academy of Sciences
DOI 10.1007/s12039-017-1342-y
REGULAR ARTICLE
Thermotropic liquid crystalline polyesters derived from 2-chloro
hydroquinone
NAGESH MANURKAR, SAYAJI MORE, KHUDBUDIN MULANI, NITIN GANJAVE
and NAYAKU CHAVAN∗
Polymer Science and Engineering Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road,
Pashan, Pune, Maharashtra 411 008, India
E-mail: nn.chavan@ncl.res.in
MS received 6 March 2017; revised 22 June 2017; accepted 5 July 2017; published online 24 August 2017
Abstract. Synthesis of thermotropic liquid crystalline polyesters derived from bis[4-hydroxy benzoyloxy]-
2-chloro-1,4-benzene (BHBOCB) and aliphatic dicarboxylic acid chlorides by interfacial polycondensation
methodology is presented. Synthesised polyesters consist of bis[4-hydroxy benzoyloxy]-2-chloro-1,4-benzene
as a mesogen and aliphatic diacid chloride as flexible spacer. The length of oligomethylene units in the polymer
was varied from the trimethylene to the dodecamethylene groups. Synthesized polyesters were characterized
by differential scanning calorimetry and optical microscopy. The transition temperatures and thermodynamic
properties were studied for all these polymers. These polyesters exhibited thermotropic liquid crystalline
behavior and showed nematic texture except decamethylene spacer. Decamethylene spacer based polyester
showed marble texture of smectic C. Mesophase stability of these polyesters was higher than 123◦C (except
first heating cycle of PE-1).
Keywords. Thermotropic liquid crystalline polyesters; aromatic mesogen; aliphatic flexible spacers.
1. Introduction
type, flexible spacers are incorporated in side chain as
substituent.
The synthesis of thermotropic liquid crystalline aro-
matic polyesters (TLCP) have received more attention
in the field of engineering polymers due to their attrac-
tive properties such as low melt viscosities, thermal
endurance, chemical stability and excellent mechani-
cal properties. Liquid crystalline polymers (LCP’s) are
extremely useful for various applications including fab-
rication of numerous optical and electronic devices,
flame retardance and solvent resistance.1–5 The ther-
motropic liquid crystalline behavior of polymers is
of strong current interest among polymer scientists,
because of their outstanding thermal and chemical resis-
tance, high strength as well as low linear viscosity in
liquid crystalline state.6–13 Basically, thermotropic liq-
uid crystalline polymers are of two types. In the first
type, aliphatic flexible spacer incorporated into the main
chain to reduce transition temperature as well as rigid-
ity and improve the solubility, whereas in the second
In TLCP, the substituent effects are of great impor-
tance because crystal to liquid crystal transition temper-
aturesoffullyextendedrigidrodpolymersareextremely
high. Thus, it is desirable to lower the crystal to liq-
uid crystal transition temperature of liquid crystalline
polymers from processing point of view. Kleinschuster
et al., first reported the effect of alkyl substitution and
the use of monosubstituted hydroquinones in random
copolyesters results in a considerable decrease in the
crystal to liquid crystal transition temperature and per-
mits melt spinning from the liquid crystalline states.14
Lenz et al., described the effect of alkyl substitution on
mesogenic unit with respect to glass transition temper-
ature, melting temperature, thermal stabilities, etc. and
inferred that polymer with alkyl substitution more than
eight carbon atoms or longer does not form a liquid
crystalline phase.15
Over the last few decades, several strategies have
been used to lower the liquid crystalline phase transi-
tion temperature and one of the most popular strategies
*
For correspondence
Electronic supplementary material: The online version of this article (doi:10.1007/s12039-017-1342-y) contains supplementary material,
which is available to authorized users.
1461

1462
Nagesh Manurkar et al.
used for this purpose is incorporation of substituent 2. Experimental
on mesogenic unit. Since melting temperatures of fully
2.1 Materials
extended rigid rod polymers are very high, majority of
such polymers degrade before its isotropization temper-
ature. Substitution lowers phase transition temperature
substantially due to decrease in axial ratio of rigid
mesogenic unit. So, it is desirable to lower the melt-
ing temperature of processing by conventional methods.
