Bergaya F. Handbook of Clay Science

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interlayer space depends on such factors as the nature of the exchangeable cation

and the water content (Hackett et al., 2000).

The technique of melt intercalation (Vaia et al., 1993 ) of polymers into phyllo-

silicates, is considered as one of the major practical steps forward in the ﬁeld of

nanocomposite preparat ion. This procedure was successfull y applied to prepare PEO

nanocomposites (Vaia et al., 1995; Giannelis, 1996). PEO is intercalated in homo-

ionic alkaline cations exchanged montmorillonites after 6 h of treatment at 80 1C.

This temperature is only about 10 1C above the melting point of PEO, but is ap-

parently sufﬁcient to ensure the mobility of the polymer chains so that they can

migrate into the interlayer space of the clay mineral.

The effectiveness of melt intercalation could be further improved by using mi-

crowave (MW) irradiation. The energy is mainly absorbed by the water molecules

coordinated to the interlayer cations. As a result the polymer and eventually the clay

mineral are heated, promoting rapid entry of the polymer into the interlayer space of

the montmorillonite. The MW-assisted melt intercalation procedure is successfully

applied for the ﬁrst time to the preparation of montmorillonite–PEO nanocompos-

ites ( Aranda et al., 1998, 2003), saving time and energy compared to the conven-

tional method of oven-heating.

PEO is also adsorbed on sepiolite, either from a solution or from the melt by MW

irradiation. Only preliminary results were published (Ruiz-Hitzky, 2001). The salient

feature is that the polymer chains can partially penetrate into the structural tunnels

of 0.4 by 1.1 nm .This pr ocess apparently involves the irreversible displacement of

water molecules ﬁlling the tunnels of the natural sepiolite.

Until recently, the direct intercalation of polymers into kaolinite appears to be

unfeasible (Sanz and Serratosa, 2002). However, if the kaolinite layers are previously

expanded, using polar molecules like dimethylsulphoxide (DMSO), the interlayer

space becomes accessible to polar polymers like PEG (Tunney and Detellier, 1996).

The resulting kaolinite–PEG nanocomposites show Dd values of about 0.4 nm in-

dicating that monolayers of polyoxyethylene chains in planar zig-zag conforma tion

interlayer cation

intercalated

PEO-salt complex

layer silicate

8 Å

Fig. 10.3.3. Structural model of montmorillonite-PEO nanocomposites prepared from ace-

tonitrile solutions. After Ruiz-Hitzky and Aranda (1990).

10.3.3. Preparation of Clay Mineral– Polymer Nanocomposites 589

were intercalated. In contrast, a distorted helical structure for PEG is observed in

modiﬁed PEG intercalated in layered double hydroxides (LDH). This is attributed to

strong interactions between surface hydroxyl groups of LDH and the oxygen atoms

of the polymer (Leroux et al., 2003).

B. In Situ Polymerisation

This process consists of the intercalation of monomers as precursor species, followed

by their polymerisation inside the interlayer space of the pristine clay mineral.

The ﬁrst polymerisation in the interlayer space of a clay mineral was reported by

Blumstein (1961) who demonstrated the possibility of achieving the homopolymer-

isation of unsaturated monomers, such as acetonitrile and methyl methacrylate,

previously intercalated in smectites. Numerous nanocomposite materials were pre-

pared by the so-called ‘in situ intercalative polymerisation’ (Kanatzidis et al., 1986).

For instance, the polymerisation of pyrrole and aniline in the interlayer space of a

clay mineral was reported by Cloos and co-workers (Cloos et al., 1979; Moreale

et al., 1985). In this process, interlayer exchangeable Cu

cations in smectites pro-

mote the formation of the aniline radical (by oxidation), inducing its polymerisation

into polyaniline (PANI). The interaction of pyrrole with iron-rich smectites, con-

taining structural Fe(III), exchangeable Fe

species or associated oxyhydroxyde Fe

species, spontaneously gives clay mineral–PPy nanocomposi tes Letaı

ef et al., 2005).

