Materials Sciences and Applications, 2011, 2, 1520-1527
doi:10.4236/msa.2011.210205 Published Online October 2011 (http://www.SciRP.org/journal/msa)
Synthesis and Characterization of Cholesteric
Thermotropic Liquid Crystalline Polyesters
Based on Isosorbide
Nayaku N. Chavan
Polymer Science and Engineering Division, National Chemical Laboratory, Pune, India.
Received July 23rd, 2011; revised August 20th, 2011; accepted September 27th, 2011.
ABSTRACT
Background: Generally main chain cholesteric thermotropic liquid crystalline polymers are prepared form chiral dia-
cid or diol monomer. But these monomers are costly. Isosorbide is chiral cycloaliphatic diol accessible from renewable
resources in the form of pure enantiomers. Thus it is used to synthesize main chain cholesteric thermotropic liquid
crystalline polymers. Incorporation of phenyl hydroquinone into the backbone of the main chain frustrates chain pack-
ing, thus lowering the crystallinity and depressing the melting point below the degradation temperature, also improves
the solubility due to disruption of packing and maintains the mechanical and thermal performance. Results: Optical
microscopy study reveals that more than 50% of isosorbide content with phenyl hydroquinone and terephthalic acid
showed “yellow iridescent oily streaks” with a background of mosaic/marble texture. These are the typical textures of
cholesteric liquid crystalline phase. Copolyesters based on phenyl hydroquinone, isosorbide and terephthalic acid are
soluble in aprotic solvents like N,N-dimethylacetamide (DMAC), dimethyl sulfoxide (DMSO), N,N-dimethylformamide
(DMF), and N-methyl-2-pyrrolidone (NMP). Solubility increases with the content of isosorbide percent. Thermal stabil-
ity of all copolyesters was more than 300˚C on the basis of 10 wt% loss. Conclusions: It was found that main chain
cholesteric thermotropic liquid crystalline polymers can be prepared form chiral cycloaliphatic diol such as isosorbide.
Main chain cholesteric thermotropic liquid crystalline polyesters are prepared from phenyl hydroquinone, isosorbide
and terephthalic acid showed thermal stability more than 300˚C. Main chain cholesteric thermotropic liquid crystalline
polymers are soluble in aprotic solvents like DMAC, DMSO, DMF and NMP.
Keywords: Liquid Crystalline Polymers, Thermotropic, Cholesteric, Differential Scanning Calorimetry, Optical
Microscopy, Thermo Gravimetric Analysis, Wide Angle X-Ray Diffraction, Crystallinity, Inherent
Viscosity, Solubility and Solution Polycondensation
1. Introduction
Cholesteric liquid crystalline polymers (ChLCPs) have
ability to produce supermolecular structures character-
ized by a helical organization with certain identity period.
This special organization is responsible for unique opti-
cal properties of cholesteric polymer liquid crystals such
as selective reflection of light at different wavelengths,
very high optical activity, high sensitivity of selective
reflection of light to temperature variations and to the
action of external electrical and magnetic fields [1-5].
The requirement of principal condition to develop a cho-
lesteric phase is associated with the presence of an chiral
(asymmetric) center in monomer which produces a heli-
cal supermolecular structure [6]. Cholesteric phase is
characterized by circular dichroism: the light selective
reflected by cholesteric liquid crystal is circularly polar-
ized. The direction of circular polarization of the re-
flected beam coinsides with the direction of the twisting
of cholesteric helix. The principal feature of cholesteric
structure is associated with a strong rotation of polariza-
tion plane, which may be as high as 104 deg·mm–1. In
conventional organic liquids this parameter is provided
by only chirality of molecules and it does not exceed 102
deg·mm−1. The helical ordering is a result of the frustra-
tion between molecular chirality and layer ordering and
is unusual in the fact that the twist is manifested by a
regular array of screw dislocations. High sensitivity of

Synthesis and Characterization of Cholesteric Thermotropic Liquid Crystalline Polyesters Based on Isosorbide1521
the pitch of the helix changes with respect to temperature
as well as the action of electrical and magnetic fields
offers substantial advantages for practical applications of
such ChLCPs are as thermometers and thermoindicators
for techniques and medicine.
