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Enhanced Thermal and Mechanical Properties of Polyimide/GrapheneComposites
Wen Dai, Jinhong Yu*, Yi Wang,Yingze Song, Hua Bai, Kazuhito Nishimura,Huiwei Liao*, and Nan Jiang*
Macromol. Res., 22, (2014)
The incorporation of graphene sheets increased the thermal conductivity, thermal sta-bility and mechanical properties of polyimide matrix.
Macromolecular Research
The Polymer Society of Korea
www.springer.com/13233
pISSN 1598-5032 eISSN 2092-7673
Enhanced Thermal and Mechanical Properties of Polyimide/Graphene
Composites
Wen Dai1,2, Jinhong Yu*,2,3, Yi Wang2, Yingze Song2, Hua Bai2, Kazuhito Nishimura2,
Huiwei Liao*,1, and Nan Jiang*,2
1School of Materials Science and Engineering, Southwest University of Science and Technology,
Mianyang, 621010, P.R. China2Key Laboratory of Marine New Materials and Related Technology, Zhejiang Key Laboratory of Marine
Materials and Protection Technology, Ningbo Institute of Material Technology & Engineering,
Chinese Academy of Sciences, Ningbo, 315201, P.R. China3Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, Ministry of Education,
School of Material Science and Engineering, Guilin University of Technology, Guilin 541004, China
Received January 20, 2014; Revised April 7, 2014; Accepted June 6, 2014
Abstract: Polyimide (PI)/graphene sheets (GSs) composites were prepared by solution blending. The incorporation
of GSs increased the thermal conductivity, thermal stability and mechanical properties of PI. The thermal conductivity
of PI/GSs composites was significantly improved compared with that of neat PI from 0.254 W/mK to 1.002 W/mK
attribute to the homogeneous dispersion of graphene and the formation of heat conduction pathway. Furthermore,
the Young modulus of PI/GSs was raised up to 4.04 GPa, approximately a two-fold enhancement in comparison with
that of neat PI. In addition, the incorportation of GSs in PI indicated excellent optical transparency at the lowest
weight fractions of GSs and modified wettability of PI films.
Keywords: polyimide, graphene, composites, thermal properties, mechanical properties.
Introduction
Aromatic polyimides (PI) are widely used in microelec-
tronics and aerospace industries as a material for electronic
packaging and electrical insulating due to their superior
mechanical properties, high thermal and chemical stabilities
and low dielectric constant.1-4 However, the PI polymers are
thermally insulating and have a thermal conductivity of only
about 0.2 W/mK,5 which cannot meet the heat-dissipating
requirements of modern electronic and electrical systems.
Therefore, improvement of the thermal conductivity of polymer
becomes important. In recent years, what has been paid much
attention is that various of fillers including alumina (Al2O3),6 alu-
minum nitride (AlN),7 boron nitride (BN),8 carbon nanotube
(CNT)9 and graphene sheets (GSs),10 with high intrinsic thermal
conductivity, are used as filler to strengthen the thermal conductiv-
ity of polyimides. Among these fillers, graphene have received
great attention due to their exceptional thermal, mechanical
and electrical properties.11-14
Graphene is a monatomic thick quasi two dimensional
nanostructure consisting of sp2 hybridized six membered carbon
rings with micrometer sized horizontal dimension.15 Prossessing
extremely high aspect ratio and high thermal conductivity
(5300 W/mK),16 graphene is considered to be the most potential
filler to enhance the thermal conductivity of polymer com-
posites. However, it is difficult to obtain composites with
excellent thermal properties and mechanical due to the poor
dispersion and weak interfacial interactions of the polymer/
GSs.4,11,17 Though much work have focused on how to improve
the thermal and mechanical properties of the composites by
changing the interface properties of the composites,18,19 it is
still one of the hardest challenges in polymer composite design
to achieve the desired performance.