Other polar species like thiophene and its derivatives can also form nanocom-

posites by interlayer polyme risation induced by Cu

and other transition metal ions

in smectites (Cloos et al., 1973; Soma et al., 1987). Not every polar monomer gives

nanocomposites by this process. For example, vinylcarbazole gives clay–polymer

compounds after treatment of a smectite by the monomer in benzene solut ion (or in

presence of the molten polymer, at 65 1C) (Biswas and Ray, 1998). The authors

indicate that the formation of the poly(N -vinylcarbazole) (PNVC) is directly induced

by the action of the clay mineral involving cationic proton species, without any

addition of polymerisation initiators. Similarly, Lewis or Brønsted acid sites in clay

minerals such as montmorillonite and kaolinite can induce polymerisation of dif-

ferent vinyl monomers such as styrene (Solomon, 1968; Solomon et al., 1971; Haw-

thorne et al., 1974). These reactions occur by simply heating a blend of the clay

mineral and the monomer in soft conditions, giving in this case non-intercalated clay

mineral–polymer materials (micro-composites). Further, XRD shows a very weak

peak at 1.46 nm assigned to the intercalated PNVC in additio n to the peak at 1.0 nm

characteristic of the pristine clay mineral. These results are not conclusive for the

formation of true nanocomposites.

AN can also easily intercalate into smectites where it is directly associated with the

interlayer cations such as Li

or Na

, forming complexes through –CRN?cation

(ion-dipole) interactions (Blumstein et al., 1974; Bergaya and Kooli, 1991; Sanz and

Serratosa, 2002). Heat treatment or gamma-irradiation induces the polymerisation

of AN in the interlayer space, giving PAN. Similarly , AN is incorporated into the

Chapter 10.3: Clay Mineral– and Organoclay– Polymer Nanocomposite590

channels of sepiolite replacing zeolitic water and interacting with coordinated water

molecules by hydrogen bonding (Ferna

ndez-Saavedra et al., 2004). In this case the

polymerisation of AN to PAN is achieved using a radical initiator such as 2,2

azobisisobutyronitrile.

In contrast to hydrophilic monomers, which polymerise in the interlayer space,

low polarity monomers such as styrene, can only polymerise in the interlayer space if

the clay mineral was pr eviously modiﬁed. This will be discussed later in this chapter.

The preparation of sepiolite–polymer nanocomposites based on isoprene and

styrene entering into the tunn els of this clay mineral was reported (Inagaki et al.,

1995). On the basis of adsorption isotherms, the authors propose that the monomers

are adsorbed on both the external surface and in the tunnels of the mineral. The

strong Brønsted acid sites of sepiolite being assum ed to catalyse the process of the in

situ polymerisation. However, the occurrence of acid sites in sepiolite is uncertain

because adsorbed basic species such as pyridine are not protonated (Ruiz-Hitzky,

2001; Kuang et al., 2003). The colour changes observed after adsorption of isoprene

suggest instead that the transition metals such as iron, are involved in the polym-

erisation reaction.

Other molecules such as pyrrole and thiophene polymerise incompletely when

adsorbed on sepiolite (Inagaki et al., 1995). The formation of polymers from such

monomers, however, is of potential interest for the development of conducting

nanowires, having the size and shape imposed by the geometry of the sepiolite.

Under drastic conditions, it is possible, to polymerise unsaturated monomers such as

ethylene in the tunnels of sepiolite (Sandi et al., 1999). If the sepiolite–PE nano-

composites are subsequently carbonised by pyrolysis, carbon nanoﬁbres useful for

solid-state lithium batteries, can be produced. Similarly, carbonisation of sepio-

lite–PAN nanocomposites can give rise to sepiolite-carbon materials useful as elect-

ro-active components in electrochemical devices. The elimination of the sepiolite

moiety by treatment with HCl and HF acids gives rise to carbon nanoﬁbres of ca.

