Synthesis and characterization of liquid crystalline polyester/graphene and a study of their properties

Article abstract of DOI:10.1166/jnn.2012.5777

A new rod-like thermotropic liquid crystalline polyester (TLCP) material and its nanocomposites based on different concentrations of graphene were synthesized by in-situ high-temperature solution polymerization. The resulting nanocomposites were characterized using XRD, microscopic analysis (POM, SEM, and TEM), spectroscopic analysis (FT-IR, UV-Vis, and fluorescence), and thermal analysis (TGA and DSC). The XRD and POM methods showed that the composite materials exhibited only the nematic phase. The TEM images revealed that the graphene were distributed in the polymer with sizes ranging from 100 to 200 nm. The absorption spectroscopy data showed that the electronic properties of graphene were mostly retained without damaging their two-dimensional electronic properties, together with the analysis of the maximum absorption spectrum and concentration of the composites in terms of the Lambert-Beer law. The fluorescence from the TLCP moiety was almost completely quenched and red shifted by graphene, indicating that the linkage mode facilitated effective energy and electron transfer between the rod-like TLCP and the extended .-system of graphene. Therefore, this novel nanocomposite material exhibits excellent thermal properties based on the thermogravimetric analysis. Copyright

Full text of DOI:10.1166/jnn.2012.5777

Copyright © 2012 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 12, 2477–2483, 2012
Synthesis and Characterization of Liquid Crystalline
Polyester/Graphene and a Study of Their Properties
Wu-Quan Hu, Zong-Zhen Zhu, Jun Jin, Zheng-Ping Dong, Xing Zhong,
∗
∗
Jing-Hui Zhou, Shi-Jun Da , and Rong Li
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R. China;
College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000, P. R. China
A new rod-like thermotropic liquid crystalline polyester (TLCP) material and its nanocomposites
based on different concentrations of graphene were synthesized by in-situ high-temperature solution
polymerization. The resulting nanocomposites were characterized using XRD, microscopic analy-
sis (POM, SEM, and TEM), spectroscopic analysis (FT-IR, UV-Vis, and fluorescence), and thermal
Delivered by Ingenta to:
analysis (TGA and DSC). The XRD and POM methods showed that the composite materials exhib-
Main CID is 80004805 (JPP)
ited only the nematic phase. The TEM images revealed that the graphene were distributed in the
IP : 93.80.83.126
polymer with sizes ranging from 100 to 200 nm. The absorption spectroscopy data showed that
Fri, 29 Jun 2012 14:37:14
the electronic properties of graphene were mostly retained without damaging their two-dimensional
electronic properties, together with the analysis of the maximum absorption spectrum and con-
centration of the composites in terms of the Lambert-Beer law. The fluorescence from the TLCP
moiety was almost completely quenched and red shifted by graphene, indicating that the linkage
mode facilitated effective energy and electron transfer between the rod-like TLCP and the extended
ꢀ
-system of graphene. Therefore, this novel nanocomposite material exhibits excellent thermal
properties based on the thermogravimetric analysis.
Keywords: Liquid Crystalline Polyester, Graphene,
ꢀ
Conjugation, Thermal Stability,
Conductivity.
1
. INTRODUCTION
crystal solid and isotropic liquid states.14 Thermotropic liq-
uid crystalline polyester (TLCP) has aroused great interest
Since the discovery of graphene by Geim et al. in
due to its properties as a high-performance engineering
1
2
004, interest regarding the fundamental properties of
15ꢀ16
material.
In 2003, Windle et al. first reported the
graphene has grown especially in recent years. Graphene
is a new two-dimensional material composed of a mono-
layer of carbon atoms. Furthermore, its consists of
sp carbon hexagonal networks in which strong cova-
lent bonds are formed between two adjacent carbon
atoms. Due to its large specific surface area, thermal
conductivity, and good electrical conductivity, this two-
dimensional material is becoming important in various
liquid crystalline behavior of an aqueous suspension of
acid-oxidized multi-walled carbon nanotubes (MWNTs)
and analyzed the phase diagram using the simple rigid-
2
2
17
rod steric theory. The liquid crystalline behavior of
carbon nanotubes has already been reported in several
3
4
18–22
systems.
