19 August 2010

POLYMER NANOCOMPOSITES

POLYMER NANO COMPOSITES: PREPARATION, PROPERTIES AND

APPLICATIONS.

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Abstract



The field of material science became quite popular and pragmatic with a tremendous lust for composite materials that exhibit the positive characteristics of both the components. Of late polymer nanocomposites have been making a large splash in the media and throughout several industries. Worldwide there has been a lot of interest to tailor the structure and composition of materials on sizes of nanometer scale. Hence a systematic review on the preparation, properties and applications of polymer nanocomposites is extremely important. Polymer nanocomposites are classified into different categories according to various parameters. The preparation techniques include sol gel process, in-situ polymerization and in-situ intercalative polymerization. The properties of nanocomposites such as mechanical, optical, rheological, flame retardancy and dielectric behavior have been extensively reviewed. Their important applications have also been described.



Contents




1. Introduction

2. Nanocomposites

3. Classification

4. Preparation techniques

5. Properties of nanocomposites

5.1 Physical properties

5.2 Mechanical properties

5.3 Barrier properties

5.4 Thermal properties

5.5 Optical properties

5.6 Rheological properties

5.7 Flame retardancy

5.8 Dielectric properties

6. Applications of nanocomposites

7. Future Outlook

8. References.



1. Introduction



Combining and orienting materials to achieve superior properties are old and well-proven concept; examples of this synergism abound in nature (1). Wood contains an oriented hard phase for toughness. Other natural composites are found in teeth, bones, bird feathers and plant leaves. Synthetic composites are found in artifacts and in recorded history. The use of chopped straw by the Israelites to control the residual cracking in bricks is an example (2). More representative of the modern structural composites are Mongolian bows, which were laminates of wood, animal tendons and silk, and Japanese Samurai swords, formed by the repeated folding of a steel bar back upon itself. The resulting structures contain as many as 215 alternating layers of hard oxide and tough, ductile steel.

Fillers have important roles in modifying the properties of various polymers. Mineral fillers, metals and fibers have been added to thermoplastics and thermosets for decades to form composites. The effect of fillers on properties of the composite depends on their concentration and their particle size and shape as well as on the interaction with the matrix. A composite can be defined as the material created when two or more distinct components are combined, but this definition is too broad to be useful; even if limited to polymers, it would include copolymers and blends, reinforced plastics and materials such as carbon-black filled rubber. Generally composites are defined as materials consisting of two or more distinct phases with a recognizable interface or interface boundary. In other words, it is a combination of two or more materials (reinforcing elements, fillers and composite matrix binders) differing in form or composition on a macroscale. In a strict sense composites are those materials formed by aligning extremely strong and stiff continuous fibers in a polymer matrix or binder. Compared to neat resins, these composites have a number of improved properties including tensile strength, heat distortion temperature and modulus. Thus, for structural applications, composites have become very popular and are sold in billion pound quantities. The materials in this class have exceptional mechanical properties and are often termed advanced composites to distinguish from chopped fillers or otherwise filled polymers. Composites are used in a wide variety of applications, as there is a considerable scope for tailoring their structure to suit the service conditions together with other advantages such as high strength to weight ratio, low cost etc. The composite materials combine beneficial properties of its component materials, which are not obtained in one component by itself.

Composites can be classified with respect to different parameters. The important ones are described below. The composite material can be classified broadly by their constituent components. There are mainly three categories of composites:

(a) Natural composite materials: These include wood, bone, bamboo, muscle and other tissues.

(b) Micro composite materials: These comprise of metallic alloys, toughened thermoplastics, Sheet molded compounds and reinforced thermoplastics.

© Macro composite (Engineering materials): These include galvanized steel, reinforced concrete beams, helicopter blades etc.




The polymeric composites are mainly micro-composites. They are further classified according to the reinforcement forms into particulate reinforced, fiber reinforced and laminar composites.

(i) Particulate reinforced composites: these include materials reinforced by spheres, rods, beads, flakes and many other shapes of roughly equal axes. The examples of polymeric materials incorporating fillers such as glass spheres or finely divided powders, polymers with rubber particles etc.

(ii) Fiber reinforced composites: These contain reinforcements having much greater strength than their cross-sectional dimensions. e.g.: glass fiber reinforced plastics, carbon fibers in epoxy resins etc.

(iii) Laminar composites: These are composed of two or more layers held together by the matrix binder. These have two of their dimensions much larger than the third. e.g.-wooden laminates, glasses, plastics etc.

Composites are also classified according to the nature of the matrix. Thus, composites can be classified as metal, ceramic or polymer based. Metal matrices of iron, Nickel, Tungsten, Titanium, Aluminium and Magnesium are used for high temperature usage in oxidizing environment. Ceramic matrices are often used with carbon, metal and glass fibers, and are used in rocket engine parts and protective shields. Glass matrices are mostly reinforced with carbon and metal oxide fibers. Heat resistant parts of engine, exhausts and electrical components are their primary applications. Polymer matrix composites are the subject of the present study and hence will be discussed in detail.

Another classification of particulate composites is based on the particle size of the dispersed phase. More recently, with advances in synthetic techniques and the ability to readily characterize materials on an atomic scale has lead to interest in nano-meter size materials. Since nanometre -size grains, fibers and plates have dramatically increased surface area compared to their conventional-size materials, the chemistry of these nanosized materials is altered compared to conventional materials. This can be micro composite, nanocomposites and molecular composites.

One of the oldest questions in natural philosophy and science is the subject today of intense research. The ancient philosopher 'DEMOCRITOS' posed the question, 'What happens as a macroscopic material is split over and over again?' Democritos reasoned that eventually the macroscopic parts derived from the multiple divisions would be distinct from the starting material; indeed at some point the process of successive divisions would lead to the indivisible atom. Interest today is focussed on nanocrystals; crystalline matter that is finely divided and has often been described as artificial atoms. Nanometer is an atomic dimension and hence the properties of nanoclusters or particles are reflective of atoms rather than bulk materials. Moreover adjusting the size can control the energy level spacing and other properties, but it is still large compared to the atomic limit. Recent studies show that it may be possible to combine the nanocrystal into nanocrystal molecules and nanocrystal solids in the same way as one does with real atoms; and these solids comprise tens to thousand of atoms and have dimensions in nanometer (<10 nm) range.

Since early 1980's scientists began taking interest in these materials. The other names that have been used for nanostructured materials were nanocrystalline, nanophase, cluster-assembled materials, quantum dots and quantum boxes. As none of these names seem to account fully for the variety of atomic structures that may form in this class of materials, the term nanostructured materials had been proposed and widely accepted.

