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Review ArticleStructure and properties of polymer clay

nanocomposites: A Report

Aniagbaoso Ikenna Kingsley

Department of Polymer and textile Engineering,School of Engineering and EngineeringTechnology, Federal university of Technology Owerri,P.M.B 1526. Owerri, Imo State , Nigeria.

ABSTRACT

An overview of the progress in polymer nanocomposites is presented in thisseminar paper with an emphasis on the different structure and properties ofpolymer/ clay nanocomposites and the extent to which these properties have beenenhanced. Other related areas that are also discussed include the differentmethods used for preparing polymer-layered silicate (PLS) nanocomposites usingthe aluminum silicate clays, the types of PLS nanocomposites morphologies thatare most commonly achieved, the methods used for examining their structure andtheir possible relevance and application in the industry.

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INTRODUCTION

Polymer/Clay Nanocomposites which can be abbreviated to ‘PCNC’ are a new

class of composites that consists of dispersed particles in a polymer matrix. The

dimensions/geometry of these dispersed particles is in the nano-range. The use of

nanometre grains, fibres, and plates have dramatically, increased the surface area

(i.e bulkness) of the pristine polymer when incorporated into the polymer matrix.

In addition studies have shown that polymer/clay nanocomposites have a number

of significantly improved properties compared to their convectional material such

as; tensile strength, modulus, heat destruction temperature, gas barrier properties,

flame retardant properties e.t.c.

Polymer/clay nanocomposite (PCNC) materials are of interest as a result of their

wide range of novel physical properties. They often have properties that are

superior to conventional microscale composites because of the strong interaction

between components and can be synthesized using simple and inexpensive

techniques. This aspect of nanotechnology in particular has strategically set a new

pace in the areas of engineering , science and technology over the last decade and

has found applications as engineering plastics, polymer product, rubbers, adhesives

and coatings.

The reinforcing filler material for a polymer-matrix nanocomposites can be made

up of Particles (e.g. minerals such as graphite, glass), fibres (e.g. carbon nanotubes

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or electro-spun fibres), or Sheets (e.g. exfoliated clay stacks). The reinforcing sheet

made from layered silicates (silicate clay) is going to be the major case study in

this seminar paper. Invariably, it means that polymer layered silicate (PLS)

nanocomposites is a type of polymer/clay nanocomposites.

Over the past years, Interest in polymer layered silicate (PLS) nanocomposites has

rapidly been increasing at an unprecedented level, both in industry and in

academia, due to their potential for enhanced physical, chemical, and mechanical

properties compared to conventionally filled composite (Gilman, W. et al, 1999).

They have the potential of being a low-cost alternative to high-performance

composites for commercial applications in both the automotive and packaging

industries.

The most heavily used filler material is based on the smectite class of aluminum

silicate clays, of which the most common representative is montmorillonite

(MMT). MMT has been employed in many PLS nanocomposite systems because it

has a potentially high-aspect ratio and high-surface area that could lead to

materials which could possibly exhibit great property enhancements. In addition, it

is environmentally friendly, naturally occurring, and readily available in large

quantities. The earliest motivation for the use of nanoparticles seems to have been

stimulated by the Toyota research group, where the first practical application of

nylon-6–montmorillonite (MMT) nanocomposite was commercialized. With only a

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small MMT loading (4.2 wt%), the modulus doubled, the tensile strength increased

more than 50%, the heat distortion temperature increased by 100◦C, and

combustion heat release rate decreased by up to 63% (Fornes, D, 2001). However,

in general, all the promises and claims that the addition of nanoparticles to polymer

matrices will miraculously lead to exceptional mechanical properties have not been

completely fulfilled because the improvements in properties seem to plateau at

levels of about 4 wt%. In nylon-6 (N6), levels of 7wt% have been reached because

of hydrogen bonding between the amide groups and the nanoclay particles (Fornes,

D, 2001).

