effects of silica nanoparticles on copper nanowire
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© 2016 The Korean Society of Rheology and Springer 111
Korea-Australia Rheology Journal, 28(2), 111-120 (May 2016)DOI: 10.1007/s13367-016-0010-y
www.springer.com/13367
pISSN 1226-119X eISSN 2093-7660
Effects of silica nanoparticles on copper nanowire dispersions in aqueous PVA solutions
Seung Hak Lee, Hyeong Yong Song and Kyu Hyun*
School of Chemical and Biomolecular Engineering, Pusan National University, Busan 46241, Republic of Korea
(Received March 10, 2016; final revision received May 2, 2016; accepted May 3, 2016)
In this study, the effects of adding silica nanoparticles to PVA/CuNW suspensions were investigated rhe-ologically, in particular, by small and large amplitude oscillatory shear (SAOS and LAOS) test. Interesting,the SAOS test showed the complex viscosities of CuNW/silica based PVA matrix were smaller than thoseof PVA/CuNW without silica. These phenomena show that nano-sized silica affects the dispersion ofCuNW in aqueous PVA, which suggests small particles can prevent CuNW aggregation. Nonlinearity (thirdrelative intensity ≡ I3/1) was calculated from LAOS test results using Fourier Transform rheology (FT-rhe-ology) and nonlinear linear viscoelastic ratio (NLR) value was calculated using the nonlinear parameter Qand complex modulus G*. Nonlinearity (I3/1) results showed more CuNW aggregation in PVA/CuNW with-out silica than in PVA/CuNW with silica. NLR (= [Q0(ϕ)/Q0(0)]/[G*(ϕ)/G*(0)]) results revealed an optimumconcentration ratio of silica to CuNW to achieve a well-dispersed state. Degree of dispersion was assessedthrough the simple optical method. SAOS and LAOS test, and dried film morphologies showed nano-sizedsilica can improve CuNW dispersion in aqueous PVA solutions.
Keywords: copper nanowire, silica nanoparticle, SAOS, LAOS, FT-Rheology, nonlinearity I3/1, Nonlinear
Linear viscoelastic Ratio (NLR)
1. Introduction
Conductive thin films are of interest to many industries.
They have many applications, such as, skin sensors for
robotics, wearable electronics, solar cells, and touch pan-
els (Cheng et al., 2014; Lee et al., 2013). In particular, the
development of touchable devices has triggered research
in these fields. To develop conductive thin films for such
application, films must have high conductivity, low thick-
ness, and have appropriate tensile strengths and flexibili-
ties. There are three well known ways to make conductive
thin films, producing coating material, coating, and drying
processes, and each process can markedly affect the prop-
erties of final products. Among these processes, producing
coating material is the most important, because the mixing
protocol, polymer composition, and the additives used,
such as, nano- and micro-sized particles and crosslinking
and dispersing agents affect the coating and drying pro-
cesses and film properties.
Conductivity is one of the most important requirements
of conductive films. Carbon nanotubes (CNT), graphene,
silver, gold, and copper are usually used as conductors in
electronic devices. Of these materials, copper is around
1000 times more abundant than the others, has a resistivity
comparable with that of silver (ρAg = 1.59×10−8 Ω·m, ρCu
= 1.68×10−8 Ω·m at 20°C), and is 100 times cheaper.
Therefore, copper is a material with high performance at
a reasonable price. Nevertheless, despite these advantages,
comparatively little research has been performed on the
use of copper in conductive films.
Conductivity is determined by particle size and shape as
well as material properties. There are many different can-
didates for particle shapes such as tubes, rods, wire (1-D
thread-like shape), 2-D plates, spheres, and cubes. Nano-
particles have been investigated to improve mechanical
and conductive properties by forming internal structure
(Pashayi et al., 2012). Although spherical nano- and micro-
particles have been more investigated than others (Ishida
and Rimdusit, 1998; Ohashi et al., 2005; Zhang et al.,
2010), nanowires exhibit best properties among various
particle types. Long particle lengths decrease percolation
thresholds, which allows amounts of particles to be reduced
and improves transparency (Pashayi et al., 2012; Wang et
al., 2014; Woo et al., 2010; Wu et al., 2006).
Degree of dispersion and percolation threshold are other
considerations (Kang et al., 2013; Wang et al., 2014).
When particles are poorly dispersed, concentrations must
be increased to obtain desired electrical percolation thresh-
olds. However, high particle concentrations increase the
risks of aggregation and product defects and increase
costs. Therefore, researcher have tried to develop method
to increase particle dispersion (De et al., 2010; Deepak et
al., 2006; Gelves et al., 2006; Gelves et al., 2008; Guo et
al., 2003; Islam and Alam, 2011; Leblanc and Nijman,
2009; Lee et al., 2015a), by changing solvent systems,
particle material types, particle structures, and by using
chemical surface treatments. Interestingly, Lee et al. (2015a)
showed particle size distribution can affect degree of dis-
persion and structure development. Small particles can
separate large particles and promote dispersion. Methods*Corresponding author; E-mail: kyuhyun@pusan.ac.kr
Seung Hak Lee, Hyeong Yong Song and Kyu Hyun
112 Korea-Australia Rheology J., 28(2), 2016
based on modifications of particles without resorting to
chemical modification reactions are referred to as physical
particle dispersion methods, which have received compar-
atively little research attention.