This could be accomplished by introducing substituent
on the mesogenic unit of the polymer.16 In our previous
papers, we reported the synthesis and characterization of
2-Chloro hydroquinone, 4-hydroxy benzoic acid, dibasic
acids such as glutaric, adipic, pimelic, suberic, azelaic,
sebacic and dodecanedioic were purchased from Aldrich. 2-
Chloro hydroquinone and 4-hydroxy benzoic acid were puri-
fied by recrystallisation from acetone and hot distilled water,
respectively. Acetic anhydride, dichloromethane, methanol,
pyridine, chloroform, thionyl chloride, pet ether and potas-
sium hydroxide were procured from Merck and were used as
thermotropic liquid crystalline polyesters derived from received.
hydroquinone and methyl hydroquinone-based rigid rod
2.2 Synthesis of trimesogenic diol [BHBOCB]
diolsasmesogenicunitsandaliphaticdicarboxylicacids
as flexible spacers.17,18
Bis[4-hydroxy benzoyloxy]-2-chloro-1,4-benzene [trimeso-
gen] was synthesized from p-hydroxy benzoic acid as start-
ing material and 2-chloro-hydroquinone as central moiety
as reported in our earlier studies.19 The synthesis route
of BHBOCB trimesogen is depicted in Scheme 1 and
thermotropic liquid crystalline polyesters were synthesized
according to Scheme 2.
The present investigation demonstrates synthesis and
characterisation of TLCP derived from bis[4-hydroxy
benzoyloxy]-2-chloro-1,4-benzene (BHBOCB) as
mesogenic moiety and aliphatic diacid chloride as flex-
ible spacer. Flexible spacer was inserted into the back-
bone of the rigid polymer to dissociate the greater order
of the main-chain and decouple the motion of mesogenic
moiety from polymer backbone. We also describe syn-
thesis, thermal transition temperatures, thermodynamic
propertiessuchasentropy/enthalpy, mesomorphicrange
and morphology in the mesophase of the liquid crys-
talline polymers.
2.2a 4-Acetoxy benzoic acid [4-ACBA]: In a 500 mL
beaker, 4-hydroxy benzoic acid (13.81g, 0.1 mole) and dis-
tilled water (250 mL) were added and stirred to make uniform
slurry. In another flask, sodium hydroxide (4.4 g, 0.11 mole)
was dissolved in 50 mL distilled water and was added to
Scheme 1. Synthesis of bis-(4-hydroxy benzoyloxy)-2-chloro-1,4-benzene (BHBOCB).
Scheme 2. Synthesis of polyesters.

Thermotropic liquid crystalline aromatic polyesters
1463
the slurry. The reaction mixture was stirred till 4-hydroxy was distilled off and the solid was extracted with chloro-
benzoic acid dissolved completely, and to it distilled acetic form. Chloroform was distilled off to get solid trimesogenic
anhydride (11.23 g, 0.11 mole) was added slowly. After the diol product. This solid product was washed with methanol
addition of acetic anhydride, stirring was continued for 4 h and dried under reduced pressure at 80◦C for 4 h. Yield:
at room temperature to obtain a precipitate of 4-acetoxy ben- 75%. M.p.: 296◦C. IR (Nujol, cm−1): 3393 (OH stretch-
zoic acid. The precipitate obtained was filtered and washed ing), 3072 (C-H stretching), 1907 (anti symmetric C=C
several times with cold dilute hydrochloric acid and distilled stretching), 1708 (C=O stretching), 1609 and 1591 (aro-
water. The crude product was recrystallized from methanol matic C=C stretching), 1512 (C-H bending), 1447 (symmetric
and dried under reduced pressure. Yield: 80%. M.p.: 190◦C, –C=C stretching), 1276, 1162 (symmetric –C=O stretch-
IR (Nujol, cm−1):1730 (C=O stretching); 1H NMR (DMSO- ing), 1066, 762 (C-Cl substitution) and 846 cm−1 (para-
d6,δ): 8.00(d, 2H); 7.30(2H, d); 2.30(s, 3H); 10.10(s, 1H).
substitution). 1H NMR (DMSO-d6): δ 6.94–6.98 (a, dd, 4H),
7.34-7.51 (b, dd, 3H), 7.96 (c, S, 1H), 8.00-8.04 (d, dd, 4H),
10.64(e, s, 2H). 13C NMR (DMSO-d6,): δ 115.86, 122.44 -
126.58, 132.71, 144.70, 148.87, 163.29, 164.29.