However, studies of the covalent or non-
5
6ꢀ7
covalent incorporation of TLCP with graphene have been
23ꢀ24
limited.
Some simple and versatile non-covalent meth-
8
fields such as high-performance nanoelectronic devices,
∗
ods through supermolecular interactions such as ꢁ–ꢁ
stacking, van der Waals, electrostatic interaction, and
hydrogen bonding have the advantage of preserving the
unique electronic properties of graphene without seri-
ous damage. Recently, Novoselov et al. reported a
graphene-based liquid crystal device that has excellent
9
10
composite materials, biosensors, and energy storage and
batteries.1
1ꢀ12
These results indicate that graphene has great
potential in the fabrication of organic and inorganic materi-
als. People are just now starting to discover this interesting
research topic.13
Liquid crystal can be considered as a fourth state of
matter following solid, liquid, and gas. Therefore, liq-
uid crystal phases, as the name implies, exist between
25
23
24
performance. More recently, Behabtu et al. reported
how graphite spontaneously exfoliates into a single-layer
graphene in chlorosulphonic acid, dissolves at isotropic
concentrations, and spontaneously forms liquid crystalline
phases at high concentrations.
∗
Authors to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2012, Vol. 12, No. 3
1533-4880/2012/12/2477/007
doi:10.1166/jnn.2012.5777
2477

Synthesis and Characterization of Liquid Crystalline Polyester/Graphene and a Study of Their Properties
Hu et al.
In this paper, we report the use of graphene as an
excellent additive to polyester matrix. Great efforts have
been made to study the thermal, dissolution, and electri-
cal properties of the TLCP/graphene nanocomposites. Our
objective is to improve the physical properties of TLCP
nanocomposites by incorporating a very small quantity of
graphene.
thionyl chloride was removed under reduced pressure. It
was received the white solid (5.550 g, 94.0%) and dried
in a vacuum oven at 60 C for 24 h, then stored in a
ꢁ
desiccator with drying agent. In this study, neat TLCP,
TLCP with 0.1 wt.% to 1.0 wt.% of graphene were synthe-
sized according to the method of high-temperature solu-
tion polycondensation and in-situ polymerization. The neat
TLCP chemical formula was shown in Scheme 1 (Part I).
A typical procedure was as follow: A mixture consisting
2. EXPERIMENTAL DETAILS
ꢀ
of 4,4 -oxydibenzoyl chloride (0.590 g, 2 mmol), resorci-
2
.1. Materials and Preparation
nol (0.220 g, 2 mmol) was charged into a round-bottomed
three-neck flask, equipped with a spherical adaptor lead-
ing to a water-cooled condenser, a nitrogen gas inlet, a
heater, and a magnetic stirrer while a solution consisting
of diphenylether (15 ml) was added rapidly. The solu-
ꢀ
4
,4 -Oxybis (benzoic acid) (OBA, 98+%) were purchased
from A. Johnson Matthey Company. Resorcinol (99.5%,
ꢁ
m.p.: 110–180 C) was obtained from Tianjin Guangfu
Fine Chemical Research Institute, China. Diphenylether
ꢁ
(
99.5%, b.p.: 256–259 C) were purchased from Tianjin
ꢁ
tion was stirred at 180 C for 6 h and then poured into
Chemical Reagent Ltd. Co., China. We used commer-
cially available graphene sheets (Jiansin Scientific and Tra-
ꢁ
methanol at 0 C to coagulate the product. The precipi-
tated polyester was washed with methanol, acetone, and
ding Ltd. Co.) which were produced by the a Dr c e- dl ii sv c eh ra er gd eby Ingenta to:
method.26
water in sequence. Finally, the white powdery solid was
Main CID is 80004805 (JPP)
ꢁ
dried in a vacuum oven at 60 C for 24 h, and then stored
The synthesis of the polyester monomer wa sI Ps h: o 9w 3n .80.83.126
in Scheme 1 (Part I). The polyester acid Fc hr li o, r 2i d9 eJ wu na s2012 14:37:14
in a desiccator with drying agent. The yield was 87.0%.