To prepare ChLCPs different polymerization methods
are applied: 1) homopolymerization of chiral mesogenic
monomers; 2) copolymerization of nematogenic mono-
mers with the chiral mesogenic monomers; 3) copoly-
merization of chiral mesogenic monomers with non-
mesogenic monomers. The most advantageous procedure
for the development of cholesteric mesophase is copoly-
merization of mesogenic and chiral monomers. This ap-
proach offers a convenient means for the preparation of
the cholesteric copolymers and allows one to control
phase and optical properties of cholesteric polymer by
varying their composition [7-9]. Addition of flexible ali-
phatic spacer increases solubility but simultaneously it
reduces the thermal and mechanical performance of
polymers. According to structure property relationship
study terephthalic acid is highly rigid diacid based
monomer and it enhances performance of polymers due
to para-para orientation. Hydroquinone is also highly
rigid diol based monomer and it improves performance
of polymers due to para-para orientation. Homopolymer
of hydroquinone and terephthalic acid is insoluble due to
highly integrated para-para orientation. Incorporation of
aromatic unit such as phenyl group on the backbone of
the main chain frustrates chain packing, thus lowering
the crystallinity and depressing the melting point below
the degradation temperature, also improves the solubility
due to disruption of packing and maintains the mechani-
cal and thermal performance [10].
Prime aim of the study was to synthesize high per-
formance main chain cholesteric thermotropic liquid cry-
stalline polyesters using an isosorbide as chiral cy-
cloaliphatic diol. Isosorbide is an attractive building
block because it is easily accessible from renewable re-
sources in the form of pure enantiomers [11-15]. It is
well known that aliphatic spacer reduces thermal and
mechanical performance of polymers. Hence it is not
advisable to use aliphatic spacer even though it improves
solubility. Isosorbide structure contains two cycloalip-
hatic rings and ether bridges. It improves the rigidity as
well as performance of polymers due to cycloaliphatic
group. In our earlier communication isosorbide based
main chain cholesteric thermotropic liquid crystalline
polyesters were reported [5].
In this communication, we attempted to use an isosor-
bide as chiral building block to prepare main chain cho-
lesteric thermotropic liquid crystalline polyesters. Ther-
mal, spectral, mesomorphic and physical properties of
polyesters were characterized by thermogravimetric ana-
lysis (TGA), 1H nuclear magnetic resonance (1H NMR),
polarizing optical microscopy (POM), wide angle X-ray
diffraction (XRD), solubility and inherent viscosity. The
influence of the concentration of the chiral monomer on
cholesteric thermotropic liquid crystalline phase behavior
of the polyesters is discussed.
2. Experimental
2.1. Materials
Phenyl hydroquinone purchased from Aldrich and was
recrystallized from chloroform. Terephthalic acid re-
ceived from Sisco and was used as received. Thionyl
chloride procured from Loba and was distilled prior to
use. Isosorbide received from Aldrich and was used as
such. Benzyl triethyl ammonium chloride (BTEAC)
purchased from Spectrochem and was used without puri-
fication. Analytical reagent grade sodium hydroxide re-
ceived from Merck and was used as received. Dichloro-
methane procured from Merck and was distilled over
calcium hydride. Dimethylacetamide (DMAC) purchased
form Aldrich and was distilled under reduced pressure on
calcium hydride.
2.2. Measurements
Inherent viscosities of copolyesters were measured at a
concentration of 0.5 dL/g in dimethyl acetamide using an
Ubbelhode viscometer at 30˚C. 1H NMR spectra were
recorded on a Brucker AC300 spectrometer at room
temperature in deuterated solvents. Thermo gravimetric
analyses (TGA) were carried out on Perkin Elmer TGA-7
model under N2 atmosphere using heating rate of 10
˚C/min. The optical characterization was performed on a
Mettler polarizing optical microscope equipped with a
FP2 Mettler hot stage, at magnification of 50X. Wide
angle x-ray scattering (WAXS) curves were obtained by
a Philips PW X-ray diffractometer equipped with 1830
generator and 1710 adjustment with CuKα radiation 1.54
Ǻ.
2.3. Polymerization (Solution Method)
Into three necked 100 mL round bottom flask, equipped
with high speed mechanical stirrer, ice bath etc., phenyl
hydroquinone (1.0 g, 5.3 mmol) and sodium hydroxide
solution (14 mL, 10.6 mmol) were added. The solution
was stirred for 15 min at room temperature. BTEAC (20
mg) added and the solution was cooled to 5˚C. To this
solution 10 mL dichloromethane containing terephthaloyl
chloride (1.076 g, 5.3 mmol) was added at once. The
solution vigorously stirred at 2000 rpm for 30 min and
polymeric solution was poured into hot water. Filtered
the solution and washed with methanol for several times.