Another important class of nanostructured materials are nanotubes. Carbon nanotubes are fullerene-related structures which consist of graphene cylinders closed at either end with caps containing pentagonal rings. They were discovered in 1991 by the Japanese electron microscopist Sumio Iijima (3) who was studying the material deposited on the cathode during the arc-evaporation synthesis of fullerenes. He found that the central core of the cathodic deposit contained a variety of closed graphitic structures including nanoparticles and nanotubes, of a type which had never previously been observed. A short time later, Thomas Ebbesen and Ajayan (4), from Iijima's lab, showed how nanotubes could be produced in bulk quantities by varying the arc-evaporation conditions. This paved the way to an explosion of research into the physical and chemical properties of carbon nanotubes in laboratories all over the world.

Nanotubes are classified into single-layer nanotubes and nanotube "ropes". A major event in the development of carbon nanotubes was the synthesis in 1993 of single-layer nanotubes. The standard arc-evaporation method produces only multilayered tubes. It was found that addition of metals such as cobalt to the graphite electrodes resulted in extremely fine tube with single-layer walls. The availability of these structures should enable experimentalists to test some of the theoretical predictions which have been made about nanotube properties. An alternative method of preparing single-walled nanotubes was described by Smalley's group in 1996 (5). Like the original method of preparing C60, this involved the laser-vaporisation of graphite, and resulted in a high yield of single-walled tubes with unusually uniform diameters. These highly uniform tubes had a greater tendency to form aligned bundles than those prepared using arc-evaporation, and led Smalley to christen the bundles nanotube "ropes" (5). Initial experiments indicated that the rope samples contained a very high proportion of nanotubes with a specific armchair structure. Subsequent work has suggested that the rope samples may be less homogeneous than originally thought. Nevertheless, the synthesis of nanotube ropes gave an important boost to nanotube research, and some of the most impressive work has been carried out on these samples. Also among the highlights of nanotube research to date is the demonstration that tubes can be opened and filled with a variety of materials including biological molecules. The other major application is nanoelectronics. Theorists have shown that nanotubes can be conducting or insulating depending on their structure. Theory also suggests that nanotubes should be immensely strong, so perhaps they will become "the ultimate carbon fibres".

In nanoscale crystals quantum size effects and the large number of surface atoms influence the chemical, electronic, magnetic and optical behavior. It is driven by both the needs to further miniature electronic components and the fact that at the nanometer scale the properties are strongly size dependent and thus can be tuned sensitively. Nanoparticles themselves exhibit different properties from their larger ones in the areas of optical, electrical, magnetic and mechanical properties. Since nanoparticles are much smaller than the wavelength of visible light, the composites may be transparent, although the same matrix with larger particles may not. Polymer nanocomposites have been making a large splash in the media and throughout several industries of late. Given all the interests in these materials, it is perhaps surprising to find that actual applications are few and far between. There are no vehicles manufactured in the US with nanocomposite parts, no nanocomposite lawn chairs, and no computers with nanocomposite cases. But the total US market, which is only pilot amounts today, is expected to exceed 55 million pounds in production by 2004.The value of this market is forecast to reach $195 million by then (6).

2. Nanocomposites

Nanocomposites have been studied for nearly 50 years. They were first referenced as early as 1950 (7). Polyamide nanocomposites were reported in 1976(8). It was the efforts of Toyota Research group that laid the foundation stone for the interest in this area (9,10). By a strict definition of nanocomposites, i.e., any filler submicron in size, there already are significant volumes of nanocomposites being produced. These amount to more than 20 million pounds. However, since these fillers are on the upper end of nanocomposites size range, most sources have excluded them from consideration. The field of material science has become quite popular and pragmatic with a tremendous lust for composite materials that exhibit the positive characteristics of both the components. World-wide, there has been a new desire to tailor the structure and composition of materials on sizes of the nanometer. This resulted in the generation of nanocomposites. Polymer nanocomposites are polymers that have been reinforced with small quantities (less than10%) of nanosized filler particles. Nanocomposites have been found to exemplify even more positive attributes than the predecessors do and thus we are trying to understand what occurs when nanocomposites of a polymer and inorganic components are produced.

Although particle filled polymer composites have been extensively studied because of their wide spread applications in the automobile, household and electrical industries, recently nanocomposites generate much interest among the various scientists principally, because of their potential they offer for applications in high performance coatings, catalysis, electronics, magnetic and biomedical materials. These nanocomposites are a new class of matrix filled with nanosize fillers. This study is based on some of the advantages of the nanocomposites over the conventional composites. Several advantages of these nanocomposites have been identified. They include efficient reinforcement with minimal loss of ductility and impact strength, heat stability, flame retardance, improved abrasion resistance, reduced shrinkage and residual stress and altered electronic and optical properties. The decrease in size of the domain to less than 100 nm enables good optical transparency. e.g., ultrafine TiO2 produces pearlscent effects(11). High surface area in comparison with small pore size can be used as catalysts for a wide variety of chemical reactions. For example, porous silica by pyrolysis of polymer hybrid. In addition to this Lithium, Calcium and Zinc salts can also be used to form homogeneous metal containing polymer hybrids for interesting ion conductive properties (12,13). Thirdly, molecular aggregates and boundary structure differs for nanosized particles as compared to conventional ones. The number of grain boundaries, pore density and the boundary energies are high for nanocrystals and hence exhibit novel electrical, magnetic and improved mechanical behavior. e.g.; ferric oxide and cadmium sulphide (14).

Another example for nanocomposite in nature is the natural bone (15) - bone consists of approximately 30% matrix material and 70% nanosized mineral. Here the matrix material is collagen fibers (polymer) and the mineral is hydroxyapatite crystals 50nmx25nmx3nm (ceramic). In the SEM image given (Fig.1), one can visualize the nanocomposite nature. The high mechanical properties of bone are supposed to be due to the nanocomposite material.

In polymer nanocomposites research, the primary goal is to enhance the strength and toughness of polymeric components using molecular or nanoscale fillers. Composites that exhibit a change in composition and structure over a nanometre scale have shown remarkable property enhancements relative to conventional composites. Most notable are increased modulus, increased gas barrier, increased heat distortion temperature, resistance to small molecule permeation, improved ablative resistance, increase in atomic oxygen resistance and retention of impact strength etc. Interestingly, these performance improvements are achieved without increasing the density of the base polymer, without degrading its optical qualities and without making it any less recyclable.

Nanocomposites are a combination of two or more phases containing different compositions or structures where atleast one of the phases is in the range of 10-to100 nm. Fillers with a particle size in the nanometer range have small number of atoms per particle and for this reason may have different properties than the bulk material and strong interactions with the matrix. The separation of filler particles is of the order of molecular dimensions, which may modify the properties of polymers. Nanostructured composites based on clay and polymers from methyl methacrylate, styrene, acrylonitrile and pyrrole have been receiving much research attention in view of many improved bulk properties of these composites compared to those of the base polymer. Considerable research is also being conducted towards the preparation and evaluation of aqueous colloidal dispersions of conducting polymer and inorganic oxide nanocomposite particles.

