ethylene/vinyl acetate copolymer/clay nanocomposites

HIGHLIGHT

Ethylene/Vinyl Acetate Copolymer/Clay Nanocomposites

S. K. SRIVASTAVA, M. PRAMANIK, H. ACHARYADepartment of Chemistry, Indian Institute of Technology, Kharagpur 721302, India

Received 21 June 2005; revised 12 September 2005; accepted 10 October 2005DOI: 10.1002/polb.20702Published online in Wiley InterScience (www.interscience.wiley.com).

Suneel Kumar Srivastava was born in 1954 in Ismilepur (Sitapur), Uttar Pradesh. He

received his M.Sc. degree in Inorganic Chemistry from Lucknow University, Lucknow.

Subsequently, he completed his post graduate diploma in ‘‘High pressure technology and

technical gas reactions’’ and Ph.D. under late Professor B.N. Avasthi in the field of ‘‘Solid

state chemistry’’ from Indian Institute of Technology, Kharagpur in 1979 and 1986,

respectively. He also carried out his post-doctoral work in Germany at the University of

Karlsruhe in 1988–89 and 2002, at University of Siegen in 1994 and 1999 as aDAADFel-

low, and at theUniversity of Nantes, France in 2003. He is currently anAssociate Professor

in the Department of Chemistry, Indian Institute of Technology, Kharagpur. His research

interests are in the field of layered materials specially clay and thermoplastic elastomer,

elastomer, plastic, polymer blends and solid polymer electrolyte based nanocomposites:

structure-property relationships. His research field also includes growth of transition metal

dichalcogenides, ternary and quaternary chalcogenides in micro and nanocrystalline forms.

Correspondence to: S. K. Srivastava (E-mail: [email protected])

ABSTRACT: This article high-

lights the history, synthetic routes,

material properties, and scope of

ethylene/vinyl acetate copolymer

(EVA)/clay nanocomposites. These

nanocomposites of EVAs are ac-

hieved with either unmodified or or-

ganomodified layered silicates with

different methods. The structures

of the resulting polymer/inorganic

nanocomposites have been char-

acterized with X-ray diffraction,

scanning electron microscopy, and

transmission electron microscopy.

The addition of a small amount of

clay, typically less than 8 wt %, to

the polymer matrix unusually pro-

motes the material properties, such

as the mechanical, thermal, and

swelling properties, and increases

the flame retardancy of these hy-

brids. VVC 2005 Wiley Periodicals, Inc.

J Polym Sci Part B: Polym Phys 44: 471–

480, 2006

Keywords: ethylene/vinyl ace-

tate copolymer (EVA); montmor-

illonite; nanocomposites; scanning

electron microscopy (SEM); swel-

ling; TEM

Journal of Polymer Science: Part B: Polymer Physics, Vol. 44, 471–480 (2006)VVC 2005 Wiley Periodicals, Inc.

SUNEEL KUMARSRIVASTAVA

471

INTRODUCTION

Polymer/layered silicate nanocomposites, known since

the 1990s, are two-phase materials with unexpectedly

improved properties normally unavailable with conven-

tional composites or pure polymers.1–5 Because of this,

the development of these nanocomposites has really

turned the research of advanced materials to a new direc-

tion with a special interest. First, a polymer/clay nano-

composite6 obtained from nylon-6/montmorillonite

(MMT) was reported by a Toyota group of researchers;

clay loadings on a very small scale resulted in thermally

and mechanically high-performance nylon-6. Second,

such a nanocomposite was synthesized by Giannelis

et al.7 through the melt blending of polymers with clays.

Since then, a lot of research work on the development of

high-performance (i.e., industrially applicable) polymer

nanocomposites has been done, and it has exhibited dra-

matically enhanced properties of many matrix materials

as a result of the nanodispersion of silicate layers. These

investigations have led to an interesting point, that sili-

cate layers can be dispersed in a wide range of polymers

on a nanometer level to achieve highly reinforced poly-

mer composites. The extent of the improvement is

directly related to the aspect ratio and the dispersion of

the clay layers in the polymer matrix.8 Depending on the

degree or nature of the dispersion of the clay layers in

the polymer matrix, either intercalated nanocomposites,

in which there is a regular insertion of the polymer

chains between the clay layers, or delaminated nano-

composites, in which the clay layers are randomly and

homogeneously dispersed in the polymer matrix, can be

obtained.9 Because of the structural feature, the latter

type offers the maximum improvement in mechanical

and other physical properties of the polymer. Some-

times, the intercalated and delaminated configurations

coexist.10 Figure 1 displays the different classes of poly-

mer/clay nanocomposites.