Degree of dispersion can be investigated in many ways,
especially, scanning electron microscopy (SEM) and trans-
mission electron microscopy (TEM) are usually used.
Both methods have advantages of direct observation and
give an intuitive sense of micro- or nano-structure. How-
ever, optical methods provide information on very small
specimen surface areas. On the other hand, mechanical
methods, such as, rheological measurements, provide
information about entire specimens. Rheological proper-
ties are sensitive to internal structural change or differ-
ences. In particular, small amplitude oscillatory shear
(SAOS) and large amplitude oscillatory shear (LAOS) test
results have attracted considerable interest. The SAOS test
provides a well-known means of measuring the linear vis-
coelasticity of complex fluids (Ferry, 1980; Larson, 1999;
Morrison, 2001), and the use of the LAOS test as a method
of characterizing complex fluids is a topic of recent inter-
est. Variations in strain amplitude (γ0) and frequency (ω)
allow access to a broad spectrum of time- and non-linear
rheological responses. Fourier transform (FT)-rheology,
which converts time domain stress data into frequency
domain stress data, is one of the many methods used to
analyze nonlinear stress based on LAOS test results (Hyun
and Wilhelm, 2009; Hyun et al., 2003; Hyun et al., 2006;
Hyun et al., 2011; Hyun et al., 2013; Kim et al., 2006;
Wilhelm, 2002; Wilhelm et al., 1998; Wilhelm et al.,
1999). Lim et al. (2013) introduced a new parameter, the
nonlinear linear viscoelastic ratio (NLR ≡ normalized non-
linearity determined by the LAOS test/normalized com-
plex modulus determined by the SAOS test), to investigate
morphological differences using a rheological approach.
By using the NLR, degree of dispersion is studied by
comparing between nonlinear and linear viscoelasticity.
Salehiyan et al. demonstrated that NLR values obtained
by dynamic oscillatory shear test are significantly related
to morphological changes (Salehiyan and Hyun, 2013;
Salehiyan et al., 2014; Salehiyan et al., 2015a; Salehiyan
et al., 2015b).
In this study, we investigated polyvinyl alcohol (PVA)/
copper nanowire (CuNW) suspensions with or without sil-
ica nanoparticles using rheological measurements, partic-
ularly dynamic oscillatory shear (SAOS and LAOS) tests,
and optically using an inverted microscope. Because the
morphologies of large particles in a polymer matrix can be
changed by adding nanoparticles, such as, silica or poly-
styrene nanoparticles (Gondret and Petit, 1997; Lee et al.,
2015a), we studied the effect of silica nanoparticles on
CuNW morphologies in a PVA matrix from the perspec-
tive of degree of dispersion. Rheological properties, espe-
cially I3/1 and NLR values, were used to quantify the
degrees of dispersion of CuNW in PVA aqueous solutions,
and microscopic images were used to investigate CuNW
morphologies in PVA films.
2. Experimental
2.1. MaterialsPolyvinyl alcohol (PVA) was selected as a polymer
matrix, because it was anticipated that it would enhance
the electrochemical properties of anodes (Park et al.,
2011). PVA and LUDOX HS-30 colloidal silica were pur-
chased from Sigma-Aldrich. The molecular weight and
degree of hydrolysis of the PVA was 31,000-50,000 and
98-99%, respectively. LUDOX HS-30 colloidal silica con-
tained 30 wt.% of spherical silica nanoparticles (NPs) of
diameter 12 nm in H2O; it was considered that silica could
have function as a dispersing agent because it is well dis-
persed and stabilized by surface charge. Copper nanowires
(CuNWs; diameter 100-200 nm and length > 5 μm) were
purchased from CNVISION Co., LTD. CuNWs were
coated with polyvinyl pyrrolidone (PVP) to facilitate sta-
bilized dispersion in the polymer solution.
2.2. Sample preparationInitially, PVA was dissolved in deionized water at 90°C
for 4 hours to make a 30 wt.% PVA stock solution. Fillers
were added to polymer stock solution and it is diluted into
20 wt.% at 90°C for 2 days. Containers of PVA suspen-
sion were sealed with parafilm to prevent evaporation
during sample preparation. Proportional concentrations for
each material are provided in Table 1. Sample preparation
and experiments were performed under strict conditions
because PVA is easily affected by ambient conditions
(Finch, 1992; Gao et al., 2010a; Gao et al., 2010b).
2.3. MeasurementsThe rheological properties of PVA/CuNW suspensions
with or without silica NPs were measured using a strain-
Table 1. Lists of sample compositions and abbreviations.
Name of samples Abbreviations
PVA 20 wt.% P20
PVA 20 wt.% + silica 1 wt.% P20 S1
PVA 20 wt.% + CuNW 0.1 wt.% P20 Cu0.1
PVA 20 wt.% + CuNW 0.3 wt.% P20 Cu0.3
PVA 20 wt.% + CuNW 0.5 wt.% P20 Cu0.3
PVA 20 wt.% + CuNW 0.7 wt.% P20 Cu0.3
PVA 20 wt.% + CuNW 1.0 wt.% P20 Cu0.3
PVA 20 wt.% + silica 1 wt.% + CuNW 0.1 wt.% P20 S1 Cu0.1
PVA 20 wt.% + silica 1 wt.% + CuNW 0.3 wt.% P20 S1 Cu0.3
PVA 20 wt.% + silica 1 wt.% + CuNW 0.5 wt.% P20 S1 Cu0.5
PVA 20 wt.% + silica 1 wt.% + CuNW 0.7 wt.% P20 S1 Cu0.7
PVA 20 wt.% + silica 1 wt.% + CuNW 1.0 wt.% P20 S1 Cu1.0
Effects of silica nanoparticles on copper nanowire dispersions in aqueous PVA solutions
Korea-Australia Rheology J., 28(2), 2016 113
controlled rheometer (ARES-G2, TA Instruments) at 25oC.
