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© 2016 The Korean Society of Rheology and Springer 41
Korea-Australia Rheology Journal, 28(1), 41-49 (February 2016)DOI: 10.1007/s13367-016-0004-9
www.springer.com/13367
pISSN 1226-119X eISSN 2093-7660
Effect of VA and MWNT contents on the rheological and physical properties of EVA
Jong-Ho Kim1, Seungwon Lee
2, Byoung Chul Kim
2, Bong-Seob Shin
1, Jong-Young Jeon
1 and Dong Wook Chae
1,*1Department of Textile Engineering, Kyungpook National University, Sangju 37224, Republic of Korea2Department of Organic and Nano Engineering, Hanyang University, Seoul 04763, Republic of Korea
(Received November 12, 2015; final revision received December 21, 2015; accepted January 10, 2016)
Ethylene vinyl acetate (EVA) copolymers with two different VA contents (15 and 33 wt.%, denoted byEVA15 and EVA33, respectively) were melt compounded with multi-walled carbon nanotubes (MWNTs)and the effect of VA and nanotube contents on the rheological, thermal and morphological properties wasinvestigated. The addition of nanotubes into both EVAs increased the onset temperature of crystallizationand broadened the peak, but further addition from 3 wt.% slightly decreased the temperature with increasingnanotube contents. In the wide angle X-ray diffraction patterns the peak of EVA15 was little affected bythe presence of nanotubes but that of EVA33 slightly shifted to higher degree and became sharper withincreasing nanotube contents. Dynamic viscosity (η') increased with nanotube contents giving abruptincrease at 2 wt.% nanotubes. Loss tangent decreased with increasing nanotube contents exhibiting the pla-teau-like behavior over most of the frequency range from 2 wt.% nanotubes. In the Casson plot, yield stressincreased with nanotube content and its increasing extent was more notable for more VA content. In theCole-Cole plot, the presence of nanotubes from 2 wt.% gave rise to the deviation from the single mastercurve by decreasing the slope. The deviated extent of EVA33 became more remarkable with increasingnanotube contents than that of EVA15. The stress-strain curve showed that more improved tensile modulusand yield stress were achieved by the introduction of MWNTs for EVA 33 than for EVA15. Tensile strengthof EVA33 increased with increasing nanotube contents, while that of EVA15 decreased.
Keywords: EVA, MWNTs, rheology, thermal properties, tensile properties
1. Introduction
Ethylene-vinyl acetate (EVA), the copolymer of eth-
ylene and vinyl acetate, is an elastomeric material to be
designed to have various characteristics by varying VA
contents. The weight percent of VA usually ranges from
10 to 40, leading to the difference of the physical prop-
erties such as processing temperature, mechanical proper-
ties, and crystal structures. The material has excellent
properties including light weight, easy to foam, high flex-
ibility, high impact strength and good light UV radiation
resistance. These beneficial properties make EVA favor-
ably applied in various industries such as compounding,
coating and adhesives, photovoltaics, biomedical materi-
als, and sporting goods (Yu and Kim, 2013). In addition,
due to its thermoplasticity and low price, EVA is used as
a substitute of natural rubbers and various vinyl products.
However, EVA does not sometimes meet the requirements
such as thermal stability and mechanical properties in
some specific areas. The combination of EVA with other
inorganic materials might be one of the ways to extend its
applications by overcoming some drawbacks and provid-
ing some functionalities (Chaudhary et al., 2005; Fang et
al., 2012; Lim et al., 2010; Yu and Kim, 2013).
In recent years, various inorganic nanoparticles are incor-
porated to endow polymeric systems with their function-
alities as a substitute for conventional micrometer-scale
fillers. Since carbon nanotubes (CNTs) were discovered in
1991 by Sumio Iijima, they attracts considerable interests
in industry and research fields due to outstanding proper-
ties such as exceptional tensile strength and modulus, and
high thermal and electrical conductivities (Collins and
Avouris, 2000; Iijima, 1991; Kim et al., 2001; Lu, 1997;
Ruoff and Lorents, 1995; Wong et al., 1997). The polymer
composites with CNTs are expected to exhibit some func-
tional properties including improved rigidity, electrical
conductivity and electromagnetic shielding properties.