Layered silicates in their pristine state are hydrophilic. Most of the engineering

polymers are hydrophobic. Therefore, dispersion of native clays in most polymers

is not easily achieved due to the intrinsic incompatibility of hydrophilic-layered

silicates and hydrophobic engineering polymers. In order to have a successful

development of polymer-clay nanocomposites, it is necessary to chemically

modify natural clay so that it can be compatible with a chosen polymer matrix.

According to Krishnamoorti, this can be done through ion-exchange reactions that

replace interlayer cations with quarternary alkylammonium or alkylphosphonium

cations. It is well established that when layered silicates are uniformly dispersed

(exfoliated) in a polymer matrix, the composite properties can be improved to a

dramatic extent. Hence, in order to capitalize on the potential offered by

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nanoparticles in areas such as reinforcement, barrier, and electrical conductivity,

higher loadings of fully dispersed nanoparticles must be obtained. In this seminar, I

will review the techniques that have been used to exfoliate nanoparticles, the

structure and properties and the many relevance of this nanotechnology to our

engineering.

1.0 DIFINATION OF TERMS

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Composites: This is a material that is made up of multiple components, compound

or complex substances.

Nanotechnology: This is the science and technology of creating particles and of

manufacturing machines which have sizes within the range of nanometers (i.e 10-

9).

Nanoparticles: These are particles between 1 and 100 nanometers in size. It can

be defined as a small object that behaves as a whole unit with respect to transport

and properties.

Polymer: is a large molecule (macromolecule) built up by the repetition of small

chemical units.

2.1 NANOCOMPOSITES

Nanocomposite is a multiphase solid material where one of the phases has

dimensions of about 1-100 nanometers (nm), or structures having nanoscale repeat

distances between the different phases that make up the material. In the broadest

sense this definition can include porous media, colloids, gels and copolymers, but

is more usually taken to mean the solid combination of a bulk matrix and nano-

dimensional phase(s) differing in properties due to dissimilarities in structure and

chemistry.

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In mechanical terms, nanocomposites differ from conventional composite materials

due to the exceptionally high surface to volume ratio of the reinforcing phase

and/or its exceptionally high aspect ratio. The area of the interface between the

matrix and reinforcement phase(s) is typically an order of magnitude greater than

for conventional composite materials. The matrix material properties are

significantly affected in the vicinity of the reinforcement. There are three major

matrix material used in the nanocomposites formation. Namely;

1) Metal-matrix nanocomposites: Metal matrix nanocomposites can also be

referred to as reinforced metal matrix composites. This type of composites

can be classified as continuous and non-continuous reinforced materials.

One of the most important nanocomposites is Carbon nanotube metal matrix

composites, which is an emerging new material that is being developed to

take advantage of the high tensile strength and electrical conductivity of

carbon nanotube materials.

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Fig.1 Nanocomposites formed from a metal matrix

2) Ceramic-matrix nanocomposites: In this group of composites, nanofillers

are incorporated into the interstitial spaces of a ceramic, i.e. a chemical

compound from the group of oxides, nitrides, borides, silicides etc. In most

cases, ceramic-matrix nanocomposites encompass a metal as an extra

reinforcing component. Ideally both components, the metallic one and the

ceramic one, are finely dispersed in each other in order to elicit the particular

nanoscopic properties. Nanocomposites from these combinations were

demonstrated in improving their optical, electrical and magnetic properties

(F. E Kruis, 1998) as well as tribological, corrosion-resistance and other

protective properties.

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Fig.2 Ceramic material incorporating Nano-fillers

3) Polymer-matrix nanocomposite: This is the simplest case; just by

appropriately adding nanoparticles to a polymer matrix, the performance can

be enhanced, often dramatically, by simply capitalizing on the nature and

properties of the nanoscale filler (these materials are better described by the

term nano-filled polymer composites). The reinforcing material can be made

up of particles (e.g. minerals), sheets (e.g. exfoliated clay stacks) or fibres

(e.g. carbon nanotubes or electro-spun fibres). This strategy is particularly

effective in yielding high performance composites, when good dispersion of

the filler is achieved and the properties of the nanoscale filler are

substantially different or better than those of the polymer matrix.