A parallel plate (50 mm diameter) geometry was adopted
and a Peltier system was used to control bottom plate tem-
perature uniformly. Because PVA/CuNW suspensions with
or without silica NPs were prepared with DI water, water
evaporation occurred during rheological measurements.
Thus, silicone oil was applied at the edge of the suspen-
sion exposed to air to prevent solvent evaporation; the sil-
icon oil used had a viscosity too low to have affected test
results. The test procedure is described in Fig. 1. Small
amplitude oscillatory shear (SAOS) and large amplitude
oscillatory shear (LAOS) tests were conducted after pre-
shearing to remove shear history in the sample. Pre-shear-
ing was conducted clockwise and counterclockwise to
prevent structure align, which can have significant influ-
ence on rheological behavior. SAOS test was conducted
by changing frequency at a fixed strain amplitude, and
LAOS test by changing strain amplitude at a fixed fre-
quency. SAOS test was performed before and after LAOS
test to determine whether or not large strain amplitude
oscillatory shear impacted on SAOS results.
Photographs of films were taken using an inverted sys-
tem microscope (IX-71, Olympus) at various magnifica-
tions, by coating samples on a slide glass to a constant
thickness of ~150 µm and drying in a convection oven at
80°C.
3. Results and Discussion
3.1. SAOS testsStorage and loss moduli (G' and G'', respectively) obtained
by small amplitude oscillatory shear (SAOS) test at fixed
strain amplitude are shown in Fig. 2. According to vis-
cosity models when fillers are added to polymer solutions,
viscosities should increase in proportion to filler concen-
tration (Einstein, 1905; Krieger and Dougherty, 1959).
However, the linear rheological properties, G', G'' and
complex viscosities, of CuNW/PVA suspension contain-
ing silica were smaller than those of CuNW/PVA suspen-
sion without silica. This phenomenon is shown on Fig. 3
for complex viscosity |η*| at a frequency 0.1 rad/s (G' and
G'' showed the same effect). Differences of complex vis-
cosity between suspensions with silica and suspensions
without silica could be explained by the internal structure
morphology of CuNWs. When CuNW are added to poly-
mer solution, they disrupt normal polymer flow behavior.
In addition, CuNW aggregates (or clusters) are formed
that tend to increase moduli and complex viscosity. On the
other hand, the addition of nano-size silica (12 nm) to PVA/
CuNW suspensions decreased complex viscosity (Fig. 3).
Notably, complex viscosity decreased when nano-sized
Fig. 1. (Color online) Rheological measurement procedure. Pre-
shear was applied clockwise and counterclockwise sequentially.
SAOS tests were performed before and after LAOS test.
Fig. 2. (Color online) Relations between storage modulus (G') and loss modulus (G'') as a function of frequency. (a) G' and (b) G''
values of PVA suspensions 20 wt.% containing different concentrations of CuNWs, respectively (c) G' and (b) G'' for PVA/silica sus-
pensions containing different concentrations of CuNWs, respectively.
Seung Hak Lee, Hyeong Yong Song and Kyu Hyun
114 Korea-Australia Rheology J., 28(2), 2016
filler was added. Since nano-sized silica particles act as
dispersants, they induce the dispersion or alignment of
CuNWs and reduce CuNW aggregation. Lee et al. (2015a)
found large polystyrene (PS) particles are dispersed by
adding silica nanoparticles, because silica nanoparticles
form bridges between PS particles.
However, viscosity suddenly dropped when the CuNW
concentration was increased to 1.0 wt.% with and without
silica, which may have been due to the sedimentation of
CuNW aggregates.
3.2. LAOS test and FT-rheology analysisNon-linear viscoelastic properties, G', G'' and I3/1 at the
high strain amplitude (> 1.0 [-]), were obtained under
large amplitude oscillatory shear (LAOS) flow. Storage
(G' ) and loss moduli (G'') obtained by LAOS test are
shown on Fig. 4. These results show that suspensions con-
taining silica had lower G' and G'' than suspensions with-
out silica.
During the LAOS experiment, strain and stress data was
obtained from the LAOS test, and these were used to draw
Lissajous curves (normalized stress versus strain curves;
Fig. 5) as functions of strain amplitude and CuNW con-
centration. However, Lissajous curves provide the shapes
of stress responses and not quantitative values. To better
quantify degree of nonlinearity, we used normalized inten-
sities of third-harmonic [I3/1≡I(3ω)/I(ω), where ω is exci-
tation frequency] obtained by FT-rheology. I3/1 data of
different samples are compared as functions of strain am-
plitude in Figs. 6a and 7c. Relative third intensities (I3/1) of
P20 and other suspensions shows remarkable differences
in Figs. 6a and 6c, indicating addition of filler signifi-
cantly affect nonlinear viscoelastic properties change by
inducing heterogeneous flow behavior. Interestingly, Figs.