2.2b 4-Acetoxy benzoyl chloride: 4-Acetoxy benzoic
acid (9.0 g, 0.05 mole) was placed in a single necked 100 mL
round bottom flask. To this flask, thionyl chloride (5 mL, 0.07
mole) was added drop wise and refluxed the reaction mixture
gently for 8 h. The initial heterogeneous mass was homog-
enized. Excess thionyl chloride was removed by distillation.
The crude acid chloride was purified by double distillation
under reduced pressure. Yield: 90%. M.p.: 29◦C. IR (Nujol,
cm−1): 1780 (COCl), 1730 (C=O stretching).
2.2e Preparation of aliphatic diacid chlorides: Glu-
taroyl chloride, adipoyl chloride, pimeloyl chloride, suberoyl
chloride, azeloyl chloride, sebacoyl chloride and dodecane-
dioyl chloride were synthesized from the parent diacids. The
aliphatic diacids were gently refluxed with thionyl chloride
at 80◦C for 8 h till a clear solution was obtained. The acid
chloride solution was initially distilled under lower reduced
pressure (100–150 mm Hg) to remove the excess of thionyl
chloride and was double distilled under reduced pressure (1-2
mm Hg) at 150◦C to get pure acid chloride, prior to use.
2.2c Bis
[4-acetoxy
benzoyloxy]-2-chloro-1,4-
benzene: Three-necked 500 mL round bottom flask was
equipped with magnetic stirrer, nitrogen inlet and calcium
chloride guard tube. To the reaction flask, 2-chloro hydro-
quinone (5.78 g, 0.04 mole) and pyridine (25 mL, 0.3 mole)
were added and stirred till dissolution. In another flask, 4-
acetoxy benzoyl chloride (23.82 g, 0.12 mole) and 300 mL
dry 1,2-dichloroethane were placed and the acid chloride
solution was added to the reaction mixture of 2-chloro-
hydroquinone solution. This reaction mixture was stirred
under nitrogen blanket for 48 h at room temperature. The
reaction mixture was then washed sequentially with 5%
sodium carbonate solution, 5% hydrochloric acid and distilled
water. 1,2-Dichloroethane layer was evaporated to dryness on
rotavapor. The crude product obtained was recrystallised from
chloroform/petroleum ether [60:80] solvent mixture and dried
in vacuum oven at 80◦C for 4 h. Yield 90%. M. p.: 181◦C, IR
(Nujol, cm−1): 1740 (C=O stretching), 760 (C-H bending).
1H NMR (CDCl3,δ): 2.3 (s, 6H); 7.1 (m, 6H); 7.3 (d, 1H); 8.2
(dd, 4H).
2.3 Polycondensation
2.3a Interfacial polycondensation: Recipe used for
interfacial polycondensation is presented in Table 1. Two-
necked 100 mL round bottom flask was equipped with
mechanical stirrer, nitrogen inlet, and calcium chloride guard
tube. To the reaction flask, trimesogen BHBOCB (1.539
g, 0.04 mole), sodium hydroxide [0.008 mole], benzyl tri-
ethyl ammonium chloride (0.02 g) and 15 mL distilled water
were added and stirred for 5 min. Into the another 100 mL
stoppered flask, 0.04 mole of acid chloride and 15 mL dry
1,2-dichloroethane were placed and stirred for 5 min. The
acid chloride solution was added into the reaction flask. The
reaction mixture was vigorously stirred at 6000–7000 rpm
for 30 min at 0−5◦C. The polyester was precipitated by pour-
ing it into 500 mL methanol. The solution was filtered and
2.2d Bis[4-hydroxy
benzoyl
oxy]-2-chloro-1,4-
Table 1. Recipe of BHBOCB and aliphatic diacid chlorides
benzene [BHBOCB]: Single-necked 250 mL round bot-
tom flask was equipped with magnetic stirrer and ice bath. To
the reaction flask, potassium hydroxide (1.32 g, 0.02 mole)
and 100 mL methanol were placed, stirred till dissolution
and the solution was cooled to 0−5◦C. To the alcoholic
potassium hydroxide solution, bis[4-acetoxy benzoyloxy]-
2-chloro-1,4-benzene (4.34 g, 0.01mole) was added. The
reaction mixture was stirred at 0−5◦C for 4 h and then
acidified with 4N hydrochloric acid. Methanol was removed
under reduced pressure. The crude product obtained was
dissolved in 400 mL ethyl acetate and ethyl acetate layer
was washed three times with distilled water. Ethyl acetate
for interfacial polycondensation.