Hence, we report a rapid and simple method to prepare
polymer/GSs composites with excellent thermal and mechanical
properties. The GSs is mixed with polyamic acid by using
speedmixer (DAC 150.1 FVZ-K FlackTek, Inc., Germany),
which is a machine applicable to mix viscous liquid. High-
speed blend coul achieve good dispersion. To the best of our
knowledge, it is the first time to report to prepare polyimide/
GSs composite by using speedmixer. In this paper, uniform
polyimide-based composite composites containing GSs were
successfully synthesized by the speedmixer. Meanwhile, we
focus on the examination of the relationship between structure
and properties of the polyimide/GSs composites. In particu-
DOI 10.1007/s13233-014-
*Corresponding Author. E-mail: yujinhong@nimte or
W. Dai et al.
Macromol. Res.
lar, we correlate the role of GSs leading to the thermal and
mechanical properties of the polyimide matrix, the mor-
phology and macroscopic response of the composites with
respect to their tensile properites, thermal conductivity, ther-
mal stability, transparency and wettability properties.
Experimental
Materials. Commercial graphene sheets (GSs) were produced
by Ningbo Institute of Materials Technology and Engineering
(Zhejiang, China). Poly(amic acid) (PAA) synthesized by pyro-
mellitic dianhydride (PMDA) and 4,4-oxidianiline (ODA) were
purchased from Ningbo Cen electrical material co., Ltd
(Zhejiang, China). N,N′-dimethlylacetamide (DMAc) were
purchased Sinopharm Chemical Reagent co., Ltd, China.
Praparation of GSs/Polyimide Composites by Speedmixer.
A solution method was adopted to prepare the PI/GSs com-
posites. Firstly, GSs powder was dissolved in DMAc (1 mg/mL)
and dispersed by applying the ultrasonic wave for 48 h at
room temperature. At the same time, poly(amic acid) (PAA)
was dissolved in DMAc at room temperature. Then, the GSs
was added to a solution of PAA/DMAc, and the resulting
mixture was put into speedmixer to stirred for 4 min at 3200
rpm to obtain homodisperse mixture. The SpeedMixer is the
ideal laboratory-sized instrument for the rapid mixing and
deaeration of a wide range of materials. It quickly and effi-
ciently creates highly uniform mixtures with repeatable results.
Then the prepared mixture was casted on a clean glass sub-
strate followed by thermally imidization in a vacuum oven
at 80 oC for 2 h and 120 oC, 150 oC, 200 oC, 250 oC, 300 oC
for 1 h, respectively. The experiment details of the process
of PI/GSs composites are shown in Scheme I. Eight sets of
films were prepared with 0, 0.1, 0.5, 1.0, 2.0, 4.0, 7.0, and
11.0 wt% of GSs loading and were named as PI, PI-0.1, PI-0.5,
PI-1.0, PI-2.0, PI-4.0, and PI-11.0, respectively.
Characterization. X-ray photoelectron spectroscopy (XPS)
was carried out with a Kratos AXIS ULTR DLD spectrome-
ter, using Al Kα excitation radiation (hv=1253.6 eV). Fourier-
transform infrared (FTIR) was carried out with a NICOLET
6700 (Thermal scientific Inc. USA) instrument over the range
of 4000-400 cm-1. The Raman spectra was recorded using a
Reflex Raman System (RENISHAW plc, Wotton-under-Edge,
UK) employing a laser wavelength of 532 nm. Atomic force
microscope (AFM) measurement was conducted on a Mul-
timode SPM from Digital Instruments with NanoscopeIa
controller. The fracture surfaces of the samples were inves-
tigated using field emission scanning electron microscopy
(FE-SEM, QUANTA FEG250, USA) at an accelerating voltage
of 20 kV, and the fracture surfaces were previously coated with
a conductive layer of gold. Thermal conductivities of the
composites were measured with LFA 457 Nanoflash (NETZSCH,
Germany) according to ASTM E1461, using the measured
heat capacity and thermal diffusivity, with separately entered den-
sity data. Samples were prepared in square shape of 10.0×10.0
mm and 0.1 mm in thickness. Thermogravimetric analyses
(TGA) were performed with a TA 50 (TA Instruments, USA)
under air gas flow at a heating rate of 3 oC/min from room
temperature to 800 oC. The tensile strength was examined on
an electron omnipotence tester of universal testing machine
(UTM, 5567A, Instron, USA). The five samples in each group
with lengths of 10 cm and widths of 10 mm were prepared
with the speed of the crosshead was 1 mm/min according to
National Standard of China (GB1040-92). Ultraviolet-visible
absorption (UV-Vis) spectra were measured by a Perkin Elmer
Lambda 750S spectrometer. Sessile drop contact-angle mea-
surements were taken at room temperature with an OCA 20 opti-
cal contact-angle instrument (Dataphysics GmbH, Filderstadt,
Germany).