6a and 6c shows significantly different behaviors at large
strain amplitude (strain amplitudes from 2.0 to 40.0 [-]).
Relative third intensities for pure polymer solutions with
or without filler were found to be quadratically related to
strain amplitude at medium strain amplitudes (0.1-1.0 [-]),
and I3/1 became constant at higher strain amplitudes. How-
ever, for PVA/CuNW suspensions without silica nanopar-
ticles (Fig. 6a), I3/1 results showed a local maximum and
minimum at large strain amplitudes (3.0-34.0 [-]) region.
The I3/1 results of PVA/CuNW suspensions containing sil-
ica nanoparticles (Fig. 6c) were constant in the large strain
amplitude region. These phenomena have been well inves-
tigated (Hyun et al., 2011; Hyun et al., 2012; Kallus et al.,
Fig. 3. (Color online) Complex viscosity development as func-
tion of CuNW concentration at a frequency of 0.1 rad/s.
Fig. 4. (Color online) Storage and loss moduli for (a), (b) PVA/CuNW suspensions without silica and (c), (d) PVA/CuNW suspension
with silica.
Effects of silica nanoparticles on copper nanowire dispersions in aqueous PVA solutions
Korea-Australia Rheology J., 28(2), 2016 115
2001; Leblanc, 2008; Leblanc and Nijman, 2009; Lee et
al., 2015b). Lee et al. (2015b) simulated the rheology and
microstructure of non-Brownian hard sphere suspensions,
and suggested local dumping of I3/1 in the large strain
amplitude region is a characteristic of suspension systems.
Leblanc and Nijman (2009) investigated the nonlinear vis-
coelastic properties of rubber compounds containing high
silica loadings. At above a critical percolation filler load-
ing, third torque harmonics T(3/1) as function of strain
amplitude exhibit a local maximum or “bump” in the high
strain amplitude region. They inferred that the “bump”
represents the superimposition of two responses, one from
the “pure” polymers and another come from the filler.
These phenomena may disappear at high strain ampli-
tudes, because the filler’s contribution is removed. Inter-
estingly, the addition of montmorillonite (MMT) as a
compatibilizer in a PLA/PCL/mLLDPE blend system
resulted in the two separated phases behaving as a single
phase system (Salehiyan and Hyun, 2013). In other words,
the secondary phase effect was removed by the compati-
bilizer and local minimum and maximum disappeared due
to improved dispersion of the dispersed phase.
Especially, metal nanowires tend to aggregate in the poly-
mer matrix due to the strong interaction between nanow-
ires and high specific surface area (Zhao et al., 2015).
Nonconductive nanoparticles are usually used to over-
come these kinds of aggregation. In particular, silica nano-
particle is one of the mostly used candidates for this
mechanism (Nam et al., 2011; Nam et al., 2012). Nam et
al. (2011, 2012) elucidated that there are attractive inter-
action between silica nanoparticle and metal nanowire and
these interaction hinder the local aggregation of metal
nanowire. Furthermore, in PVA/CuNW/silica nanoparticle
suspension case, intermolecular hydrogen bond between
PVA and silica nanoparticle involving hydroxyl group can
make another types of steric hindrance of CuNWs which
make better dispersion (Sarkar and Deb, 2008). In other
words, the metallic attractive force involving only PVA/
CuNW suspensions are replaced by metal-silica-polymer
interactions. Therefore, the aggregation of CuNW caused
local maximum and minimum of I3/1 (Fig. 6a), and this
behavior was removed by adding silica nanoparticles (Fig.
6c), indicating the aggregation of CuNWs was prevented
by silica nanoparticles.
Figs. 6a and 6c show that normalized third relative
intensity (I3/1) is related to strain amplitude with a power
law manner, log(I3/I1) = a + b logγ0. As a result of scaling
behavior, Hyun et al. (2009) proposed a new nonlinear
coefficient obtained by FT-rheology. In this
definition, they used absolute strain amplitude and not
percent strain amplitude. In Figs. 6b and 6d, Q values are
represented as function of absolute strain amplitude. In
addition, the zero-strain nonlinear parameter,
was defined as the asymptotic value of Q at small strain
amplitudes. It is defined like the zero shear viscosity (η0)
at relatively low shear rate. Therefore, the nonlinear vis-
coelastic parameters can be mathematically fitted using
the modified Carreu-Yasuda viscosity equation (Eq. (1))
(Lim et al., 2013).
. (1)
Lim et al. (2013) suggested a new parameter, the non-
linear-linear viscoelastic parameter (NLR), and defined it
as follows:
NLR = (2)
Q0 I3/1/γ 0
2≡
Q0 limγ0
0→ Q≡
Q Q0≡ 1+ C1γ0( )C
2
⎩ ⎭⎨ ⎬⎧ ⎫
C3
1–( )/C2
Q0 ϕ( )/Q0 0( )
G*ϕ( )/G*
0( )------------------------------
Fig. 5. (Color online) Lissajous curves of normalized strains and
stresses of (a) PVA/CuNW suspensions without silica nanopar-
ticles and (b) PVA/CuNW suspensions with silica nanoparticle as
a function of CuNW concentration (x-axis) and strain amplitude
(y-axis). The almost horizontal lines in the curves represent the
elastic portions of stress curves.
