Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

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13.1 Introduction 405

Table 13.1 Groups, formula, structural description and speciﬁ c minerals of the clay group.

Group Formula Description Mineral(s)

Clays Kaolinite Al

(OH)

Silicate sheets (Si

)

bonded to aluminum

oxide/hydroxide layers

(Al

(OH)

)

Kaolinite

Dickite

Nacrite

Smectite (Ca, Na, H)(Al, Mg,

Fe, Zn)

(Si,

Al)

(OH)

– x H

S ilicate layers sandwiching

a g ibbsite [Al

(OH)

] (or

brucite [Mg

(OH)

]) layer in

between, in an s - g - s

stacking sequence. Variable

amounts of water

molecules lie between the

s - g - s sandwiches

Pyrophyllite

Talc

Vermiculite

Sauconite

Saponite

Nontronite

Montmorillonite

Illite (Mica) (K, H)Al

(Si,

Al)

(OH)

– x H

S ilicate layers sandwiching

a g ibbsite - like layer in

between, in an s - g - s

stacking sequence. The

variable amounts of water

molecules would lie

between the s - g - s

sandwiches as well as the

potassium ions

Biotite

Lepidolite

Muscovite

Paragonite

Phlogopite

Zinnwaldite

Chlorite X

4 – 6

(OH, O)

S ilicate layers sandwiching

a b rucite or brucite - like

layer in between, in an

s - b - s stacking sequence

similar to the above groups.

There is an extra weakly

bonded brucite layer in

between the s - b - s

sandwiches. This gives the

structure an s - b - s b s - b - s b

sequence. The variable

amounts of water

molecules would lie

between the s - b - s

sandwiches and the brucite

layers

Amesite

Baileychlore clinochlore

Cookeite corundophilite

Daphnite

Delessite

Gonyerite

Nimite

Odinite

Orthochamosite penninite

Pannantite

Rhipidolite (prochlore)

Sudoite

Thuringite

etc.

a The X represents either aluminum, iron, lithium, magnesium, manganese, nickel, zinc or, rarely, chromium.

The Y represents either aluminum, silicon, boron, or iron but mostly aluminum and silicon.

406 13 Nanocomposites Based on Phyllosilicates

Montmorillonite ( MMT ) a member of the smectite group, is commonly used in

polymer – clay nanomaterials. The layered structure of MMT (Figure 13.1 ) consists

of two silica tetrahedral (corner - shared) sheets fused to one (edge - shared) octahe-

dral sheet of alumina (aluminosilicate) or magnesia (magnesium silicate). The

lateral dimensions of these layers vary from 100 nm to a few microns, and about

1 nm in thickness, depending on the particular silicate. Due to an isomorphic

substitution of alumina into the silicate layers (Al

for Si

) or magnesium for

aluminum (Mg

for Al

), each unit cell has a negative charge of between 0.5 and

1.3. The layers are held together with a layer of charge - compensating cations, such

as Li

, Na

, K

, and Ca

; the charge - compensating cations help the intercalation

and surface modiﬁ cation of clays, which is required to disperse clays at the

Table 13.2 Charge/unit formula for different phyllosilicates minerals.

Mineral species Interlayer cations Charge/unit formula

Kaolinite - serpentine Kaolinite, Halloysite – 0.0

Pyrophillite - talc Antigorite, Chrysotile – 0.0

Smectite Montmorillonite,

Beidelite, Nontronite,

Saponite, Hectorite

Na, Ca 0.25 – 0.6

Vermiculite Dioctahedral - vermiculite Mg 0.6 – 0.9

Trioctahedral - Vermiculite

Mica Muscovite, Biotite,

Phlogopite

K 1.0

Figure 13.1 Structure of montmorillonite clay.

13.1 Introduction 407

Table 13.3 Structure and physical characterization of two organoclays.

Organic moiety CEC

(mEq kg

− 1

)

Moisture

content (wt%)

Loss on

ignition

Basal Spacing

(nm)

HDTM

Hexadecyltrimethylammoniumbromide

0.91 1.13 29.98 1.9

SMB

Stearyldimethylbenzylammoniumchloride

0.91 1.05 34.19 2.5

nanoscale into polymers. The cation - exchange capacity ( CEC ), which is indicative

of the intercalation capacity, is deﬁ ned as the number of exchangeable interlayer

cations, and has units of mEq 100 g

− 1

. Typically, MMT has a CEC ranging from

approximately 75 to 115 mEq 100 g

− 1

[11] .