Clay minerals are layer-structured materials11–13 and

are formed by the assemblage of tetrahedral and octahe-

dral sheets. These layers are not electrostatically neutral

but rather are negatively charged; this results from the

Monoj Pramanik received his B.Sc.(Hons.) degree in Chemistry from Midnapore

College, Vidyasagar University (V.U.), India in 1996. He received his M.Sc. degree

in Chemistry with specialization in Inorganic Chemistry from V.U. in 1998. Mr. Pra-

manik has received his Ph.D. in Chemistry from the Indian Institute of Technology,

Kharagpur, India in 2005. He has worked on ‘‘Inorganic Layered Materials-Polymer

Nanocomposites’’ with Prof. Suneel Kumar Srivastava, Department of Chemistry,

Indian Institute of Technology, Kharagpur, India. Presently, he is a Postdoctoral

Research Fellow working in ‘‘Coatings’’ in Thames-Rawlins Research Group,

School of Polymers and High Performance Materials, University of Southern Missis-

sippi. He is interested in polymer nanocomposites and coating technology.

Himadri Acharya was born in 1979 at Contai (Midnapore), West Bengal, India.

He received his B.Sc. (2000) and M.Sc. (2002) degree in Chemistry from the

Vidyasagar University, Midnapore, India. Subsequently, he joined the Depart-

ment of Chemistry, Indian Institute of Technology, Kharagpur, for Ph.D. with

Professor Suneel K. Srivastava. His research focuses on nanocomposites sys-

tems with layered inorganic nanomaterials.

Figure 1. Different configurations of polymer/clay nano-

composites.

MONOJ PRAMANIK

HIMADRI ACHARYA

472 J. POLYM. SCI. PART B: POLYM. PHYS.: VOL. 44 (2006)

isomorphous substitution of Al3þ in the octahedral sheet

by Mg2þ and Fe2þ and the substitution of Si4þ in the tet-

rahedral sheet by Al3þ. The negative charge is balanced

by interlayer cations (usually Mg2þ, Naþ, Ca2þ, etc.).Both natural clays and synthetic layered silicates are

being used in the development of polymer/clay nano-

composites and hybrid materials.14,15 Clay minerals used

for the synthesis of polymer nanocomposites are mostly

organomodified. A very limited number of unmodified

clays have been used to develop polymer nanocompo-

sites16,17 because the polymers are to be hydrophilic and

sufficiently polar.18 The organomodification of clay

could change the hydrophilic nature to a hydrophobic

one to create an appropriate environment for the interac-

tion with the polymer and additionally expand the gal-

lery height;19 this may lead to complete exfoliation of

silicate layers during the processing of polymer nano-

composite. That means organomodification makes clays

compatible with a polymer for the synthesis of nano-

composites. In most cases, to render clays compatible

with other polymers, interlayer cations are exchanged

with cationic–organic surfactants, such as alkylammo-

niums.10,19

Poly(ethylene-co-vinyl acetate) polymers have a

large commercial impact because of their broad spec-

trum of practical applications in different fields: electri-

cal insulation, cable jacketing and repair, component

encapsulation and waterproofing, corrosion protection,

packaging of components, and shoes.20 Different vinyl

acetate contents indeed make ethylene/vinyl acetate

copolymers (EVAs) available as rubbers, thermoplastic

elastomers, and plastics, which in turn provide this broad

spectrum of uses. Because of the high extent of commer-

cial importance, recently researchers have made an

intense focus on EVA composites filled with nanolevel-

dispersed layered silicates. EVA/clay nanocomposites

were first developed contemporarily by Mulhaupt

et al.21 and Srivastava et al.22 After this, many research

groups9,18,23–25 synthesized EVA/clay nanocomposites.

