Research Article
www.acsami.org
Enhanced Electrical Networks of Stretchable Conductors with Small
Fraction of Carbon Nanotube/Graphene Hybrid Fillers
Jae Young Oh,† Gwang Hoon Jun,† Sunghwan Jin,§ Ho Jin Ryu,*,‡ and Soon Hyung Hong*,†
†
Department of Material Science and Engineering and ‡Department of Nuclear and Quantum Engineering, Korea Advanced Institute
of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea
§
IBS Center for Multidimensional Carbon Materials (CMCM), Ulsan National Institute of Science and Technology (UNIST), 291
Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea
S Supporting Information
*
ABSTRACT: Carbon nanotubes (CNTs) and graphene are
known to be good conductive fillers due to their favorable
electrical properties and high aspect ratios and have been
investigated for application as stretchable composite conductors. A stretchable conducting nanocomposite should have
a small fraction of conductive filler material to maintain
stretchability. Here we demonstrate enhanced electrical
networks of nanocomposites via the use of a CNT−graphene
hybrid system using a small mass fraction of conductive filler.
The CNT−graphene hybrid system exhibits synergistic effects
that prevent agglomeration of CNTs and graphene restacking and reduce contact resistance by formation of 1D(CNT)−
2D(graphene) interconnection. These effects resulted in nanocomposite materials formed of multiwalled carbon nanotubes
(MWCNTs), thermally reduced graphene (TRG), and polydimethylsiloxane (PDMS), which had a higher electrical conductivity
compared with MWCNT/PDMS or TRG/PDMS nanocomposites until specific fraction that is sufficient to form electrical
network among conductive fillers. These nanocomposite materials maintained their electrical conductivity when 60% strained.
KEYWORDS: carbon nanotubes, graphene, hybrid materials, nanocomposite, stretchable conductor
1. INTRODUCTION
Electrical devices are typically rigid, are often heavy, and can be
damaged by physical impact. For these reasons, there has been
much recent research interest in stretchable conductors, which
maintain their conductivity following physical deformation.1
These properties can enable electronic devices including
wearable devices,2 flexible organic light-emitting diodes
(OLEDs),3 stretchable energy storage devices,4 and portable
digital information devices.5 However, it is difficult to satisfy the
required mechanical and electrical properties simultaneously.6
Many previous studies have attempted to overcome the
limitations of stretchable conductors. The methods used in
these studies can be classified into two types.6 The first involves
structural development of inorganic conductive materials;7−9
the second involves mixing a conductive filler with an
elastomer.6,10,11 The former has limits in terms of the
anisotropic stretchable properties, and the fabrication process
required for flexible inorganic materials is typically complex.12
The latter provides isotropically stretchable materials, and the
fabrication processes are simpler. This method can be classified
into two categories: techniques that create patterned
stretchable conductors consisting of patterned conductive
material on a stretchable material,11,13,14 and techniques that
create composites of conductive fillers with a stretchable
matrix.10,15,16 Stretchable conductors formed of composites
typically have a greater variety of potential applications, as the
© 2016 American Chemical Society
stretchable electrodes are fully conductive, unlike patterned
stretchable conductors.11,13,14
Carbon-based nanomaterials, including carbon nanotubes
(CNTs) and graphene, have particularly promising properties
as conductive fillers due to the high electron mobility (∼10 000
cm2 V−1 s−1) and high aspect ratio (>1000).8,17 However, to
date, stretchable conductors formed of CNTs and graphene via
solution mixing processes have demonstrated poor electrical
performance.15 One of the main challenges in the solution
mixing of nanocomposites formed using CNTs and graphene is
preventing agglomeration or restacking. Most studies have
focused on improving the dispersibility of the conductive filler
via functionalization of the surface of the CNTs or of the
graphene. Chua et al. reported enhanced dispersibility via the
use of the diphenyl-carbinol and silanized diphenyl-carbinol
functionalization of multiwalled carbon nanotubes
(MWCNTs).15 However, nanocomposites consisting of
functionalized MWCNTs have a lower conductivity than
those formed of pristine MWCNTs due to degradation of the
electrical properties of MWCNTs following covalent functionalization. Noncovalent functionalization, however, maintains
the electrical properties of the conductive fillers. Hwang et al.