Polymer obtained was dried under reduced pressure at

Synthesis and Characterization of Cholesteric Thermotropic Liquid Crystalline Polyesters Based on Isosorbide 1522
are reported in Table 2. All these cholesteric thrmeo-
tropic liquid crystalline polyesters were insoluble in com-
mon organic solvent like chloroform due to high rigidity
80˚C for 8 h.
3. Results and Discussion
Chiral polymers have been attracting much more interest
because of their chirooptical and stereospecific properties.
The synthesis of cholesteric liquid crystalline polyesters
was performed with sugar diol such as isosorbide an at-
tractive chiral building block. Several isotropic polyes-
ters of this cycloaliphatic diol were synthesized by other
research groups [11-15].
0˚C-5˚C
Synthesis of cholesteric thermotropic liquid crystalline
polyesters from phenyl hydroquinone, isosorbide and
terephthalic acid is depicted in Scheme 1 and composi-
tions used for the syntheses of copolyesters are listed in
Table 1. Polycondensation was performed by interfacial
polymerization method using BTEAC as a phase transfer
catalyst at 5˚C.
3.1. Solubility Scheme 1. Synthesis of cholesteric thermotropic liquid cry-
stalline polyesters based on phenyl hydroquinone, isosor-
bide and terephthaloyl chloride.
Solubility of cholesteric liquid crystalline polymers based
on phenyl-hydroquinone, isosorbide and terephthalic acid
Table 1. Compositions of copolyesters based on phenyl-hydroquinone, isosorbide and terephthaloyl chloride.
Polymer code ph-Hq:Iso:TPCl (Compositions) ph-Hq (mmol)Isosorbide (mmol)TPCl (mmol) NaOH (mmol) BTEAC (mg)
NNC-1 100:00:100 5.300 - 5.300 10.600 20
NNC-2 85:15:100 4.505 0.795 5.300 10.600 20
NNC-3 70:30:100 3.710 1.590 5.300 10.600 20
NNC-4 50:50:100 2.650 2.650 5.300 10.600 20
NNC-5 35:65:100 1.855 3.445 5.300 10.600 20
NNC-6 20:80:100 1.060 4.240 5.300 10.600 20
NNC-7 00:100:100 - 5.300 5.300 10.600 20
ph-Hq = phenyl-Hydroquinone; Iso = Isosorbide; TPCl = Terephthaloyl chloride; BTEAC = Benzyl triethyl ammonium chloride; NaOH = Sodium hydroxide.
Table 2. Solubility of liquid crystalline homo/copolymers based on phenyl-hydroquinone, isosorbide and terephthaloyl chlo-
ride.
Solvent
Polymer code
CHCl3 DMAc DMSO DMF NMP
NNC-1 – – – – –
NNC-2 – – – + +
NNC-3 – + – + +
NNC-4 – + + + +
NNC-5 – + + + +
NNC-6 – + + + +
NNC-7 – + + + +
+
= soluble at room temperature; – = insoluble at room temperature.

Synthesis and Characterization of Cholesteric Thermotropic Liquid Crystalline Polyesters Based on Isosorbide1523
of the structure.
Solubility of cholesteric thermotropic liquid crystalline
copolyesters was tested in aprotic solvents like dimethyl
acetamide (DMAC), dimethyl sulphoxide (DMSO), di-
methyl formamide (DMF) and N-methyl-2-pyrrolidone
(NMP). Solubility increases with isosorbide content [5,
16]. Homopolymer based on phenyl hydroquinone and
terephthalic acid is insoluble in chloroform as well in
aprotic solvents like DMAC, DMSO, DMF and NMP. It
clearly indicates that introduction of phenyl ring on hy-
droquinone does not improve solubility in aprotic sol-
vents also. Homopolymer of isosorbide and terephthalic
acid is also insoluble in chloroform. It dictates that iso-
sorbide contain cycloaliphatic ring which acts as rigid
moiety. Incorporation of terephthalic acid increases ridi-
gity due to para-para orientation and reduces solubility of
homopolyestser also.
3.2. Thermal Properties
The thermo gravimetric analyses (TGA) of cholesteric
liquid crystalline copolymers based on phenyl-hydro-
quinone, isosorbide and terephthaloyl chloride are sum-
marized in Table 3. Thermo gravimetric analyses (TGA)
of cholesteric liquid crystalline copolyesters were deter-
mined in nitrogen atmosphere using 10˚C/min heating
rate. Higher the molar fraction of phenyl hydroquinone
stabilizes cholesteric liquid crystalline phase and rises the
isotropization temperature above 300˚C. TGA thermo-
gram of NNC-1 showed 5 wt% loss at 350˚C and 10 wt%
loss at 395˚C. DTG curve of the same polyester showed
no decomposition up to 375˚C and maximum decompo-
sition was at 509˚C. TGA thermogram of NNC-2 showed
maximum wt. loss at 536˚C, but DTG peak was broad.