Result and Discussion
Characterization of GSs. The morphology of GSs was
investigated by SEM, TEM, and AFM analysis as shown in
Figure 1. Figure 1(a) shows that the SEM image exhibited
visual appearance of GSs, displaying a variety of different
size of it. The TEM and selected area electrical diffraction
(SAED) patterns of GSs revealed the at least two-layer structure
of GSs, and between each layer structure are independent of
each other, from the analysis of the two sets of diffraction
spots emerged in SAED pattern, as shown in Figure 1(b).
The morphology and thickness of GSs were further charac-
terized by AFM and the micrographs were shown in Figure
1(c). It can be clearly seen that GSs are well exfoliated, cor-
responding to a particle size range from 0.1-4 μm and the
average thickness around 1 nm. The thickness of GSs was higher
than theoretical value (0.34 nm), which was corresponded to
the multilayered structure of GSs from TEM analysis. Although
there is no unambiguous model describing the precise chemical
structure of GSs, it is known that the basal planes of GSs areScheme I. Preparation process of the PI/GSs composites.
Enhanced Thermal and Mechanical Properties of Polyimide/Graphene Composites
Macromol. Res.
decorated with epoxide and hydroxyl groups, in addition to
carbonyl and carboxyl groups. These oxygen functionalities
will alter the van der Waals interactions between the layers
and make them hydrophilic, thus facilitating their exfoliation in
aqueous media and polar solvents.20 The length (L) of 178
pieces GSs were measured by analysing AFM images. The length
data is presented as a histogram in Figure 1(d). The GSs lengths
range from 100 nm to 4 µm with a mean of <L>=1.91 µm.
Raman spectroscopy is a efficient tool to characterize car-
bon-based materials to understand the behavior of electrons
and phonons in graphene.21 Figure 2(a) shows the Raman
spectroscopy of GSs which exhibits a weak band, the D-band
nearby 1350 cm-1 arising from a defect in the carbon struc-
ture,22,23 and two strong bands: the G-band at around 1580 cm-1
that was attribute to the first order scattering of the E2g phonon
of sp2 carbon, the 2D-band at around 2750 cm-1. Therefore
the intensity ratio from the D-band to the G-band (ID/IG) provides
standard to measure disordered degree of graphene,24 which
is corresponded to the proportion of sp3-hybirdized carbon
atoms in sp2 conjugated carbon structures. The value of ID/
IG (only 0.08) demonstrates there is litter defect of this GSs
and it possess a high intrinsic thermal conductivity. 2D-band
originate from two phonon participating in resonance Raman
processes with contrary momentum.25 The researchers found
that the intensity ratio from the 2D-band to the G-band (I2D/
IG) associated with the layer of GSs. If I2D/IG>2, the graphene is
monolayer.26 The smaller the value of I2D/IG, the more lay-
ers. In this investigation, I2D/IG=0.57. It revealed the multi-
layered structure of GSs agree with the above TEM analysis.
The FTIR spectrogram were used to investigate the GSs.
From Figure 2(b), the adsorption bands appeard at 1728 cm-1
and 1651 cm-1 can be assigned to the C=O stretching vibration
of carboxyl and C=C stretching vibration of aromatic ring.
In addition, the peak at 1080 cm-1 and 1380 cm-1 are attributed
to the stretching vibrations of C-O and C-OH,31 respectively.
Furthermore, the broad band from 3400 to 3500 cm-1 can be
originated -OH stretching vibration of hydroxyl groups. The
results shows that the GSs possess some oxygen-containing
functional group. In order to further understand the atomic
percentages and the group distribution, the GSs was ana-
lyzed by X-ray photoelectron spectroscopy (XPS). The XPS
survey spectra of GSs was showed in Figure 2(c), which
exhibited three peaks assigned to C1s (95.77%), O1s (2.56%)
and N1s (1.67%). The deconvolution of the C1s peaks was
shown in the Figure 2(d). It revealed peaks at 284.58,
284.99, 286.28, 287.10, and 287.80 eV correspond to C=C10
(61.32%), C-C10 (26.77%), C-O-H32 (10.29%), C-O-C (0.16%),
and C=O(1.46%), respectively. The percentage of conjugated
carbon ring structure containing C=C and C-C is 88.09%.