1 mm by 20–30 nm (Ferna

ndez-Saavedra et al., 2004).

C. Template Synthesis

This recent procedure (Carrado and Xu, 1998; Carrado, 2000; Carrado et al., 2000)

is based on the in situ hydrothermal crystallisation of gels from a mixture of silica

sol, magnesium hydroxide, LiF and selected water-soluble polymers in solution (Fig.

10.3.4). The synthetic clay mineral formed under these conditio ns is considered as a

poorly ordered ﬂuorohectorite.

By this method, polymers like PANI, PAN and PVP give hybrid materials with a

controllable silicate/polymer ratio. This value determines both the structural ar-

rangement and properties such as the electrical conductivity. The clay mineral–PANI

nanocomposites prepared following this procedure are semi-delaminated, showing

clay/polymer ratios smaller than those of their counterparts prepared by in situ

polymerisation. One interesting characteristic of these nanocomposites prepared by

10.3.3. Preparation of Clay Mineral– Polymer Nanocomposites 591

this method is their ability to form well-dispersed systems in an aqueous medium

without any loss of the polymer back into the solution (Carrado et al., 2000). Un-

fortunately, this method does not work with certain polymers such as PEO.

The compounds obtained by template synthesis are of great interest in view of the

preparation of inorganic porous materials (Fig. 10.3.4). When the PVP is removed

from the nanocomposite by calcination (at 500 1C in air), the rearrangement of

individual clay mineral layers gives nanoporous materials with a narrow distribution

of mesopores in the range of 4 to 10 nm (Carrado, 2000); in general, the pore size of

such materials is related to the MW of the polymer involved (Carrado and Xu,

1999). This is then a very interesting method to design inorganic materials with pre-

determined mesoporous dimensions representing new applications of clay–polymer

nanocomposites. An alternative approach to preparing of clay mineral–silica nano-

composites from organoclays uses the sol-gel processes involving the controlled hy-

drolysis of alkoxysilanes such as tetramethylorthosilicate (TMOS) (Lataı

ef and

Ruiz-Hitzky, 2003). In this case the exfoliation of montmorillonite occurs as a con-

sequence of the form ation of silica, which is in fact an inorganic polymer.

10.3.4. HOST-GUEST COMPATIBILITY

Most polar monomers or polymers can be intercalated in the pristine clay mineral.

Because clays are hydrophilic, polymers of low polarity are in general incompatible

with unmodiﬁed raw clays. The less polar compounds can be intercalated only if

the clay minerals are previously modiﬁed by exchange with alkylammonium ions

polymer-clay

nanocomposite

SiO

+ Mg (OH)

+ LiF + polymer

(GEL)

100ºC, 2 days

calcination

(500ºC)

mesoporous solid

Fig. 10.3.4. Template synthesis of clay-polymer nanocomposites as intermediates for pre-

paring mesoporous solids. After Carrado (2000).

Chapter 10.3: Clay Mineral– and Organoclay– Polymer Nanocomposite592

(organoclays) or by grafting with suitable species. For intercalation of non-polar

polymers, it is necessary to render the mineral surface organophilic, or modify the

polymers.

A. Organoclays

The principal method used to modify the interlayer space of smectites is based on

cation exchange with alkylammonium ions (Jordan, 1949a, 1949b). Such a procedure,

well known since several decades, was thoroughly investigated by numerous research-

ers (Theng, 1974; Lagaly, 1986; Raussell-Colom and Serratosa, 1987; Yariv and Cross,

2002; Ruiz-Hitzky et al., 2004), and the resulting materials, the so-called organoclays,

are used commercially in many industries under the generic name of ‘Bentones’. The

alkylammonium species used to prepare organoclays are quaternary ammonium

compounds containing alkyl, phenyl, benzyl and pyridyl groups (Table 10.3.2).