Polymer Code Aliphatic diacid chloride Polyester [m/ru]*
PE-1
PE-2
PE-3
PE-4
PE-5
PE-6
PE-7
Glutaryl
Adipoyl
Pimeloyl
Suberoyl
Azeloyl
Sebacoyl
Dodecyl
480.83
494.86
508.88
522.91
536.94
550.96
579.01
* mole per repeat unit.

1464
Nagesh Manurkar et al.
Table 2. Transition temperature and thermodynamic data of BHBOCB polyesters (First heating cycle).
Polymer Code [CH2]n n K-N (◦C) N-I (◦C) ꢀT (◦C) ꢀH K-N ꢀH N-I ꢀS K-N ꢀS N-I DOC %
PE-1
PE-2
PE-3
PE-4
PE-5
PE-6
PE-7
3
4
5
6
7
8
10
217.5
224.9
170.0
173.6
173.2
155.2
120.4a
278.1
345.9
278.2
303.8
261.5
283.8
204.8b
60.6
121.0
108.2
130.2
88.3
128.6
84.4
4.52
6.88
6.62
1.68
12.30
2.65
3.42a
6.35
4.45
5.24
3.09
3.40
3.47
2.90b
9.22
13.81
14.95
3.76
11.52 40.81
7.19
9.50
5.36
6.36
6.24
6.07b
40.00
46.07
37.50
50.00
40.42
48.13
27.57
6.19
8.70a
K-N (◦C): Crystal – liquid crystal transition temperature, N-I (◦C): Liquid crystal – isotropic transition
temperature, ꢀT(◦C): Temperature range of mesophase stability, ꢀH (K-N) and ꢀH (N-I): Enthalpy change
in kJ mol−1 of repeat unit (mru), ꢀS (K-N) and ꢀS (N-I): Entropy change in Joule mru−1 (K), DOC: Percent
degree of crystallinity. aCrystal to smectic transition; bSmectic to isotropic transition.
Table 3. Transition temperature and thermodynamic data of BHBOCB polyesters (Second heating cycle).
Polymer Code [CH2]n n K-N (◦C) N-I (◦C) ꢀT (◦C) ꢀH K-N ꢀH N-I ꢀS K-N ꢀS N-I DOC %
PE-1
PE-2
PE-3
PE-4
PE-5
PE-6
PE-7
3
4
5
6
7
8
10
162.9
178.8
159.6
139.6
166.0
162.8
104.3a
341.3
367.7
348.2
337.8
282.8
303.8
227.3b
178.4
188.9
188.5
198.4
116.8
141.0
123.0
2.12
5.88
3.10
0.73
5.37
5.06
0.70a
5.72
9.80
6.92
8.74
4.67
6.34
2.44b
4.86
13.04
7.18
9.32
40.81
15.30 40.00
11.15 46.07
25.85 37.50
8.41
10.63 40.42
4.87b
48.13
1.78
12.23
11.63
1.85a
50.00
K-N (◦C): Crysta◦l – liquid crystal transition temperature, N-I (◦C): Liquid crystal – isotropic transition
temperature, ꢀT ( C): Temperature range of mesophase stability, ꢀH (K-N) and ꢀH (N-I): Enthalpy change
in kJ mol−1 of repeat unit (mru), ꢀS (K-N) and ꢀS (N-I): Entropy change in Joule mru−1 (K), DOC: Percent
degree of crystallinity. aCrystal to smectic transition; b Smectic to isotropic transition.
the obtained solid polymer was washed with 5 weight per- room temperature. Inherent viscosities were determined with
cent sodium carbonate solution, dilute hydrochloric acid and 0.5% solution in chloroform at 30◦C.
distilled water. The obtained polymer was further purified by
soxhlet extraction using methanol as solvent and dried under
reduced pressure at 80◦C for 8 h.
3. Results and Discussion
3.1 General properties
2.4 Characterization
The transition temperatures and thermodynamic prop-
erties of polyesters are presented in Tables 2 and 3.