The FTIR spectra of neat PI in different temperature were
investigated to monitor the imidization process. As shown
in Figure 3, the decrease of characteristic peaks in the PAA
spectra including -NH2 at 1650 cm-1 and N-H or O-H in the
range 2500-3500 cm-1 is observed with a rise of temperature.
Simultaneously, the intensities of characteristic imide peak
at 725 cm-1 (cyclic C=O bending), 1380 cm-1 (C-N stretching),
1720 cm-1 and 1780 cm-1 (symmetric and asymmetric C=O
stretching) enhance with increasing temperature. The char-
acteristic imide peak at 1380 cm-1 (C-N stretching) was chosen
to estimate the degree of imidization at different tempera-
tures of PAA. The intensity of an internal standard at 1500 cm-1
(C=C stretching in benzene ring) was selected to eliminate
the effect of test condition. The ratio of intensity at 1380 cm-1 to
that at 1500 cm-1 was calculated at various temperatures. It
was observed that rate at 300 oC and 350 oC almost equally.
This is a direct evidence to prove the fully curing of PI.
The Morphology of PI Composites. In order to understand
the relationship between structure and properties of neat PI
and PI composites, the fracture microstructure of neat PI and PI
Figure 1. (a) SEM, (b) TEM, and (c) AFM images of GSs, (d)
histograms of measured values for 178 nanosheets length.
Figure 2. The Raman spectra (a), FTIR spectra (b) and (c,d) XPS
for the GSs.
W. Dai et al.
Macromol. Res.
composites was observed by SEM, as shown in Figure 4.
The Figure 4(a) exhibit that the fracture surface of neat PI is
quite smooth with only a few dots in it, which may be attributed to
not be elastically relapsed to smooth plane after stretching
out upon mechanical deformation. The morphology and dis-
persion of GSs in PI matrix was evaluated by SEM, as shown
in Figure 4(b-h). Compared to the neat PI, the fracture surfaces of
PI composites is rough and ridged, owing to the two dimensional
geometry of GSs sheets. It is clear that the GSs were present
in the PI composites as multi-layered platelets. According to
Figure 4(b-h), the GSs platelets are isolated by PI matrix at
a lower concentration (less than 1 wt%) ,interconnected with
each other at a middle concentration (2-4 wt%) and a higher
concentration (more than 7 wt%). In addition, the GSs are curved
or even interwoven in the PI matrix, indicating that the GSs
are extremely flexible. It was obvious noted from Figure
3((b)-(d)) that the PI with a low GSs loading (less than 1 wt%)
form sea-island structure, and the PI is continuous phase.
Under this kind of structure, the fracture surface was ductile
fracture and some GSs appeared in the root of the fracture.
Figure 4((e),(f)) show less bare polyimide and the fracture
surface is more like brittle fracture with the increase of GSs
loading result in the continuity of PI broken gradually. It can
be also seen from Figure 4((g)-(h)) that the GSs platelets are
dispersed well even in PI-11.0 and some large platelet agglomer-
ates are observed. Meanwhile, it is hard to see polyimide
matrix in cross section because the GSs become a continuous
phase forming cross-linked network. The good dispersion of
GSs platelets in PI matrix should be attributed to the functional
groups on the reduced GSs, which could enhance the inter-
action between GSs and PI matrix. The oxygen functionalities
could form hydrogen bonding between the hydrogen atom
in GSs with nitrogen atoms in PI matrix, and also between
the oxygen atoms in GSs with hydrogen atom in PI matrix,
thus facilitating their dispersion in the composites. Especially, it
is worth noting that highest GSs loading (11 wt%) in Figure 4(h)
show more GSs origenting along with film surface direction
because of stereo-hindrance effect.
Thermal properties of PI Composites. The experimen-
tally determined thermal conductivities, displayed as a func-
tion of weight fraction are shown in Figure 5. It can be seen
that the addition of GSs is very effective in increasing thermal
conductivity of PI matrix. A significant enhanced thermal
conductivity is observed at high GS loading levels. When
the concentration of GSs reaches 11.0 wt% the thermal con-
ductivity of the composite sample achieves 1.002 W/mK,
which is about 4 times increase when compared with the
neat PI (0.254 W/mK) attribute to the homogeneous disper-
sion of graphene and the formation of heat conduction path-
way. It is known that phonons are responsible for thermal
conduction in amorphous polymers,27,28 The free path of phonon
Figure 3. FTIR spectra of neat PI in different temperature.
