Bergaya F. Handbook of Clay Science

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Handbook of Clay Science

Edited by F. Bergaya, B.K.G. Theng and G. Lagaly

Developments in Clay Science, Vol. 1

r 2006 Published by Elsevier Ltd.

583

Chapter 10.3

CLAY MINERAL– AND ORGANOCLAY–POLYMER

NANOCOMPOSITE

E. RUIZ-HITZKY

AND A. VAN MEERBEEK

Instituto de Ciencia de Materiales de Madrid, Campus de Cantoblanco,CSIC,

ES-28049 Madrid, Spain

Advanced Elastomer Systems SA, B-1140 Bruxelles, Belgium

The development of inorgani c-organic hybrid materials was especially trendy in

the last decad e. This is attested by the ever growing number of symposia, books, and

specialised journals that are devoted to this subject. They parallel the intensive

research activity in complex systems at the atomic/molecular scale (nanometer scale)

of organic species of diverse functionality with inorganic entities, generally based on

silica or silicates. The application of the concepts of molecular eng ineering enables

new materials with a wide range of predete rmined properties to be obtained (Ruiz-

Hitzky, 1988).

By definition, composite materials are solids resulting from the combination of

two or more simple mate rials that develop a continuous phase (polymer, metal,

ceramic, etc.), and a dispersed phase such as glass ﬁbres, carbon particles, silica

powder, clay minerals, etc. In addition they have properties that are essentially

different from the components taken separately. Within the vast collection of in-

organic-organic hybrid materials, nanocomposites are an emerging group that re-

ceived a great deal of attention not only because of their potential in indust rial

applications but also from their fundamental point of view (Ruiz-Hitzky, 2003,

2004). Recent reviews on this topic were published by Alexandre and Dubois (2000)

Sinha Ray and Okamoto (2003) and Ruiz-Hitzky et al. (2004).

A well-accepted definition of nanocomposites is that the dispersed particles have

at least one dimension in the nanometer range (nan oﬁllers). Although it could be

confusing to the clay scientists, nanocomposites are usually classiﬁed according to

the number of the nanodimensions of the ﬁller (Table 10.3.1). Thus, in the case of

phyllosilicates like smectites integrated into nanocomposites, the clay–polymer

DOI: 10.1016/S1572-4352(05)01018-4

‘‘Polymer-clay nanocomposites’’ is more widely used than ‘‘clay-polymer nanocomposites’’ (and

thereby ‘‘organic-inorganic hybrids’’ vs. ‘‘inorganic-organic-hybrids’’) but the last term was retained in

this Chapter for coherence with the book style.

nanocomposite could be considered as ‘‘one-nano-dimensional’’ (1ND) because the

clay ﬁller has one dimension at the nano meter scale, although the clay ﬁller is two

dimensional (2D) in the microscopic sense.

This chapter is not an exhaustive summary of the published literature on nano-

composites. Rather, the aim is to call the attention of clay scientists and engineers to

the numerous opportunities that this type of new materials offer. Another aim of this

contribution is to establish a base for further studies, rationalise and comment on the

processes used to prepare those compounds, and ﬁnally to illustrate some properties

of these materials such as thermal, mechan ical, ﬂammabili ty, and conductivity.

10.3.1. EXFOLIATION OR INTERCALATION?

Many authors claim to achieved exfol iation (also called delamination) when pre-

paring a nanocomposite. In fact only a fraction (often not quantiﬁed) of the clay

mineral is truly exfoliated.

There is probably nothing to gain in entering into the futile debate of the clas-

siﬁcation of polymer-clay nanocomposites in intercalated or exfoliated compounds.

It should sufﬁce to say that there is a continuum of morphologie s and physical/

mechanical properties associated with either one or the other.

Purely exfoliated clay–polymer compounds are by definition those in which the

clay mineral particles are individually dispersed in the polymer. Only a few examples

of this type can be found in the literature, but many published microphotographs

show small regions in a compound where complete exfoliation occurred. Consistent

orientation of these particles during processing is certainly a chall enge that was not

Table 10.3.1. Definition of dimensionality in ﬁne particles used as ﬁllers in nanocomposites

Size/dimensionality Scheme Examples

a,b,c: 1–100 nm

3ND

silica nanoparticles

carbon black

3-nano-dimensional fullerenes

allophanes

a,b: 1–100 nm

nanowires (metals)

c > 100 nm nanoﬁbers (sepiolite)

nanotubes (carbon)

2-nano-dimensional

c : 1–100 nm

2ND

smectites, kaolinites

a,b > 100 nm layered double hydroxides (LDH)

1-nano-dimensional 1ND

Chapter 10.3: Clay Mineral– and Organoclay– Polymer Nanocomposite584

addressed: the organoclay particles are folded during processing (Fig. 10.3.1). Per-

fectly exfoliated compounds constitute one end of the spectrum; the other end is

represented by clay mineral–polymer compounds in which the polymer is interca-

lated as a monolay er into the clay mineral. The reality of nanocomposites is that

even in cases where exfoliation appears to have taken place, little if any individual

platelets can be found in the compound, and most often exfoliation only leads to one

layer or few layers (Akelah, et al., 1994; Gilman et al., 2000; Bafna et al., 2003).

