effects of silica nanoparticles on copper nanowire

© 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: [email protected]

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






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.



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.



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.



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.



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



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



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).

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