Seung Hak Lee, Hyeong Yong Song and Kyu Hyun
116 Korea-Australia Rheology J., 28(2), 2016
where Q0 (0) and G* (0) are nonlinear and linear visco-
elastic coefficients, respectively, for suspensions without
secondary filler. Q0 (ϕ) and G* (ϕ) are nonlinear and linear
viscoelastic coefficients for suspensions with a secondary
filler concentration of ϕ, respectively. NLR was devised
to access the relation between degree of nonlinearity and
degree of linearity on adding filler at a concentration of ϕ.
Therefore, although many additives are added to the sus-
pension, NLR can be used for comparing degree of dis-
persion of one different matter. For example, if one wanted
to compare the effect of silica on a PVA/CuNW suspen-
sion, one could use Q0 (S1) and G* (S1) for a P20 S1
Cu1.0 suspension and Q0 (S0) and G* (S0) from P20 Cu1.0
suspension. Q0 and G* for suspensions with or without sil-
ica nanoparticles are shown on Figs. 7a and 7b.
For NLR values > 1, the nonlinear viscoelastic coeffi-
cient is more amplified than the linear viscoelastic coef-
ficient, meaning is that droplets or aggregates are more
dispersed. However, when NLR is < 1, the nonlinear vis-
coelastic coefficient is less amplified than the linear vis-
coelastic coefficient, which means interfacial tension or
the polymer network is too strong to influence the non-
linear viscoelastic parameter (Lim et al., 2013; Salehiyan
et al., 2014).
To compares the effects of silica, each NLR value should
be calculated with different result. For example, NLR at a
CuNW concentration of 0.7 wt.% is calculated using the
linear and nonlinear viscoelastic parameters of P20 S1
Cu0.7 and P20 Cu0.7. Using this method, it is easily
understood how silica affects the degree of dispersion of
CuNW in PVA solution. NLR values of PVA/CuNW with
or without silica are shown in Fig. 8. At low CuNW con-
centrations (0.1 or 0.3 wt.%), NLR was < 1, indicating sil-
ica barely affect the degree of dispersion of CuNW.
However, the NLR of the suspension containing CuNW at
Fig. 6. (Color online) Comparisons of relative intensities (I3/1) and of the nonlinear viscoelastic parameter (Q) as functions of strain
amplitude. (a) I3/1 and (b) Q values of PVA suspensions containing concentrations of CuNWs, respectively, (c) I3/1 and (b) Q values
of PVA/silica suspension containing different concentrations of CuNWs, respectively. The asymptotes of Q value were plotted to cal-
culate Q0 values.
Fig. 7. (Color online) Calculated (a) Q0 and (b) G* values for
PVA suspension containing different CuNW concentrations with
or without silica.
Effects of silica nanoparticles on copper nanowire dispersions in aqueous PVA solutions
Korea-Australia Rheology J., 28(2), 2016 117
0.7 was > 1. These findings suggest that at some optimum
ratio of silica to CuNW, CuNW would be well dispersed.
3.3. Morpholoies of dried filmsIn order to compare degrees of dispersion intuitively, we
used microscopic images. Figs. 9 and 10 show morphol-
ogies of PVA/CuNW film prepared by coating and drying.
Dried films were assumed to reflect dispersion qualities in
suspension.
Photomicrographs of P20 Cu0.7 and P20 S1 Cu0.7 films
are shown in Figs. 9a and 9b at low magnification. The
film shown in Fig. 9a appeared more transparent and well-
dispersed than that in Fig. 9b. However, this transparency
is affected by particle flocculation, and the PVA/CuNW
suspension has many empty regions and appeared more
transparent. This phenomenon is well shown in Figs. 9c
and 9d. Adding silica to the CuNW/PVA suspension dis-
persed aggregates in dried films (Fig. 9d). Well-dispersed
nanowires in the PVA/CuNW/silica film disrupt light pen-
etration and the film appeared darker than that without sil-
ica. Fig. 10 shows images of P20 Cu1.0 and P20 S1 Cu0.7
films. Here, a comparison of low magnification images
(Fig. 10a and 10b) shows similar trends as those shown by
Figs. 9a and 9b. Interestingly, a distinct difference was
observed between 0.7 wt.% and 1.0 wt.% CuNW contain-
ing films. As shown by the high magnification image
(Fig. 10d) films were still produced with many aggregates,
indicating silica did not effectively disperse CuNWs at 1.0
wt.%, although Fig. 10b appears darker than Fig. 10a,
which suggests there is critical concentration ratio to main-
tain a well-dispersed CuNW state when silica is added to
the suspension. The number average ratio of diameters of
aggregates of suspensions without silica, P20 Cu0.7 and
P20 Cu1.0, and suspensions with silica, P20 S1 Cu0.7 and
Fig. 8. (Color online) NLR values for the PVA/CuNW suspen-
sions containing silica NPs using PVA/CuNW suspensions with-
out silica.
Fig. 9. (Color online) Photographs of dried films of CuNW 0.7 wt.% in PVA 20 wt.% suspension (P20 Cu0.7) with or without silica.