It is a remarkable fact that in addition to the profound changes in physical properties, which materials display when they are nanometer in scale, the chemical behavior is profoundly altered as well. When an inorganic solid is composed of only a few thousands of atoms, it has a great deal of surface area. By binding an appropriate organic molecule to this inorganic surface, it is possible to make nanocrystals behave chemically just like an organic macromolecule. Typically an inorganic nanocrystal will be coated with a monolayer of surfactant, rendering the nanocrystals hydrophobic. In this configuration the nanocrystals are soluble in non-polar solvents. If the solvent is removed the nanocrystals aggregate but not fuse, since a layer of surfactant separates them. These nanocrystals can be redissolved. Further the surfactant can be exchanged of with another organic molecule, enabling the nanocrystals to be placed in almost any chemical environment.

The design of organic-inorganic nanocomposites is a fascinating topic for science and technology and many applications are expected in the fields of optics, mechanics, iono-electronics, biosensers and membranes. Unexpected enhancements of properties such as barrier properties, fire resistance and increase of mechanical properties are reported. One of the most promising approaches to synthesise these materials consists in dispersing an inorganic mineral in an organic polymer on a nanometre scale. However there is a strong tendency of nanoparticles to get agglomerated and in turn prevents a homogeneous dispersion in polymer melt, which are characterised by high viscosities. This results in number of loosened clusters of particles and exhibit properties even worse than conventional particle / polymer systems. However, one can break the nanoparticles agglomerates and can produce nanostrustured composites by the addition of organically modified nanoparticles to a polymer solution, and by the in-situ polymerisation of monomers in the presence of nanoparticles (10). The interaction between the filler and polymer matrix is schematically represented in the Fig. 2.

3.Classification

Nanocomposites are classified into thermoplastic and thermoset nanocomposites.

1. Thermoplastic nanocomposites: these materials are divided into two major categories, i.e., commodity resins and engineering resins. Thermoplastics filled with nanometer-size materials have different properties than thermoplastics filled with conventional materials. Some of the properties of nanocomposites, such as increased tensile strength, may be achieved by using higher conventional filler loading at the expense of increased weight and decreased gloss. Other properties of nanocomposites such as clarity or improved barrier properties cannot be duplicated by filled resins at any loading.

Polymer nanocomposites were developed in the late 1980s in both commercial research organizations and academic laboratories. The first company to commercialize these nanocomposites was Toyota, which used nanocomposites parts in one of its popular car models for several years. Most commercial interest has focussed on thermoplastics. Thermoplastics can be broken into two groups: less expensive commodity resins and more expensive (and higher performance) engineering resins. One of the goals of nanocomposites was to allow substitution of more expensive engineering resins with a less expensive commodity resin nanocomposite. Substituting a nanocomposite commodity resin with equivalent performance, as a more expensive engineering resin should yield overall cost savings.

2. Thermoset nanocomposites: these have received less commercial interest in their development than thermoplastic nanocomposites, but these materials may be relatively straightforward to bring into production. Furthermore, thermoset nanocomposites can offer some significant advantages over conventional thermosets. At this point of time, there has been much less commercial interest in thermoset nanocomposites compared to thermoplastics. This neglect may not continue much longer since thermoset nanocomposites have some distinct advantages over neat thermoset resins.

Nanocomposites can also be classified based on the filler into three, viz., clay (silica) based, inorganic-polymer layered and inorganic-polymer hybrids. In the clay variety considerable work was done in the recent years. The filler particles are the individual layers of a lamellar compound, most typically clay. Since a single clay layer is only 10 A thick, it has a very large aspect ratio, usually in the range of 200-2000. This makes it possible to use very small amounts (i.e., a few weight percent) of clay to interrupt the structure of a polymer matrix on a nanometer length scale. The resulting nanocomposites can exhibit dramatically altered physical properties relative to the pristine polymer. The key to forming such novel materials is understanding and manipulating the guest-host intercalation chemistry occurring between the polymer and the layered compounds. Pioneering advances at Toyota research during the early 1990's has stimulated the development of various polymer/organoclay nanocomposites with attractive property profiles (16). There are two end members that define the realm of structures possible in such nanocomposites (17) and these are shown schematically in Fig.3. At one end are well ordered, stacked multilayers that result from intercalated polymer chains within host silicate clay layers. At the other end are delaminated materials, in which the host layers have lost their registry and are randomly dispersed in a continuous polymer matrix. The organoclays as precursors to nanocomposites formation has been extended into various systems including epoxies, polyurethanes, polyimides, nitrile rubber, polyester, polystyrenes, and siloxanes (18-23). For true nanocomposites, the clay nanolayers must be uniformly dispersed (exfoliated) in the polymer matrix, as opposed to being aggregated as tactoids or simply intercalated (Fig. 4).

The second type of nanocomposites focuses on layered compounds such as transitional metal dichalcogenides (24-27), hybrid metal oxides (28-30) and layered metal polymer chalcogenides (31). Layered silicates dispersed as a reinforcing phase in an engineering polymer matrix are one of the important forms of such ''hybrid organic-inorganic nanocomposites.'' Although the high aspect ratio of silicate nanolayers is ideal for reinforcement, the nanolayers are not easily dispersed in most polymers due to their preferred face-to-face stacking in agglomerated tactoids (32).

Giannelis and co-workers (27, 30-39) did a lot of work on polymer layered silicate nanocomposites. The static and dynamic properties of these systems are thoroughly investigated. Despite the topological constraints imposed by the host lattice, mass transport of the polymer, when entering the galleries defined by adjacent silicate layers, is quite rapid and the polymer chains exhibit mobilities similar to or faster than polymer self-diffusion. However, both the local and global dynamics of the polymer in these nanoscopically-confined galleries are dramatically different from those in the bulk. On a local scale, intercalated polymers exhibit simultaneously a fast and a slow mode of relaxation for a wide range of temperatures, with a marked suppression of co-operative dynamics typically associated with the glass transition. On a global scale, relaxation of polymer chains either tethered to or in close proximity ((1nm as in intercalated hybrids) to the host surface are also dramatically altered.