Their fiber-like structure in a nano-sized diameter and tens
of micron-sized length creates extremely high surface
areas. Due to great surface area of the nanotubes their
addition is expected to have a significant effect on the
physical properties of the polymeric systems even at a
small loading level (Bae et al., 2012; Chae and Hong,
2012; La Mantia and Tzankova Dintcheva, 2006; Prasad
et al., 2006).
EVA random copolymer is a polar polyolefin, whose
polarity depends on the VA content (Marini et al., 2010).
The introduction of bulky VA groups into the backbone by
copolymerization decreases the regularity, which has a
great effect on the thermal and rheological properties. An
understanding of the relationships between chemical
structure and physical properties gives important clue to*Corresponding author; E-mail: [email protected]
Jong-Ho Kim, Seungwon Lee, Byoung Chul Kim, Bong-Seob Shin, Jong-Young Jeon and Dong Wook Chae
42 Korea-Australia Rheology J., 28(1), 2016
develope appropriate polymeric systems which can meet
the diverse application requirements. The modification of
EVA with inorganic nanoparticles may have a consider-
able effect on the viscoelastic, thermal, and mechanical
properties of the polymeric system in combination with its
degree of polarity. The viscoelastic properties are strongly
affected by molecular structure and the presence of addi-
tives. Thus, the microstructure of these nanocomposites
under a given processing condition might be understood
through rheological characteristics. In other words, the
study on the viscoelastic properties of composite system is
prerequisite to determine optimum processing condition
and to solve the problems occurring in the process. While
many researches have done on EVA/clay nanocomposites,
there are few studies on the combined influence of the VA
and nanotube contents on the rheological and physical
properties of the EVA/CNT composites (George and
Bhowmick, 2009). In this work, EVA composites filled
with MWNTs were prepared from melt compounding, and
the rheological behavior and other physical properties
such as thermal, morphological and mechanical properties
were examined varying VA and nanotube contents.
2. Experimental
2.1. Materials and sample preparationTwo different commercially available EVAs (VC640,
melt flow index = 6; VC710, melt flow index = 25) were
obtained from Lotte Chemical, Korea. VA contents of
VC640 and VC710 were 15 and 33 wt.%, respectively and
these samples were designated EVA15 and EVA33,
respectively. High purity multi-walled carbon nanotubes
(MWNTs) (purity = 95%, average diameter = 10-15 nm,
length = 20 μm) were supplied by Hanwha Nanotech,
Korea. All specimens were vacuum-dried prior to use. To
prepare the composite specimen EVA pellets were first
crushed into a fine powder in liquid nitrogen using
mechanical blender, which prevented them from being
molten by frictional heat. A given amount of powdered
EVA and nanotube were then dry-mixed for 1 minute
using the mechanical blender. Premixtures of EVA and
MWNTs were melt compounded at 110oC with a rotor
speed of 50 rpm for 7 minutes using a Haake mixer
(Haake PolyDrive, Thermo Electron Corp.). The loading
levels (X) of MWNTs in EVA matrices were 0.5, 1, 2, 3,
4, and 5 and they were coded EVA15-X or EVA33-X.
2.2. Physical propertiesThe fractured surface of the composites was examined
with a field emission scanning electron microscope (FESEM;
JSM-6340F, JEOL) to evaluate the dispersion state of
MWNTs in EVA. The samples were sputter-coated with a
thin Pt layer to avoid charging. The thermal properties of
EVA and EVA/MWNT composites were measured by dif-
ferential scanning calorimetry (DSC; TA Instruments, DSC-
2010) in nitrogen atmosphere. Cooling scan was per-
formed from 110 to −30°C at a rate of 20oC/min, followed
by the heating scan. 5 min holding prior to cooling or
heating scan was applied to remove any thermal history.
The crystal structure of EVA and EVA/MWNT compos-
ites was examined by wide-angle X-ray diffractometer
(WAXD; Rigaku Denki Co.) with Nickel filtered CuK
radiation of 40 kV and 60 mA. Scanning was carried out
on the equator over the 2θ range of 5° to 50° at a scan
rate of 7°/min. Measurements were recorded at every
0.05°. Dynamic rheological properties were measured by
advanced rheometric expansion system (ARES, Rheom-
etric Scientific) in nitrogen atmosphere. Parallel plate
geometry with a diameter of 25 mm was adopted. The
plate gap and strain level were 0.6 mm and 5%, respec-
tively. Dynamic frequency sweep tests were performed at
110oC over the frequency range of 0.05 to 500 rad/s. The
specimens were kept at the same temperature for 5 min
prior to measurement to remove the residual stress. Ten-
sile properties were measured on dog bone-shaped spec-
imens (ASTM D638 Type V) at room temperature using
universal testing machine (Instron tensile tester model
4465). Crosshead speed was 10 mm/min and gauge length
was 20 mm. Average values of ten measurements were
taken as the data.
