Properties of thermotropic liquid crystalline polyester nanocomposites derived from multi-walled carbon nanotubes

Article abstract of DOI:10.1166/jnn.2011.4135

A series of nanocomposites containing multi-walled carbon nanotubes (MWNTs) and terephthaloyldi- p-oxybenzoyl chloride-triethylene glycol copolyester (T-T) were prepared by in-situ hightemperature solution polycondensation. The liquid crystalline properties and thermal stability of the resulted MWNTs/T-T nanocomposites were investigated by wide-angle X-ray diffraction (WAXD), polarized optical microscopy (POM), differential scanning calorimetry (DSC), and thermogravimetric analyzer (TGA), respectively. The results showed that MWNT doping at a low concentration improved the thermal stability of T-T matrix and expanded the nematic temperature range without largely changing its conformation, making it more suitable for processing. The dispersion of MWNTs and interfacial interactions between the thermotropic liquid crystalline polymer (TLCP) molecules and the carbon nanotubes (CNTs) were investigated by scanning electron microscopy (SEM) and optical spectroscopies. The UV-vis spectroscopy and fluorescence spectra supported a strong π-π* interaction between the polymer and the nanotube. Copyright

Full text of DOI:10.1166/jnn.2011.4135

Copyright © 2011 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 11, 5018–5023, 2011
Properties of Thermotropic Liquid Crystalline
Polyester Nanocomposites Derived from
Multi-Walled Carbon Nanotubes
Wen-Feng Zhao, Ji-Xin Liu, Zhen-Kai Cui, Wu-Quan Hu, Jian Jiang, Shi-Jun Da∗, and Rong Li∗
College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, P. R. China
A series of nanocomposites containing multi-walled carbon nanotubes (MWNTs) and terephthaloyl-
di-p-oxybenzoyl chloride-triethylene glycol copolyester (T–T) were prepared by in-situ high-
temperature solution polycondensation. The liquid crystalline properties and thermal stability of the
resulted MWNTs/T–T nanocomposites were investigated by wide-angle X-ray diffraction (WAXD),
polarized optical microscopy (POM), differential scanning calorimetry (DSC), and thermogravimet-
ric analyzer (TGA), respectively. The results showed that MWNT doping at a low concentration
improved the thermal stability of T–T matrix and expanded the nematic temperature range with-
out largely changing its conformation, making it more suitable for processing. The dispersion of
MWNTs and interfacial interactions between the thermotropic liquid crystalline polymer (TLCP)
molecules and the carbon nanotubes (CNTs) were investigated by scanning electron microscopy
(SEM) and optical spectroscopies. The UV-vis spectroscopy and fluorescence spectra supported a
strong ꢀ−ꢀ∗ interaction between the polymer and the nanotube.
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Keywords: MWNTs/T–T Nanocomposites, In-Situ Polymerization, Liquid Crystalline Properties,
Copyright: American Scientific Publishers
Thermal Stability, Interactions.
1. INTRODUCTION
The discovery of carbon nanotubes9 has attracted a
great deal of scientific interest because of their excel-
lent electrical, mechanical, thermal and magnetic prop-
erties, such as low density, high surface area and high
chemical stability. CNTs have been envisioned as promis-
ing materials for potential applications in many fields.10ꢀ11
Intensive research has focused on CNTs composite mate-
rials with enhanced mechanical,12 electrical13 and thermal
properties,14 which suggests that CNTs are regarded as
effective reinforcements in the polymer composites15 due
to the combination of their unique properties with high
aspect ratio16 and the efficient load transfer from the host
matrix to the tubes.17 Recently, much work has been done
in preparing polymer/MWCNTs nanocomposites.18–20
Because of their special physical and chemical prop-
erties of nanometer materials (e.g., quantum size effect,
small size effect, surface and interface effect),21 Poly-
mer nanocomposites have become the object of intensive
research during the last decade. TLCPs with rigid-rod-
like molecules have the long range orientation order and
can be preferentially orientated to form fibrils under elon-
gational or shear flow during melt processing, termed as
the Freedericksz transition.22 While carbon nanotubes are
exceptionally anisometric particles with diameters in the
During the last several decades, a new branch of polymer
science, i.e., the synthesis and study of thermotropic liquid
crystalline polymers (TLCPs), appeared and has received
intensive studies.1–3 TLCPs could form a thermally sta-
ble nematic mesophase due to their excellent mechani-
cal properties, such as superior chemical resistance, low
controllable coefficient of thermal expansion, low gas and
liquid permeability, excellent dimensional stability under
high temperature or other adverse environments, and easy
to process with high precision by extrusion and injection
molding.4–7 Therefore, TLCPs have received considerable
attention both in the neat state and as reinforcing fillers
of thermoplastic polymers. TLCPs have also been used
to displace conventional thermoplastic polymers and to
develop commercial TLCPs and their composites such as
high performance engineering plastics or fibers.8 Among
these, main chains TLCPs have become useful structural
materials, either as high precision mouldings, or as high
performance fibres.