The Tmax temperature was on higher side due to the
broadness of DTG peak. All copolyesters showed two
stage degradation. It clearly indicates that liquid crystal-
line phases are formed. Homopolyester based on isosor-
bide and terepathalic acid showed first stage degradation
above 300˚C due to degradation of ether linkages of iso-
sorbide and second stage degradation above 400˚C due to
degradation of polymeric unit.
Thermal stability decreases with isosorbide content in-
crease. It is due to two ether linkages are present in isosor-
bide unit. Bridging group like ether degrade faster than
aromatic ring due to bond energy of C=C (145 kcal/mol) is
higher than C-O linkage (78 kcal/mol). The thermal stabil-
ity of all copolyesters was more than 300˚C on the basis of
10% wt. loss. Even though 10 wt%. loss of homopolyester
based on isosorbide and terephthalic acid (NNC-7) was at
299˚C. It clearly indicates that homopolyester based on
isosorbide and terephthalic acid is also highly thermally
stable. Maximum decomposition temperature (Tmax) de-
creases with isosorbide content increase.
3.3. Inherent Viscosity
Inherent viscosities of liquid crystalline copolyesters
were determined in dimethyl acetamide (DMAC) at 30˚C
using 0.5% concentration in Ubbelhode viscometer.
Polyester NNC-1 and NNC-2 are insoluble in DMAC.
Inherent viscosities of liquid crystalline copolyesters
were in the range of 0.11 to 0.15 dL/g. It indicates that
molecular weights of copolyesters were not high. It may
be due to interfacial polycondensation method is not
suitable for the polymerization of isosorbide with acid
chloride in the presence of strong base like sodium hy-
droxide. During interfacial polycondensation, isosorbide
containing ether linkages may not be stable and lead to
imbalance the stoichiometry of diols to acid chloride
ratio. Polycondensation of diols with acid chlorides may
be carried out with weak base like pyridine, and solution
polycondensation may be carried out at high temperature
using toluene or o-dichlorobenzene as solvent [5].
Table 3. Physical properties of liquid crystalline homo/copolymers based on phenyl-hydroquinone, isosorbide and tere-
phthaloyl chloride.
Polymer code T5 (˚C) T10 (˚C) Tmax (˚C) ηinha (dL/g) Crystallinity (%)
NNC-1 350 395 509 b 31.17
NNC-2 301 330 536 (broad peak) b 37.81
NNC-3 286 362 514 0.15 28.37
NNC-4 303 326 545 (broad peak) 0.12 38.75
NNC-5 299 321 512 (broad peak) 0.13 33.71
NNC-6 344 369 464 0.12 33.44
NNC-7 272 299 418 0.11 37.62
T
5 = 5 wt% loss, T10 = 10% weight loss; Tmax = maximum decomposition; a = inherent viscosity determined in DMAC at 30˚C; b = insoluble in DMAC.

Synthesis and Characterization of Cholesteric Thermotropic Liquid Crystalline Polyesters Based on Isosorbide 1524
3.4. 1H NMR Spectrum
1H NMR spectrum of isosorbide was carried out in CDCl3
and is presented in Figure 1. The protons of isosorbide
are observed in 3.50 - 4.75 δ ppm range and hydroxyl
protons are observed at 2.45 and 5.58 δ ppm. 1H NMR
spectrum of cholesteric thermotropic liquid crystalline co-
polyester NNC-4 in deuterated DMSO based on phenyl
hydroquinone:isosorbide:terephthaloyl chloride (50:50:100)
is presented in Figure 2. It showed that the incorporation
of isosorbide protons which are observed at 4.0 - 5.5 δ
ppm range. The position of hydroxyl groups also shifted. It
clearly indicates that the copolymer formation. The inten-
sity of isosorbide peaks was low because inherent viscosi-
ties of these copolyesters were low.
3.5. Optical Microscopy
Higher concentrations of phenyl hydroquinone like 85% or
100% did not melt up to 350˚C on polarizing microscope,
hence photomicrographs were not performed. The most
interesting textures of cholesteric phase observed for co-
polymer NNC-5 and NNC-6 and are presented in Figures
3-10. It clearly showed “yellow iridescent oily streaks”
having a background of mosaic texture which is a special
characteristic of cholesteric liquid crystalline phase.