Other clays are saponite , a synthetic material that closely resembles MMT, and

hectrite , which is a magnesium silicate (CEC 55 mEq 100 g

− 1

) and its synthetic

equivalent, Laponite ® . Typically, sheets of Laponite are 25 – 30 nm wide, whereas

MMT has sheets that are approximately 200 nm in width.

The dispersion of a hydrophilic nanoﬁ ller into a hydrophobic polymer matrix

requires a preliminary modiﬁ cation of the clay to render it more organophilic.

Consequently, a treatment is often carried out with suitable chemicals to replace

the inorganic exchange ions in the galleries between the layers with alkylammo-

nium surfactants or other organic cations. Clays treated in this way are referred

to as “ organoclays ” . The number of onium ions that can be packed into the gal-

leries depends on the charge density of the clay and the CEC. In fact, at lower

charge densities the surfactants pack in monolayers while, as the charge increases,

bilayers and trilayers can be formed with different orientations of packed alkyl

chains. The intercalation of organophilic cations into the layers can be followed

using X - ray diffraction ( XRD ) analysis, with the distance between a plane in the

unit layer and the corresponding plane in the next unit layer being deﬁ ned as the

“ basal plane spacing ” d

001

, often known as the “ interlayer distance ” . When a sur-

factant replaces sodium in the galleries, an increase in interlayer distance can be

observed. The organic moiety used for preparing organoclays depends on the host

polymer to be used as a matrix for the nanocomposites. Both, the structural and

physical characteristics of two common ammonium salts (hexadecyltrimethylam-

monium bromide and stearyldimethylbenzylammonium chloride) are reported in

Table 13.3 [7] .

408 13 Nanocomposites Based on Phyllosilicates

The aim of the ion exchange process is to enlarge the interlayer distance so as

to promote the accommodation of monomers or polymers, thus affording an

organophilic surface and improving wetting between the clays and polymers.

13.1.2

Morphology of Composites

Polymer – clay composites based on layered silicates can be classiﬁ ed into three

types, depending on the extent of separation of the silicate layers [12] , namely

conventional microcomposites, intercalated nanocomposites, and exfoliated nano-

composites (these are shown schematically in Figure 13.2 ). If the polymer does

not enter the galleries, the d

001

of the clay will remain unchanged, in agreement

with the achievement of a microcomposite. If an organic species enters the galler-

ies and causes an increase in d

001

, but the clay layers remain stacked, then the

composite is considered “ intercalated. ” However, if the clay layers are completely

pushed apart to create a disordered array, the composite is termed “ exfoliated. ”

Usually, a composite for which d

001

> 10 nm (a spacing that cannot be determined

by conventional XRD analysis) is designated as exfoliated. Recently, Sinha Ray

et al . [13] deﬁ ned another type of nanocomposite within the set of “ intercalated

nanocomposites ” that they termed intercalated - and - ﬂ occulated nanocomposites,

and which contained aggregates of intercalated silicate layers due to the hydroxy-

lated edge – edge interaction of the silicate layers. In fact, most intercalated tactoids

include both single stacks and several connected stacks of clay layers, such that

the distinction between ﬂ occulation and intercalation will rest on an electron

microscopic analysis of the structure. Moreover, the “ ﬂ occulation ” could be attrib-

uted to the long molecular chains that intercalate into two or more clay galleries

and play a bridging role. Electron microscopy studies have often shown that

most nanocomposites are both intercalated and exfoliated, but this cannot be

readily deduced from XRD measurements. This situation may occur because the

preparation method has not allowed sufﬁ cient time for adsorption and penetration

Figure 13.2 Classiﬁ cation of morphologies for polymer – clay composites.

13.1 Introduction 409

of the galleries to be completed, or because the dispersive mixing has been less

effective and that, if more time were available, the polymer – clay system might

develop either fully intercalated or exfoliated structures. The extent of clay disper-

sion in polymer is dependent on the intrinsic properties of the polymer and clay,

including the aspect ratio of the clay platelets, the volume fraction of the clay, the

nature/structure of polymer, the interactions between the polymer, clay and clay

modiﬁ er, and the processing conditions used. Fully exfoliated polymer – clay nano-

composites are, in theory, only found for volume fractions of clay less than 3%,

as calculated by considering the small size of clay platelets [14] .