It is beneﬁcial for the discussion, however, to agree and assign the most widely

acceptable definitions to the different polymer nanocomposite types, and above all,

to differentiate them clearly from the micro-composites. The latter are clay–polymer

compounds wherein clay is predominantly present as large size aggregates (dimen-

sions larger than 1 mm). For the nanocomposites, interesting sets of definitions were

been proposed to whic h even clay mineral specialists may partially adhere as follows:

(i) intercalated structures are formed when the polymer chains are inserted into the

interlayer space; and (ii) a delaminated (or exfoliated) structure results when the

silicate layers are no longer close enough to interact with each other.

Many authors proposed artiﬁcial limits to deﬁne the type of structure, based on the

presence or absence of diffraction peaks in a X-ray diffraction (XRD) diagram. In

particular, exfoliation was often hastily concluded from wide angle X-ray diffraction

Fig. 10.3.1. Transmission electron microscopy (TEM) image of a section of a clay-polysty-

rene nanocomposite. From (Hoffmann et al., 2000).

10.3.1. Exfoliation or Intercalation? 585

(WXRD) or small angle X-ray scattering (SAXS) measurements of the interlayer

spacing, overlooking the fact that interlayer expansion can occur without disaggre-

gation of the mineral (Moet, 1990). XRD diagrams do not show reﬂections for in-

tercalated structures having an interlayer space larger than 8 nm, corresponding to a

few mono-layers of polymer in the interlayer space. Distinguishing between the two

types of structures on the basis of their mechanical properties is often not possible as

some interesting properties were found also for intercalated structures, and mechanical

properties are not always improving with increasing interlayer expansion (Yano et al.,

1993; Kodgire et al., 2000).

Further, exfoliated nanocomposites can return to the status of intercalated mor-

phology under certain conditions (Manias et al., 2001). In the melt, mass transport

of the polyme r, entering the interlayer space is rapid, and the polymer chains exhibit

a mobility similar to or faster than the polymer self-diffusion (Giannelis et al., 1999).

If the thermodynamic conditions are favourable for intercalation, the polymer can

crawl in and out of the interlayer space until equilibrium is reached. Changes in

physical conditions (external pressure, temperature, shear, etc.) disturb this equilib-

rium, and the polymer may eventually leave the interlayer space. The mechanical and

physical properties of nanocomposites, and the relation to their morphology will be

discussed later.

10.3.2. CLAY MINERAL-POLYMER INTERACTIONS AND STRUCTURES

The interaction of phyllosilicates with polymers is an old theme ( Theng, 1979),

but it is only recently that innovative new uses of these materials emerged.

By the beginning of the 1960s (Blumstein, 1961), the ability of smectites to induce

the polymerisation of certain unsaturated monomers in the interlayer space of clay

minerals were already demonstrated (see Chapter 7.3). Once the polymer is formed,

it remains strongly associated to the mineral substrate, a nd is clearly located in the

interlayer space of the clay mineral. The resulting material displays a widely different

behaviour from either the original clay mineral or the neat polymer. From this

pioneering work to the recent commercialisation of TPO (modiﬁed polypropylene)

nanocomposite rocker panels in the 2002 models of General Motors Safari and

Astro vans, the clay–polymer interaction was intensively investigated in both basic

research and technological aspects. These developments always require a high degree

of creativity.

The most intense research devoted to clay–polymer nanocomposi tes concerns

natural (montmorillonite, hectorite, saponite) or synthetic (Laponite, ﬂuorohecto-

rites) smectites. These 1D nanoﬁllers have a layer thickness of the order of one

nanometer. The other dimensions of the clay mineral (except for Laponite) are about

1000 times larger than the thickness, i.e. they are at the micrometer scale. The

association of these clay minerals with polymers gives materials with different

structures (Fig. 10.3.2). The dispersion of clay aggregates in polymer corresponds to

Chapter 10.3: Clay Mineral– and Organoclay– Polymer Nanocomposite586

conventional ﬁller-polymer micro-composites. On the other hand, in nanocompos-

ites the single clay mineral layers or nanometer particles are dispersed in the polymer.

If the layers are regularly intercalated with polymers the nanocomposites are called

‘intercalated’. If the clay mineral layers or the particles are irregularly dispersed in

the polymer, the nanocomposites are called ‘delaminated’ or ‘exfoliated’. All these

structures can be characterized by XRD and TEM.