Figure 4. SEM micrographs of fractured surface of (a) PI-0, (b)
PI-0.1, (c) PI-0.5, (d) PI-1.0, (e) PI-2.0, (f) PI-4.0, (g) PI-7.0, and
(h) PI-11.0.
Enhanced Thermal and Mechanical Properties of Polyimide/Graphene Composites
Macromol. Res.
of polymer is tiny result in a lower thermal diffusivity due to
the amorphous structure of the polymer materials. Figure 5
shows that the PI-0.1 could improve thermal diffusivity by
47.6% in comparison with the neat PI. The improvement of
thermal conductivity may GSs acted as the nucleus of PAA
crystallization lead to improve the crystallization degree of
PI. The enhancement of the crystallization degree increased
the phonon free path result in enhancing the thermal diffu-
sivity. As known the factors which have influences on the
thermal conductivity of the composite are very complicated.
The contributions of the thermal conductivity enhancement
of a composite include the thermal conductivity of materials
in the composite and the interaction between them. GSs filled
in continuous PI matrix could not form a covalent bond lead
to phonon scattering serious. Thus, the interface thermal resis-
tance of composites increase with the increase of contact
area between GSs and PI. The enhancement ratios of the
thermal conductivity of the composite are much higher than
the value predicted by the Maxwell-Eucken model,29,30 which
may be attributed to the extremely high intrinsic thermal
conductivity of GSs.
Thermal Stability of PI Composites. As a sort of high-
performance engineering plastics, thermal stability is one
important property for PI-based nanocomposites. As reported
in other literature, the thermal stability of PIs are usually
improved by incorporating into inorganic filler.33 Figure 6
illustrates the typical TGA curves of PI and PI composites.
It is noted that the thermal stability (Td-10%, the 10% decom-
position temperature) of PI was only improved slightly up to
4.0 wt% GSs loading and then decreased with the increase of
GSs content. The hydroxyl grafted the GSs may act as cata-
lyst or inducement to promote PI decomposition which was
a predominant in a high GSs loading. For neat PI, the Td-10% is
541.3 oC. The maximum Td-10% value achieved 548.2 oC
with the incorporation of 4.0 wt% GSs loading, correspond-
ing to the increases by 6.9 oC. This result can be ascribed
the following reason: (1) efficient thermal capacity of the
GSs, (2) the wrinkle structure of GSs which may inset the
polymer matrix restricting the relaxation of segments, (3) the
carbon surface of GSs in the PI might play the role of a rad-
ical scavenger to delay the thermal degradation temperature
and result in improving thermal stability of PI.34,35 However,
the Td-10% value of PI-11.0 decrease to 539.5 oC due to the
heavy agglomeration of GSs in PI matrix, which can not
affect the performance of PI with the decline of less than
2 oC. In a word, the thermal stability of PI was not signifi-
cant change with incorporation of GSs. Hence, this rapid
preparation method is a practicable way to prepare excellent
thermal conductivity of composite materials. In addition,
our results also showed that the weight of residue of the PI
composites after pyrolysis is irregular when the GSs content
is higher that 1.0 %, indicating that GSs might facilitate the
flux of the degradation products.
Mechanical Properties of PI Composites. Representa-
tive tensile stress-strain curves and tensile properties of the
neat PI and PI composites for 0.1, 0.5, 1.0, 2.0, 4.0, 7.0, and
11.0 wt% of GSs additives are plotted in Figure 7 and Figure 8.
The tensile specimens are shown in the inset of Figure 7.
From Figure 7, the tensile strength and Young’s modulus of
PI-0.5 increased by 5.8% and 15% in comparison with the
neat PI, respectively. It is attributed to form sea-island struc-
ture of PI composites. PI matrix was the continuous phase,
and the GSs disperse homogeneously in PI matrix, as shown
in Figure 4(b). It is the direct evidence to prove that GSs can
prevent the fracture of PI attributing to the efficient load
transfer from PI to GSs. In addition, GSs can also occur slippage
to dissipate stress to increase the elongation at break. In a
word, the enhancement effect was due largely to good dis-
persion and effective stress transfer. From Figure 8, when
the addition 11.0 wt%, the maximum modulus value is achieved,
up to 4.04 GPa, which is 2 times increase when compared
with the neat PI (2.01 GPa). Meanwhile, the tensile strength
of the PI composites increased with the increase of GSs content
up to 0.5 wt% then decreased with the increase of GSs con-
tent. The maximum value of tensile strength of the PI com-
posites achieved 98.5 MPa with the incorporation of 0.5 wt%.