−1
ꢀ
IR (KBr): aromatic ether C–O–C: 1127 and 1060 cm ,
prepared by refluxing 4,4 -Oxybis (benzoic acid) (5.165 g,
0 mmol) with excess thionyl chloride (20 ml) in the pre-
− −1
1
2
C
C: 1592, 1499 and 1442 cm , C O: 1739 cm ,
−
1
−1
sence of a few drop of dry DMF for 8 h. Then excess
C–O–C: 1009 cm , COO: 1238 cm .
Scheme 1. Preparation steps and schematic diagram.
2478
J. Nanosci. Nanotechnol. 12, 2477–2483, 2012

Hu et al.
Synthesis and Characterization of Liquid Crystalline Polyester/Graphene and a Study of Their Properties
The synthesis of nanocomposites based on 0.1 wt.%
defects. The intermolecular spacing can be estimated from
the radial position of the large angles using the equation
graphene was given as an example. The same method
ꢀ
14
was adopted for others. Monomers 4,4 -oxydibenzoyl chlo-
(2 · d · sin ꢃ = 1ꢄ117 · ꢅꢆ. In this paper, the representa-
ride (0.590 g, 2 mmol), resorcinol (0.220 g, 2 mmol)
and graphene with 0.1 wt.% were added in diphenylether
tive XRD curves are shown in Figure 1. The data showed
ꢁ
a diffraction peak at a 2ꢃ of about 20.5 , indicating an
(
15 ml), followed by supersonic treatment for 2 h. Then
average distance of 4.83 Å between two neighbor TLCP
molecules within the layers of the mesophase. The sharp
ꢁ
the mixture was heated to 180 C for 6 h under nitrogen
and anhydrous conditions in a three-necked flask, and then
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
diffraction angles at 2ꢃ = 12ꢄ7 , 15.2 , 16.5 , and 20.5 ,
ꢁ
poured into methanol at 0 C to coagulate the product.
and the amorphous diffuse angle at about 2ꢃ of 25.5
The precipitated polymer was washed with methanol, ace-
tone, and water in sequence for several times. The gray
corresponds to the characterization of the semi-crystalline
27–29
polymer.
Besides these, the intensity was increased
ꢁ
powdery solid was dried in a vacuum oven at 60 C for
with the increase of graphene. However, there was no
sharp peak at the low angle region. This result reveals that
TLCP and related nanocomposites exhibited only nematic
mesophases, which was consistent with their DSC curves
in later analysis. Furthermore, the positions of the diffrac-
tion peaks of TLCP and the nanocomposites were similar,
indicating that the introduction of graphene in TLCP did
not affect the nematic type polymer. As shown in Figure 1,
2
4 h, and stored in a desiccator with drying agent. The
yield was 85.6 wt.%. IR (KBr): aromatic ether C–O–C:
− −1
1
1
1
128 and 1059 cm , C C: 1593 and 1499 cm , C O:
−
1
−1
−1
740 cm , C–O–C: 1009 cm , COO: 1238 cm .
.5 wt.% graphene: The yield was 80.0 wt.%. IR (KBr):
0
−
1
aromatic ether C–O–C: 1128 and 1060 cm , C C: 1593,
−1
−1
1
1
499 and 1443 cm , C O: 1740 cm ,
C–O–C:
−
1
−1
009 cm , COO: 1239 cm .
Delivered by Ingenta to:
no graphene peaks were detected, presumably because the
1
.0 wt.% graphene: The yield was 84.0 w Mt . %a .i nI RC( IKDB ri )s : 80004805 (JPP)
content of graphene doping was very low and the doped
aromatic ether C–O–C: 1129 and 1060 cm , C CI P: 1 :5 99 33 ,.80.83.126
−1
graphene were uniformly distributed into the TLCP matrix.
−
1
−1
1
1
499 and 1446 cm , C O: 1741 cm ,F ri, C2 9– OJ –u Cn : 2012 14:37:14
1
−
−1
008 cm , COO: 1238 cm .
A typical procedure for preparing TLCP and nanocom-
3.2. Morphological Characterization
posites base on the graphene sheets is given in Scheme 1
Part I).