In some cases, a mixture of alkyl- and alkenylammonium ions is used. The most

important application of these organoclays is to control the rheological behaviour of

dispersions in organic solvents where they function as thickeners and thixotropic

agents (Cody and Kemnetz, 1987; Finlayson et al., 1987 ).

Sepiolite has a much lower cation exchange capacity (CEC) than smectites (about

15 vs. about 100 cmol/kg). Nevertheless, treatment with quaternary ammonium salts

also produce organophilic solids (Alvarez et al., 1987). As in the case of smectites

additional surfactant ion pairs (cations plus the counterions) can be adsorbed (see

Chapter 7.3). These types of materials also show an excellent compatibility with low

polar organic media, and are used as paint thickeners and other industrial appli-

cations (Alvarez et al., 1984, 1985).

Several interlayer structures were proposed for alkylammonium-exchanged

phyllosilicates, mainly smectites and vermiculites. The alkyl chains are generally

lying ﬂat on the clay mineral surface as mono- or bilayers. Other arrangements are

the pseudo-trimolecular and parafﬁn-type arrangements (Lagaly, 1986; Ruiz-Hitzky

et al., 2004). The factors controlling the interlayer arrangement of the alkyl chains

Table 10.3.2. Quaternary ammonium cations frequently used to prepare organoclays

Quaternary cation Abbreviation Formula

Tetramethylammonium TMA (CH

)

Trimethyl phenylammonium TMPA C

(CH

)

Benzyl trimethylammonium BTMA C

(CH

)

Hexadecylpyridinium HDPY C

)

Benzyl dimethyl tetradecylammonium BDTDA C

)(CH

)

Hexadecyl trimethylammonium HDTMA C

(CH

)

Dioctadecyl dimethylammonium DODMA (C

)

(CH

)

10.3.4. Host-Guest Compatibility 593

are discussed in detail in Chapter 7.3. The surface properties of the organoclays

predetermines their potential uses.

The most important new application of organoclays today is the preparation of

various types of nanocomposites. As shown above, PAN is not intercalated by un-

treated clay minerals but vermiculite exchanged with n-butylammonium is able to

form nanocomposites after treatment with PAN in dimethylformamide (Aviles et al.,

1993; Pe

rez-Rodrı

guez and Maqueda, 2002). Alternatively, this type of nanocomposite

was prepared by intercalation of AN monomer into untreated clay mineral, followed

by polymerisation using a radical initiator such as benzoyl peroxide at 50 1C during

24 h (Pe

rez-Rodrı

guez and Maqueda, 2002). As already indicated, PAN is a polymer

of interest as it may form carbon ﬁbres by an adequate thermal treatment. When

heated, clay mineral–PAN nanocomposites give graphitised carbonaceous clay ma-

terials, which are transformed at high temperature into sialons, after carboreduction of

the silica and other metal oxides composing the silicate (Aviles et al., 1993, 1994;

rez-Rodrı

guez and Maqueda, 2002).

Low polarity polymers such as PS, in the molten state, can be directly intercalated

in alkyl-ammonium-exchanged smectites. Typical experimental conditions (Vaia

et al., 1993) require heating at 1651C in vacuum for more than 24 h. The resulting

organosmectite–PS nanocomposites show a basal spacing of 3.2 nm, which decreases

to the value of the pristine organosmectite (2.52 nm) after toluene extraction. This

type of nanocomposites displays a heterogeneous structure with well ordered inter-

calated layers and some disordered layers. This structure depends on processing

conditions. Ultrasonication and other experimental procedures were proposed to

obtain more homogeneous systems (Vaia et al., 1993).

Organoclay–PS materials have also been prepared by a novel procedure consist-

ing of modifying the clay mineral (Na

–montmorillonite) by ion-exchange with an

ammonium salt containing a styrene group such as CH

QCH–C

(CH

)



(Moet and Akelah, 1993). In this case, the PS remains partially

attached to the exchangeable cation s of the clay mineral, and only the excess polymer

can be extracted with appropriate organic solvents. The insertion of grafted PS

between the clay mineral layers is clearly monitored by XRD. The basal spacing

of the exchanged clay mineral (1.5 nm) increases to 2.45 nm after react ion with

styrene.

