effect of va and mwnt contents on the rheological and ... · thin pt layer to avoid charging. the...

© 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


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.



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.



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.



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



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.



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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effect of va and mwnt contents on the rheological and ... · thin pt layer to avoid charging. the...

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