∗Authors to whom correspondence should be addressed.
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doi:10.1166/jnn.2011.4135

Zhao et al. Properties of Thermotropic Liquid Crystalline Polyester Nanocomposites Derived from Multi-Walled Carbon Nanotubes
order of nanometers, their lengths range from microns
to centimeters.23 Furthermore, it was expected that the
aromatic structure of the TLCP could interact strongly
with the graphene sheet of the nanotube surface through
intermolecular overlap of ꢁ-orbitals (ꢁ-stacking).24 So
it is very interesting for carbon nanotubes in combina-
tion with liquid crystals to form a new research area.25
Recently, Kim and coworkers20 demonstrated that the
thermal, mechanical and rheological properties of aro-
matic polyester PEN/carbon nanotube nanocomposites
were strongly dependent on the homogeneous dispersion
of CNTs and the interactions between CNTs and PEN,
which could be enhanced by slight chemical modification
of CNTs. Kim et al.26 also reported that a very small
quantity of CNTs could remarkably improve thermal sta-
bility and mechanical properties of the TLCP nanocompos-
ites. Jyongsik Jang et al.27 reported that surface oxidation
of CNTs could improve the mechanical durability and
thermal stability of LCE-CNT comoposites. The self-
organizing properties of anisotropic, calamitic nematic liq-
uid crystals have been exploited to align a minute number
of CNTs.28 Evidence of MWNT alignment was found in
the studied polyazomethine/MWNT composites.29 Wern-
Shiarng Jou et al.30 reported that the electromagnetic
shielding effectiveness of CNTs/LCPs was higher than that
of CNTs/melamine formaldehydes (MF) composites. Sim-
ilarly, electromagnetic interference shielding effectiveness
of carbon nanofiber/liquid crystal polymer (LCP) compos-
ites was also discussed.31
4-Hydroxybenzoic acid, terephthaloyl dichloride and tri-
ethylene glycol were used as received Diphenylether and
thionyl chloride were AR grade and used as supplied. The
TLCP and TLCP/MWNTs nanocomposite were synthe-
sized as follows.
2.2. Preparation of TLCP and the Nanocomposites
The terephthaloyl-di-p-oxybenzoyl chloride (TOBC) was
synthesized according to Bilibin et al.33 A solution of
TOBC (0.885 g, 2 mmol) and triethylene glycol (0.300 g,
2 mmol) in diphenylether (20 mL) was purged with nitro-
ꢀ
gen and then stirred at 180 C for 10 h under a slow but
steady nitrogen flow. After being cooled to the room tem-
perature, the mixture was poured into large methanol to
precipitate the polymer. The white powdery polymer was
filtered and washed carefully with acetone and water. The
ꢀ
final product was dried at 80 C in a vacuum oven. The
steps are shown in Scheme 1. IR (TOBC): 1778.14 cm−1
(COCl), 1738.05 cm−1 (COOR), 1597.69 cm−1
,
1409.46 cm−1 (C C), 1266.41 cm−1 (C–O); IR (T–T):
2891.16 cm−1 (–CH–), 1731.28 cm−1 (–COO), 1271.34
cm−1 (–O–C C), 1603.30 cm−1 (–C C).