All the polyesters exhibit major two endothermic
melting peaks which correspond to the crystal to liq-
uid crystal and liquid crystal to the isotropic transitions
along with small melting peaks during the first heating
cycle. This is due to solid-solid transitions, formed in
rapid succession when more than one liquid crystalline
phase is present. All the polyesters exhibited a rather
narrow endothermic transition for the mesophase at the
isotropization temperature, which suggests that poly-
dispersity was rather narrow. To eliminate the thermal
history of the polyester samples generated during sol-
vent evaporation and drying, the second DSC heating
Synthesized polymers were soluble in chlorinated organic
solvents such as chloroform, carbon tetrachloride, dichlo-
romethane, dichloroethane, 4-chlorophenol or phenol:TCE
(50:50 v/v) mixture. The thermal properties were determined
with a Mettler DSC-30 thermal analyzer under a nitrogen
atmosphere. Indium and solid mixture of Indium-Lead-Zinc
were used as calibration standards for enthalpy (ꢀH) calcula-
tion and temperature scale, respectively. The endotherm peaks
(maxima) were taken as phase transition temperatures. The
enthalpies and entropies of liquid crystalline phase transitions
were determined. A Leitz Ortholux polarizing microscope
with a Mettler FP-52 hot stage controlled by Mettler FP-5
temperature controller was used for visual examination of
phase changes in the polymers. Wide angle X-ray diffrac-
tometer was used to determine the degree of crystallinity at was applied for all polyester samples to determine the

Thermotropic liquid crystalline aromatic polyesters
melting transitions of the liquid crystalline polymers.
1465
In the second heating cycle, all polyesters display two
small endothermic peaks which indicate that thermal
history of polyester sample is eliminated.
Synthesized polyesters are well ordered and soluble
in the polymerization solvent medium. A broadening
of isotropic transition may be due to polydispersity, as
indeed observed for the polyesters based on unsubsti-
tuted trimesogen, wherein insolubility intervened. The
peak maxima were selected to make comparative esti-
mation of structure-property relationship.
No definite trend was observed in crystal to liquid
crystal transition temperature with respect to the num-
ber of methyelene units (n) incorporated in aliphatic
spacer, but crystal to liquid crystal or liquid crys-
tal to isotropic transition temperature decreases with
number of methylene units due to the dilution of meso-
gen or decrease in axial ratio (Table 2). Mesophase
stability of hydroquinone, methyl hydroquinone and
chlorohydroquinone-based polyesters showed signifi-
cant difference in mesophase stability on the basis of
second heating cycle in which thermal history was
eliminated.17,18 Mesophase stability of the substituted
hydroquinone (e.g., methyl or chloro)-based rigid-rod
flexible spacer polymers showed higher than unsusbti-
tuted polymers on the basis of second heating cycle.
Maximum mesophase stabilities were observed 198.4,
163.1 and 107.8◦C for chloro, methyl and unsubstituted
hydroquinone-based polymers, respectively, whereas
minimum mesophase stabilities were observed 123.0,
89.8 and 99.8◦C for chloro, methyl and unsubstituted
hydroquinone-based polymers, respectively. It shows a
considerable decrease in crystal to liquid crystal tran-
sition temperature but no decrease in liquid crystal to
isotropic temperature. This may be due to the difference
in the pKa values of hydroquinone and chlorohydro-
quinone which are 10.2 and 8.6, respectively. It clearly
indicates that chlorohydroquinone posseses higher acid-
ity than hydroquinone as such.
Figure 1. Photomicrograph of polyester PE-7 at 187.7 ◦C
observed on cooling from isotropic state.
Figure 2. Photomicrograph of polyester PE-7 at 134.8 ◦C
observed on cooling from isotropic state.
units are linked in the main chain of repeating unit of
the polymer, displays the nematic schlieren texture from
the isotropic phase to liquid crystalline phase on cooling
underpolarizingmicroscopeat187.7◦CasshowninFig-
ure 1, and on further cooling showed small droplets of
texture of smectic C at 134.8◦C under crossed polarizer
as seen in Figure 2.20 This is because when the number
of methylene units is more than 8 side chain crystalliza-
tion occurs and thus, nematic liquid crystal phase turns
to more ordered smectic phase.
It is known that deisotropization in main liquid crys-
talline polyesters exhibit a smaller degree of super
cooling. In other words, isotropization is thermodynam-
ically more of an equilibrium (reversible) process, rather
than crystal to liquid crystal transition. Due to this effect,
crystal to liquid crystal temperature of polyesters was
decreased by 50◦C and liquid crystal to isotropic tem-
3.2 Substitution effects
perature of polyesters was increased by 25 to 50◦C. Effect of different substituents on liquid crystalline
Thus, mesophasic stability of polyesters was consider- behaviour of low-molecular mass liquid crystals has
ably higher (> 120◦C).