3. Results and Discussion
Fig. 1 shows FESEM images of the fractured surface of
EVA composites with 5 wt.% MWNT. Nanotubes are
globally dispersed in both EVA matrices without large
agglomerates. It is ascertain that premixing of powered
EVA and MWNTs at a dry state and subsequent melt com-
pounding is an efficient way to obtain good dispersion.
The difference in the dispersion state caused by varying
VA contents is not discernible from the FESEM images. It
was reported that an increased VA content gave rise to a
large free volume available and improved affinity between
polymer chain and filler, which might allow easy disper-
sion of nanotubes in the amorphous rubbery phase
(George and Bhowmick, 2009). However, there is no such
indication from the FESEM images for EVA matrices
used in this study.
DSC cooling and subsequent heating scans of EVA and
EVA/MWNT composites are shown in Fig. 2 and their
thermal properties are summarized in Table 1. EVA33
presents smaller crystalline peaks and lower crystallization
temperature in the cooling scan by about 30oC than
EVA15. The introduction of bulky side groups by copo-
lymerization serves to reduce the regularity and hence the
ability to crystallize. That is, EVA15 having greater mobil-
ity than EVA33 can be crystallized at a high temperature,
the unfavorable condition for the crystallization, resulting
Effect of VA and MWNT contents on the rheological and physical properties of EVA
Korea-Australia Rheology J., 28(1), 2016 43
in larger crystallites. The addition of nanotubes increases
the onset temperature of crystallization and broadens the
peak, indicative of heterogeneous nucleation. Nanotubes
give rise to the formation of less uniform crystal , which
requires longer crystallization time. The movements of the
EVA chains are confined by the high aspect ratio of CNTs,
hence there is great contact probability between EVA and
nanotubes, which might reduce their involvement toward
the crystal lattice (Li et al., 2004). The introduction of 3
wt.% nanotubes increases the Tc by about 1.5oC for
EVA15 and 1.9oC for EVA33, resulting from their nucle-
ation effects. It is worth noting that such a small increase
Fig. 1. FESEM images of the fractured surfaces of EVA/MWNT composites; (a) EVA15-5 and (b) EVA33-5.
Fig. 2. DSC cooling thermograms of (a) EVA15/MWNT composites and (b) EVA33/MWNT composites, and subsequent heating ther-
mograms of (c) EVA15/MWNT composites and (d) EVA33/MWNT composites.
Jong-Ho Kim, Seungwon Lee, Byoung Chul Kim, Bong-Seob Shin, Jong-Young Jeon and Dong Wook Chae
44 Korea-Australia Rheology J., 28(1), 2016
in Tc of EVA occurs over the loading level of MWNTs
considering nucleation ability of nanotubes in the polymer
matrix. This implies that nanotubes are selectively placed
in amorphous region, that is, the free volume between
bulky VA groups rather than between ethylene groups.
However, further addition slightly decreases Tc with
increasing nanotube contents. This suggests that above
some critical loading level of nanotubes their physical hin-
drance effects are dominant over the nucleation ones. In
addition, for higher VA contents the nanotube role retard-
ing polymer mobility seems to be accelerated in combi-
nation with low chain mobility of more bulky group,
resulting in dull exothermic peak. As shown in Figs. 2c
and 2d, EVA33 shows lower and broader melting point
(Tm) than EVA33. In general, the width and shape of the
melting endotherm reflect the types of crystalline phases
present as well as the distribution of crystallites sizes,
which depend on the crystallization conditions (Chae et
al., 2004). EVA33 whose VA content is close to the tran-
sition concentration (40-50 wt.%) from partial to complete
amorphous gives unclear endotherm peak, which is a typ-
ical pattern of basically amorphous polymer. The presence
of bulky units decreases the crystal size and its perfections
by interfering crystallization. Tm of both EVA15 and
EVA33 slightly decreases with increasing nanotube con-
tents by 2.2 and 1.4oC, respectively, which might be
attributed to the reduced crystal size caused by nanotube
addition. However, the shape difference of melting peak is
not clearly observed over all loading levels of nanotubes
exhibiting broad endotherm for both EVAs.