Clay mineral–nylon nanocomposites were prepared at the Toyota Central Re-

search Laboratories (Fukushima and Inagaki, 1987; Fukushima et al., 1988; Usuki

et al., 1993). The original method involves three steps: (i) ion exchange of Na

for

protonated 12-aminolauric acid

–(CH

)

–COOH; (ii) intercalation of epsilon-

caprolactam into the modiﬁed clay mineral; and (iii) thermal polymerisation of the

monomer, generating nylon-6 nanocomposites. These materials show interesting

mechanical properties. Although they are not the ﬁrst clay–polymer nanocomposite,

they are the ﬁrst to were used commercially. Since the ﬁrst publication in 1993, the

preparation method for clay–nylon nanocomposites was further improved by adding

small amounts of 6-aminocaproic to accelerate the polymerisation process

Chapter 10.3: Clay Mineral– and Organoclay– Polymer Nanocomposite594

(Usuki et al., 1993). Researchers at Toyota laboratory made various attempts to

prevent the partial co-polymerisation of the epsilon-caprol actam with the interca-

lated 12-aminolauric acid. They found that the epsilon-caprolactam can be directly

intercalated in the unmodiﬁed clay mineral at 2001C. The lactam is then reacted with

the 6-aminocaproic acid, yielding also a nylon-6 nanocomposite (in situ polymer-

isation) (Kojima et al., 1993a).

Numerous papers were published, showing that the choice of the clay modiﬁer is

important in reaching optimum mechanical properties and nowadays, commercial

nylon nanocomposites exist, wherein the 12-aminolauric acid was eventually replaced

by alkylammonium cations (Vaia et al., 1993; Krishnamoorti et al., 1996; Fornes et al.,

2002). Further, it was reported that clay–nylon nanocomposites can be synthesised in

one step, instead of the original three steps process (Kojima et al., 1993b).

B. Grafting of Clay Minerals

Silane coupling agents were extensively used to prepare micro-composites based on

clays, silica, ﬁbreglass, etc. These agents can also be used effectively to prepare

nanocomposites, since they make the clay mineral surfaces organophilic. Organosi-

lane coupling agents contain RSi–X groups (X ¼ OR, Cl) and are able to react

with silanol groups on the clay mineral surface, givi ng stable siloxane bridges.

½surfaceRSi2OH þ X2 SiR½R

!

½surfaceRSi2O2SiR½R

þHX (1)

Smectites and vermiculites have a low content in silanol groups because these groups

only occur at particle edges. In contrast, sepiolite is rich in such reactive hydroxyls,

due to the discontinuity of the silicate layers (Ahlrichs et al., 1975). The silanol

groups located on the external surface, at the edges of the struc tural channels, are

directly accessible to reagents. After grafting with organosilanes (Ruiz-Hitzky and

Fripiat, 1976), the hydrophilic surface becomes organophilic, and sepiolite can be

easily dispersed in low-polarity compounds including polymers (Ruiz-Hitzky, 1974).

The stability of these organic derivatives is excellent; the attached groups are only

eliminated after heating at elevated temperatures by combustion or pyrolysis, re-

spectively in the presence or absence of oxygen.

Organosilanes containing unsaturated groups (vinyl and methacryloxi) or thiol

functions (3-propylmercapto) give organic derivatives of sepiolite capable of further

co-polymerisation reactions. Nanocomposites in which the polymer is covalently

bonded to the modiﬁed clay mineral were prepared (Ruiz-Hitzky, 1974).