Three principal methods are available to determine the exfoliated fraction:

• Quantitative XRD, using a strong and independent reﬂ ection from an internal

standard, can potentially be used to track the decrease in the ratio d

001

for the clay

to the standard peak as exfoliation proceeds.

• A method based on NMR, which was recently established [15] .

• Transmission electron microscopy ( TEM ) provides an indication, but is difﬁ cult

to use for precise measurement unless very thin specimens are prepared, a large

number of images are captured, and quantitative microscopy coupled with

stereology is used.

Recently, Luo et al . [16] developed a statistical TEM image analysis methodology

to evaluate the dispersion parameters D

0.1

, based on measurement of the free - path

spacing distance between the single clay sheets. Drummy et al . [17] reported a

morphological characterization of layered silicate nanocomposites by using both

electron tomography and small - angle X - ray scattering ( SAXS ). The latter method

has the advantage of providing complementary results with respect to the electron

tomography. In fact, it can be used to probe materials with subnanometer resolu-

tion while at the same time sampling a large number of nanoparticles, thus pro-

viding more than adequate statistics.

Rheological testing [18] might also be developed to measure the degree of exfolia-

tion, but at present this is only used for semi - quantitative analysis. Hence, the

morphological characterization of layered silicate nanocomposites requires the use

of different techniques in order to achieve a relevance not only in the three -

dimensional ( 3 - D ) representation of the nanometer - scale part of the sample, but

also regarding the average distribution of platelets in the systems.

The characterization of layered silicate polymer blends is complicated by the

need to study both the phase and ﬁ ller distribution. In miscible systems, such as

poly(methyl methacrylate) ( PMMA )/ poly(ethylene oxide) ( PEO ) [19, 20] , the mor-

phological analysis can be carried out as in a general polymeric matrix. However,

in immiscible systems, such as poly(propylene) ( PP )/ poly(styrene) ( PS ), both XRD

patterns and TEM observations have shown that the silicate layers were either

intercalated or exfoliated, depending on their interactions with the polymer pair,

and were located at the interface between the two polymers [21] . In this case, the

compatibilizing action of an organically modiﬁ ed layered silicate resulted in a

decrease in the interfacial tension and particle size, and in a remarkable increase

410 13 Nanocomposites Based on Phyllosilicates

in the mechanical properties of the modiﬁ ed immiscible blends. A similar result

was obtained by Fang et al . [22] , who investigated the morphology of nanocom-

posites based on 80/20 and 20/80 (w/w) poly( ε - caprolactone) ( PCL )/PEO immis-

cible blends and organophilic layered silicates prepared by melt extrusion. From

the TEM analysis, it was observed that the exfoliated silicate platelets were located

preferentially at the interface between the two blend phases. However, when the

blend - based nanocomposites were prepared via a two - step process, in which the

silicates were ﬁ rst premixed with the PEO component or with the PCL component,

the silicate layers migrated from the PEO phase or PCL phase to the interface. The

emulsifying capability of layered silicates in immiscible blends depends on the

structure and physical properties of the couple of polymer components. In fact, in

PP/ poly(amide) ( PA ) blends [23] , it was shown, using electron microscopy, that in

all cases the inorganic ﬁ ller was enriched in the PA phase, and this resulted in a

phase coarsening in comparison with the unﬁ lled PP/PA blend. In contrast, in

blends of a poly(vinylidene ﬂ uoride)/nylon - 6 (PVDF/PA6) 30 : 70 melt compounded

with various organoclays either directly or sequentially [24] , the nanocomposite

with the best mechanical properties was characterized by a good dispersion of

particles throughout the matrix (PA6) and at the PVDF/PA6 interface. The authors

ascribed this good result to a suppression of the coalescence of PVDF domains.

Moreover, the crystallization of the PVDF domains was suppressed, ultimately

creating a blend nanocomposite that was stiffer, stronger, and tougher than the

blend without nanoparticles.