Fig. 3 shows the variation of WAXD patterns of EVA/
MWNT composites with VA and nanotube contents. EVA15
gives two notable crystalline peaks while EVA 33 gives
only one broad peak indicating that VA contents in the
range have a significant effect on the crystal morphology.
The two peaks of EVA15 at 21o and 23.5o superimposed
on the amorphous halo are associated with the crystallo-
graphic planes (110) and (200) of the orthorhombic form
of polyethylene, respectively. The WAXD patterns of EVA
15 are little affected by the presence of MWNTs, while the
crystallization temperature increases a little (Chae et al.,
2006; Chae et al., 2007). However, the peak of EVA33
around 21o slightly shifts to higher degree and becomes
sharper with increasing nanotube contents. This might be
associated with relatively greater nucleation effect of nano-
tube. Higher VA content gives more free volume between
polymer chains which is efficiently packed by nanotube,
resulting in closer distance and more interaction between
nanotube and polymer chain.
Fig. 4 exhibits the variation of dynamic viscosity (η') of
EVA and EVA/MWNT composites with VA and nanotube
contents. The viscosity increases with nanotube contents
exhibitng no plateau in the low frequency range at a high
nanotube content. Up to 1 wt.% loading pseudoplastic flow-
Table 1. Crystallization and melting temperatures of EVA/MWNT
composites.
MWNT contents
(wt.%)
Tc (
o
C) Tm (
o
C)
EVA15 EVA33 EVA15 EVA33
0 71.13 40.85 90.74 63.63
0.5 71.73 42.14 90.63 63.51
1 71.85 42.34 90.39 63.38
2 71.93 42.59 90.05 63.34
3 72.70 42.78 89.11 62.49
4 72.63 42.60 88.86 62.34
5 72.56 42.37 88.56 62.29
Fig. 3. WAXD profiles of (a) EVA15/MWNT composites and
(b) EVA33/MWNT composites.
Effect of VA and MWNT contents on the rheological and physical properties of EVA
Korea-Australia Rheology J., 28(1), 2016 45
like behavior is observed for both EVAs, followed by sud-
den viscosity drop, representing power-law fluid. Above
the concentration, however, no lower Newtonian flow
region in the low frequency is observed giving a contin-
uous shear thinning profile. The presence of finite yield
stress is attributed to a promoted physical association in
the polymeric system. It is worth noting that more pro-
moted η' by nanotube addition is observed for EVA33 than
for EVA15 exhibiting even higher η' from 4 wt.% MWNTs.
This suggests that EVA with higher VA content, that is,
more free volume, is efficiently filled with MWNTs result-
ing in more restricted molecular mobility by increased
contact between nanotube and polymer chain. At 2 wt.%
nanotube loading both EVAs show abrupt increase of η'.
This might be attributed to the presence of rheological per-
colation around the concentration. The sharp increase of η'
can be explained by the fact that the network structure of
nanotubes is formed from the critical loading level
restricting the mobility of polymer chain considerably. At
high frequency range, however, the viscosity difference
caused by nanotube addition is greatly reduced. The extent
of hindered chain mobility by MWNTs is diminished,
which might be attributed to the breakdown of the net-
work structure under high shear force. Moreover, high
aspect ratio of nanotubes seems to accelerate the orienta-
tion of polymer chains along the shear direction, giving
rise to a high degree of shear thinning for high nanotube
content.
The loss tangent (tan δ) against frequency is plotted in
Fig. 5. tan δ, defined as ratio of loss modulus (G") to stor-
age modulus (G'), is a quantitative measure of solid-like
elastic or liquid-like viscous properties of a system. The
Fig. 4. The viscosity curves of (a) EVA15/MWNT composites
and (b) EVA33/MWNT composites at 110oC.
Fig. 5. The tan δ curves of (a) EVA15/MWNT composites and
(b) EVA33/MWNT composites at 110o
C.