Received: November 19, 2015
Accepted: January 19, 2016
Published: January 19, 2016
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DOI: 10.1021/acsami.5b11205
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Figure 1. (a) Schematic illustration of interaction between CNTs and graphenes. (b) CNT/graphene/PDMS nanocomposites fabrication.
then filtered and washed using 10% HCl solution and deionized water.
The filtered GO was dried under a vacuum for 4−5 days.
The steps for the fabrication of TRG from GO were as follows.
First, GO was heated to 900 °C in an N2 atmosphere at a rate of 20
°C/min, which caused it to undergo volume expansion and thermal
reduction. The temperature was maintained at 900 °C for 1 h; during
the first 30 min, an N2 atmosphere was used, whereas an H2
atmosphere was used for the final 30 min, which induced the chemical
reduction of GO.
2.2. Fabrication of MWCNT/PDMS, TRG/PDMS, and MWCNT/
TRG/PDMS Nanocomposites. Nanocomposites formed of PDMS,
MWCNTs, and TRG were fabricated via solution mixing. MWCNT or
TRG was dispersed in tetrahydrofuran (THF) via sonication for 2 h.
The PDMS was dissolved in THF by stirring, and then the MWCNT
(or TRG) and the PDMS solutions were mixed. Solution mixing and
removal of the solvent proceeded via heat treatment at 70 °C with
stirring. To remove the residual solvent from the solution, an
additional heat treatment step was carried out at 90 °C for 3 h under a
vacuum. A curing agent was mixed with the solution containing
PDMS, MWCNT, or TRG with a PDMS/curing agent mass ratio of
10:1. Nanocomposites were also formed with MWCNT/TRG mass
ratios of 7:3 and 9:1. The mixtures were poured into dog-bone shaped
molds to fabricate samples for electrical conductivity and strain
measurements. The curing proceeded at 90 °C for 3 h.
2.3. Microstructural Characterization. The microstructures of
the MWCNT, TRG, and MWCNT−TRG samples were observed
using scanning electron microscopy (SEM) (Hitachi S-4800), atomic
force microscopy (AFM) (SPI 3800N, Seiko), and transmission
electron microscopy (TEM) (Tecnai TF30 ST). Samples were
prepared via solvent dispersion and spin-coated onto Si wafers to be
observed using SEM, and the samples for TEM characterization were
drop-dried onto copper grids. The fractured surfaces of the
nanocomposites were also observed using SEM following coating
with osmium.
2.4. Measurement of the BET Surface Area of the
Conductive Fillers. The BET surface area of the MWCNT, TRG,
and hybrid MWCNT−TRG (1:1) samples was characterized by
measuring N2 adsorption. Variations in surface area due to the
interaction between the MWCNTs and TRG were examined by
dispersing the samples in THF via sonication and heat-drying to
evaporate the solvent.
2.5. Electrical Conductivity Measurements. Electrical conductivity was measured using a four-point probe (Loresta-P MCP-
reported a lower percolation threshold via noncovalent
functionalization of MWCNTs using poly(3-hexylthiophene).16
A critical issue when fabricating such nanocomposites is the
electrical properties of the functionalizing materials. Most
functionalizing materials that result in enhanced dispersibility of
the conductive fillers in elastomers are insulators, so the contact
resistance between the individual conductive filler particles is
large. Accordingly, a key challenge when using CNTs and
graphene in stretchable conductors is identifying and
incorporating conductive functionalizing materials using a
noncovalent functionalization processes.
Hybrid CNT−graphene materials are bonded by π−π
interactions, which induce functionalization due to the
differences in geometry between the graphene and the CNTs.
The most important advantage of hybrid CNT−graphene
materials is that they offer functionalization without requiring
an additional insulating material. Several studies have reported
enhanced electrical properties through hybrid CNT−graphene
materials. Tang et al. reported that the electrical conductivity of
hybrid CNT−graphene films increased due to a decrease in the
contact resistance as well as the formation of an efficient
percolating network.18 However, no previous studies have
focused on fabricating stretchable conductors via solution
mixing of CNT−graphene hybrid fillers with an elastomer
matrix. Here we describe stretchable nanocomposites formed
by solution mixing of hybrid CNT−graphene conductive fillers
with polydimethylsiloxane (PDMS) and present the enhanced
electrical networks of the resulting materials.