SYNTHESIS AND STRUCTURE OF EVA/CLAY NANOCOMPOSITES

EVAs have been processed into their nanocomposites

with clay minerals by either melt blending or solution

blending by different research groups. In the melt-blend-

ing process, the polymer chains lose their conforma-

tional entropy because of their intercalation between the

clay layers, and this loss of conformational entropy is

probably compensated by enthalpic interactions between

the clay and the polymer, which in turn drive the pro-

cess.26,27 However, in the case of solution blending,

polymer intercalation from a solution is associated with

the entropy gained by the desorption of solvent mole-

cules, which is compensated by the decrease in the confor-

mational entropy of the intercalated polymer chains.26

Two different EVAs containing 12 and 19% vinyl

acetate, abbreviated as EVA-12 and EVA-19, respec-

tively, were processed into their nanocomposites with

synthetic clay fluorohectorite (FH) organomodified by

octadecyl ammonium ion (ODA) and aminododecanoic

acid.21 These materials were melt-blended at 120 8C in a

DACA twin-screw minicompounder to obtain nanocom-

posites. Octadecyl ammonium ion intercalated FH (FH-

ODA) formed a delaminated nanostructure, whereas

ammoniumdodecanoic acid intercalated FH formed a

microcomposite. This means that the octadecyl ammo-

nium ion is more compatible than ammoniumdodeca-

noic acid when intercalated in FH during the synthesis

of EVA-19/FH nanocomposites.

Alexandre et al.9 prepared EVA/clay nanocomposites

from different EVAs with organomodified MMT. EVAs

containing 4.2, 7.1, and 10.8 mol % vinyl acetate were

separately melt-blended (at 130 or 160 8C) with MMT

organomodified by a series of alkylammonium ion and

amine containing carboxylic acids. In this case, di(hy-

drogenated tallow) dimethyl ammonium and dimethyl-

2-ethylhexyl (hydrogenated tallow) intercalated (MMT)

layers exfoliated in the polymer matrix to form nano-

composites, but ammonium ion containing carboxylic

acid moiety intercalated MMT formed microcomposites.

Other EVA/clay nanocomposites23–25 were synthe-

sized by a melt-blending method. Tang et al.24 melt-

blended EVA with organophilic MMTs such as hexa-

decyl and octadecyl ammonium ions at about 145 8C to

obtain EVA/clay nanocomposites. In these EVA/clay

composites, individual silicate layers were exfoliated in

the polymer matrix in a nanometer range along with

some large, intercalated tactoids. Zhang et al.25 also syn-

thesized EVA/clay nanocomposites through a melt-

blending method, taking different EVAs ranging from

thermoplastic elastomers to rubbers with octadecyltri-

methyl ammonium, dioctadecyldimethyl ammonium,

and tricetadecymethyl ammonium ion exchanged MMT.

Interestingly, EVA chains intercalated into the organo-

modified MMT sheets, but in the case of sodium mont-

morillonite (Na-MMT), there was no such intercalation.

However, Li and Ha23 selected maleic anhydride grafted

EVA containing 18 mol % vinyl acetate to process its

nanocomposites with organomodified Cloisite through

melt blending at 175 8C. The dispersion of Cloisite in

the maleic anhydride grafted EVA was much better than

the simple EVA matrix, probably because of the polarity

of maleic anhydride.

EVA/clay nanocomposites22,28–30 were also devel-

oped by the solution blending of EVAs (EVA-12, EVA-

28, and EVA-45 containing 12, 28, and 45% vinyl ace-

HIGHLIGHT 473

tate, respectively) with organomodified MMTs such as

dodecyl ammonium ion intercalated MMT (12Me-

MMT). To observe the dispersion of the clay layers and

morphology, X-ray diffraction (XRD) patterns of an

EVA-12/12Me-MMT composite system and a transmis-

sion electron microscopy (TEM) photograph of an EVA-

12/12Me-MMT composite are displayed in Figures 2

and 3, respectively.30 Na-MMT and 12Me-MMT show

basal reflection peaks at 2h values of 7.4 and 5.68,respectively, corresponding to their respective interlayer