Direct reaction of organochlorosilanes with vermiculite gives essentially organ-

osiloxanes where the silane is not grafted to the silicate (Arago

n de la Cruz et al.,

1973). In the absence of reactive silanol groups on the surface, the RSi–Cl groups of

10.3.4. Host-Guest Compatibility 595

the reagent are quickly hydrolysed by the water molecules directly coordinated to the

interlayer cations.

X2SiRðRÞ

þ H

O !½ðRÞ

RSi2OH!ðRÞ

RSi2O2SiRðRÞ

(2)

Unstable intermediates are formed which polycond ense to siloxanes.

Nanocomposites were prepared by co-hydrolysis involving silicates, organosilanes

and strong acids. The clay minerals involved are chrysotile, sepiolite, and vermiculite

(Fripiat and Mendelovici, 1968; Zapata et al., 1972; Ruiz-Hitzky and Van Meerbeek,

1978; Van Meerbeek and Ruiz-Hitzky, 1979). The extraction of octahedral cations

such as Mg

ions by acid attack yields silica containing fresh silanol groups,

RSi2O2Mg2O2SiR þ½H

O!½surfaceRSi2OH þ Mg

2þ

(3)

which reacts with the organosilanols produced by hydrolysis of the chloro- or

alkoxy-organosilanes as shown in Eq. (1). Chrysotile and sepiolite preserve their

morphology even after complete removal of the octahedral sheet. When this sheet is

eliminated the particles disintegrate, and individual inorganic-organic micro- or even

nanoﬁbres are produced. When the surface was made compatible, nanocomposites

can be prepared with many polymers.

Surface-grafting with vinylsilanes further allows co-polymerisation with various

unsaturated monomers such as methylacrylate, methylmethacrylate, n-butylacrylate,

vinylacetate and styrene (Zapata et al., 1973). The resulting nanocomposites show

improved mechanical properties; for example, the dynamic modulus is nearly 100

times higher than the neat polymer (Zapata et al., 1973). These and other grafted

phyllosilicates, and especially their blending with pol ymers merit thorough inves-

tigation.

After reaction of sepiolite with vinylsilanes, the unsaturated groups are homog-

enously distributed. This is visualised by TEM by reacting the vinyl groups on the

sepiolite with OsO

(Barrios-Neira et al., 1974), conﬁrming the arrangement of the

grafted species to silanol sites along the edge of the structural channels.

Clay minerals such as sepiolites and vermiculites can also be grafted using co m-

pounds of various functionalities such as isocyanates and epoxides (Table 10.3.3).

Isocyanates react with silanol grou ps forming silyl-urethane bridges:

RSi2OH þ R2NCO ! R Si2O2CO2NH2R (4)

as demonstrated by IR spectroscopy (Ferna

ndez-Herna

ndez and Ruiz-Hitzky,

1979).

The stability of the grafted species depends on the nature of the alkyl groups (R),

which stabilise the bridges by their positive inductive effect whereas phenyl groups

Chapter 10.3: Clay Mineral– and Organoclay– Polymer Nanocomposite596

give less stable bridges. A competitive reaction is the hydrolysis of both the is-

ocyanate reagents and the silylurethane bridges.

2R2NCO þ H

O !ðNH2RÞ

CO þ CO

(5)

RSi2O2CO2NH-R þ H

O ! RSi2OH þ CO

þ R2NH

(6)

The formation of N,N

-disubstituted ureas (R–NH)

CQO, by reaction of isocyanates

with water molecules adsorbed on the surface of clay minerals is a further process that

predominates when R is a phenyl group. When sepiolite is reacted with phenyl is-

ocyanate diphenyl urea is almost quantitatively formed (Ferna

ndez-Herna

ndez and

Table 10.3.3. Organic derivatives of silicates by grafting of isocyanates and epoxides

Function Grafted groups Reagent Substrate

Isocyanates

Butyl isocyanate Sepiolite



Phenyl isocyanate Sepiolite



R–CQNQO RSi–O–CO–NH–R Hexamethylene

di-isocyanate

Sepiolite

Vermiculite

2,4-Toluene di-

isocyanate

Sepiolite

Vermiculite

Epoxides

1,2-Epoxybutan Sepiolite

y,z,J

1,2-Epoxyethyl

benzene

(epoxystyrene)