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Nanoparticles such as graphene, carbon nanotubes, molybdenum disulfide

and tungsten disulfide are being used as reinforcing agents to fabricate

mechanically strong biodegradable polymeric nanocomposites for bone

tissue engineering applications.

1.1 POLYMER/CLAY COMPOSITES

Polymer-clay nanocomposites are formed through the union of two different

materials with organic and mineral pedigrees (origin). This hybrid composition

however exhibit large increases in tensile strength, modulus, and heat distortion

temperature as compared to pristine polymer (polymer in its purest form).

Fig.3 Polymer Matrix Incorporating Nano-sheets

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2.0 SYNTHESIS

The most popularly used filler material is based on the smectite class of aluminum

silicate clays, of which the most common representative is montmorillonite

(MMT). Other layered silicates in this same general family that can be used are;

hectorite, mica, talc, vermiculite, kaolinite, saponite (Bridley S. et al, 1980). The

MMT crystal structure is made up of a layer of aluminum hydroxide octahedral

sheet sandwiched between two layers of silicon oxide tetrahedral sheets (Fig. 1).

The nominal composition of MMT is Na1/3(Al5/3Mg1/3)Si4O10(OH)2. The layer

thickness of each platelet is on the order of 1 nm, and the lateral dimension is

approximately 200 nm (Pinnavaia T, 2000). These clay platelets are stacked on

each other and held together through van der Waal forces and are separated from

each other by 1 nm gaps (galleries). These galleries are usually occupied by

cations, normally alkali and alkaline-earth cations such as Na+ and K+, which

counterbalance the negative charges generated from isomorphic substitution within

the layers (for montmorillonite, Al3+ replaced by Mg2+).

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Fig 1. Structure of 2:1 layered silicate showing two tetrahedral sheets of silicon oxide fused to anoctahedral sheet of aluminum hydroxide (Quang t. et al, 2006)

It is well established that main objective in preparing PLS nanocomposites is to

obtain exfoliation of the large stacks of silicate nanoplatelets into individual layers.

By analogy with polymer blends, the physical mixture of silicate layers and

polymer matrix may not form a nanocomposite due to the unmatched chemical

affinity between the two (based on their polarity). Thus, in order to have a

successful development of clay-based nanocomposites, it is necessary to

chemically modify a naturally hydrophilic silicate surface to an organophilic one

so that it can be compatible with a chosen polymer matrix. Generally, this can be

done through ion exchange reactions by replacing interlayer cations with

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quarternary alkylammonium or alkylphosphonium cations (Fig. 4). Ion-exchange

reactions with cationic surfactants such as the ones mentioned above render the

normally hydrophilic silicate surface organophilic, thus making it more compatible

with non-polar polymers. These cationic surfactants modify interlayer interactions

by lowering the surface energy of the inorganic component and improve the

wetting characteristics with the polymer. Furthermore, they can provide functional

groups that can react with the polymer or initiate polymerization of monomers and

thereby improve the strength of the interface between the polymer and inorganic

component.

Fig.5 Schematic representation of a cation-exchange reaction between the silicate and an

alkylammonium salt (after Zanetti et al, 2000).

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Complete dispersion of clay platelets in a monomer or polymer matrix involves

three steps, such as;

I. Wetting the surface of clay platelets by monomer or polymer

molecules,

II. Intercalation of the monomer into the clay galleries, and

III. Exfoliation of clay layers.

In polymer nanocomposites, the following routes can be taken to incorporate clay

into the polymer matrix at nanolevel. They are;

2.1 Solution Induced Intercalation Method: This involves the solubilizing of

polymer in an organic solvent, then the clay is dispersed in the obtained solution

and subsequently either the solvent is evaporated or the polymer precipitated. This

approach leads to poor clay dispersion, besides other problem like: high costs of

solvents required, large amount of solvent needs to be used to achieve appreciable

filler dispersion, technical phase separation problem, and health and safety

problem. Solvent route technique is used in the case of water-soluble polymers for

clear reasons.