In the third category the focus is on the nanocomposites formed from inorganic fillers in polymer matrix. These are materials in which nanoscopic inorganic particles, typically 10-100 angstrom in atleast one dimension, are dispersed in an organic polymer matrix in order to improve dramatically the performance properties of the polymer. In this process first we have to prepare the nanosized particles of inorganic moiety and then to incorporate it in the matrix. One of the primary objectives of the various synthesis techniques is to control the particle size either by spatial conditions, such as size of pores and entities in the media, or by reaction kinetics. Stabilising nanosize metal or semiconductor particles are critical. Several advantages have been reported for the usage of polymer as the matrix. There are three major techniques for preparing nanoparticles in a polymer matrix. These are

(1) in situ generation of the particles: This method involves two steps. First, incorporation of a metal ion in the polymer by immersion of the polymer matrix, or polymer membrane, in an aqueous solution containing the metal ions. The ion(s) is absorbed or adsorbed to the polymer matrix. The second step is the formation of particles in the polymer matrix, by reacting the product of the first step with the proper reactants; e.g., reducing compounds. An example of such synthesis is formation of Copper Sulphide particles in poly(vinyl alcohol)-poly(acrylic acid) matrix(40).The polymer mixture was immersed into copper sulphate aqueous solution, where the acidic groups of poly(acrylic acid) serve as complexation sites for cuprous ions. Subsequently, the ions in the polymer were reduced using sodium sulphide to form ~10nm CuS particles. As another example, PbS was synthesised by milling Pb(CH3COO)2 with poly(ethylene-methacrylic acid)(E-MAA)copolymer. The metal cations form polar clusters with the carboxylate groups of the E-MAA.The Pb containing E-MAA film was then reacted with H2S to form 2.5-7 nm particles. This PbS-EMAA system had good mechanical and optical properties (41). A third example is the synthesis of nanoparticales in a polymer blend membrane (42). The membrane was prepared from cellulose acetate (CA) and poly(styrenephosphonate diethylester) (PSP) which had phosphonate ester functionality, -PO(OR)2. The blend membranes were immersed in an aqueous solution of CdNO3 and then exposed to H2S, which diffused into the polymer matrix forming nanoparticles.

Another method of interest was the preparation of nanoparticle particulate film. In this method, a polymer monolayer was spread on an aqueous solution of metal salt, e.g., Cadmium, Zinc and Lead ions. Injection of a reactant gas, e.g., H2S, into the gas phase of the enclosed system initiates particle growth. The polymer monolayer acts a matrix for the size-controlled growth of semiconductor particles that can be transferred, essentially intact, to a solid substrate. The CdS particles were formed in the poly(styrenephosphonate ethyl ester)particulate monolayer (43).The observed images had particles with 20-30 A widths and heights (Fig.5).This methodology demonstrated very unique nanoparticle-polymer system that allows the construction of a nanosized electronic device by transferring multiple ultrathin polymer particulate films and constructing a stack of different films containing different band gap nanoparticles.

(ii) Formation of nanoparticles via Polymerisation.

The second method of synthesising nanoparticles is via polymerisation of colloidal solutions containing metal ions and monomers. The particle size can be controlled by the reaction temperature and properties of the colloidal solution, thermal coagulation and Ostwald ripening. One example of such synthesis of PbS nanoparticles in polymer matrices is the polymerisation of Pb(MA)2(lead methylacrylic acid) with styrene. Lead methyl acrylate was prepared from PbO and methylacrylic acid. Since there are two C=C bonds in each Pb(MA)2 molecule, it is easier to copolymerise with styrene to form Pb-polymer microgel. This was subsequently treated with H2S gas to obtain PbS nanoparticles in the polymer matrix and is evenly distributed through the matrix. Almost all the particles are spherical and uniform in size. The average particle diameter shown in the figure is ~4nm (Fig.6). These materials showed large optical non-linearities, i.e., the third order optical susceptibility that was as high as 10-8 esu (44).

(iii) Mechanical mixing of nanoparticles with polymers.

This method involves the direct mechanical mixing of a polymer solution with a pre-synthesised, highly dispersive nanoparticle solution. Several authors reported the synthesis of these types of composites. Li et al. (28) reported the synthesis of CdS/ polyacrylamide nanocomposites in a single step by gamma-irradiation. Harmer et al. (29) prepared Cobalt oxide /PMMA nanocomposite. Room temperature synthesis of Mn-Ferrites in a polymer matrix has been the work of Shen and Egerton (30). A non-aqueous solution route to prepare polyacrylamide silver nanocomposite at room temperature has been reported by Giannelis et al. (23) This has been done by the (gamma-irradiation to the substrates and the products obtained were transparent. Using Ziegler-Natta polymerisation a novel magnetic PE nanocomposite was synthesised by Pinnavia group (45). He found that the activity of ethylene polymerisation is unaffected by the polymerisation time and temperature.

Giannelis et al. (33-39) extensively studied about polymer nanocomposites. In their systems, the inorganic particles are the individual layers of a lamellar compound, most typically smectite clay. Since a single clay layer is only 10 angstrom thick, it has a very large aspect ratio, usually in the range of 200-2000. This makes it possible to use very small amounts (i.e., a few weight percent) of clay to interrupt the structure of a polymer matrix on a nanometer scale. The resulting nanocomposites can exhibit dramatically altered physical properties relative to the pristine polymer. The key to forming such novel materials is understanding and manipulating the guest-host intercalation chemistry occurring between the polymer and the layered compounds. As an example of a new generation nanocomposite, they have reported an epoxy-clay nanocomposite prepared by the polymerization of the diglycidyl ether of bisphenol A with diamines in the galleries of acidic alkylammonium ion exchanged forms of montmorillonite. X-ray powder diffraction and TEM confirmed delamination of the montmorillonite clay in the cured epoxy. Electron micrographs for a 5 wt% [H3N(CH2)11COOH]+ clay-polymer nanocomposite revealed that the micron sized clay tactoids had been exfoliated by the polymer into single platelet assemblies in which the interlayer spacings range upto 2000A. Polymer-clay hybrids represent another type of polymer-clay nanocomposites that have been investigated by them. These nanocomposites have been prepared by intercalation of the organoclay with polyamic acid. In contrast to the completely exfoliated epoxy-clay system, the polyimide system contains regularly intercalated clay aggregates in the polymer matrix. Although face-to-face clay layer aggregation is extensive, the clay-polyimide hybrid composite films exhibit greatly improved CO2 barrier properties at low clay content as shown by this graph: Less than 8.0 vol. % Clay results in almost a ten-fold decrease in permeability. A self-similar or fractal dispersion of the clay platelets in the polyimide matrix may explain the barrier property enhancement (Fig.7).

4. Preparation of Nanocomposites.

A polymeric particle/ polymer nanocomposite contains a rigid polymer component dispersed within a flexible polymer matrix on a nanoscale level. The rigid polymer, with high modulus and high strengths, usually has high melting temperature, is insoluble in organic solvents, and combining it with the flexible polymer is thermodynamically unfavorable. Therefore it is very difficult to prepare a nanocomposite, and phases may undergo segregation during processing and end use. Hydrodynamic effects and physi- or chemisorption of matrix at filler surface governs the reinforcement.