Jong-Ho Kim, Seungwon Lee, Byoung Chul Kim, Bong-Seob Shin, Jong-Young Jeon and Dong Wook Chae
46 Korea-Australia Rheology J., 28(1), 2016
addition of nanotubes decreases tan δ with increasing their
loading level. From 2 wt.% nanotubes, the plateau-like
behavior, where tan δ is little affected by the frequency,
begins appearing over most of the frequency range observed.
This might be associated with the presence of percolated
structure of nanotubes in EVA matrix resulting in gel-like
behavior of nanofilled systems. In particular, the fre-
quency-independent plateau behavior caused by nanotube
addition is more notable for EVA33 than for EVA15. As
the free volume caused by bulky VA group increases, the
nanotubes are more efficiently dispersed in the less dense
structure leading to an increased contact probability and
interaction between nanotube and polymer chain. Thus, an
increased chain rigidity is represented by an increase of
elastic response leading to a decrease of tan δ.
The yield behavior of heterogeneous system can be well
expressed by the following Casson plot;
(1)
where G" stands for loss modulus, yield stress, and K
constant. As shown in Fig. 6, the Casson plot demon-
strates a non-zero positive intercept for all the samples.
Their yield stress is tabulated in Table 2. The addition of
nanotubes increases the heterogeneity with increasing the
content, giving high value of the intercept. In particular, a
significant change of yield stress is observed around 2
wt.% nanotube loading where percolated structure of
nanotube seems to be developed, in a good agreement
with the aforementioned rheological data. As nanotube
contents increase, the effects of the interaction between
nanotubes become greater in the polymeric systems, even-
tually leading to the formation of their interconnected
structure (Peeterbroeck et al., 2005). This entanglement
between nanotubes gives the material with enhanced resis-
tance against the applied deformation, resulting in the sub-
stantial increase of G'. At high concentrations of nano-
tubes, an increasing extent of yield stress is more notable
for EVA33 than for EVA15, showing even higher value
from 3 wt.% at the corresponding nanotube contents. This
might be explained that filler effects promoting the het-
erogeneity of polymeric systems overwhelm the negative
effects of free volume caused by bulky VA group.
A log-log plot of G' versus G", so-called Cole-Cole plot,
is shown in Fig. 7. It is well known that the homogeneous
and isotropic polymer melts or solutions exhibit the slope
of 2 because G' and G'' are proportional to the second
order and the first order of frequency, respectively (Han
and Jhon, 1986). The degree of heterogeneity for poly-
meric systems, that is, the change in microstructure of the
composites is evaluated by the deviated extent from the
slope 2. The introduction of MWNTs decreases the slope,
indicative of an increased heterogeneity by the association
between MWNT and polymer matrix. The nanotube load-
ing up to 1 wt.% seems to have little effect on the slope,
exhibiting almost single master curve. However, above the
concentration the curves start to be deviated from the sin-
G″
1
2---
= Gy″
1
2---
+ Kω
1
2---
Gy″
Fig. 6. (Color online) Casson plots of (a) EVA15/MWNT com-
posites and (b) EVA33/MWNT composites at 110oC.
Table 2. Yield stress of EVA/MWNT composites obtained from
Casson plot.
MWNT contents
(wt.%)
EVA15
(Pa)
EVA33
(Pa)
0 177.4 21.81
0.5 220.5 45.43
1 289.0 83.54
2 835.2 623.5
3 2314 2631
4 3563 5107
5 5711 7994
Effect of VA and MWNT contents on the rheological and physical properties of EVA
Korea-Australia Rheology J., 28(1), 2016 47
gle master curve and the deviated extent increases with
nanotube contents. In addition, the deviated extent of
EVA33 becomes more notable with increasing nanotube
contents than that of EVA15. EVA with high VA content
undergoes the structural change at relatively low concen-
tration of nanotube or more notable change at the corre-
sponding nanotube content. As mentioned above, the
nanotubes are efficiently mixed with EVA33 to have less
dense structure than EVA15 resulting in more steric hin-
drance by dispersed nanotubes.