2.2 In situ intercalative polymerization: Using this technique, polymer formation

can occur in between the intercalated sheets. In situ polymerization is based on the

following procedure:

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I. Swelling of the layered silicate within the liquid monomer and the

polymerization can be initiated either by heat or radiation.

II. By the diffusion of a suitable initiator, or by an organic initiator.

This approach has been successfully applied in manufacturing of nylon–6-

montmorrillonite nanocomposite, and later it was extended to other thermoplastics.

This is a convenient method for thermoset– clay nanocomposites.

3.3 Melt Intercalation: In this technique, no solvent is required and the layered

silicate is mixed within the polymer matrix in the molten state. A thermoplastic

polymer is mechanically mixed by conventional methods such as extrusion and

injection molding with organophillic clay at an elevated temperature. The polymer

chains are then intercalated or exfoliated to form nanocomposites. This is a popular

method for preparing thermoplastic nanocomposites. The polymers, which are not

suitable for adsorption or in situ polymerization, can be processed using this

technique (Ray, S.S. et al, 2003)

2.4 Structure of Polymer/Clay Nano Composites

In general, the degree of dispersion of the clay platelets into the polymer matrix

determines the structure of nanocomposites. Depending on the interaction between

the clay and the polymer matrix, two main idealized types of polymer–clay

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morphologies can be obtained. Namely; intercalated and exfoliated (Fig. 6). The

intercalated structure results from penetration of a single polymer chain into the

galleries between the silicate layers, resulting in formation of alternate layers of

polymer and inorganic layers. An exfoliated structure results when the individual

silicate layers are completely separated and dispersed randomly in a polymer

matrix. Usually exfoliated nanocomposites are preferred because they provide the

best property improvements (Masnelli-varlot K. et al, 2002). Since the remarkable

improvements in the material properties in a nylon-6/clay nanocomposite

demonstrated by the research group (Okada A, 1998) numerous other polymers

have been investigated by many researchers around the world. These include, but

are not limited to, polypropylene, polyethylene, polystyrene poly (ethylene oxide),

polycaprolactone, polyimides, polyamide, poly (ethyleneterephthalate),

polycarbonate, polyurethane, and epoxy resins (Lan T. et al, 1995). Often, a

nanocomposite may not be intercalated or exfoliated in structure but a combination

of both where the polymer is unable to intercalate between the silicate sheets and a

phase separated composite is obtained.

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Fig.6 Schematic illustrations of two types of polymer-layered silicate morphologies: (left) intercalated

and (right) exfoliated (Quang t. et al, 2006).

The complete dispersion of clay particle into the polymer micronsize matrices is

possible because of their nanosizes and this allows their dispersion throughout the

polymeric resin. In developing and optimizing nanocomposites, one needs to know

the degree of exfoliation of a particular sample and compare it to other samples.

The dispersion of clay in the polymer matrix to form intercalated or exfoliated is

often investigated using X-ray diffraction (XRD) and transmission electron

microscopy (TEM) techniques. The nanocomposite structure, namely, intercalated

or exfoliated, may be identified by monitoring the position, shape, and intensity of

the basal reflections from the distributed silicate layers. XRD can offer a

convenient method to determine the interlayer spacing of the silicate layers in the

original layered silicates and in the formed polymer-clay nanocomposites, but not

much can be concluded about the spatial distribution of the silicate layers in the

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matrix of the polymer composite (Yalcin B. et al, 2004). In addition, because some

layered silicates actually do not exhibit well-defined basal reflections, their

structures are very difficult to study systematically. Thus, conclusions based solely

on XRD patterns are only tentative when concerning the mechanism of

nanocomposite formation and their structure. To supplement the deficiencies of

XRD, TEM can be used. TEM allows a qualitative understanding of the internal

structure, spatial distribution of the various phases, and views of the defect

structure through direct visualization (Wang, Z. et al 1998). Together, TEM and

XRD are essential tools for evaluating nanocomposite structure. TEM is time

consuming and gives qualitative information on selected regions of the sample,

whereas low-angle peaks in XRD allow quantification of changes in layer spacing.