The T–T/MWNTs nanocomposites were synthesized
by the following steps: the MWNTs were added into
the solution of triethylene glycol and terephthaloyl-di-p-
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oxybenzoyl chloride in diphenylether, followed by super-
IP: 113.208.64.211 On: Mon, 12 Oct 2015 12:27:09
sonic treatment for 2 h. The subsequent procedure was
Copyright: American Scientific Publishers
the same as the T–T preparation. In order to investigate
the effect of different MWNTs content on the properties
of T–T/MWNTs nanocomposites, we prepared a series of
nanocomposites with the CNT content ranging from 0.1 to
5.0 wt%. For simplicity, the composites were referred to
as 0.1, 0.3, 1.0, 5.0 wt% and so on.
In the present work, efforts were made to study ther-
mal properties and liquid crystalline behaviors of the
TLCP/MWNTs nanocomposites. We first prepared the
TLCP nanocomposites with various concentrations of
MWNTs by in-situ polymerization and then investigated
the interactions between MWNTs and host matrix, as well
as the profound effects of MWNTs on the nanocomposites
properties.
2.3. Characterization
The Fourier transform infrared spectrum (FT-IR) of TLCP
was recorded from 400 to 4000 cm−1 to identify the struc-
ture of synthesized samples using a Nicolet Nexus 670 FT-
IR from the Thermo Nicolet Inc, USA. Wide-angle X-ray
diffraction measurements were performed at room tem-
perature on a Rigaku (D/Max-IIIB) X-ray Diffractome-
ter using Ni-filter Cu-Kꢂ radiation. The scanning was
2. EXPERIMENTAL DETAILS
2.1. Materials
MWNTs (diameters: 20–40 nm, purity: 95–98%) prepared
by the catalytic decomposition of CH4 were provided by
Shenzhen Nanotech Port Co. Ltd. (China). In a typical
experiment, raw MWNTs (1 g) were added to a mix-
ture (160 ml) of concentrated sulfuric acid and nitric acid
ꢀ
(3:1 volume ratio) and sonicated in a water bath at 50 C
for 12 h to remove mineral and catalyst additives and to
increase some –COOH or –OH groups on the CNTs sur-
face, which may decrease the ꢁ−ꢁ stacking effect among
the aromatic rings of the nanotubes, leading to the forma-
tion of their agglomeration.32 After filtration, the remain-
ing black power was washed with deionized water until
the pH value reached neutral.
Scheme 1. Preparation steps and structure of T–T.
J. Nanosci. Nanotechnol. 11, 5018–5023, 2011
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Properties of Thermotropic Liquid Crystalline Polyester Nanocomposites Derived from Multi-Walled Carbon Nanotubes Zhao et al.
8ꢀ/min over a range of 2ꢃ = 2–70ꢀ. The morphology of
the samples was investigated using a JSM-6701 Scanning
Electron Microscopy (JEOL, Japan). The thermal and ther-
mogravimetric analyses of TLCP/MWNTs were carried
out under N2 atmosphere on Netzsch STA 449C equip-
host. Fortunately, the mesophase structure of T–T in the
nanocomposite might seems not significantly affected.
3.2. Morphological Characterization
ꢀ
SEM micrographs in Figure 2 clearly show that the
MWNT loading can significantly affect the morphology
of the nanocomposites.29 It is evident that nanotubes are
aligned along the LC director at a low concentration and
fiberlike nanostructures are formed in the T–T/MWNTs
composites (Fig. 2(a)), demonstrating that liquid crys-
tals could induce the CNT alignment. However, with
the increase of MWNT content, the pull-apart shape of
MWNTs appears (Fig. 2(b)). Figure 2(c) shows that at high
MWNTs content,35 the degree of polymer structural orga-
nization lost and disruption of the orientation and aggre-
gation of the MWNTs appeared.
ment. The samples were heated at 10 C/min. A polarized
optical microscope (XS-402P) equipped with a hot stage
was used to examꢀine the liquid crystalline behavior, at a
heating rate of 10 C /min The characterization of interac-
tions between CNTs and LCPs was performed on a Perkin
Elmer Lamda 35 UV-VIS Spectroscopy and a Hitachi
F-4500 Fluorescence Spectrophotometer.