In agreement with the latter results, organoclay strongly inﬂ uences the dynamic

evolution of phase morphology, both in partially miscible and immiscible blends.

A partially miscible blend was examined by investigating the poly(vinyl methyl

ether) ( PVME )/ poly(styrene) ( PS ) [25] demixing in the presence of organically

modiﬁ ed Laponite. The phase separation of these near - critical blends proceeds by

a spinodal decomposition, even with added nanoparticles. However, the presence

of nanoparticles slowed the phase - separation kinetics. In immiscible systems,

such as poly(ethylene ) ( PE )/PA6 blends, the presence of an organoclay, preferen-

tially distributed in the polyamide phase, effectively stabilized a co - continuous

phase morphology [26] which persisted for more than 500 s. The authors explained

this experimental evidence by suggesting that the co - continuous morphology

could evolve to a phase - separated morphology in the timescale of their experi-

ments, as the organoclay framework would radically slow down the melt state

dynamics of the percolating network formed during the melt processing.

The presence of a compatibilizer [e.g., ethylene – propylene random copolymers

(EPM), functionalized with maleic anhydride (EPM - g - MAH), or maleic anhydride -

functionalized PP (PP - g - MAH)] in PP/PA6 blends containing organoclay, improves

the phase morphology of the blends and promotes the formation of exfoliated

nanocomposites, with the nanoplatelets preferentially distributed in the polyamide

phase [27, 28] . In particular, atomic force microscopy ( AFM ) studies [29] carried

out after selective chemical or physical etching of PP - g - MAH compatibilized PA6/

PP/organoclay polished sample surface, showed that the organoclay was embed-

ded in the PA6 - g - PP phase of the PA6/PP blends compatibilized with PP - g - MAH.

The preferential location of the clay in the PA6 - g - PP phase was attributed to pos-

13.2 Polyoleﬁ n-Based Nanocomposites 411

sible chemical interactions between the PA6 and the organic surfactant of the clay.

The use of a proper organoclay, modiﬁ ed with the reactive (glycidoxypropyl)tri-

methoxy silane, in poly(lactic acid) ( PLA )/ poly(butylene succinate) ( PBS ) blends

[30] , allowed an exfoliated blend to be obtain contemporarily, and improved the

phase adhesion between the two polymers. The authors ascribed this result to the

formation of a grafted hybrid in the interphase region, based on the reaction of a

glycidyl unit with the terminal hydroxyl or carboxylic groups of the polyesters.

In general, the phase morphology development and the compatibilization of

nanoﬁ lled blends requires more studies to be conducted, in order to create a

rational scheme concerning the effect of nanoscaled interactions, physical fea-

tures, and rheological properties on ﬁ ller and phase distribution, with the aim of

modulating the ﬁ nal properties of the composites.

13.1.3

Properties of Composites

The dispersion of phyllosilicates into polymer or blends at nanometer scale allows

improvements to be made to the properties of the polymer matrix [31] . In

particular:

• the barrier properties to gases are enhanced [32] , not only when an exfoliated

morphology is obtained [33] , but also in an intercalated morphology [34] . In fact,

the results of ﬁ tting of experimental data with proper models showed that the

chain conﬁ nement enhanced the barrier properties of the intercalated

nanocomposites [35] .

• Flame retardancy is improved, based on the dispersion of nanoclays, such that

char formation is favored on the burning surface [36, 37] .

• Stiffness is improved, with an increase in tensile modulus frequently being

observed in phyllosilicate – polymer nanocomposites [21] .

• Thermal stability, as degradation temperature, determined via thermogravimetric

analysis ( TGA ), shows an increase of at least 7 – 8 ° C when an exfoliated

nanocomposite is tested [38] .

Other properties of nanocomposites are currently the object of much research,

and in some cases – an example is that of thermal behavior [39, 40] – these inves-

tigations have begun to establish new properties that might be exploited such that

the nanocomposites can be used for a variety of industrial applications.

13.2

Polyoleﬁ n - Based Nanocomposites

The polyoleﬁ n s ( PO s) represent one of the most widely used classes of thermo-

plastic materials, as they offer a unique combination of high thermomechanical

properties and good environmental compatibility, at low cost.