In addition to smectites, other clay minerals such as sepiolite and kaolinite can be

used to prepare nanocomposites. Polymers not only interact with the external sur-

face of sepiolite, but it can also penetrate into the structural tunnels of the mineral

(Inagaki et al., 1995; Sandi et al., 1999; Ruiz-Hitzky, 2001). Since these cavities are

organised as nanostructured pores, the resulting materials are considered as

clay–polymer nanocomposites. There are other cavities of nanometric dimensions

along the ﬁbre axis, mainly attributed to defects in particle growth that are accessible

to organic species (Kuang et al., 2003). Kaoli nite is also of interest in the preparation

of clay–polymer nanocomposites (Tunney and Detellier, 1996).

10.3.3. PREPARATION OF CLAY MINERAL–POLYMER

NANOCOMPOSITES

Smectites could form nanocomposites using one of the three preparation methods

(i) direct incorporation of the polymers; (ii) in situ polymerisation of the monomers;

and (iii) template synthesis of clay minerals.

micro-composite

polymer-clay nanocomposites

intercalated delaminated

polymer

single layers: 1nm

thickness

CLAY microcrystal

length 1000 nm

Fig. 10.3.2. Different structures proposed for clay-polymer materials arranged as nano- and

micro-structured solids.

10.3.3. Preparation of Clay Mineral– Polymer Nanocomposites 587

The clay mineral selected to prepare nanocomposites can occur in different forms

such as powder, ﬁlms, colloidal dispersion in water or in other polar liquids. De-

pending on the polymer or monomer to be intercalated, the clay mineral can be used

in the raw (untreated) state, or after speciﬁc modiﬁcation.

A. Direct Intercalation from Solution or Melt

Polar polymers, like poly (vinyl alcohol) (PVA), poly (ethylene glycol) (PEG) (Parﬁtt

and Greenland, 1970a, 1970b), poly (N-vinyl pyrrolidone) (PVP) (Levy and Francis,

1975), poly (ethylene oxide) (PEO) (Ruiz-Hitzky and Aranda, 1990; Aranda and

Ruiz-Hitzky, 1992; Wu and Lerner, 1993; Aranda and Ruiz-Hitzky, 1994; Vaia et al.,

1995; Aranda and Ruiz-Hitzky, 1999; Aranda et al., 2003), and chitosan (Darder

et al., 2003 ; 2005) that can be directly intercalated into smectites from solutions of

the polymers in polar solvents (water, alcohols, nitriles, etc.), and in some cases from

the melt.

The mechanism of this interaction is based in each case on the formation of

hydrogen bonds between the hydroxyl groups (in PVA, PEG), or the oxyethylene

groups (in PEO, PEG), or nitrogen heteroatoms (in PVP), of the polymers with the

mineral surface. This interaction occurs directly with the siloxane surface of the clay

mineral layer or with the water molecules coordinated to the interlayer cations,

through water bridges. Interactions of the ion-dipole type between the oxygen atoms

of the oxyethylene (–CH

–CH

–O–) groups and the interlayer cations were also

proposed (Aranda and Ruiz-Hitzky, 1992, 1994; Ruiz-Hitzky and Aranda, 2000).

The cation biopolymer chitosan is intercalated in homoionic smectites from solu-

tions through a cation exchange mechanism giving nanocomposites with a mono-

layer of the polymer. At high concentration, a second layer of chitosan is intercalated

by hydrogen bonding between the ﬁrst layer of polymer and the clay mineral surface

(Darder et al., 2003; 2005).

The synthesis of montmorillonite–PEO nanocomposites is a representative model

of the direct adsorption method. These functional nanocomposites show an attrac-

tive ion-conducting behaviour as reported for the ﬁrst time by Ruiz-Hitzky and

Aranda (1990). PEO of different molecular mass in acetonitrile solutions or other

polar solvents, are intercalated by smectites, forming stable interlayer coordination

complexes with the exchangeable cations. XRD shows an increase of the interlayer

distance (Dd), of about 0.8 nm, compatible with two possible arrangements of PEO:

(i) a bilayer of polymer chains in a planar zig-zag disposition or (ii) a monolayer of

polymer with helical conformation. The latter arrangement occurs when Li

or Na

is the exchangeable cation (Fig. 10.3.3). This is also consistent with IR,

C and

NMR spectroscopic data indicating that the oxygen atoms of PEO interact with

these cations (Aranda and Ruiz-Hitzky, 1992, 1994). Wu and Lerner (1993) de-

scribed two phases for Na

–montmorillonite–PEO nanocomposites prepared from

aqueous dispersions of the clay minerals suggesting either a mono- or bi-layer ar-

rangement of the polymer in zig-zag conformations. The arrangement of PEO in the

Chapter 10.3: Clay Mineral– and Organoclay– Polymer Nanocomposite588