Images were obtained using an inverted microscope (IX-71, Olympus). (a), (b) Original magnification × 6.4; scale bars represent 0.3
mm. (c), (d) Original magnification × 32; scale bars represent 0.06 mm
Seung Hak Lee, Hyeong Yong Song and Kyu Hyun
118 Korea-Australia Rheology J., 28(2), 2016
P20 S1 Cu1.0) are shown in Fig. 11. We assumed most
aggregates were spherical and that largest aggregates were
connected by two or three spherical aggregates. This sug-
gests P20 S1 Cu0.7 did not contain aggregates larger than
12 μm in diameter. In addition, number average ratio of
P20 S1 Cu0.7 represents the high proportion of small par-
ticle diameter (2-8 μm).
These results agree well with SAOS and LAOS results,
especially with respect to NLR comparisons. As shown in
Fig. 8, a dramatic fall-off in NLR development was
observed on increasing CuNW concentration from 0.7
wt.% to 1.0 wt.%. At 1.0% of CuNW concentration, many
aggregated lumps were not dispersed, which would make
it difficult to improve nonlinear viscoelastic properties.
Thus, NLR may represent degree of dispersion in suspen-
sions as well as it does for nanocomposites or blends (Lim
et al., 2013; Salehiyan and Hyun, 2013; Salehiyan et al.,
2014; Salehiyan et al., 2015a; Salehiyan et al., 2015b).
4. Conclusions
The effects of adding silica nanoparticles to PVA/CuNW
suspensions were investigated from the rheological per-
spective. Adding filler to polymer solutions or suspensions
generally increases viscosities, but SAOS test revealed
that the complex viscosities of PVA/CuNW suspensions
with silica nanoparticle were smaller than those without
silica. Relative third nonlinearity (I3/1) showed a local
maximum and minimum in the large strain amplitude
region (> 1.0 [-]) for PVA/CuNW suspensions without sil-
ica nanoparticles, suggesting that internal structure was
changed at high strain amplitude due to the presence of
aggregates. However, PVA/CuNW/silica suspensions did
not exhibit a local maximum or minimum at high strain
amplitude, indicating silica nanoparticles had improved
CuNW dispersion.
Nonlinear and linear viscoelastic parameters were used
to calculate NLR values. When NLR values were deter-
mined for suspensions with different CuNW concentrations,
it was found that dispersion was greatest at an optimum
concentration ratio of silica to CuNW. Furthermore, micro-
scopic images of dried films supported these results. Sub-
sequent comparison of photomicrographs of dried films of
P20 Cu0.7 and P20 S1 Cu0.7 showed the addition of silica
reduced aggregates. On the other hand, comparisons of the
photomicrographs of P20 Cu1.0 and P20 S1 Cu1.0 films
revealed the presence of large aggregates even after add-
ing silica. Therefore, it appears that NLR results accorded
Fig. 10. (Color online) Photographs of CuNW 1.0 wt.% in PVA 20 wt.% suspension (P20 Cu1.0) with or without silica. Images were
obtained using an inverted microscope (IX-71, Olympus). (a), (b) Original magnification × 6.4; scale bar represents 0.3 mm. (c), (d)
Original magnification × 32; scale bar represents 0.06 mm
Effects of silica nanoparticles on copper nanowire dispersions in aqueous PVA solutions
Korea-Australia Rheology J., 28(2), 2016 119
closely with microscopic results with respect to morpho-
logical differences.
SAOS and LAOS results confirmed that nano-sized sil-
ica can promote the dispersion of Cu nanowires in aque-
ous PVA solutions. Furthermore, the morphologies of dried
films support this conclusion. Nevertheless, further studies
are required to investigate the effects of small particles on
dispersion of large particles.
Acknowledgment
This work was supported by a 2-Year Research Grant
from Pusan National University (2014-2016).
References
Cheng, Y., S. Wang, R. Wang, J. Sun, and L. Gao, 2014, Copper
nanowire based transparent conductive films with high stability
and superior stretchability, J. Mater. Chem. C 2, 5309-5316.
De, S., P.J. King, M. Lotya, A. O'Neill, E.M. Doherty, Y. Her-
nandez, G.S. Duesberg, and J.N. Coleman, 2010, Flexible,
transparent, conducting films of randomly stacked graphene
from surfactant-stabilized, oxide-free graphene dispersions,
Small 6, 458-464.
Deepak, F.L., P. Saldanha, S.R.C. Vivekchand, and A. Govinda-
raj, 2006, A study of the dispersions of metal oxide nanowires
in polar solvents, Chem. Phys. Lett. 417, 535-539.
Einstein, A., 1905, Eine neue bestimmung der moleküldimen-
sionen, Annu. Phys.-Berlin 324, 289-306.
Ferry, J.D., 1980, Viscoelastic Properties of Polymers, 3rd ed.,
Wiley, New York.
Finch, C.A., 1992, Polyvinyl Alcohol: Developments, 2nd ed.,
Wiley, Chichester.
Gao, H., J. He, R. Yang, and L. Yang, 2010a, Characteristic rhe-
ological features of high concentration PVA solutions in water
with different degrees of polymerization, J. Appl. Polym. Sci.
116, 2734-2741.
Gao, H.W., R.J. Yang, J.Y. He, and L. Yang, 2010b, Rheological
behaviors of PVA/H2O solutions of high-polymer concentra-
tion, J. Appl. Polym. Sci. 116, 1459-1466.