Sepiolite

y,z,J

- CH-R

RSi–O–CH(CH

OH)–R 1-Allyloxy-2,3-

epoxypropane

(allylglycidyl

ether)

Sepiolite

y,z,J

2,3-Epoxypropyl

methacrylate

Sepiolite

3-Vinyl-7-

oxabycycle (4,1,0)

heptane

Sepiolite



Ferna

ndez-Herna

ndez and Ruiz-Hitzky (1979).

Ruiz-Hitzky et al. (1979).

Siffert and Biava (1976).

Casal and Ruiz-Hitzky (1977).

Casal and Ruiz-Hitzky (1984).

Casal et al. (1980).

10.3.4. Host-Guest Compatibility 597

Ruiz-Hitzky, 1979). By contrast, alkyl isocyanates, like butyl isocyanate, give stable

grafted sepiolite compounds.

The reaction of di-isocyanates, such as hexamethylene di-isocyanate and 2,4-

toluylene di-isocyanate, with vermiculite were also reported (Siffert and Biava,

1976). By heating the clay mineral to remove the associated water before starting the

chemical reaction, urea formation is avoided. The yield of grafted species depends on

the temperature of the pre-treatment. The potential interest of these grafted minerals

is to introduce free –CQNQO functions on the mineral, which are able to react

further with polyols and polyesters in view of preparing inorganic-organic polycon-

densates. Nanocomposites of isocyanate grafted silicates and polyurethane foams

have promising industrial applications (Casal et al., 1980).

Grafting of 1,2-epoxides to silanol groups present on mineral surfaces was well

established by IR spectroscopy. The epoxy functions are opened at 80–100 1C, in

vapour phase or in solution using aprotic solvents. The coupling reaction of the

epoxide group with the RSi–OH groups in sepiolite (Casal and Ruiz -Hitzky, 1977,

1984) is shown below :

- CH-R

Si-O-CH (CH

OH) - R

Si-OH +

ð7Þ

In this case the organic R groups are linked to the surface through RSi–O–CR

bonds. The reaction takes place only in the absence of physically adsorbed water.

Vapour phase reactions can be made easily under dynamic vacuum. Reactions in

solution are carried out after elimination of ad sorbed water molecules, using a Dean-

Stark system to separate the azeotrope during the grafting reactions.

IR and chemical analysis show that the extent of grafting, related to the same

available surface area, is greater for sepiolite than for silica (Casal and Ruiz-Hitzky,

1977, 1984). This is attributed to the better ordering of RSi–OH groups on the clay

mineral compared to amorphous silica.

The reaction of allylglycidyl ether with sepiolite in organic solvents gives a

monomolecular surface coverage even when the concentrations of the starting epox-

ide are low (Casal and Ruiz-Hitzky, 1977, 1984). The treatment of clays by epoxides

with unsaturated groups such as allylglycidyl ether, 2,3-epoxypropyl methacrylate or

3-vinyl-7-oxabicycle (4,1,0) heptane (Casal et al., 1980) give rise to materials that can

be used as reinforcing ﬁllers for elastomers and thermoplastics. The grafted groups

are very stable, and are eliminated only by burning. However, the Si–O–C bonds

could be hydrolysed by hot water. Seconda ry reactions giving carbonyl compounds

by the catalytic activity of silico-alumina centres can be produced in the reactions of

epoxides with clay minerals (Ruiz-Hitzky and Casal, 1985).

The opening of epoxy rings can be applied to forming smectite–polyether nano-

composites. This chemistry is therefore useful for the preparation of thermoset

nanocomposites that have potential applications (Wang et al., 2000).

Chapter 10.3: Clay Mineral– and Organoclay– Polymer Nanocomposite598