The POM and SEM micrographs are shown in
Figures 2(A) and (B). As shown in Figure 2(A), a birefrin-
gent texture can be observed at the phase transition tem-
(
2
.2. Characterization
ꢁ
perature in neat polyester (250 C), which illustrates that
The Fourier transform infrared spectrum (FT-IR) of TLCP
was recorded on a Nicolet Nexus 670 from 400 to
the synthesized polyester exhibits a nematic phase. Mean-
while, the SEM micrographs in Figure 2(B) clearly show
the features of the polyester surface. To investigate the
degree of dispersion of graphene in the polyester matrix,
as an example, Figures 2(a) to (c) shows the morphologies
of the samples with different concentrations of graphene
observed through TEM. Based on these images, the size
−
1
4
000 cm . X-ray diffraction (XRD) measurement was
performed at room temperature on a Rigaku (D/Max-
IIIB) X-ray diffractometer using Ni-filter Cu-Kꢂ radia-
ꢁ
ꢁ
tion. The scanning was 10 /min over a range of 10–50 ,
and the scanning type was continuous scan. Transmission
electron microscopy (TEM) measurements were taken on
a Hitachi-600 electron microscope, with an accelerating
voltage of 100 kV. The thermogravimetric analysis (TGA)
and differential scanning calorimetry (DSC) of TLCP and
nanocomposites were carried out under N atmosphere
2
ꢁ
over the temperature range of 25–800 C at a heating rate
of 10 C/min on TGA/DSC1 equipment (Mettler Toledo
ꢁ
Inc.) to investigate the thermal degradation and decom-
position. Fluorescence spectra were recorded in the form
of thin film on a Hitachi F-4500 fluorescence spectropho-
tometer. UV-vis spectra were recorded on a Lambda 35 in
ethanol solution over a range of 200–500 nm.
3. RESULTS AND DISCUSSION
3
.1. X-Ray Diffraction Analysis
The information obtainable from X-ray diffraction studies
includes the degree of crystallinity, crystal lattice, crys-
tal size, molecular orientation, and a variety of structural
Fig. 1. XRD patterns of TLCP and its nanocomposites based on
graphene from 0.1 to 1.0 wt.%.
J. Nanosci. Nanotechnol. 12, 2477–2483, 2012
2479

Synthesis and Characterization of Liquid Crystalline Polyester/Graphene and a Study of Their Properties
Hu et al.
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Fri, 29 Jun 2012 14:37:14
ꢁ
Fig. 2. The structure morphology of TLCP and its nanocomposites. (A) The POM of pure TLCP in 250 C. (B) The SEM image of TLCP and the
TEM of nanocomposites with different concentration of graphene (a) 0.1 wt.%, (b) 0.5 wt.%, (c) 1.0 wt.%.
of graphene was found to be 100–200 nm. Aside from
that, the TLCP/graphene nanocomposites with low densi-
ties (0.1 and 0.5 wt.%) were observed to have uniformly
distributed graphene in the composite matrix [Figs. 2(a)
and (b)]. However, the aggregation of graphene occurred
when the graphene content was increased to 1.0 wt.%
TLCP and its nanocomposites were characterized by TGA
and the curves are compared in Figure 3(b). At tempera-
ture of 5.0 wt.% mass loss increased after adding graphene
in the polyester. This could be explained that dispersed
graphene might hinder the flux of degradation product
and thereby delay the onset of degradation. The thermal
decomposition temperatures were the lowest for the result-
ing product of the polymerization of TLCP in the presence
of little graphene. The thermal decomposition temper-
atures of the TLCP/graphene nanocomposites increased
with increasing amounts of graphene in the experimen-
tal range. That is, the results show that the degradation
of pure TLCP was faster than that of its hybrids (0.1–
1.0 wt.% graphene). Accordingly, the thermal stability of
[
Fig. 2(c)], which is consistent because graphene eas-
ily forms agglomerates irreversibly due to their strong ꢁ
stacking tendency.3
0ꢀ31
3
.3. Thermal Behaviors
Both the curves of TGA and DSC detected thermal and
phase behaviors, including weight loss, glass transition,
and phase transitions. Figure 3(a) is a representative DSC
of samples that has a nematic phase above melting. As
shown in Fig. 3(a), the polyester has a melting transition
∗
the nanocomposites with the interaction of ꢁ–ꢁ conjuga-
tion between the graphene and the TLCP matrix are better
than the pure TLCP and those without this band. There-
fore, the interface properties had been improved with the
addition of a coupling agent.