Gelves, G.A., B. Lin, J.A. Haber, and U. Sundararaj, 2008, Enhanc-
ing dispersion of copper nanowires in melt-mixed polystyrene
composites, J. Polym. Sci. Pt. B-Polym. Phys. 46, 2064-2078.
Gelves, G.A., B. Lin, U. Sundararaj, and J.A. Haber, 2006, Low
electrical percolation threshold of silver and copper nanowires
in polystyrene composites, Adv. Funct. Mater. 16, 2423-2430.
Gondret P. and L. Petit, 1997, Dynamic viscosity of macroscopic
suspensions of bimodal sized solid spheres, J. Rheol. 41, 1261-
1274.
Guo, Y.G., J.S. Hu, H.P. Liang, L.J. Wan, and C.L. Bai, 2003,
Highly dispersed metal nanoparticles in porous anodic alumina
films prepared by a breathing process of polyacrylamide hydro-
gel, Chem. Mater. 15, 4332-4336.
Hyun, K. and M. Wilhelm, 2009, Establishing a new mechanical
nonlinear coefficient Q from FT-Rheology: First investigation
of entangled linear and comb polymer model systems, Mac-
romolecules 42, 411-422.
Hyun, K., H.T. Lim, and K.H. Ahn, 2012, Nonlinear response of
polypropylene (PP)/Clay nanocomposites under dynamic oscil-
latory shear flow, Korea-Aust. Rheol. J. 24, 113-120.
Hyun, K., J.G. Nam, M. Wilhelm, K.H. Ahn, and S.J. Lee, 2003,
Nonlinear response of complex fluids under LAOS (large
amplitude oscillatory shear) flow, Korea-Aust. Rheol. J. 15, 97-
105.
Hyun, K., K.H. Ahn, S.J. Lee, M. Sugimoto, and K. Koyama,
2006, Degree of branching of polypropylene measured from
Fourier-transform rheology, Rheol. Acta 46, 123-129.
Hyun, K., M. Wilhelm, C.O. Klein, K.S. Cho, J.G. Nam, K.H.
Ahn, S.J. Lee, R.H. Ewoldt, and G.H. McKinley, 2011, A
review of nonlinear oscillatory shear tests: Analysis and appli-
cation of large amplitude oscillatory shear (LAOS), Prog.
Polym. Sci. 36, 1697-1753.
Hyun, K., W. Kim, S.J. Park, and M. Wilhelm, 2013, Numerical
simulation results of the nonlinear coefficient Q from FT-Rhe-
ology using a single mode pom-pom model, J. Rheol. 57, 1-25.
Ishida, H. and S. Rimdusit, 1998, Very high thermal conductivity
obtained by boron nitride-filled polybenzoxazine, Thermochim.
Acta 320, 177-186.
Islam, S.N. and M.S. Alam, 2011, Characterization of dispersion
properties of silicon nanowire considering different core geom-
etry, TENCON 2011-2011 IEEE Region 10 Conference, Bali,
Fig. 11. (Color online) Aggregates distributions for suspensions
without silica (P20 Cu0.7 and P20 Cu1.0) and suspensions with
silica (P20 S1 Cu0.7 and P20 S1 Cu1.0). Number average diam-
eters (Dn) of total aggregates of suspensions are shown.
Seung Hak Lee, Hyeong Yong Song and Kyu Hyun
120 Korea-Australia Rheology J., 28(2), 2016
Indonesia, 638-641.
Kallus, S., N. Willenbacher, S. Kirsch, D. Distler, T. Neidhöfer,
M. Wilhelm, and H.W. Spiess, 2001, Characterization of poly-
mer dispersions by Fourier transform rheology, Rheol. Acta 40,
552-559.
Kang, M.H., H.Y. Yeom, H.Y. Na, and S.J. Lee, 2013, Compar-
ative study of physical dispersion method on properties of
polystyrene/multi-walled carbon nanotube nanocomposites,
Polym. Kor. 37, 526-532.
Kim, H., K. Hyun, D.J. Kim, and K.S. Cho, 2006, Comparison
of interpretation methods for large amplitude oscillatory shear
response, Korea-Aust. Rheol. J. 18, 91-98.
Krieger, I.M. and T.J. Dougherty, 1959, A mechanism for non-
Newtonian flow in suspensions of rigid spheres, Trans. Soc.
Rheol. 3, 137-152.
Larson, R.G., 1999, The Structure and Rheology of Complex Flu-
ids, Oxford University press, New York.
Leblanc J.L., 2008, Large amplitude oscillatory shear experi-
ments to investigate the nonlinear viscoelastic properties of
highly loaded carbon black rubber compounds without cura-
tives, J. Appl. Polym. Sci. 109, 1271-1293.
Leblanc, J.L and G. Nijman, 2009, Engineering performance and
material viscoelastic analyses along a compounding line for sil-
ica-based compounds, part 2: Nonlinear viscoelastic analysis,
J. Appl. Polym. Sci. 112, 1128-1141.
Lee, J., P. Lee, H.B. Lee, S. Hong, I. Lee, J. Yeo, S.S. Lee, T.S.
Kim, D. Lee, and S.H. Ko, 2013, Room-temperature nanosol-
dering of a very long metal nanowire network by conducting-
polymer-assisted joining for a flexible touch-panel application,
Adv. Funct. Mater. 23, 4171-4176.