3. RESULTS AND DISCUSSION
3.1. X-Ray Diffraction
Figure 1 shows the representative WAXD patterns of pure
T–T and the T–T/MWNTs nanocomposites in the region
of 2ꢃ = 2–70ꢀ. The sharp diffraction peak is indicative of
the presence of crystallites in the copolyester. From the
WAXD plots, it can be seen that there is a sharp diffraction
peak around 2ꢃ = 20ꢀ, while a little more diffuse diffrac-
tion peak appears around 2ꢃ = 23ꢄ5ꢀ for the neat TLCP,
demonstrating that T–T is a semicrystalline TLCP. This
can be further evidenced from the DSC thermograms. With
3.3. Thermal Behaviors
The TGA curves for neat T–T and the prepared
T–T/MWNTs nanocomposites are presented in Figure 3.
The two phases in the thermal decomposition curves are
attributed to the fact that the main chain of the polymer is
composed of flexible diol segments and thermally stable
rigid aromatic units.36 The diol segments decomposed first,
followed by the heat resistant aromatic chain. The weight
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the increase of MWNT concentration from 0.1 wt% to
IP: 113.208.64.211 On: Mon,lo1s2sOdcute2t0o1t5he12d:e2c7o:m09position of TLCP and its hybrids
5.0 wt%, the strong reflections observed for pure T–T were
Copyright: American Scientific Publishers
(0.1–5.0 wt% MWNTs) wasꢀnearly the same until the tem-
also seen for T–T/MWNTs nanocomposites. Compared to
neat T–T, the T–T/MWNTs nanocomposites showed vir-
tually featureless XRD patterns. This observation seems
to suggest the presence of strong attractive interactions
between T–T matrix and MWNTs, as well as the highly
dispersion of MWNTs in the nanocomposites.34 However,
the crystalline peaks become weaker with the increase
of MWNT contents, demonstrating that the introduction
of MWNT caused a reduction in crystallinity of TLCP
perature reached about 390 C. After this point, the initial
degradation temperature was influenced by MWNT load-
ing in hybrids. When the content of MWNTs increased
from 0 wt% to 1 wt%, the initiꢀal degradation temperature
ꢀ
increased from 399 C to 403 C, correspondingly. From
Figure 3, it seems that the degradation rate of pure T–T
was faster than its nanocomoisites (0.1–1.0 wt% MWNTs).
In this work, an extreme case (with 5.0 wt% MWNTs)
was also created to investigate the negative effects of
excessive content of MWNTs. Though the incorporation
of MWNTs into TLCP increased the thermal stability and
the residual weight for T–T/MWNTs nanocomposites, the
initial degradation temperature of 5.0 wt% MWNTs/T–T
decreased sharply. This may be due to the poor MWNT
dispersion in the TLCP matrix and the CNTs segrega-
tion and reagglomeration when the composite was melted,
which could have caused more deterioration on the crys-
tallinity of the TLCP. In the meantime, the induced LC
defects actually decreased the thermal stability. Another
possible mechanism was that the presence of MWNTs
reduced crosslinking of the matrix chain.37 The results
demonstrate that the presence of MWNTs could help
stabilize the T–T matrix at low concentrations, thereby
enhancing the thermal stability of TLCP nanocompos-
ites. The introduced CNT could induce protective barri-
ers against thermal decomposition and retard the thermal
decomposition of T–T nanocomposites, as a result of the
Fig. 1. WAXD of the pure TLCP and representative TLCP/MWNTs
nanocomposites.