Nanocomposites are prepared mainly by three methods:

i) Sol- gel process, This includes two approaches: hydrolysis of the metal alkoxides and then polycondensation of the hydrolyzed intermediates. This process provides a method for the preparation of inorganic metal oxides under mild conditions starting from organic metal alkoxides, halides, esters etc (46-47).

Wei et al. (48) prepared PT nanoparticles and composite thin films by this method using a precursor (hydrate lead acetate and titanium n-butoxide). The ultrafine particles obtained were spheroid in shape and have size in the range 40-80 nm. The PT/PEK-C nano composite formation was done by spin coating method using chloroform as the solvent.

The formation of transparent films of organic-inorganic hybrid materials derived via co hydrolysis and polycondensation of alkyltrimethoxysilane-tetramethoxysilane mixtures are notable (49). This opens the possibility of structural and morphological variations of hybrids by utilizing both the molecular-assembling property of long chain alkyltrialkoxysilanes and the network-forming ability of tetraalkoxysilanes. The sol-gel process using metal alkoxides is an effective way to produce inorganic-organic hybrids (50-53). Kuroda et al. synthesized these types of nanocomposite thin films using alkyltrimethoxysilanes with various alkyl chain lengths and tetramethoxysilane (49). A typical XRD pattern of the product is shown in Fig.8. The SEM image of the edge of the film shows the stacked multilayers, and the TEM image of the product clearly shows the layers indicating the presence of an inorganic-organic interstratified structure (Fig.9&10). The 29Si MAS NMR and thermal analyses indicated copolymerisation between C12TMS units and TMOS units.

Schmidt and coworkers (54) controlled the polymerization rate and stresses in metal alkoxides through the concept of chemically controlled condensation, where competitive esterification reactions were used to slow the elimination of water. In addition to the manipulation of the processing parameters, another approach toward dealing with the stress associated with drying involves the modification of the inorganic metal oxide with an appropriately functionalised polymer. Such inorganic-organic hybrids or composites can be designed to offer a range of properties depending on the relative composition of each component, size scale of phase separation, and reactivity between the components. Chujo and coworkers (55) have reported that nanometer phase separation is obtained for only when there is inorganic functionality on the organic component and there is a strong interaction (i.e. Hydrogen bonding) between the components. Examples of such polymers include triethoxysilyl functional polyoxazolines, poly(methylmethacrylate), poly(vinyl acetate), poly(N,N-dimethylacrylamide) etc. (56-59). Hedrick et al. (60) prepared polyimide-modified poly(silsesquioxane) hybrids using functionalised poly(amic acid alkyl ester) precursors.

Using sol-gel technique Iyoku et al. (61-63) introduced methyltriethoxysilane (MTES) and phenyl triethoxysilane (PhTES) into the polyamic acid. Furthermore, for polyimide hybrids the MTES component was partially replaced with dimethyl diethoxysilane (DMDES). The motive behind this was to improve the properties of polyimide in terms of better mechanical strength and uniform nanocomposite. Whang et al. (64) developed a novel low dielectric polyimide / poly(silsequioxane) nanocomposite material using this technique.

ii) in-situ intercalative polymerization, which is a good method for the preparation of polymer/clay mineral hybrids. A novel class of fillers is anisotropic layered silicates of the montmorillonite type, which can be modified by cation exchange with organic ammonium salts, thus producing organophilic clays, further called organoclays. Organophilic modification affords compatibility between filler and polymer. Different methods have been introduced to achieve matrix-filler compatibilization: melt or solution intercalation of organoclay with polymers, cation exchange of montmorillonite with polymers bearing quartenary ammonium groups, or cation exchange and subsequent polymerization with monomers containing quaternary ammonium groups. These compatibilisation techniques account for improved interfacial adhesion and effective dispersion of either intercalated silicate layer aggregates or even individual exfoliated silicate layers. Such nanocomposites exhibit superior stiffness, impact, strength and heat distortion temperature. In this method the mostly used clay is montmorillonite (MMT) because of the large surface area (about 750m2/g) and large aspect ratio (greater than 50), with a platelet thickness of 10 A. Strawhecker and Manias (65) synthesized poly(vinyl alcohol )/ Na+ Montmorillnite nanocomposites by the solution intercalation film casting method. Hybrid films were casted from an MMT/water suspension where PVA was dissolved. A suspension of sodium montmorillonite at a concentration of less than 2.5 wt%. The suspensions were stirred and sonicated. Then low-density, fully hydrolyzed atactic poly(vinyl alcohol) was added to the stirring suspensions so that the total concentration of the solids were kept at less than 5 wt%. The mixtures were heated to 900C and sonicated. Then nanocomposite films were casted in varying thickness of 0.001 to 0.1cm.Bright field TEM is used for the study of dispersion of inorganic layers. A typical TEM image is given in Fig.11 for the 20wt% MMT nanocomposite. Extensive TEM observations revealed that a coexistence of silicate layers in the intercalated and the exfoliated states. XRD studies also revealed the existence of exfoliated inorganic layers throughout the polymer matrix. The system becomes mostly intercalated as silicate loading increases beyond f mmt >60 wt%. This exfoliation of layers is attributed to the water casting method used, since the water suspended layers become kinetically trapped by the polymer and cannot aggregate.

Gronski et al. (66) observed that when compared to porous silica, organophilic layered silicates showed excellent dispersion in rubber matrix. This occurs due to their surface modification via cation exchange of intergallery sodium ions for organic ammonium ions. But the mechanism of reinforcement for anisotropic fillers on a nanoscale is not yet established.

Brittain et al. (67). prepared PMMA-layered silicate nanocomposite by this method. For this, the layered silicates were dispersed individually into water.

iii) In situ polymerization, which is a method where nanometer scale inorganic fillers or reinforcements are dispersed in the monomer first; then this mixture is polymerized using a technique similar to bulk polymerization.

Krishnamoorti et al. (68) prepared an important class of polymer layered silicate nanocomposites by this method. The end tethered polymer layered silicate nanocomposites were prepared by in situ polymerization of (e-caprolactone. For this the silicate surface was converted from a hydrophilic to an organophilic surface by an ion exchange of the metal cations by 12-aminolauric acids. The carboxyl groups of the aminolauric acid initiate the polymerization of the monomer and the polymerization proceeds via a ring opening of the (e-caprolactone. The layered silicates were highly anisotropic with a thickness of 1nm and lateral dimensions (length and width) ranging from several 100nm to a few microns. The polymer chains are tethered to the surface via ionic interactions between the silicate layer and the polymer end. A schematic representation is given in the Fig.12. The same authors also studied end-tethered nylon-6 silicate nanocomposites (37). Giannelis et al. (69) also produced polyurethane-nanosilica composites by mixing polyol with the filler. After removing the solvent by distillation curing was done using diisocynate at 1000C. The filler concentration was varied from 0% to 50% in different steps and thin films were made. From the SEM studies the particle size was found in the range 10-20nm. Mauritz et al. (70) prepared surlyn/titanate nanocomposites materials through polymer-in situ sol-gel reaction.