spacings of 1.194 and 1.578 nm. This increase in the

interlayer spacing by 0.384 nm can be attributed to the

intercalation of dodecyl ammonium ions. EVA-12

exhibits no peak in the angle range (2h ¼ 3–108) of

study up to the filler loading of 6 wt %. The disappear-

ance of this peak indicates the delamination or exfolia-

tion31 of aluminosilicate layers of MMT. Upon further

filler loading up to 8 wt %, the EVA-12/12Me-MMT

composite shows a peak in the angle range of 2h ¼ 5–

68, probably because of the aggregation of silicate layers

of clay.8,32 A TEM photograph of this composite shows

that the aluminosilicate layers of MMT are randomly

and homogeneously dispersed on a nanometer scale in

the EVA-12 matrix, supporting directly the formation of

nanocomposites. A similar observation was made for

EVA-19/FH–ODA nanocomposites.21

MATERIAL PROPERTIES

Mechanical Properties

In most of the EVA/clay nanocomposites reported so far

by different research groups, the mechanical properties

in terms of the tensile strength (TS), elongation at

break (EB), elongation modulus, and Young’s modulus

as a function of the clay content have been stud-

ied;9,18,22,24,28–30 these nanocomposites have exhibited

improved mechanical properties. As a typical example,

the tensile properties of EVA-12/montmorillnite nano-

composites30 are described in Figures 4 and 5. TS and

EB of the composites increase significantly with the

introduction of the organoclay into the EVA-12 copoly-

mer matrix. However, this enhancement is maximum

when the clay concentration is only 2 wt %. Upon a fur-

ther increase in the clay concentration up to 4 wt % in

the polymer matrix, TS remains almost constant. This

improvement in TS and EB of the nanohybrids indicates

that the EVA-12 matrix is strengthened by the incorpo-

Figure 2. X-ray diffractograms of (a) Naþ-MMT, (b) 12Me-

MMT, (c) EVA-12, (d) EVA-12 and 2 wt % 12Me-MMT, (e)

EVA-12 and 4 wt % 12Me-MMT, (f) EVA-12 and 6 wt %

12Me-MMT, and (g) EVA-12 and 8 wt % 12Me-MMT.

Reproduced from M. Pramanik et al., Macromol Res 2003, 11,

260, with permission from Polymer Society of Korea.

Figure 3. TEM photograph of a nanocomposite containing

2 wt % 12Me-MMT in an EVA-12 matrix. Reproduced from

M. Pramanik et al., Macromol Mat Eng 2004, 289, 56, with

permission from Wiley-VCH Verlag.


ration of 12Me-MMT dispersed homogeneously and

randomly in the EVA-12 matrix. The delamination of

the aluminosilicate layers of 12Me-MMT to the nanole-

vel in the polymer matrix offers the whole surface of the

silicate layers for strong interaction with the polymer

chains. This strong interfacial interaction between the

polymer chains and nanolevel-dispersed layers of 12Me-

MMT forms the shear zones when the hybrids are under

stress and strain. The formation of such shear zones is

distinctly supported by scanning electron microscopy

(SEM) images30 [shown in Fig. 6(a,b)] of fracture surfa-

ces of the composites containing 0 and 2 wt % organo-

clay: the fracture surface of the composite containing

0 wt % organoclay exhibits some cracks, whereas no

crack but rather some shear zones on the fracture surface

of the nanocomposite containing 2 wt % organoclay can

be observed, as shown in Figure 6(b). Because of this

strong interaction and development of shear zones, TS

of the nanocomposites is increased. With an increase in

the organoclay loading from 4 to 8 wt % in the EVA-12

matrix, it can be clearly observed that both TS and EB

decrease, and this decrease in TS and EB is probably

due to the aggregation of aluminosilicate layers of clay.

This means, the degree of delamination of aluminosili-

cate layers starts to decrease beyond the 4 wt % filler

loading in the EVA-12 matrix. The aggregation of orga-

noclay particles in the polymer matrix is evidenced by

XRD of the composite with 8 wt % organoclay. Figure 5

depicts the relationship between the modulus at different

elongations and the organoclay content in the EVA-12

matrix. The modulus at 100, 200, and 300% elongations

of the nanocomposites also increases linearly with the

organoclay content in the polymer matrix because of the

strong interaction of aluminosilicate layers with the pol-

ymer chains. Similar improvements in the tensile proper-

ties were observed in EVA/clay nanocomposites,22,28

but this was at a comparatively higher filler loading for

higher vinyl acetate content EVAs such as EVA-28 and

EVA-45. EVA/clay nanocomposites prepared by a melt-

intercalation method9 show that the Young’s modulus of

the nanocomposites increases, but at the same time, both

TS and EB of the nanocomposites slightly fall. It is very

difficult to compare the extent of the improvement of the

mechanical properties of different EVA/clay nanocom-

posites reported so far because EVAs of different vinyl

acetate contents have been processed into their nano-

composites with different clays and different modifying

agents by different methods.