5020
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Zhao et al. Properties of Thermotropic Liquid Crystalline Polyester Nanocomposites Derived from Multi-Walled Carbon Nanotubes
Fig. 2. SEM images of (a) 0.3 wt.% MWNTs/T–T, (b) 1.0 wt.% MWNTs/T–T and (c) 5.0 wt.% MWNTs/T–T.
effective function of CNTs as physical barriers to hinder
the transport of volatile decomposed products out of TLCP
nanocomposites during thermal decomposition.38ꢀ39
3.4. Liquid Crystalline Properties
Figure 5 shows the representative POM micrographs of
T–T and T–T/MWNTs nanocomposites during heating
from the solid state. Polarized optical microscopy stud-
ies revealed the presence of a nematic mesophase, typi-
cal birefringence of T–T (Fig. 5(a)). When the content of
MWNTs was 0.3 wt%, we could not see the CNT agglom-
eration, demonstrating that MWNTs were dispersed very
well in the hybrid (Fig. 5(b)). From Figures 5(b and c),
it could be seen that MWNTs were aligned along the
vector of liquid crystal phase. However, the agglomera-
tion of MWNTs began to appear for the 1.0 wt% sample
(Fig. 5(c)). Some disorientation of the nematic domains
was also observed, due to the many LC defects induced by
the nanotube aggregates.35 The liquid crystallinity of the
nanocomposites was not obvious when the MWNT content
was up to 5.0 wt%. Here we only chose a local glimpse for
5.0 wt% sample to illuminate the interfacial interactions
of the nanocomposites. It was observed that at the temper-
ature of the phase transition the birefringence texture was
still stable in the proximity of large bundles of nanotubes
(Fig. 5(d)), indicating the persistence of the nematic phase
in these regions. In other areas, the phase turned black
and thus became isotropic. However, when the temperature
was decreased, the first signs of birefringence appeared
The DSC traces of T–T and T–T/MWNTs nanocompos-
ites are shown in Figure 4 and some parameters are sum-
marized in Table I. Neat T–T is a semicrystalline TLCP
ꢀ
having a melting temperature (Tmꢅ of ca. 173.1 C and a
ꢀ
clearing (isotropization) temperature (Tiꢅ of ca. 385.3 C.
It could be seen that incorporation of MWNTs into T–T
matrix had little effect on Tm, while Ti of the nanocompos-
ites was obviously increased with MWNT concentration.
This is probably because rigid aromatic units of T–T could
interact strongly with the CNTs, mainly through ꢁ−ꢁ
conjugation, restricting the movement of rigid mesogenic
units.40 Thus Ti of T–T/MWNTs is logically enhanced.
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However, there is no ꢁ-stacking between CNTs and the
Copyright: American Scientific Publishers
flexible diol spacers, and hydrogen bond interaction is
the most possible one.26 Due to the very few carboxylic
acid groups or hydroxyl groups on the nanotube surface,
the interactions across the liquid crystal-carbon nanotube
interface are too weak to limit the diol segment motion.
Finally, no significant change in the melting point of the
T–T phase was detected in the hybrids. The melting and
isotropization associated enthalpy changes (ꢆHm, ꢆHiꢅ
were also given (Table I), which decreased as MWNT con-
tent increased.
Fig. 3. TGA thermograms of the pure TLCP and TLCP/MWNTs
Fig. 4. DSC thermograms of the pure TLCP and TLCP/MWNTs
nanocomposites.
nanocomposites.
J. Nanosci. Nanotechnol. 11, 5018–5023, 2011
5021

Properties of Thermotropic Liquid Crystalline Polyester Nanocomposites Derived from Multi-Walled Carbon Nanotubes Zhao et al.
Table I. DSC results of the pure TLCP and TLCP/MWNTs
nanocomposites.
Sample (wt%)
TmꢇꢀC)
Ti (ꢀC)
ꢆHm (J/g)
ꢆHi (J/g)
0.0
0.1
0.3
1.0
5.0
173.1
174.6
175.1
175.3
177.2
385.3
410.9
412.3
428.0
430.6
23.6
22.9
21.7
21.2
19.3
117ꢄ2
63ꢄ3
57ꢄ2
35ꢄ1
25ꢄ6
again around the nanotube bundles. These observations
confirmed that the T–T molecules interact strongly with the
MWNTs surface, making the degree of order in the vicin-
ity of large-scale aggregates of MWNTs increased. The
CNT bundles could also be regarded as nucleation centres
for liquid-crystalline organization.24ꢀ41 The temperature
ranges of all nematic phases we observed during heating
were basically consistent with the DSC thermograms.