been systematically described by Gray21–23 and oth-
All polyesters, except PE-7, display the formation ers.16,24,25 It is desirable to lower the melting temperature
of nematic schlieren texture from the isotropic phase of polymers for processing point of view. This is accom-
to liquid crystalline phase upon cooling under polariz- plished by introducing the substituent into mesogenic
ing microscope. Polyester PE-7, wherein 10 methyelene unit of thermotropic polymers. The first description

1466
Nagesh Manurkar et al.
of substituted main chain thermotropic liquid crys-
talline polymers was made by Roviello and Sirigu.16
However, the highly polar substituents (-Cl, -Br, -CN,
-NO2) are known to be very effective in depressing
the crystal to liquid crystal transition temperature. This
partial decrease in transition temperature arises from
steric effects, which limits the molecular packing effi-
ciency in both crystalline and liquid crystalline states.
The Van der Waals radii of chloro, bromo, cyano
and nitro substituents are larger than that of methyl
group.
Chloro substitution based liquid crystalline polymers
significantly lower the transition temperature compared
to the unsubstituted polymers. The reduction in crystal
to liquid crystal transition temperatures of chlorohydro-
quinone based thermotropic liquid crystalline polyesters
may be due to bulkiness of the chloro substituent com-
pared to the corresponding polyesters prepared from
unsubstituted or methyl substituted series. It can be con-
firmed by bond lengths of C-H, C-Me and C-Cl are 1.07,
1.54 and 1.76 Å, respectively.23
Substitution plays important role on mesophase sta-
bility. If the substituent is electron-withdrawing group
such as chloro may substantially change acidity of
the resulting polyesters. The ionization constant of
chlorohydroquinone is several times greater than that
of unsubstituted hydroquinone, which greatly stabilizes
the phenoxide ion by permitting a portion of the neg-
ative charge to be carried by its own oxygen. Due
to this effect, reduction in melting transitions of liq-
Figure 3. DSC thermograms of BHBOCB polyesters (First
heating cycle).
uid crystalline phases was observed but isotropization cycles of these polyesters are shown in Figures 3 and 4.
temperatures were not affected. Thus, mesomorphic sta- The transition temperatures and thermodynamic data, as
bility of chlorohydroquinone based liquid crystalline determined by DSC thermograms, corresponding to the
polymers was higher than unsubstituted and methyl sub- first and second heating cycles of synthesized polyesters
stitutedhydroquinonebasedliquidcrystallinepolymers. are presented in Tables 2 and 3. Looking at the liquid
In our previous communication, we have reported that crystal transition temperatures, it can be stated that the
synthesis and thermal properties of polyesters derived odd-even effect is weakly expressed in this homologues
frommethylsubstitutedhydroquinoneascentralmoiety. series of polyesters.
The incorporation of methyl substituent on hydro-
Rath and Ponrathnam described the nature of lat-
quinone central moiety of triad mesogen unit resulted eral substituent on the mesogen has very marginal
in lower isotropisation temperature with respect to the influence on Tm in random thermotropic liquid crys-
similar number of methylene units.18 In short, crystal to talline polyesters.26 Ordered polyesters with bulkier
liquid crystal transition temperatures of unsubstituted substituent on the mesogen usually show greater depres-
hydroquinone based polyesters were higher than those sion in Tm.27 The attraction due to greater electroneg-
of the methyl and chlorohydroquinone based polyesters. ativity of chlorine is outweighed by steric repulsion in
The trend of crystal to liquid crystal transition tempera- ordered systems and resulted in a reduced lateral pack-
ture of polyesters based on hydroquinone is as follows: ing of chains. The greater effect of steric repulsion is
H > Me > Cl.
not easily realized in randomized structures. Unlike the
methyl series, the ordered chloro series shows some odd
fluctuations in isotropic transition temperatures Ti.26
In general, these temperatures are lower than those in
3.3 Thermodynamics of LC States
The differential scanning calorimetry (DSC) thermo- methylene homologues. Partial decomposition during
grams corresponding to the first and second heating isotropization and non-equilibrium nature of crystal to

Thermotropic liquid crystalline aromatic polyesters
1467
polyesters as well.29 The origin of such high values indi-
cates that some conformational ordering of spacers in
the liquid crystalline state.