Occasionally, small angle X-ray scattering (SAXS) can also be used to characterize

the structure of nanocomposites. SAXS is useful when layer spacing exceed 6–7

nm in intercalated nanocomposites or when the layers become relatively disordered

in exfoliated nanocomposites. Recently, it was discovered that simultaneous SAXS

and WAXD studies yielded quantitative characterization of nanostructure and

crystallite structure in nylon-6 based nanocomposites.

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Fig. 7. Transmission electron microscopy of a melt-intercalated organoclay tactoid in a

PP matrix. Courtesy of Dr M. Bacia, UST Lille.

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3.0 PROPERTIES OF POLYMER/CLAY NANO COMPOSITES,

RELEVANCE AND APPLICATION

1. Mechanical Behaviour: Being able to improve strength and stiffness with

limited alteration of toughness is a goal readily achievable with polymer–

clay nanocomposites. The first commercial example of polymeric

nanocomposites in automotive applications was clay–nylon-6

nanocomposites used for making timing belt covers (the Toyota Motor

Company, 1991). The creep and fatigue properties of polymer

nanocomposites have drastically led to the overall improvement of the

mechanical performance of automobiles.

Fig 8. Nanocomposites application for automotive parts [Mehdi Hojjati et al, 2006]

2. Ballistic performance: This is an important issue for the survivability and

damage tolerance studies for aerospace and automotive structures. Polymer-

clay Nanocomposites have been studied and seen to have ability to maintain

a course through the air determined by its initial orientation and engine

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thrust. The US Army research laboratory investigated the ballistic impact

strength of polycarbonate-layered silicate nanocomposites. Boeing, USA

demonstrated the potential for aerospace application in a workshop in 2004

in FL, USA. According to their description, nanocomposites can play an

important role in longer-range missiles and a greater payload for aircraft.

3. Fire-Retardant Behaviour: Controlling polymer flammability remains a

key issue in numerous applications of engineering plastics and commodity.

The fire-retardant additive approach provides cost-effective solutions, but

generally at the expenses of some physical and mechanical properties. There

is also growing pressure for environmentally safe products and processes,

including recyclability and use of halogen-free compounds. For these

reasons, recognition of improved flammability properties in the case of

polymer–clay nanocomposites has triggered the development of extensive

research programs on a large variety of materials.

Koo and Pilato investigated the polymer nanocomposite for high-

temperature applications using cyanate ester, epoxy, phenolic, nylon 11, etc.

and described the feasibility of using these materials for fire retardant

coatings, rocket propulsion insulation, rocket nozzle ablative materials,

damage tolerant performance, etc. Ablatives are required to protect

aerospace launching systems against solid rocket exhaust plumes (3600oC)

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at very high velocity. They demonstrated that nanoclay plays a key role in

reducing the flammability on coating systems. Flammability is another

important issue for many applications. Other studies show that

nanocomposites prepared from the nylon family, epoxy, polystyrene or vinyl

ester, exhibit reduced flammability compared to their pure polymers.

Cone calorimetry is used to evaluate the flammability under fire-like

conditions. Relevant parameters such as the heat release rate (HRR) and its

peak value, heat of combustion (Hc), smoke yield (specific extension area,

SEA), and carbon monoxide yield are obtained. The table below shows some

typical data for layered silicate nanocomposites based on organically treated

montmorillonite, with polyamide 6, poly(propylene-graft-maleic anhydride),

and polystyrene as the host matrix. Nanocomposites under investigation

have a substantial reduction in peak HRR value (50–75%), whereas Hc and

CO formation shows little variation.

The table also compares the PS–clay nanocomposite with a PS–clay mix for

which intercalation does not occur. The peak HRR value remains identical to

that of pure PS while the intercalated PS–clay nanocomposite shows a 50%

reduction.