Several synthetic routes were examined to produce polystyrene/ organoclay nanocomposites, aiming at improving exfoliation of organoclay. Moet and coworkers (71) reported in-situ bulk and solution polymerization of styrene using coreactive organophilic montmorillonite, obtained via ion exchange of sodium montmorrilonite with vinylbenzyltrimethyl ammonium chloride, in order to achieve interfacial grafting of polystyrene onto clay and to promote swelling of clay in styrene and various solvents.

In nature, polymer/inorganic nanocomposite materials are frequently encountered (for example, bone, tendon, dentin and bamboo) and represent some of the finest examples of the optimized interfacial interaction. However, it is still inherently difficult to reproducibly generate polymer/inorganic composite architectures with the level of nanometer-scale sophistication responsible for the remarkable properties of biological composites. Consequently, one of the frontiers in nanotechnology is the advancement of viable methods for the efficient design and synthesis of polymer-inorganic nanocomposites with architectural control and improved properties as a result of this sophistication. A wide variety of methodologies have been employed to synthesize polymer/inorganic nanocomposite. Depending on whether the inorganic component is grown in presence of polymer (monomer) matrix or pre-fabricated, these methods can be basically divided into two categories ''in situ'' and ''ex situ''. Among all these synthetic strategies, the assembly of inorganic nanoparticles into polymer matrix appears to be one of the most promising approaches. Nanoparticles are readily obtained and have potentially useful optical, optoelectronic and material properties deriving from their small nanoscopic size. These properties might lead to wide applications including chemical sensors, spectroscopic enhansers, quantum dot and nanostructure fabrication and micro imaging methods. Again, the interfacial interaction between inorganic nanoparticles and polymer matrix exerts important influence on the properties of resulted nanocomposite. As a result, tailoring and manipulating interfacial interaction becomes a particular preparative challenge.

By using a controlled/ living radical polymerization to grow polymer chains from the nanoparticle surface, one can completely control the structure of resulting composite by manipulating the polymer grafting density, chain length and molecular weight. The synthesis of polystyrene/ SiO2 nanocomposite was accomplished by this technique. Spherical silica particles with an average diameter of 70nm, determined from TEM micrographs and dynamic light scattering (DLS) measurement, were used to these experiments. From the TEM analysis one can observe that the distance between the nanoparticles within the domains increased from approximately 10 to 30 and 40nm, respectively, in correlation with the molar mass of the grafted polystyrene chains. When this synthesis was repeated with starting silica nanoparticles that had a narrow size distribution, the nanopatricles within the film domains were observed to pack into hexagonal arrays.. This method is useful for generating nanoparticles film arrays with interesting magnetic and optical properties (72).

Another method, which involves the in situ polymerisation, is the photo deposition. This process provides the flexibility to polymerize by a UV laser and create patterned films with controlled layer thickness. Also the films obtained by this method are of high optical quality, and the process is extremely straightforward and very reproducible. The photo deposition of polydiacetylenes has proven to be a robust and versatile process for depositing polymer thin films onto a variety of substrates (14).

5. Properties of nanocomposites

Nanocomposites offer much different properties than conventional composites. The most important ones are enhanced mechanical strength, optical transparency, improved thermal stability, improved barrier properties, improved flexibility, novel electrical properties etc.

5.1 Physical properties

According to the linear mixture equation the density of a composite, re, is a linear combination of densities of the matrix and filler and their respective volume fractions. Petrovic et al (73) studied the effect of filler concentration on density of polyurethane filled with microsilica and nanosilica, which is given in the Figure 13. It was found that the density of the samples increased with filler concentration in both series, but more so in the series with microsilica. The increase in density is attributed to the increase in volume of the polymer matrix on incorporation of nanoparticles. In the same system swelling studies were done. The degree of swelling for two series is given in figure 14. Since a lower degree of swelling indicates better curing, it is obvious that the sample with 50% nanosilica stands out as less cured. Also the glass transition temperature increased with increased filler concentrations both in nanocomposites and microcomposites. It seems that although there was a strong interaction between the matrix and the filler (which should have increased Tg), an opposing effect came from incomplete curing of the matrix.

5.2 Mechanical properties

Novak (74) reported that there exists an increase in hardness and scratch resistance with the addition of nanoparticles to polymer matrix. But in polyurethane nanosilica composites first there is an increase and then it decreases (73), Fig. 15. From the tensile graph the nanosilica shows a 600% improvement in elongation at break. The tensile strength of the composites with nanosilica and microsilica, shown in Fig.16, indicated that up to a 20% filler concentration there was not muchdifference between the nanosilica and microsilica effect. But experiments with nanosilica in PDMS Elastomers showed that the elongation decreased and strength increased with increasing filler concentration (75). On the other hand, nanotitania filled PDMS networks showed partial increase in elongation at break with increasing filler content (76)-that is no regular pattern can be said to be emerging.

Gao et al. (77) reported that intercalating as little as 2 vol% silicate into nylon-6 on a nano-scale, the tensile strength and modulus of the hybrid were improved more than twice than that of the virgin polymer. Kojima et al. (78) showed that only 10phr of organic clay was necessary to achieve tensile strength comparable to a compound loaded with 40 phr of carbon black. It was shown that tensile strength and elongation at break was improved depending on filler loading and compatibilization.

Exfoliated polymer/silicate systems have been found to exhibit superior mechanical properties than the conventionally filled systems. Strawhecker et al. (65) studied the mechanical properties of PVA/Na+ Montmorillonite nanocomposites. The variations in properties were plotted in the Fig.17. The mechanical/tensile properties of these nanocomposites were studied for low silicate loadings, and Young’s modulus was found to increase by 300% for 5-wt% silicate, with only a 20% decrease in toughness, and no sacrifice of the stress at break compared to the case of neat PVA. In addition, for these low loadings, thermal stability from TGA measurements was shown to be slightly enhanced, and high optical purity was retained.

Ganter et al. (79) studied the mechanical properties of rubber nanocomposites using BR and SBR vulcanizates reinforced by organophilic-layered silicates. It was shown that in the absence of coupling agent, the compounds containing layered silicates show increased tensile strength and strain at break with respect to unfilled vulcanizates. When compared to silica compounds, both SBR and BR vulcanizates showed good enhancement in mechanical properties. But when the matrix is chemically bonded to silica by TSEPT, tensile strength remains constant, but strain at break decreases. A similar effect is observed for anisotropic layered silicates also. This is owing to the fact that the reactive coupling of the elastomer matrix is also effective on the surface of silicate layers containing quaternary ammonium salts on the surface. From the hysteresis studies, improved reinforcement by 20%-50% relative to silica vulcanizates is observed for BR and SBR organoclay compounds. Chemical bonding of the rubber matrix led to further increase in reinforcing effect by 50%-90% relative to the corresponding silica systems. The hysteresis of the silicate filled compounds is much larger than that of vulcanizates filled with silica. In presence of reactive coupling, hysteresis is reduced for both filler types but is still 1.8 fold larger than that of organoclay vulcanizates.