Thermal Properties

The thermal stability of any polymer and its nanocompo-

site is an important property for designing the material

for a particular use in a specific field. In particular, in the

case of nanocomposites, the dispersion of clay particles

in a polymer matrix plays a significant role in changing

the thermal behavior. It has been well established by dif-

ferent research groups that EVAs exhibit two-step

decompositions.21,33 Figure 7 elucidates the effect of

aluminosilicate layers on the thermodegradation of

EVA-12.30 A two-step weight loss of EVA-1233 and its

hybrids can be observed because of two-step decomposi-

tions. The first step corresponds to the deacetylation

reaction, whereas the second one is associated with the

formation of transvinylenes accompanied by the main-

chain scission. The mechanism of this two-step decom-

position has been presented elsewhere.21,33 The thermal

stability of EVA-12 increases with the introduction of

12Me-MMT. The thermal stability of an EVA-12/clay

hybrid with 2 wt % 12Me-MMT is greater. Upon a fur-

Figure 4. TS and EB versus the clay content of EVA-12/

12Me-MMT nanocomposites. Reproduced from M. Pramanik

et al., Macromol Res 2003, 11, 260, with permission from

Polymer Society of Korea.

Figure 5. Variation of the modulus at different elongations

versus the 12Me-MMT content of EVA-12/12Me-MMT

nanocomposites. Reproduced from M. Pramanik et al., Mac-

romol Res 2003, 11, 260, with permission from Polymer

Society of Korea.

HIGHLIGHT 475

ther increase in the 12Me-MMT loading, the hybrids

show a decreasing trend toward their initial thermal

decomposition temperature. Thermogravimetric analysis

(TGA) of EVA-12 and its hybrids shows that the weight

loss in the first step is least when the filler loading is

only 2 wt %. The second-step weight loss decreases with

an increase in the filler content. Figure 7 also explicitly

shows that the overall weight loss of the hybrids is inver-

sely related to the filler loading; that is, the char forma-

tion and the filler loading show a one-to-one correspond-

ence because of the high heat resistance exerted by the

clay. These findings lead to the fact that the EVA-12

matrix is thermally enhanced for lower filler contents

(2 wt % 12Me-MMT) because of homogeneous exfolia-

tion and random dispersion of 12Me-MMT on a nano-

meter level. These nanometer-level-dispersed silicate

layers interact strongly with the polymer chains, and

simultaneously, the barrier effect of silicate layers inhib-

its the mobility of small molecules produced as a result

of thermal degradation. These two effects of silicate

layers together contribute toward the enhancement of

the thermal stability of the nanocomposites. It may be

added here that the higher filler loading destabilizes the

EVA-12 matrix because of the aggregation of silicate

layers (as evidenced through XRD of a hybrid contain-

ing 8 wt % 12Me-MMT), which ultimately provides a

lesser extent of surface area, which, in turn, offers rela-

tively weak interaction with the polymer chains. EVA/

clay nanocomposites22,30 containing more vinyl acetate

Figure 7. TGA of (a) EVA-12, (b) EVA-12 and 2 wt %

12Me-MMT, (c) EVA-12 and 4 wt % 12Me-MMT, (d) EVA-12

and 6 wt % 12Me-MMT, and (e) EVA-12 and 8 wt % 12Me-

MMT. Reproduced from M. Pramanik et al., Macromol Res

2003, 11, 260, with permission from Polymer Society of Korea.