ꢁ ꢁ
T ꢆ at 211 C and an isotropic transition (T ꢆ at 425 C, as
m i
(
shown by the jump in the heating curve. When doping with
different concentrations of graphene into the TLCP matrix
by the in-situ method, the Tm and T of nanocomposites
i
3
.4. Optical Spectroscopy
were shifted to higher temperatures. In addition, detailed
changes are shown in Table I. The reason for the increase
We also investigated the optical spectral properties of
TLCP and its nanocomposites. Figure 4 shows the UV-Vis
absorption spectra of the TLCP and nanocomposites in
the presence of different concentrations of graphene, and
∗
in T and T may be that the polyester with ꢁ–ꢁ conju-
m
i
gation near the graphene sheet might be degrading more
slowly.32 Subsequently, the thermal stability properties of
2480
J. Nanosci. Nanotechnol. 12, 2477–2483, 2012

Hu et al.
(a)
Synthesis and Characterization of Liquid Crystalline Polyester/Graphene and a Study of Their Properties
(a)
(
b)
(
b)
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Fig. 3. The curves of thermodynamics. (a) Differential scanning
calorimetry (DSC) and (b) Thermo gravimetric analysis (TGA).
Fig. 4. UV-Vis absorption and fluorescence emission spectra of the pure
TLCP and TLCP/graphene nanocomposites for 0.1 to 1.0 wt.%. For
UV-Vis: C = 0ꢄ02 mg/ml, Fluorescence: ꢅex = 295 nm.
samples were characterized by the absorption band of the
TLCP (202 nm) and graphene (279 nm). The absorption
band maximum (ꢅmax/nm) due to the ꢁ–ꢁ transition was
state.34 From the inserted figure in Figure 4(a), the flu-
orescence spectra of the TLCP and the nanocomposites
were considered under the same conditions in the form
of a thin film. Upon excitation at the maximal absorp-
tion wavelength, the pure TLCP showed strong fluores-
cence emission. However, the fluorescence emission of
the 1.0 wt.% graphene in TLCP was 15% that of the
pure TLCP, indicating that graphene efficiently quenched
the fluorescence of the TLCP moiety. This fluorescence
quenching has been attributed to the electron or energy
transfer from the fluorophore to the graphene sheet. Nev-
ertheless, the emission peak shifts to longer wavelengths,
from 365 to 380 nm, when the concentration of graphene is
increased. Namely, this emission peak appeared to the red
shift. The red shift proves a strong conjugate interaction
between the benzene rings of TLCP and graphene. Other-
wise, this longer emission wavelength also corresponds to
a higher overlap or closer distance of ꢁ-electrons between
∗
in the order 1.0 wt.% >0.5 wt.% >0.1 wt.% >TLCP,
which suggests that the sample with 1.0 wt.% had the
shortest conjugation length due to a twisted linkage at the
3
3
allene (C
C
C) moiety. Otherwise, the addition of
graphene to the matrix of TLCP caused not only a red
shift in the 202 nm band but also significant diversification
of the graphene absorption band at 279 nm. Furthermore,
the intensity of the absorption band increased with increas-
ing concentrations of graphene. This result indicates that
there was notable electronic communication between the
two ꢁ-systems of graphene and the TLCP in the ground
Table I. Thermal properties of TLCP/graphene nanocomposites.
ꢁ
ꢁ
Graphene content (wt.%)
0
Tm ꢇ C)
T ( C)
ꢈT
i
211
237
245
255
44
425
450
457
470
45
214
213
212
215
—
3
5–37
0
0
1
.1
.5
.0
the TLCP and graphene.
In order to make compari-
son, the content of 1.0 wt.% graphene treated in mechan-
ical mixing was employed. The direct mixing sample had
a hypsochromic shift to 201 nm shown in Figure 4(b).