Lee, J., S.J. Lee, K.H. Ahn, and S.J. Lee, 2015a, Bimodal colloid
gels of highly size-asymmetric particles, Phys. Rev. E 92,
012313.
Lee, Y.K., J. Nam, K. Hyun, K.H. Ahn, and S.J. Lee, 2015b,
Rheology and microstructure of non-Brownian suspensions in
the liquid and crystal coexistence region: Strain stiffening in
large amplitude oscillatory shear, Soft Matter 11, 4061-4074.
Lim, H.T., K.H. Ahn, J.S. Hong, and K. Hyun, 2013, Nonlinear
viscoelasticity of polymer nanocomposites under large ampli-
tude oscillatory shear flow, J. Rheol. 57, 767-789.
Morrison, F.A., 2001, Understanding Rheology, Oxford Univer-
sity press, New York.
Nam, S., H.W. Cho, S. Lim, D. Kim, H. Kim, and B.J. Sung,
2012, Enhancement of electrical and thermomechanical prop-
erties of silver nanowire composites by the introduction of
nonconductive nanoparticles: Experiment and simulation, ACS
Nano 7, 851-856.
Nam, S., H.W. Cho, T. Kim, D. Kim, B.J. Sung, S. Lim, and H.
Kim, 2011, Effects of silica particles on the electrical perco-
lation threshold and thermomechanical properties of epoxy/sil-
ver nanocomposites, Appl. Phys. Lett. 99, 043104.
Ohashi, M., S. Kawakami, Y. Yokogawa, and G.C. Lai, 2005,
Spherical aluminum nitride fillers for heat-conducting plastic
packages, J. Am. Ceram. Soc. 88, 2615-2618.
Park, H.K., B.S. Kong, and E.S. Oh, 2011, Effect of high adhe-
sive polyvinyl alcohol binder on the anodes of lithium ion bat-
teries, Electrochem. Commun. 13, 1051-1053.
Pashayi, K., H.R. Fard, F. Lai, S. Iruvanti, J. Plawsky, and T.
Borca-Tasciuc, 2012, High thermal conductivity epoxy-silver
composites based on self-constructed nanostructured metallic
networks, J. Appl. Phys. 111, 104310.
Salehiyan, R. and K. Hyun, 2013, Effect of organoclay on non-
linear rheological properties of poly (lactic acid)/poly (capro-
lactone) blends, Korean J. Chem. Eng. 30, 1013-1022.
Salehiyan, R., H.Y. Song, and K. Hyun, 2015a, Nonlinear behav-
ior of PP/PS blends with and without clay under large ampli-
tude oscillatory shear (LAOS) flow, Korea-Aust. Rheol. J. 27,
95-103.
Salehiyan, R., H.Y. Song, W.J. Choi, and K. Hyun, 2015b, Char-
acterization of effects of silica nanoparticles on (80/20) PP/PS
blends via nonlinear rheological properties from Fourier trans-
form rheology, Macromolecules 48, 4669-4679.
Salehiyan, R., Y. Yoo, W.J. Choi, and K. Hyun, 2014, Charac-
terization of morphologies of compatibilized polypropylene/
polystyrene blends with nanoparticles via nonlinear rheological
properties from FT-rheology, Macromolecules 47, 4066-4076.
Sarkar, M.D. and P. Deb, 2008, Synthesis and characterization of
hybrid nanocomposites comprising poly (vinyl alcohol) and
colloidal silica, Adv. Polym. Technol. 27, 152-162.
Wang, S., Y. Cheng, R. Wang, J. Sun, and L. Gao, 2014, Highly
thermal conductive copper nanowire composites with ultralow
loading: Toward applications as thermal interface materials,
ACS Appl. Mater. Interfaces 6, 6481-6486.
Wilhelm, M., 2002, Fourier-transform rheology, Macromol. Mater.
Eng. 287, 83-105.
Wilhelm, M., D. Maring, and H.W. Spiess, 1998, Fourier-trans-
form rheology, Rheol. Acta 37, 399-405.
Wilhelm, M., P. Reinheimer, and M. Ortseifer, 1999, High sen-
sitivity Fourier-transform rheology, Rheol. Acta 38, 349-356.
Woo, D.K., W.J. Noh, and S.J. Lee, 2010, Effect of nanotube
length on rheological characteristics of polystyrene/multi-walled
carbon nanotube nanocomposites prepared by latex technol-
ogy, Polym. Kor. 34, 534-539.
Wu, H.P., J.F. Liu, X.J. Wu, M.Y. Ge, Y.W Wang, G.Q. Zhang,
and J.Z. Jiang, 2006, High conductivity of isotropic conductive
adhesives filled with silver nanowires, Int. J. Adhes. Adhes. 26,
617-621.
Zhang, R., K.S Moon, W. Lin, and C.P. Wong, 2010, Preparation
of highly conductive polymer nanocomposites by low tempera-
ture sintering of silver nanoparticles, J. Mater. Chem. 20, 2018-
2023.
Zhao, T., C. Zhang, Z. Du, H. Li, and W. Zou, 2015, Function-
alization of AgNWs with amino groups and their application in
an epoxy matrix for antistatic and thermally conductive nano-
composites, RSC Adv. 5, 91516-91523.
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