Figure 6. SEM images of fracture surfaces of (a) EVA-12 and

(b) an EVA-12/clay nanocomposite containing 2 wt % 12Me-

MMT. Reproduced from M. Pramanik et al., Macromol Res

2003, 11, 260, with permission from Polymer Society of Korea.


keep this thermal stability improvement up to 6–8 wt %

filler loadings, probably because of the degree of disper-

sion of clay in the polymer matrices. A similar improve-

ment in the thermal stability has been observed for

EVA/18Me-MMT nanocomposites24 slightly at higher

filler loadings. According to ref. 24, the first step of the

decomposition of EVA/MMT nanocomposites is not

strongly influenced by the catalytic effect offered by the

modifier of the clay. Zanetti et al.21 studied the thermal

degradation of EVA-19/modified FH and MMT nano-

composites in air and in nitrogen. They observed that

both the EVA-19/modified FH and MMT nanocompo-

sites did not show the catalysis of deacetylation in air as

found in nitrogen. This means that exfoliated silicate

layers tend to shield EVA polymers from the action of

oxygen.

Swelling Characteristics

The knowledge of the swelling of a polymer and its

nanocomposites is an important aspect for predicting the

extent of suitability for their applications in a particular

environment. An EVA-28/MMT nanocomposite is given

as a typical example of EVA/clay nanocomposites34 to

observe the effect of the nanodispersion of clay in the

EVA matrix. The swelling indices of nanocomposites

obtained from EVA-28 and 12Me-MMT are displayed

in Figure 8. The insertion of nanometric layers of alumi-

nosilicate in the EVA matrix reduces the swelling indi-

ces of this copolymer, and it varies inversely with the

filler loading. This is due to the inhibiting capability of

clay layers to swell in a solvent. The swelling of a poly-

mer is associated with a general physical phenomenon

called diffusion, by which liquid molecules are first

adsorbed onto the polymer surface and then are trans-

ported from one region to the other. The crosslinks

between polymer chains usually resist the solvent mole-

cules to penetrate the polymer matrix.35 To observe the

effect of aluminosilicate layers on the penetration of

liquid molecules, the crosslink density (defined as the

number of moles of crosslinks per unit of volume) of

nanocomposites can be calculated by the Flory–Rehner

equation.36 Figure 9 presents the effect of aluminosili-

cate layers on the volume fraction and crosslink density

of EVA during swelling.34 Both the volume fraction and

crosslink density of the EVA-28 copolymer in swollen

EVA-28/12Me-MMT nanocomposites are greater than

those of its pure form; that is, the solvent uptake of EVA

polymers tends to decrease when the polymer matrix is

filled with a nanolevel dispersion of clay layers, which

causes a strong interaction with polymer chains because

nanolevel-dispersed clay particles make the whole sur-

face area available for the interaction with the polymer

chains. Indeed, this strong interfacial interaction

between the polymer chains and silicate layers inhibits

the penetration of xylene molecules from the surface to

the bulk region of the nanocomposites, leading to a

reduction of the xylene uptake. In addition, Figure 9

shows that the volume fraction and crosslink density of

the EVA matrix in the swollen mass is increased with

the filler content, and this, in turn, forces a reduction in

the interaction between the polymer chains and solvent

molecules. This is attributed to the fact that the presence

of more nanolevel-dispersed clay particles substantially

imparts polymer–clay interactions. Interestingly, the

crosslink density of the EVA-28/clay system increases

rapidly up to filler loadings of about 4–6 wt %, but upon

further 12Me-MMT loading, the rate of increase in the

crosslink density is quite slow. This is because, beyond

4–6 wt % clay, aluminosilicate layers probably start to

aggregate in the EVA matrix.28,30 Because of this aggre-

gation of silicate layers, the interaction between the poly-

mer chains and silicate layers tends to decrease, and this,

in turn, causes a slow increase in the volume fraction

of the EVA matrix in the swollen gel as well as a slow

increase in the crosslink density. Normally, the swel-

ling of a polymer in a solvent is favored by an increase

in entropy and by a decrease in Gibb’s free energy.

This implies that a high degree of swelling is associ-

ated with the more positive entropy and more negative

Gibb’s free energy of swelling polymers. The free

energy change (DGw) and entropy change (DSw) of

EVA/clay nanocomposites34 for swelling are displayed

in Figure 10. Figure 10 clearly depicts that DGw of

swelling increases with the 12Me-MMT insertions in

the EVA matrix. This means that clay particles dis-

persed on a nanolevel play a negative role in the swel-

ling of EVAs. Such a finding can be attributed to the

Figure 8. Effect of the 12Me-MMT content on the swelling

index of EVA-28/12Me-MMT nanocomposites.