Shifted (Tmax–Tmin
ꢆ
J. Nanosci. Nanotechnol. 12, 2477–2483, 2012
2481

Synthesis and Characterization of Liquid Crystalline Polyester/Graphene and a Study of Their Properties
a) (b)
Hu et al.
(
(
c)
(
d)
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Fig. 5. Absorption spectra of (a) to (d) represent TLCP and nanocomposites (0.1 to 0.06 mg/ml, respectively). Shown in the insets are the plots of
optical density at 200 nm versus concentration. The straight lines are a linear least-square fit to the data.
This phenomenon illustrated that the polyester wrapped
the graphene, which would cause a blue shift of TLCP’s
lowest energy absorption band because of interruption of
the ꢁ–conjugation. In the same way, the sample of direct
mixing was quenched slightly and blue shift (360 nm)
from the inserted in Figure 4(b).
and its composites in ethanol were measured, which are
shown in Figure 5. The curves from 5(a) to 5(d) repre-
sent the relationship of sample concentration and the max-
imum adsorption spectra. For example, for the first curve
in Figure 5, a continuously rising absorption was obtained
as the concentration increased. Furthermore, a linear rela-
tionship exists between the concentration and the maxi-
mum adsorption value (200 nm) (inset in Fig. 5(a) with an
R-value of 0.996). The results demonstrate that polyester
dispersions conform to the Lambert-Beer law at low con-
centrations. As shown in Figures 5(b) to (d), when the
graphene content was increased, the nanocomposite con-
centration and the adsorption value also exhibited a linear
3
8
Based on the UV-Vis and fluorescence results, we came
up with the following hypothesis: Due to the favorable
in-situ polymerization, the polyester chain gets closer to
the graphene surface, which may promote more efficient
∗
35
ꢁ
–ꢁ interaction, and such parallel stacking and inter-
3
9ꢀ40
action are quite common in many cases.
To investigate the dispersibility of the polymer and the
nanocomposites, ethanol was used as a solvent. The TLCP
and nanocomposites with 0.1 to 1.0 wt.% graphene exhib-
ited significant solubility in ethanol. The process includes
pretreatment with mild sonication for 15 min followed
by centrifugation at 3000 rpm for 10 min. There were
no obvious precipitates observed after a week, indicat-
ing that the dispersions were very stable. To further study
their solubility, the UV-Vis absorption spectra for polyester
4
1
relationship.
4. CONCLUSIONS
A series of liquid crystalline polyester based on differ-
ent concentrations of graphene nanocomposite materials
have been synthesized by in-situ high-temperature poly-
merization. The result from different analytical techniques
2482
J. Nanosci. Nanotechnol. 12, 2477–2483, 2012

Hu et al.
Synthesis and Characterization of Liquid Crystalline Polyester/Graphene and a Study of Their Properties
suggest that graphene, when added to the polyester matrix,
is uniformly distributed and it remarkably improves the
thermal stability of the nanocomposites. Improvement on
the physical properties of the nanocomposites may be
14. X. Wang and Q. Zhou, Liquid Crystal Polymer, World Scientific
Publishing Co. Pte. Ltd., London (2004).
1
1
5. J. Kim, S. Kang, and S. Kim, Fiber. Polym. 7, 358 (2006).
6. B. Volkan, A. Serpil, Y. Zekeriya, and A. Haydar, Polym. Int. 59, 749
(
2010).
∗
caused by the ꢁ–ꢁ conjugation between graphene and
17. S. Zhang and S. Kumar, Small 4, 1270 (2008).
the host matrix. Graphene, as a brilliant additive, played an
important role in enhancing the physical properties of the
polymer. Finally, we also provide a method for preparing
the TLCP/graphene nanomaterials and believe that these
graphene based TLCPs will be of great value in translating
the dispersive, electrical, thermal, and liquid properties of
nanocomposites in high performance fibers and structural
materials in future studies. Taken together, the nanocom-
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1
2
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2
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2
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Main CID is 80004805 (JPP)
2
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Received: 3 January 2011. Accepted: 19 May 2011.
J. Nanosci. Nanotechnol. 12, 2477–2483, 2012
2483