Optical textures of copolyester NNC-5 at 245, 273,
and 297˚C were depicted in Figures 3-5 respectively. It
showed “yellow iridescent streak” with background of
mosaic texture, which is a typical texture of cholesteric
phase. Optical texture of polymer NNC-5 at 297˚C de-
picted in Figure 5. It showed “yellow broad and bright
oily streak” with background of mosaic texture, which is
also a typical texture of cholesteric phase. Copolyesters
NNC-5 showed broad range of cholesteric liquid crystal-
line phase i.e. 245˚C - 297˚C. Optical texture of copoly-
mer NNC-5 at 315˚C was illustrated in Figure 6 and it
showed “yellow broad blackish oily streak” with back-
ground of mosaic texture. It clearly indicates the initia-
tion of degradation of cholesteric phase.
Optical textures of copolyester NNC-6 at 174˚C,
184˚C, 198˚C, and 231˚C are depicted in Figures 7-10.
Optical texture of copolyester NNC-6 at 174˚C showed
“small yellow oily streak” having a background of mar-
ble texture indicates the initiation of formation of cho-
lesteric phase. Optical textures of copolyester NNC-6 at
δ ppm
Figure 1. 1H NMR spectrum of Isosorbide.

Synthesis and Characterization of Cholesteric Thermotropic Liquid Crystalline Polyesters Based on Isosorbide1525
Figure 2. 1H NMR spectrum of copolyester NNC-4 [ph-HQ:Isosorbide:TPC (50:50:100)].
Figure 3. Optical texture of copolyester NNC-5 at 245˚C.
184˚C and 198˚C showed “broad yellow oily streaks”
with background of mosaic texture. It indicated that the
broad range of cholesteric liquid crystalline phase i.e.
174˚C - 231˚C. Optical texture of polymer NNC-6 at
231˚C is depicted in Figure 10. It showed blackish “yel-
low iridescent oily streaks” with background of marble
texture, which is a typical character of initiation of deg-
radation. Homopolyester of isosorbibe with terephthalic
Figure 4. Optical texture of copolyester NNC-5 at 273˚C.
acid did not show cholesteric liquid crystalline texture
due to depression of the liquid crystalline orientation
order. All the optical photomicrographs were performed
on heating cycle of optical microscope using magnifica-
tion 50×.
3.6. Crystallinity
Crystallinity percentage of cholesteric liquid crystalline

Synthesis and Characterization of Cholesteric Thermotropic Liquid Crystalline Polyesters Based on Isosorbide 1526
Figure 5. Optical texture of copolyester NNC-5 at 297˚C.
Figure 6. Optical texture of copolyester NNC-5 at 315˚C.
Figure 7. Optical texture of copolyester NNC-6 at 174˚C.
copolyesters was in the range was 31.17 to 38.75. It
shows that copolyesters are semicrystalline. The WAXD
patterns did not show definite trend.
Figure 8. Optical texture of copolyester NNC-6 at 184˚C.
Figure 9. Optical texture of copolyester NNC-6 at 198˚C.
Figure 10. Optical texture of copolyester NNC-6 at 231˚C.
4. Conclusions
The present study allows the conclusion that the choles-

Synthesis and Characterization of Cholesteric Thermotropic Liquid Crystalline Polyesters Based on Isosorbide 1527
teric thermotropic liquid crystalline polyesters can be
synthesized from isosorbide, phenyl hydroquinone and
terephthalic acid. Isosorbide is an optically active and
inexpensive cycloaliphatic diol. Due to its cycloaliphatic
nature it helps to increase the performance cholesteric
thermotropic liquid crystalline polyesters. Homopolyes-
ter of isosorbide and terephthalic acid is thermally stable
up to 300˚C. Copolyesters based on isosorbide, phenyl
hydroquinone and terephthalic acid showed thermal sta-
bility more than 300˚C on the basis of 10 wt% loss. Op-
tical microscopy study reveals that more than 50% of
isosorbide content with phenyl hydroquinone and tere-
phthalic acid showed “yellow iridescent oily streaks”
with a background of mosaic/marble texture. These are
the typical textures of cholesteric liquid crystalline phase.
Interfacial polymerization method is not suitable to get
high molecular weight polyesters, because isosorbide
containing two ether linkages may not be stable with
strong inorganic base like sodium hydroxide lead to im-
balance the stoichiometry of diols to acid chloride ratio.
High performance cholesteric liquid crystalline polyes-
ters with high molecular weight will be synthesized from
same monomers by solution polycondensation method
using weak organic base.
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