The higher values of entropy change (Tables 2 and 3)
for liquid crystal to isotropization temperature for the
polyesters having chloro substituent on hydroquinone as
central moiety in triad based mesogenic diol suggesting
poor thermal stability of mesophase and also indicating
the existence of a higher degree of order in their liquid
crystalline state.30
4. Conclusions
In this study, thermotropic liquid crystalline polyesters
with very large mesophasic stability (>123◦C) were
generated by incorporating a chloro substitution on triad
mesogen. These polyesters display somewhat complex
nematic ordering in the liquid crystalline state. Smectic
liquid crystalline phase manifests with decamethylene
spacer (n = 10). The weak odd-even effect is noted
in transition temperatures as well as in enthalpy and
entropy values. The high enthalpy and entropy changes
onisotropisationindicateconformationalorderinginthe
Figure 4. DSC thermograms of BHBOCB polyesters liquid crystalline state. Unlike in random copolyesters,
(Second heating cycle).
incorporation of chloro substituent into an ordered
polyester structure depresses the isotropisation temper-
ature.
liquid crystal or liquid crystal to crystal transitions are
the probable reasons for the non-emergence of odd-even
effect in a clear-cut fashion during isotropic and liquid
crystal transitions, respectively.
Supplementary Information (SI)
The characterization details including IR, 1H and 13C NMR,
thermogravimetric analysis (TGA) and wide angle X-ray
diffractogram of triad based mesogenic diol BHBOCB are
presented in Supplementary Information which is available
at www.ias.ac.in/chemsci.
The aliphatic methylene spacer is expected to give
more conformational freedom to the mesogenic units to
form ordered structure in melt. Jin et al., synthesised
a series of polyesters using biphenyl p-oxybenzoate as
a mesogenic unit and pentamethylene as spacer. They
observed that presence of the spacer enchances the heat
capacity of the compound to form a mesophase.27 In
general, lowering of the transition temperature occurs
due to incorporation of aliphatic spacers. Therefore,
the polyesters with the increasing length of the spacer,
a downward trend in the clearing temperature was
observed.28
References
1. Han H and Bhowmik P K 1997 Wholly aromatic liquid-
crystalline polyesters Prog. Polym. Sci. 22 1431
2. (a) Chavan N N and Mulani K B 2012 In Thermotropic
liquid crystalline polyesters (Germany: Lambert Aca-
demic Publishing Co.); (b) Jin J I and Kang C S 1997
Thermotropic main-chain polyesters Prog. Polym. Sci.
22 937
3. Negi Y S and Goyal R K 2003 Development of ther-
motropic liquid crystalline polymers and their applica-
tions Int. J. Plastics Technol. 7 99
4. Broer D J, Lub J and Mol G N 1995 Wide-band reflec-
tive polarizers from cholesteric polymer networks with
a pitch gradient Nature 378 467
5. Bunning T J and Kreuzer F H 1995 Cyclosiloxane-based
liquid crystalline materials Trends Polym. Sci. 3 318
6. Xing C, Jacky W Y, Zhao K Q and Tang B Z 2008 Synthe-
sis and liquid crystalline properties of poly(1-alkyne)s
3.4 Enthalpy and entropy changes
The enthalpy and entropy changes for the crystal to
liquid-crystal and liquid-crystal to isotropic transitions
are of the same magnitude as those presented in litera-
ture for other main chain thermotropic liquid crystalline
polyesters. The values associated with the isotropisa-
tion are somewhat on the higher side. Similar high
values have been noted in methyl substituted series of

1468
Nagesh Manurkar et al.
carrying triphenylene discogens J. Polym. Sci. Part A: 19. Mulani K, Ganjave N and Chavan N 2014 Synthesis
Polym. Chem. 46 2960
7. Zhu Z, Zhi J, Liu A, Cui J, Tang H, Qiao W, Wan
X and Zhou Q 2007 Synthesis and characterization of
and characterization of triad based rigid mesogenic diols
derived from hydroquinone and 4-hydroxybenzoic acid
Ind. J. Chem. B 53B 591
a thermotropic liquid-crystalline poly[2,5-bis(4-alkoxy 20. Pereira F V, Borsali R, Ritter O M S, Gonçalves P F,
carbonyl phenyl)styrene] J. Polym. Sci. Part A: Polym.