2. EXPERIMENTAL SECTION
2.1. Fabrication of TRG. Thermally reduced graphene (TRG) has
a moderate electrical percolation threshold due to less restacking of
graphene sheets by its wrinkled structure and the high Brunauer−
Emmett−Teller (BET) surface area.19 TRG was fabricated via thermal
expansion and reduction of graphene oxide (GO), which was
fabricated using the modified Hummers method. Highly ordered
pyrolytic graphite (HOPG) was dissolved in H2SO4 and stirred in an
ice bath. To oxidize the HOPG, KMnO4 was slowly added to the
solution. After stirring at 35 °C for 2 h, H2O2 was added dropwise to
the solution, which was maintained in the ice bath. The solution was
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Figure 2. (a) Microstructure of MWCNTs by SEM (inset: photograph of MWCNTs). (b) Microstructure of TRG by SEM (inset: photograph of
TRG) and structural information on TRG by AFM. (c, d) Interaction of hybrid MWCNT/TRG (1:1). (e) BET surface area of MWCNT, TRG,
hybrid MWCNT/TRG.
T610), and strain was measured using a universal testing machine
(Instron 8848).
ratio (∼320) than that of the TRGs (∼150), and it means that
the MWCNTs have structural advantages to form electrical
network compared to the TRGs.23 However, high aspect ratio
of the MWCNTs induces agglomeration of the MWCNTs by
strong van der Waals forces, so a bundled structure was
typically observed. When these MWCNTs are used as
conductive fillers of nanocomposites, this agglomeration
degrades the electrical networks of the nanocomposites.24 To
reduce electrical networks degradation by agglomeration of
conductive fillers, TRGs were used as hybrid fillers in our
research. In the MWCNT/TRG nanocomposites, the TRG was
bonded onto the MWCNTs via π−π interactions, which
induced steric hindrance and prevented the bundling of
MWCNTs.21
The interactions between MWCNTs and TRGs were
observed using SEM and TEM analyses of the hybrid
MWCNT/TRG materials, as shown in Figure 2, panels c and
d. MWCNTs were located between the TRG sheets, which
inhibits restacking of the TRG sheets.
The BET surface areas of the MWCNTs, TRG, and hybrid
MWCNT/TRG (1:1) were characterized to confirm effects
through the interaction between MWCNTs and TRGs, as
shown in Figure 2, panel e. We found that the hybrid
MWCNT/TRG material had a large BET surface area of 481.3
m2/g, which was greater than that of the MWCNTs (165.8 m2/
g) or the TRGs (328.7 m2/g). The increase in the surface area
of the hybrid MWCNT/TRG nanocomposite is attributed to
the inhibition of restacking of TRG and agglomeration of
MWCNTs. Some researchers reported a relationship between
the increase of BET surface area of expanded graphite and the
enhancement of electrical conductivity of nanocomposites
comprising these graphites.25 In this research, effects of surface
area increase by hybrid MWCNT/TRG materials were
confirmed through various analyses.
Additionally, electrical contact between the constituent
particles of the fillers was enhanced by introducing the
combination of TRG and MWCNTs. Interconnections
between MWCNTs (a 1D material) and TRG (a 2D material)
in the hybrid MWCNT/TRG nanocomposite material yielded
3. RESULTS AND DISCUSSION
Figure 1, panel a presents schematic illustrations of the
interactions in the hybrid CNT−graphene system, and Figure
1, panel b shows the fabrication of the CNT/graphene/PDMS
nanocomposite through CNT/graphene interaction. CNT and
graphene exhibited strong cohesiveness because of the strong
van der Waals forces due to the large surface-area-to-mass
ratio.20 In single-component dispersions of CNTs or graphene,
this leads to agglomeration and restacking, respectively.
However, the coexistence of CNTs and graphene inhibits
these interactions due to increased steric hindrance via the π−π
interactions of CNTs and graphene sheets, and instead CNTs
are inserted between graphene sheets.21 This leads to an
enhanced electrical network of the conductive filler in the
matrix. Another effect of using the hybrid CNT−graphene
system is a reduction in the contact resistance. As shown in
Figure 1, panel a, when both CNTs and graphene are used as
fillers, 1D−2D interconnections are formed, with a large
contact area, thus decreasing the electrical resistivity.22 The
relationship between CNT/TRG mass ratio and the electrical
properties is not clear, however, and it is necessary to
investigate the optimal mass ratio to provide optimized
electrical properties of the stretchable conductors.