5.3 Barrier properties

With the dispersion of the ultra thin inorganic layers throughout the polymer matrix, the barrier properties of the nanocomposites are expected to enhance strongly compared to the respective polymer. In PVA/Na+Montmorillonite nanocomposites the water vapor transmission rates were measured for the pure polymer and several of its low MMT nanocomposites. The permeabilities decreased to about 40% of the pure water vapor transmission values for silicate loadings of only 4-6-wt%. This decrease is attributed to the increased path tortuosity of the penetrant molecules and to the enhanced modulus of the polymer matrix (65).

5.4 Thermal properties

The DSC studies in poly(vinyl alcohol)/Na+Montmorillonite nanocomposites (65) established that there is a suppression of the thermal transitions (Tg and Tm) for the purely intercalated systems. Bulk PVA has a Tg at 700C and a melting transition at 2250C. But for the fully intercalated hybrids DSC does not detect any transitions between 35 and 2500C (Fig.18). For these neatly intercalated nanocomposites both Tg and Tm are too weak and/or too broad to measure, or they are suppressed due to the polymer confinement. Although the physical origin of this behaviour is still under debate, (80-81) this absence of thermal events is in agreement with the general behavior of polymers intercalated in clays and synthetic silicates. In a plethora [nylon-6, PEO,PMPS,PS,PCL,and PMMA intercalated in naturally occurring silicates and in synthetic layered aluminosilicates(82-87)]of systems studied, there exist no detectable thermal transitions for the intercalated polymers, over a wide temperature range below the Tg and above the Tm. Despite the use of methods with an increasing resolution and sensitivity no transitions can be detected in neatly intercalated systems. For example, TSC, DSC, and NMR studies (82-87) of an intercalated PEO/MMT hybrid indicated the absence of any thermal transitions between –100 and 1200C that could correspond to the vitrification or the melting of PEO. A systematic study of the DSC with MMT (Fig.19) shows that the Tg and Tm signals weaken gradually and disappear for MMT above 60-wt%. This suggests that in these systems the inorganic layers affect the entire polymer, and there seems to be no bulk like PVA present. For higher polymer concentrations there appear two distinct and overlapping melting peaks: one around the bulk Tm and another one at higher melting temperature.

5.5 Optical properties

Petrovic et al. (73) studied the optical properties of polyurethane-nanosilica composites in detail. They observed that at all filler concentrations the composites were transparent, while those of micro silica were not. UV/VIS spectra of 1mm thick samples showed total absorption below 320 nm and high transmission between 450 and 900nm in all samples with nanosilica.

Conjugated polymers show good optoelectronic properties. Poly(p-phenylenevinylene) (PPV) and its derivatives are used for this purpose. For the improvement of optical and electronic properties of PPV several attempts were made (88-91). A feasible way to improve the optical properties is to combine PPV with inorganic nanoparticles. Incorporation of Cadmium Selinide nanoparticles made PPV a blue light emitter and showed enhanced luminescence (92-93). Blends of TiO2 nanoparticles with PPV got improved photovoltaic properties (94). Zhang et al. (95) observed some peculiar properties of PPV/TiO2 nanocomposites. The TEM images showed that the TiO2 nano aggregates took the form of a sphere and finally ellipsoid with an alignment, as the TiO2 content increased. PL spectra revealed that the light emission of the PPV/ TiO2 nanocomposites was from PPV, and the relative intensity of the vibrant components changed with the formation of the TiO2 alignment structure (Fig.20).

5.6 Rheological properties

The rheological properties of in-situ polymerized nanocomposites with end tethered polymer chains were first described by Giannelis et al. (68-69). They found that the flow behavior of poly((e-caprolactone) and polyamide-6 nanocomposites differed extremely from that of the neat matrices, whereas the thermorheological properties (Arrhenius activation energy of flow) of the composites were entirely determined by that behavior of the matrix. The slope of the storage modulus G' and the loss modulus G'' versus the frequency (in the terminal region was smaller than 2 and 1 respectively. Values of 1 and 2 are expected for melts of linear monodisperse polymers and the large deviation, especially for small amounts of silicate loading in the percentage range may be due to network formation. However, such nanocomposites based on the in-situ polymerization technique exhibit fairly broad molar mass distribution of the polymer matrix, which hides the structure relevant information and impedes the interpretation of the results. Weiss and Vansant (96) established that the interlayer distance of organoclays increases with increasing chain length of alkyl groups used as substituents of alkyl ammonium cations, which represents typical modifiers of organoclays. Earlier Okada (97) observed that, who used (w-aminoacids as modifiers; interlayer distance can increase from 0.95nm upto 2nm when using such small molecular weight modifiers. But quantitative correlations between the viscoelasticity and the morphology of the composites cannot be established. Dietrich et al. (98) studied rheological and mechanical properties of polystyrene-clay nanocomposites using polystyrenes with narrow molecular distributions as continuous matrix and as ammonium-functional modifier of organoclay in order to establish correlations between morphology development and rheological behavior of polystyrene/organoclay nanocomposites. Since both the polystyrene continuous phase and the ammonium-terminated polystyrene attached to the silicate nanoparticle surface exhibited very narrow molecular weight distributions it became possible to examine rheological properties. Only when high molecular weight polystyrene was grafted onto the dispersed silicate layers, rheological investigation revealed the formation of networks via assembly of in-situ formed silicate nanoparticles.

5.7 Flame retardancy

Research in the area of condensed flame retardants for polymers usually builds upon existing technologies. They are metal hydroxides (alumina, magnesium hydroxide) or phosphorus based materials. However, these materials tend to weaken mechanical properties while improving flammability resistance. No new major flame retardant technology has emerged in this area for quite some time. Polymer-clay nanocomposites have generated a great deal of interest primarily due to improved mechanical and thermal properties. Also, they have improved flammability resistance while maintaining good mechanical properties, a key advantage over existing condensed phase flame-retardants. Morgan and co-workers (99) did extensive work on this aspect. They have shown that polymer-clay nanocomposites have greatly reduced heat release rates. Also, they have observed polymers, which normally do not char, or leave any carbonaceous residue upon burning, produce char in the presence of clay.