412 13 Nanocomposites Based on Phyllosilicates

Currently, a huge effort is being devoted to extend the ﬁ eld of application for

POs and to satisfy their market demand, the latter point being based on these

materials having speciﬁ c and advanced properties, and being adaptable to the

environment and, ideally, recyclable. In particular, the preparation of PO nano-

composites by the addition of very small amounts of layered silicate loadings is a

very common approach that provides drastic improvements in the elasticity

modulus, strength and heat resistance of the materials, as well as a substantial

decrease in their gas permeability, when compared to either virgin POs or con-

ventional microcomposites [41 – 49] .

Attempts have been made to prepare either intercalated and exfoliated nano-

composites, through different routes, and extensive studies have been conducted

to promote an understanding of the fundamental aspects associated with the clay

dispersion, the resultant physical properties, and the inter - relationship between

both. Indeed, the properties of polymer – clay nanocomposites depend largely on

the synergistic effect of nanoscale structure and interactions occurring between

the clay and polymer.

The achievement of a desired level of clay dispersion into the polymer, and

interaction between the phases, involves numerous chemical and technological

implications. Generally, the chemical modiﬁ cation of one or both system phases

is necessary. For this, the hydrophilic clay is often changed to hydrophobic either

by the cation exchange or grafting of alkoxysilanes, by exploiting the cation -

exchange capability in one case and the reactivity of silanol groups present on the

edges of the clay platelets in the other case. Typically, a compatibilizer – that is, a

functionalized PO or copolymer – is added to assist in this situation. At present, it

is not completely clear which conditions lead to exfoliated layers and which cause

only the intercalation of polymer chains into the gallery spacing of the stacked

silicate layers. However, what does appear well established is that the interface is

not only fundamental for the adhesion between the two phases and stabilization

of the attained morphology, but also plays a major role in the bulk properties, due

to its large extension [39] .

Clearly, the ability to predict and control the morphology of nanocomposites

would be of major interest, by virtue of the strict connection between structural

control and the ﬁ nal properties of the nanocomposite [49, 50] . Consequently, the

main objective of this section is to review the concepts and methods relating to

the optimization of layered silicate dispersion in PO matrices by melt intercalation,

notably with regard to the use of a compatibilizer and identiﬁ cation of the pre-

ferred process conditions.

13.2.1

Overview of the Preparation Methods

Two major synthetic strategies are generally followed for preparing PO/layered

silicates nanocomposites: (i) polymer intercalation (including melt intercalation

[51 – 56] and solution intercalation methods [57] ); and (ii) in situ intercalative

polymerization [58 – 62] .

13.2 Polyoleﬁ n-Based Nanocomposites 413

The in situ intercalative polymerization method, which is based on the thermo-

mechanical forces generated during the process of oleﬁ n polymerization inside

the interlayers of the silicate, and the energy released during the polymerization,

is reported to promote silicate layer dispersion and exfoliation in the nonpolar PO

matrix. However, a very poor interaction between the polar silicate layers and the

hydrophobic PO matrix makes the nanocomposite structure thermodynamically

instable. In fact, it has been demonstrated that melting processes of this type of

nanocomposite material cause agglomeration of the silicate layers and collapse of

the nanocomposite structure. Even the more recently developed polymerization

ﬁ lling technique , which consists of attaching the polymerization catalyst to the

surface and into the interlayers of the silicate and polymerizing the oleﬁ n in situ ,

although favoring exfoliated structures does not allow thermodynamically stable

nanocomposites to be prepared [63] . The method which should be adopted for

stabilizing the morphology is to copolymerize the oleﬁ n with a functionalized

comonomer, thus improving the necessary interaction with the clay. However,

many challenges remain for the catalytic polymerization of polar monomers with

both traditional Ziegler – Natta and metallocene catalysts, due mainly to the catalyst

deactivation.

For solution intercalation , the enthalpy of dispersion of the silicate layers in solu-

tion must balance the entropy loss of the solvent molecules which intercalate into

the silicate layers [64] . Moreover, this method is poorly sustainable because it

requires a large amount of organic solvent to ensure a good clay dispersion.