Chem. 45 830
8. Luzny W, Pomarzanska S E and Pron A 1999 Struc-
Merloa A A and da Silveira N P 2006 Structure-property
relationships of smectic liquid crystalline polyacrylates
as revealed by SAXS J. Braz. Chem. Soc. 17 333
tural properties of selected poly(azomethines) Polymer 21. Gray G W and Warrall B M 1959 The meso-
40 6611
morphic transition temperature of 3-substituted 4-n-
octyloxydiphenyl-4-carboxilic acids J. Chem. Soc. 1545
22. Gray G W, Johns B and Marson F 1957 Mesomor-
phism and chemical constitution. Part VIII. The effect
of 3’-substituents on the mesomorphism of the 4’-n-
alkyloxyphenyl-4-carboxylic acids and their alkyl esters
J. Chem. Soc. 393
9. Shiota A and Ober C K 1997 Rigid rod liquid crystalline
thermosets Prog. Polym. Sci. 22 975
10. Ortiz C, Wagner M, Bhargava N, Ober C K and Kramer
E J 1998 Deformation of a polydomain, smectic liquid
crystalline elastomer Macromolecules 31 8531
11. Ortiz C, Ober C K and Kramer E J 1998 Stress relaxation
of a main-chain, smectic, polydomain liquid crystalline 23. Branch S J, Byron D J, Gray G W, Ibbotson A and War-
elastomer Polymer 39 3713
rallBM1964Mesomorphismandchemicalconstitution.
Part XII. The preparation and mesomorphic properties
of substituted 4, 4’-di-(p-n-alkoxybenzylideneamino)
biphenyls J. Chem. Soc. 3279
12. HeinoMT, HietaojaPT, VainioTPandSeppalaJV1994
Effect of viscosity ratio and processing conditions on the
morphology of blends of liquid crystalline polymer and
polypropylene J. Appl. Polym. Sci. 51 259
13. Bualek-Limcharoen S, Saengsuwan S, Amornsakchai
T and Wanno B 2001 Rheology, Morphology and
24. Arora S L, Ferguson J L and Taylor T R 1970
Molecular structure and liquid crystallinity phenylene
Bis(alkoxybenzoates) J. Org. Chem. 35 4055
tensile properties of thermotropic liquid crystalline 25. Young W R, Haller I and Green D C 1972 Mesomorphic
polymer/polypropylene in-situ composites Macromol.
Symp. 170 189
structure of some ring-methylated phenyl benzoyloxy-
benzoates J. Chem. Soc. 37 3707
14. Zhou Q and Lenz R W 1983 Liquid crystalline proly- 26. Sonpatki M M, Ravindranath K and Ponrathnam S 1994
mers. 15. Synthesis and liquid crystalline properties of
alkyl-substituted polyesters J. Polym. Sci. Part A: Polym.
Chem. 21 3313
Segmented copoly(ether-ester) elastomers. Influence of
hard segment length and substitution on mesophase for-
mation Polym. J. 26 804
15. Kleinschuster J J, Pletcher T C, Schaefgen J R and Luise 27. Jin J, Chung B Y, Chol J and Jo B 1991 Inportance of the
R R 1994 Ger. Offen. 2 520
16. Roviello A and Sirigu A 1975 Mesophasic structures
in polymers. A preliminary account on the mesophases
roleofflexiblespacersinliquidcrystalformationbybend
dimesogenic and star-shaped trimesogenic compounds
Bull. Korean Chem. Soc. 12 189
of some polyalkanoates of p,p‘-dihydroxy-α, α’- 28. Meurisse P, Noel C, Monnerie L and Fayolle B 1981
dimethyl dibenalazine J. Polym. Sci. Polym. Lett. Ed.
13 455
17. Mulani K, Momin M, Ganjave N and Chavan N 2014
Polymers with mesogenic elements and flexible spacers
in the main chain: Aromatic-aliphatic polyesters British
Polymer J. 13 55
Thermotropic liquid crystalline polyesters based on 29. Lenz R W 1985 Synthesis and properties of thermotropic
trimesogenic diol and flexible spacers Malaysian Polym.
J. 9 54
liquid crystal polymers with main chain mesogenic units
Polymer J. 17 105
18. Mulani K, Momin M, Ganjave N and Chavan N 30. Jo B W, Lenz R W and Jin J 1982 Liquid crys-
2015 Thermotropic liquid crystalline polyesters derived
from bis-(4-hydroxybenzoyloxy)-2-methyl-1,4-benzene
and aliphatic dicarboxylic acid chlorides Bull. Mater.
Sci. 38 1301
talline polymers, 7. Thermotropic polyesters with main
chain phenyl-1,4-phenylene,4,4’-biphenlene, and 1,1’-
binaphthyl-4,4’-ylene units Makromol. Chem. Rapid
Comm. 3 23