Figure 2, panel a presents SEM images of the MWCNTs, and
Figure 2, panel b presents SEM images and AFM images of the
TRGs. Samples were dissolved in dimethylformamide (DMF)
and spin-coated onto Si wafers and dried. MWCNT diameters
ranged from 15−20 nm, and lengths are around 5 μm; thus, it
has an aspect ratio of around 320. TRGs have wrinkled and
exfoliated structures due to thermal expansion. The SEM image
of Figure 2, panel b showed stacked TRGs. To confirm accurate
structure information on each the TRG, TRGs were analyzed
by AFM after solution exfoliation process by sonication and
spin coating them onto mica. TRGs have structures of around 4
nm of thickness, 600 nm of lateral size, and thus approximately
150 aspect ratio. The MWCNTs have relatively higher aspect
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than that of the TRG/PDMS nanocomposites at all filler
fractions.
The electrical conductivity graph of MWCNT/PDMS
nanocomposites is divided into two areas as shape of curve in
Figure 4, panel a. Area 1 exhibits rapid increase of the electrical
conductivity, while gradual increase of electrical conductivity is
observed in the area 2. This rapid increase of electrical
conductivity in the area 1 is due to weak electrical network by
small fraction of conductive filler.27 The two areas showed
different variation of electrical conductivity with addition of
TRG (see Figure 4b,c). In area 1, 0.4 wt %, 0.6 wt % of
MWCNT/TRG nanocomposites show increase of electrical
conductivity at some ratios MWCNTs to TRGs unlike the
decrease of electrical conductivity with addition of TRG in area
2. These results are different from general predictions. The
electrical conductivity (σ) of nanocomposites comprising
conductive fillers is predicted by scaling law of the following
formula:28
improved electrical contact compared with nanocomposites
formed using only MWCNTs. The inclusion of TRG provided
a large contact area with the MWCNTs and resulted in an
increase in the number of contact sites among the conductive
filler particles, and hence a low electrical resistivity.22
MWCNT/TRG/PDMS nanocomposites were fabricated
using these MWCNT/TRG hybrid fillers. To confirm
dispersion of MWCNT/TRG hybrid fillers and interaction
among these conductive fillers in PDMS matrix, same fractions
(0.4 wt %) of MWCNT/TRG/PDMS and MWCNT/PDMS
nanocomposites were fabricated, and then their microstructures
were observed (see Figure 3). MWCNT/TRG/PDMS nano-
σ = σ0 × (Φ − Φc)t
By this formula, the electrical conductivity of nanocomposites is proportional to that of conductive fillers (σ0).
As rule of mixture, electrical conductivities of MWCNT/TRG
hybrid fillers are lower than single MWCNT fillers due to lower
electrical conductivity of TRGs (∼3 S/cm) than that of
MWCNTs (∼10 S/cm). Thus, it is expected that the electrical
conductivity of MWCNT/TRG/PDMS nanocomposites is
lower compared to that of MWCNT/PDMS nanocomposites
(see Figure 4d). However, experimental electrical conductivity
of 9:1 (MWNCT/TRG) has higher values than that of
MWCNT/PDMS nanocomposites at low fraction of conductive fillers (0.4 wt %, 0.6 wt %). These results were caused
by reduction of Φc (filler’s fraction to electrical percolation
threshold) according to formula. We confirmed reasons for
such results by observation of microstructures in MWCNT/
PDMS and MWCNT/TRG/PDMS nanocomposites. Although
TRG, which has lower electrical and structural properties
(intrinsic conductivity, aspect ratio) compared to those of
MWCNT, showed the dispersibility enhancement of conductive filler with formation of 1D−2D interconnection
enhanced electrical network among conductive fillers as
addition of TRG. These synergistic effects to enhance electrical
networks by MWCNT/TRG hybrid fillers are more dominant
than intrinsic properties of conductive fillers at some ratios
MWCNT to TRG in area 1 comprising small fraction of
conductive fillers. In the area, the electrical conductivity curves
exhibit different shapes with changing fraction of fillers.