Figure 5. Thermal diffusivity and thermal conductivity of neat
PI and PI composites with various contents. Figure 6. TGA curves of the neat PI and PI composites.
W. Dai et al.
Macromol. Res.
Furthermore, the elongation of the PI composites also increased
with the increase of GSs content up to 0.1 wt% then decreased
with the increase of GSs content. The maximum value of
elongation of the PI composites achieved 28.9% with the
incorporation of 0.1 wt% GSs. However, the tensile strength
and elongation of the PI composites decreased with incorpo-
ration of over 0.5 and 1.0 wt%, respectively. It is attributed to
the large agglomerations and cluster of GSs.
Other Properties of PI Composites. Figure 9 shows the
transmittance spectra of neat PI and PI composites with thick-
ness in the wavelength range of 200-800 nm. It can be seen
that the decrease in the transmittance is obvious with increasing
GSs loading, which may be due to too many black GSs in
the composite film. As expected, lower GSs loading composite
films are more optically transparent, with those GSs loading
at 0, 0.1, 0.5, 1.0, 2.0, and 4.0 wt% displaying the optical
transmittances of 76.1%, 63.5%, 11.8%, 4.2%, 3.0%, and 0%,
respectively, at a wavelength of 550 nm. The PI composites
with incorporation of over 4.0% GSs content lose the optical
transparency. The inset of Figure 9 shows the optical images of
neat PI and PI composites with the rectangular of 1 cm×10
cm and the thickness of around 40 μm. It indicates excellent
optical transparency at the lowest weight fractions of GSs.
The hydrophilicity or hydrophobicity is the natural property
of materials. PI film normally exhibits hydrophobicity, whereas
GSs exhibites hydrophilicity due to hydrophilic functional
groups, such as carboxyl groups. In comparison with neat PI
film, the PI composites become more hydrophilic. Figure 10
illustrates the typical appearances of water drop on the PI
film and PI composites. It is obvious that the contact angle
is gradually decrease with those GSs loading increasing.
The hydrophilic behavior could been ascribed as follows: 1)
From the IR spectrum and XPS C1s peaks analysis, it can
be know there are some oxygen-containing groups in the
GSs sheets, which had a certain hydrophilicity. 2) According to
another paper, hydrophilic groups (hydroxyl and epoxy) gener-
ally located in the basal plane of the GSs. One can see from
Figure 3(g) that the higher content of GSs filling had greater
probability to origent along with film surface direction, as a
result of the large aspect ratio of GSs 2D-structure and the
reduced probability of space in perpendicular to the film
surface direction.
Conclusion
PI/GSs composites were prepared by solution blending.
The incorporation of GSs increased the thermal conductiv-
Figure 7. Representative tensile stress-strain curves of neat PI
and PI composites.
Figure 8. Tensile strength, modulus, and elongation at break of
neat PI and PI composites.
Figure 9. Transmittance spectra of neat PI and PI/GSs composite
films. The inset is optical images of neat PI and PI/GSs compos-
ite films.
Enhanced Thermal and Mechanical Properties of Polyimide/Graphene Composites
Macromol. Res.
ity, thermal stability and mechanical properties of PI. The
thermal conductivity of PI/GSs composites were significantly
improved compared with that of neat PI from 0.254 W/mK
to 1.002 W/mK attribute to the homogeneous dispersion of
GSs and the formation of heat conduction pathway. Further-
more, the Young modulus of PI/GSs was raised up to 4.04 GPa,
approximately a 100% enhancement in comparison with that
of neat PI. In addition, the incorportation of GSs in PI indicated
excellent optical transparency at the lowest weight fractions
of GSs and modified wettability of PI films.
Acknowledgments. The authors gratefully acknowledge
the financial support by National Natural Science Foundation
of China (51303034), the Opening Funding of Guangxi Key
Laboratory for Advance Materials and New Preparation Tech-
nology (12KF-8), Innovation Team of Guangxi Universities’
Talent Highland and Guangxi Funds for Specially-appointed
Expert.
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Figure 10. Photographs of a water drop on the film of neat PI
and PI composites.