Fig. 6. Room-temperature solution-phase (C2H5OH) UV-visible spectra
(25 ꢈg/mL) and solid state fluorescence spectra (inset) (excitation wave-
length: 383 nm).
3.5. Optical Spectroscopies
for T–T, indicating that the ꢁ-conjugation of T–T is largely
unchanged.43
The strong fluorescence of T–T is efficiently quenched
in the nanocomposite (1.0 wt% MWNTs) by in-situ
polymerization (inset). The quenching likely arises from
efficient energy transfer between T–T and MWNTs,
The optical spectroscopies support a typical ꢁ−ꢁ inter-
action between T–T and the nanotubes (Fig. 6). Pris-
tine T–T shows a strong absorption band at around
208 nm, which is attributed to the ꢁ−ꢁ∗ transition. When
doped with MWNTs (1.0 wt%), the ꢁ−ꢁ∗ character-
istic peak of the T–T matrix in the nanocomposite are
suggesting the disruption of ꢁ-conjugation caused by a
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shifted to lower wavelength (203 nm) and a substantial
IP: 113.208.64.211 On: Mon,c1o2nfOorcmta2t0io1n5al1c2h:a2n7g:0e9was not a big issue. Energy trans-
decrease in absorbance occurs. This suggests that there are
Copyright: American Scientific Publishers
fer quenching between molecules and for molecules on
strong interactions between the aromatic rings of T–T and
MWNTs. The other possibility responsible for the blue
shift is that when the backbone conformation of poly-
mer was modified due to its interaction with a nanotube,42
the extent of ꢁ-conjugation decreased and the band gap
increased, similar to a blue shift. In 1.0 wt% MWNTs/T–T,
no significant shift was seen in the energy absorption band
graphitic carbon and metal surfaces was reported long
before.44 In this work, we prepared the reference sample
(mixing 5.0 wt% MWNTs with pristine T–T by simple
stirring) to remove the effect of MWNT itself and to make
the in-situ polymerization easier.
On the basis of the UV-vis and fluorescence results,
we come up with the following hypothesis: Owing to the
favorable in-situ polymerization, the T–T chain gets more
closer to the graphitized MWNT surface, which may pro-
mote more efficient ꢁ−ꢁ∗ interaction45 and such paral-
lel stacking and interaction are quite common in many
cases.42ꢀ46ꢀ47
4. CONCLUSIONS
A series of MWNTs/TLCP nanocomposites have been pre-
pared via in-situ polymerization. The crystal structure of
T–T matrix was not greatly perturbed by the incorporation
of MWNTs. On the basis of POM and SEM studies, we
found that MWNTs could be well dispersed and aligned
along the vector of the liquid crystal phase at low content.
At the same time, the MWNT aggregates probably acted
as the nucleation centres for liquid-crystalline organization
and could stablilize the liquid crystal phase. The enhanced
thermal properties were also observed from TGA and DSC
results. In the end, optical spectroscopies were used to
Fig. 5. POM images of (a) pure T–T at nematic phase, (b) 0.3 wt.%
MWNTs/T–T at nematic phase, (c) 1.0 wt.% MWNTs/T–T, and
(d) 5.0 wt% MWNTs/T–T at the transition from nematic to the isotropic
phase. Large bundles were chosen in order to easily investigate the tex-
ture of T–T around them.
5022
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Zhao et al. Properties of Thermotropic Liquid Crystalline Polyester Nanocomposites Derived from Multi-Walled Carbon Nanotubes
characterize the interfacial interaction between T–T and
the MWNTs.
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Received: 23 June 2010. Accepted: 9 November 2010.
J. Nanosci. Nanotechnol. 11, 5018–5023, 2011
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