Electrical conductivity of 7:3 and 9:1 (MWCNT/TRG)/
PDMS nanocomposites (2.78 × 10−5 S/cm (7:3), 6.54 ×
10−5 S/cm (9:1)) is higher in MWCNT/PDMS nanocomposite (1.88 × 10−5 S/cm) with a filler fraction of 0.4 wt
%, while electrical conductivity of only 9:1 of (MWCNT/
TRG)/PDMS nanocomposite (6.17 × 10−3 S/cm) is higher
than that of MWCNT/PDMS nanocomposite with a filler
fraction of 0.6 wt % (1.85 × 10−3 S/cm) (see Figure 4c). These
results prove that synergistic effects for improving the electrical
network among conductive fillers strengthen as the fraction of
fillers reduces.
However, the electrical conductivity of MWCNT/TRG/
PDMS nanocomposites did not continuously increase as the
ratio of TRG increased. We found that there were optimal
TRG/MWCNT mass ratios in our research. As the TRG/
MWCNT mass ratio increased beyond 1:9, the electrical
Figure 3. (a) Microstructure of MWCNT/TRG/PDMS and
MWCNT/PDMS nanocomposites. (b) Microstructure of MWCNT
(1D)/TRG (2D) interconnection in PDMS matrix.
composites exhibited homogeneous dispersion of MWCNT/
TRG hybrid fillers at their fractures, while relatively much
agglomeration of fillers existed at the fractures of MWNCT/
PDMS nanocomposites (see Figure 3a). Additionally, it was
observed that 1D−2D interconnections between MWCNTs
and TRGs were formed and maintained in the PDMS matrix
(see Figure 3b). These dispersibility enhancements of
conductivity fillers and formation of 1D−2D interconnection
can be attributed to increase the electrical conductivity and
reduction of the electrical percolation threshold of nanocomposites. To confirm how these contributions affect
electrical properties of nanocomposites, we analyzed changes
of electrical properties by controlling fraction of conductive
fillers in the matrix and mass ratio MWCNTs to TRGs.
Figure 4, panel a shows the electrical conductivity of the
MWCNT/PDMS and TRG/PDMS nanocomposites with
various filler fractions. The TRG samples typically exhibited
less electrical conductivity than the theoretical electrical
conductivity of graphene, which can be attributed to residual
oxide and defects that formed during the oxidation and
reduction processes.26 We found that the TRG films had lower
electrical conductivity (∼3 S/cm) compared to that of the
MWCNTs films (∼10 S/cm) (see Figure S3) as well as aspect
ratio of TRG (∼150) was lower than that of MWCNTs
(∼320). Because of the difference of electrical and structural
properties between MWCNTs and TRGs, the electrical
conductivity of MWCNT/PDMS nanocomposites was higher
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Figure 4. (a) Electrical conductivity of MWCNT/PDMS and TRG/PDMS nanocomposites. (b) Electrical conductivity of MWCNT/PDMS,
MWCNT/TRG/PDMS nanocomposites with changing filler’s fraction. (c) Electrical conductivity of MWCNT/PDMS, MWCNT/TRG/PDMS
nanocomposites with changing ratio MWCNTs to TRGs. (d) Comparison of electrical conductivity in MWCNT/PDMS and MWCNT(9)/
TRG(1)/PDMS nanocomposites (expected electrical conductivity and experimental electrical conductivity).
the help of TRG, although the electrical resistance rapidly
increased to out of measurement range owing to loss of
electrical networks among fillers after 60% strain. This stability
in the electrical properties in response to strain demonstrates
that MWCNT/TRG/PDMS nanocomposites can be used as
stretchable conductors. Furthermore, these stretchable conductors were fabricated using a simple solution process
involving only sonication and stirring, in contrast to the
complex chemical processing that is required with covalently
functionalized nanocomposite materials.15,16
conductivity of the nanocomposite decreased (see Figure 4c).
These results mean that when TRG ratio is above specific
values, electrical degradation by lower conductivity, and aspect
ratio of the TRG than that of MWCNT, is more dominant than
enhancement of electrical networks by addition of TRG.