HIGHLIGHT 477

fact that these nanolevel-dispersed clay particles prob-

ably create an environment like a bound polymer with

a polymer matrix, which forces the EVA matrix to

unswell in xylene. The entropy of swelling of these

nanocomposites decreases with the 12Me-MMT con-

tent, as shown in Figure 10, and this indicates the ten-

dency of unswelling in xylene. This means that the

nanolevel-dispersed clay particles hinder the EVA ma-

trix from swelling in xylene as a high degree of swel-

ling is favored by an increase in entropy. Very similar

swelling characteristics have been observed in nano-

composites of EVA-12 and EVA-45 with clay.34

Figure 10. Effect of the 12Me-MMT content on DGw and DSw of the swelling of EVA-28/

12Me-MMT nanocomposites.

Figure 9. Effect of the 12Me-MMT content on the volume fraction and crosslink density of

the swelling of EVA for EVA-28/12Me-MMT nanocomposites.


Dynamic Mechanical Properties

The dynamic mechanical properties are the mechanical

properties of materials as they are deformed under peri-

odic forces. The polymeric materials under such condi-

tions are often in a dynamic stress and strain field. The

knowledge of the dynamic mechanical properties of poly-

meric materials is therefore indispensable for the design

of these polymers and their nanocomposites. The signifi-

cant role of silicate layers of clay in the storage modulus

(E0) and damping [loss tangent (tan d)] of EVA-28/claynanocomposites37 is presented in Figures 11 and 12. E0

of the EVA-28/MMT nanocomposites increases with the

filler loading in the EVA-28 matrix. The E0 values of anEVA-28/MMT nanocomposite containing 6 wt % are

2.0 � 109 Pa at about 60 8C below the glass-transition

temperature (Tg), 4.8 � 108 Pa at Tg, and 3.5 � 107 at

25 8C above Tg; which are about 1.33, 2.28, and 2.50

times that of pristine EVA-28. This increase in the mod-

ulus results because the silicate layers interact with

EVA-28 chains, strengthening the dynamic modulus.25

Tg of EVA-28/MMT nanocomposites, accompanied by

b relaxation,38 increases slightly at lower filler contents

and decreases with the filler loading. Additionally, the

peak height of tan d decreases with the clay content in

the polymer matrix because of the increasing interaction

between the silicate layers and polymer chains. In the

case of EVA/clay nanocomposites25 prepared by melt

blending, there is also a significant improvement in E0

for EVA/modified-MMT nanocomposites but not for

EVA/Na-MMT microcomposites.

Other Properties and Scope

Fire-retardant materials must be quite resistant to igni-

tion or flame. EVA/clay nanocomposites are also good

fire retardants. Thus, EVA/clay nanocomposites pre-

pared by Duquesne et al.39 exhibited a 50% reduced

peak heat release rate in comparison with pure EVA. As

EVAs are used as packaging and dielectric materials, the

nanolevel dispersion of these silicate layers in these poly-

mer matrices is expected to cause unusual changes in the

gas-barrier, solvent-barrier, and dielectric properties.

These properties are to be investigated because it is

essential to understand them before the design of these

nanocomposites for applications. In our laboratory, we

are investigating these particular properties of these

nanocomposites.

The Ministry of Human Resource Development (New Delhi,

India) and Council of Scientific Industrial Research are grate-

fully acknowledged for providing the financial support to

carry out this investigation smoothly.

Figure 11. E0 versus the temperature for (a) EVA-28, (b)

EVA-28 and 2 wt % 12Me-MMT, (c) EVA-28 and 4 wt %

12Me-MMT, (d) EVA-28 and 6 wt % 12Me-MMT, and (e)

EVA-28 and 8 wt % 12Me-MMT.

Figure 12. Tan d versus the temperature for (a) EVA-28,

(b) EVA-28 and 2 wt % 12Me-MMT, (c) EVA-28 and 4 wt

% 12Me-MMT, (d) EVA-28 and 6 wt % 12Me-MMT, and

(e) EVA-28 and 8 wt % 12Me-MMT.

HIGHLIGHT 479

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