Peak HRR, kW/m2 MeanHRR,kW/m2

Mean Hc,MJ/kg

Total heatreleased,MJ/kg

MeanSEA,m2/kg

Nylon-6 1011 603 27 413 197

Nylon-6–clay 2% 686 390 27 406 271

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delaminated

Nylon-6–clay 5%Delaminated

378 304 27 397 296

PS 1118 703 29 102 1464

PS–clay mix 3%immiscible

1080 715 29 96 1836

PS–clay 3%intercalated

567 444 27 89 1727

PP-g-MA 2028 861 38 219 756

PP-g-MA–clay 5%intercalated

922 651 37 179 994

4. Barrier Properties: Organically modified clays dispersed in a nylon-6–

polymer matrix greatly improved the dimensional stability and the barrier

properties. Improvement in barrier resistance in nanocomposites plays an

important role in beverage applications. When the layers are laminated, it

increases the effective path length for molecular diffusion and the path

becomes highly tortuous to reduce the effect of gas and moisture

transmission through the film. Based on this barrier properties,

nanocomposite packaging films made in polyethylene terephthalate (PET)

have been studied as replacements for conventional polymer films. The

example of polyimide clay films illustrates the dramatic decrease of

permeability coefficients. Only 2 mass% montmorillonite loading reduces

the permeability by more than 50% of the pure polymer value for water

vapour, oxygen, or helium. Notwithstanding possible changes in diffusion

and/or solubility (Gao, F, 2004). It has been postulated that the major role of

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the clay platelets consists in substantially increasing the path length of the

permeant, which is by creating a highly tortuous path, due to the high aspect

ratio of the clay.

A simple theory derived by Nielsen expresses the relative permeability as

follows:

Pc/Po =1/ [1+ (L/2W) Vf]

In which Vf is the volume fraction of plates, L is the plate length, and W is

the thickness. Pc and Po stand for the nanocomposite and polymer

permeability respectively. Using equivalent loadings of clay but varying the

aspect ratio yields results in fairly good agreement with the theoretical

prediction. In the same way, a significant reduction in water vapor

permeability was observed in the case of a poly(ε-caprolactone) – organo-

montmorillonite nanocomposite, showing a fivefold reduction at only 4.8

vol% clay whereas it is only halved at best with a 20 vol% conventionally

filled silicate composite.

5. Biodegradability: Another interesting aspect of nanocomposite technology

is the significant improvements of biodegradability of biodegradable

polymers after nanocomposite preparation with organoclay (Mohanty, A.K.

et al, 2003). TOYOTA Technological Institute is commercializing layered

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silicate nanocomposites for packaging materials and also for short-term

disposable applications.

6. Thermal Stability: In recent years, thermogravimetric analysis of various

polymer–clay systems has confirmed this observation even for low

nanofiller loadings. One striking example is that of cross-linked

poly(dimethylsiloxane) incorporating 10 mass% exfoliated organo-

montmorillonite for which thermal stability under nitrogen flow is enhanced

by 140oC. For space applications, some critical issues are important, such as

temperature extremes of 196 to 125oC, higher toughness, dimensional

stability (i.e., resistance to micro cracking), etc. thus the science of polymer-

clay nanocomposites will be of optimum usefulness in space application

having fulfilled this degree of thermal stability.

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CONCLUSION

It has become apparent that nanoscale reinforcement is an attractive way of

improving the properties and stability of polymers. The development of

Polymer/Clay Nanocomposites is now a rapidly expanding multidisciplinary global

research activity. The nanoscale and high specific surface area of nanofillers

(which exhibit at least one dimension in the range of 1-100nm, such as

nanoparticles) and the resulting predominance of interfaces in PCNCs significantly

affect the structure and morphology of PCNCs at the molecular scale, influencing

their physical and material properties at scales that are inaccessible when

traditional (e.g. micron-sized) filler materials are used. The resulting PNCs exhibit

an excellent property profile that is applicable to a wide variety of industrial

applications; for example, high stiffness, chemical and thermal resistance,

dimensional stability, reduced water absorption, as well as improved electrical and

optical properties, all of which are significantly different from those provided by

conventional composites.

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