Stretchable conductors were fabricated using MWCNT/
TRG/PDMS nanocomposites with various mass fractions of
the conductive fillers for strain and conductivity tests, as shown
in Figure 5, panel a. As the filler fraction increased, the electrical
conductivity of the nanocomposites also increased; however,
the stretchability decreased, as shown in Figure 5, panel b. To
develop enhanced stretchable conductors, it is necessary to
show enhanced and stable electrical conductivity of the
conductor with strain when the mass fraction of conductive
filler is small. As shown in Figure 5, panel c, electrical enhanced
MWNCT/TRG/PDMS nanocomposites showed stable electrical conductivity with strain. The electrical conductivity of the
0.4-wt % 9:1 (MWCNT/TRG)/PDMS nanocomposites was
maintained in the range of 10−5−10−4 S/cm when a strain of
60% was applied, and the conductivity of the 0.6-wt % 9:1
(MWCNT/TRG)/PDMS and 1-wt % 9:1 (MWCNT/TRG)/
PDMS nanocomposites remained in the range of 10−3−10−2 S/
cm (0.6-wt %) and 1−2 × 10−2 S/cm (1-wt %) when a strain of
60% was applied. The stress was caused by tensile strain affects
arrangement and networks of fillers by being transferred from
the matrix to fillers.29−31 The transferred stress induces the
alignment variation of fillers along the strain direction without
damage of the electrical network under a low strain range; after
then, contact losses among fillers occur under a high strain
range.30 Our stretchable conductors containing a small fraction
of MWCNT/TRG hybrid fillers show maintenance of stable
electrical properties until 60% strain mainly due to the high
aspect ratio of MWCNT and its homogeneous dispersion with
4. CONCLUSIONS
CNTs and graphene are attractive as conductive fillers for
stretchable conductors because of their high electrical
conductivity and high aspect ratio. However, agglomeration
of CNTs and restacking of graphene sheets present challenges
for the fabrication of stretchable composite conductors. We
fabricated and characterized stretchable composite materials
using CNT−graphene hybrid fillers and a PDMS matrix. The
synergistic effects of 1D−2D interconnections inhibit restacking and agglomeration, as confirmed by observations of the
interconnected CNT−graphene fillers with the PDMS matrix.
The electrical conductivity of nanocomposites formed of
MWCNT/PDMS, TRG/PDMS, and MWCNT/TRG/PDMS
were investigated with various fractions of filler. We observed
enhanced electrical networks and conductivity of the
MWCNT/TRG/PDMS nanocomposite compared with the
MWCNT/PDMS and TRG/PDMS nanocomposites at small
fraction of conductive fillers, which is insufficient to form
electrical networks among conductive fillers. However, in
nanocomposites that contain high fraction of conductive fillers,
intrinsic properties of conductive fillers are more important to
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■
Research Article
AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected].
*E-mail: [email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work was supported by the KUSTAR-KAIST Institute,
KAIST, Korea, the ICT R&D program of MSIP/IITP, [B010115-0239, Human Friendly Devices (Skin Patch, Multimodal
Surface) and Device Social Framework Technology], a grant
(10037689) from the Fundamental R&D Program for
Technology of World Premier Materials (WPM) funded by
the Ministry of Knowledge Economy (MKE, Korea), and the
Center for Advanced Soft-Electronics funded by the Ministry of
Science, ICT and Future Planning as Global Frontier Project
(2013M3A6A5073173).
■
Figure 5. (a) MWCNT/TRG/PDMS composites. (b) Strain at break
and electrical conductivities of MWCNT(9)/TRG(1)/PDMS composites. (c) Electrical conductivity variation of 0.4 wt %, 0.6 wt %
MWCNT/TRG/PDMS composites during stretching.
enhance electrical conductivity of nanocomposites than these
synergistic effects that enhance electrical networks of nanocomposites. Additionally, nanocomposites that contain low
fraction of conductive fillers retained their electrical conductivity in the range of 10−5−10−4 S/cm (0.4 wt %) and
10−3−10−2 S/cm (0.6 wt %) when strained up to 60%, which
demonstrates their potential for applications as stretchable
conductors.
■
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ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsami.5b11205.
Structural information on MWCNT, XRD data of GO
and TRG, and electrical conductivity of MWCNT and
TRG films (PDF)
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Research Article
ACS Applied Materials & Interfaces
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