Melt intercalation is the most commonly applied process for preparing PO

nanocomposites, because it is sustainable, versatile, and inexpensive. This meth-

odology is limited by the low interaction enthalpy between the nonpolar PO

chains and the polar silicate layers. In fact, when using polymer chain intercala-

tion between the silicate layers, an effective and stable morphology is achieved

only if the interaction enthalpy balances the entropy loss of the polymer chains

conﬁ ned within the silicate interlayers. However, it has been recognized that

the entropy loss due to such polymer conﬁ nement is only partially compensated

by the entropy gain associated with the increased conformational freedom of

the surfactant tails as the interlayer distance increases with polymer intercalation

[65, 66] , whereas favorable enthalpic interactions are critical for determining the

nanocomposite structure [67] . Hence, even if a good dispersion of the clay can

be achieved simply by applying strong shear forces during the process, the

structure will be unstable and reagglomeration of the particles may occur quite

rapidly if an effective interaction between the polymer and clay surface is not

ensured [44] .

It is somewhat evident that the PO hydrophobicity makes it almost impossible

to obtain intercalation without chemical modiﬁ cation of the clay or polymer, or

both. In order to overcome the above - described thermodynamic obstacles, and to

promote the interaction between the hydrophobic PO and the polar silicate layers,

the general approach is to use either: (i) organophilic layered silicate s ( OLS s),

obtained via cation exchange; or (ii) polymer compatibilizers (i.e., functionalized

polyoleﬁ ns).

414 13 Nanocomposites Based on Phyllosilicates

Whilst the dispersal of layered silicates in polar polymers is relatively straight-

forward, the achievement of comparable results with hydrophobic POs remains a

challenge. Favorable interactions at the inorganic/organic interface are necessary

in order to separate the clay monolayers and to successively grant cohesion forces

between the exfoliated layers and polymer chains, thus achieving a stable

nanostructure.

13.2.2

Organophilic Clay and Compatibilizer: Interactions with the Polyoleﬁ n Matrix

Two main methods can be applied for clay modiﬁ cation: (i) to replace the metallic

cations that were originally present on the clay surface with large organic cations;

and (ii) to graft alkoxysilanes onto the clay by exploiting the reactivity of the silanol

groups [64] .

The ﬁ rst of these methods has been widely applied, using several types of cation

(mostly dialkyldimethyl - or alkyltrimethyl - ammonium cations). The introduction

of quaternary ammonium salts with long alkyl chains between the layers (as dis-

cussed above) allows the interlamellar space to be expanded, reduces the interac-

tion among the silicate sheets, and also facilitates the diffusion and accommodation

of the polymeric matrix. In contrast, the grafting of alkoxysilanes has been used

only rarely with layered silicates, due to the small number of reactive - edge silanols

present. Moreover, it has been shown recently, by using a combination of mor-

phological analysis and multinuclear solid - state nuclear magnetic resonance

( NMR ) experiments [68] , that the modiﬁ cation of a synthetic Laponite with a com-

bination of organic ammonium cations and alkoxysilanes will result in a disor-

dered arrangement of the clay platelets (which favors interplatelet interactions).

This effect was due to self - condensation occurring among clay silanols at the

platelet edges, thus limiting dispersion into the polymer matrix.

Several reports have described the use of OLSs modiﬁ ed with alkylammonium

surfactants of different nature and length for the preparation of PO/OLS nano-

composites [52, 69] . In particular, the effect of factors such as the length and

number of alkyl groups of the cationic surfactant was investigated. It became

apparent that the equilibrium structure of the polymer/OLS depended on the

nature of the polymer, on the charge density of the layered silicate, and also on

the chain length and structure of the cationic surfactant. Balazs et al . [70, 71]

showed that an increase in surfactant length improved the separation of the layers

by allowing the polymer to adopt more conformational degrees of freedom. Both,

exfoliated and intercalated PE/OLS nanocomposites were obtained when the

number of methylene units in the alkylammonium chain exceeded 16 [72] , whereas

it was reported that an excessive density of tethered chains would prevent the

formation of intercalated structures. The dispersion level was found to depend

strictly on the amount of OLS; in particular, it was shown that the morphology of

the PO/OLS systems could be shifted from disordered exfoliated to predominantly

intercalated by varying the OLS content from 6 to 36 wt% [73] . Finally, from a

thermodynamic point of view, OLSs modiﬁ ed with alkylammonium salts were