Fig. 8 shows the variation of stress-strain (SS) curve of
EVA and EVA/MWNT composites with VA and nanotube
contents and their tensile properties are summarized in
Table 3. The presence of nanotubes does not modify the
overall SS behavior of both EVAs. The tensile modulus of
both EVAs obtained from the initial slope in the curve,
where the stress is proportional to the strain, increases
with increasing nanotube contents. Dispersed nanotubes
provide the resistance to the movement of polymer chains
under applied stress, resulting in an increase of the chain
rigidity. In particular, the nanotube addition gives more
improved modulus for EVA33 than for EVA15. Increased
free volume and polarity by increasing the VA content
seem to provide greater levels of filler-polymer intercon-
nection. Tensile strength of EVA33 increases with increas-
ing nanotube contents while that of EVA15 decreases.
Nanotubes in EVA33 with larger free volume retard effi-
ciently the disentanglement of polymer chain under exten-
sion than that in EVA15. Thus, much energy is required to
the fracture of the specimens. However, nanotubes in
EVA15 play a role of defects resulting from the concen-
trated stress in polymer parts. The dense structure by low
Fig. 7. Logarithmic plots of G' versus G'' of (a) EVA15/MWNT
composites and (b) EVA33/MWNT composites at 110oC.
Fig. 8. (Color online) Stress-strain curves of (a) EVA15/MWNT
composites and (b) EVA33/MWNT composites at 110oC.
Jong-Ho Kim, Seungwon Lee, Byoung Chul Kim, Bong-Seob Shin, Jong-Young Jeon and Dong Wook Chae
48 Korea-Australia Rheology J., 28(1), 2016
free volume does not have sufficient room between poly-
mer chains for dispersed nanotube, leading to reduced
interconnection between polymer matrix and filler. Thus,
the specimen with higher filler contents is fractured under
less stress. The presence of MWNTs in both EVAs increases
the yield stress with nanotube contents, obtained at the
point where the slope of the curve greatly decreases in
comparison with the initial slope. EVA33 shows more
improved yield stress than EVA15, exhibiting ca. 2.9
times increase at 5 wt.% nanotube loading and ca. 1.7
times one, respectively, compared with that of each neat
polymer. This can be explained that nanotubes play a role
in preventing the molecular slip of EVA under stress and
more efficiently for higher VA contents. The incorporation
of MWNTs diminishes the elongation at break of both
EVAs with increasing nanotube contents, suggesting that
the polymer chains surrounded by nanotubes are immo-
bilized. This reduction of the ductility is more notable for
less VA content, which might be associated with less inter-
action between nanotube and EVA and more presence of
some area where nanotubes are favorably populated.
4. Conclusions
This work aims to understand the combined effects of
polymer polarity and added nanotubes on the thermal,
mechanical, and rheological properties of EVA. EVAs with
two different VA contents were melt-compounded with
MWNTs. Bulky VA group in EVA increases free volume
and polarity. Thus, as VA content increases, incorporated
nanotubes might have more chances to be dispersed between
polymer chains and more contact between polymer and
nanotubes. This gave rise to have different degree of effects
on the thermal, viscoelastic, and tensile properties of EVA.
The addition of nanotubes increased slightly the crystal-
lization temperatures by ~1.9oC, indicative of weak nucle-
ation effects. Such a small increase suggests nanotubes are
favorably placed in the free volume between bulky VA
groups rather than ethylene ones. In the XRD measure-
ment, slight shift of crystalline peak was observed for
EVA 33, while there was little change for EVA15. At a
high VA content, nanotubes are efficiently packed in the
free volume between polymer chains resulting in close
distance and great interaction between polymer chain and
nanotube. The addition of nanotubes increased dynamic vis-
cosity and yield stress and decreased loss tangent. There
was a sharp change observed around 2 wt.% nanotube,
which might be ascribed to the presence of rheological per-
colation. In addition, more promoted viscoelastic proper-
ties were observed for higher VA content. Due to the loose
structure of bulky VA group, nanotubes in EVA 33 are
more efficiently dispersed than in EVA15 resulting in
increased contact probability and interaction between
nanotube and polymer chain. The presence of nanotubes
did not modify the overall SS curve of both EVAs. How-
ever, it increased the modulus and yield strength of EVA
33 more than those of EVA 15. Tensile strength of EVA 33
increased with nanotube contents, while that of EVA 15
decreased to some degree. This suggests that increased
free volume and polarity by bulky VA group lead to
greater levels of filler-polymer interconnection.
Acknowledgements
This research was supported by Kyungpook National
University Research Fund, 2012.
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