Morgan et al. studied the flammability of polymer nanocomposites by taking two types of polymer-clay nanocomposites. Nanocomposites were prepared from polycaprolactum (PA-6) and polyethelene-co-vinylacetate (EVA). Generally flame retardants improve flammability but reduce polymer mechanical properties. But from this study it has been observed that polymer-clay nanocomposites reduced the flammability of polymers with improvement in the mechanical properties. Antonov (100) have shown that adding as little as 0.05% nano-metal particles in a polymer system can increase upto 23% fire resistance of the polymer. This led to the exploration of novel metal/ polymer nanocomposites for electronic products and the improvement of fire retardancy.

The most important difficulty in the development of clay/polymer nanocomposites with the purpose of enhancing fire retardancy is that the most efficient structure for the enhancement of fire retardancy may not result in the best mechanical properties. The enhancement of fire retardancy in layered silicate/polymer nanocomposites is achieved essentially via the formation of torturous passways to inhibit the evolution of flammable volatile pyrolysis species. This may become less effective when the silicate layers separating apart over a certain distance to cause the collapse of the torturous passways. Gao (101) developed graphite/ polymer nanocomposites, which showed better fire retardancy. The mechanisms of fire retardancy in a graphite/polymer system are achieved via not only the formation of torturous passways but also the catalytic effect of the nucleation of char formation by the aromatic sheets. Gao and his group focussed on the development of non-black colored graphite/polymer nanocomposites with better mechanical properties and fire retardancy.

Polymer-layered silicate (PLS) nanocomposites offer effective flame retardancy without creating environmental problems in terms of combustion, recycling and disposal of the end products. This is the most successful approach developed so far to produce environmental-friendly flame retarding polymers.

5.8 Dielectric properties

Dielectric spectroscopy (DEA) is a powerful tool in studying relaxation phenomenon in polymers and composites. It provides information about the location and activation energy of relaxation transitions, the dipole moments of the subunits involved, concentration and mobility of charge carriers and so on. Dielectric measurements in polyurethane-nanosilica composites showed that both the nonfilled and the filled composites exhibit an overlapping transition consisting of two subrelaxations, which become resolved at the highest frequencies only. Wei et al. (48) studied the effect of poling on the dielectric properties of the PT/PEK-C nanocomposite films. The difference between the dielectric constants of the components is very large. The figure shows the temperature dependence of dielectric constants of compacted sample of PT ultra fine particles and PEK-C polymer. From the figure Tg of the nanocomposite thin film is about 2000C,because of plasticization. Plasticization should be considered inorder to determine the poling temperature of the nanocomposite thin films. Besides the viscosity of polymer descends, and the alignment of Pt ultrafine particles is easy at high temperature. But the conductivity of PT/PEK-C composite thin films increases fast with temperature, i.e., the thin films is broken down easily at high temperature.

6. Applications of Nanocomposites

In the forgoing discussion, it has been observed that nanocomposites have set the current trend in the novel materials drawing considerable interest due to the unusual properties displayed by them. Several authors have adopted various techniques to prepare nanocomposites. However, the techniques they utilized are very cumbersome which require careful control of various parameters such as pH, moisture, temperature etc.

In recent years significant progress has been achieved in the synthesis of various types polymer-nanocomposites and in the understanding of the basic principles, which determine their optical, electronic and magnetic properties. As a result nanocomposite-based devices, such as light emitting diodes, photodiodes, photovoltaic solar cells and gas sensors, have been developed, often using chemically oriented synthetic methods such as soft lithography, lamination, spin-coating or solution casting. Milestones on the way in the development of nanocomposite-based devices were the discovery of the possibility of filling conductive polymer matrices, such as polyaniline, substituted poly(paraphenylenevinylenes) or poly(thiophenes), with semiconducting nanoparticles: CdS, CdSe, CuS, ZnS, Fe3O4 or Fullerenes, and the opportunity to fill the polymer matrix with nanoparticles of both n- and p- conductivity types, thus providing access to peculiar morphologies, such as interpenetrating networks, p-n nanojunctins or fractal p-n interfaces, not achievable by traditional microelectronics technology.

The peculiarities in the conduction mechanism through a network of semiconductor nanoparticle chains provide the basis for the manufacture of highly sensitive gas and vapor sensors. These sensors combine the properties of the polymer matrix with those of the nanoparticles. It allows the fabrication of sensor devices selective to some definite components in mixtures of gases or vapors. Magnetic phenomena, such as superparamagnetism, observed in polymer-nanocomposites containing Fe3O4 nanoparticles in some range of concentrations, particle sizes, shapes and temperatures, provide a way to determine the limits to magnetic media storage density.

Over the last decades, the polymer nanocomposites application have gained their commercial footing, due in large part to the efforts of resin manufacturers, compounding and master batch producers who now offer user friendly products. Nanocomposites differ from traditional plastic composites in that they provide these properties with minimal impact on articles weight and they do so without providing penalties. Lastly in packaging nanocomposites deliver with good clarity, a combination not possible using traditional composites approaches.

7. Future Outlook

The pace of revolutionary discoveries now in nanotechnology is expected to accelerate in the next decade worldwide. This will have a profound impact on existing and emerging technologies in almost all industry sectors, in conservation of materials and energy, in biomedicine and in environmental sustainability. Potential technological applications with high commercial impact can be expected in areas of superplastic forming of ceramics, ultra high strength and tough structural materials, magnetic refrigerators, a wide range of nanoparticle filled nanocomosites based on elastomers, thermoplastics and thermosets and ductile cements.

One question frequently asked is “how broadly can this compounds be applied in the long run?” It is impossible to give a definitive answer at this state of knowledge. Polymer nanocomposites can do much more than enhancing classic engineering properties and barriers. Range finding work provides evidence for the improvement in electrical phenomena, UV stabilization, fire retardancy and control of polymer crystallization. A decade ago nanocomposite technology was a concept with great potential. Today it is a reality and tomorrow it will flourish.

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Captions to figures and schemes.

Fig. 1. Scanning Electron Micrograph of bone, The black areas represent the

polymer, the white areas are the mineralised mineral.

Fig. 2. Schematic representation of the interaction between the filler and polymer

matrix in a polymer nanocomposite.

Fig. 3. Nanoscale polymer-clay hybrid structures: (a) well-ordered arrays of clay

platelets and polymer chains, (b) delaminated or exfoliated clay platelets

dispersed within a continuous polymer matrix, and (c) a semi-exfoliated

hybrid, with small stacks of intercalated clay layers embedded within a

polymer matrix.

Fig. 4. Schematic illustrations of (A) a conventional, (B) an intercalated, (C) an

ordered exfoliated and (D) a distorted exfoliated polymer-clay

nanocomposite.

Fig. 5. Scanning Tunneling Micrograph of Cadmium Sulphide Nanoparticle-

Polymer Monolayer Membrane.

Fig. 6. Transmission Electron Micrograph of lead sulphide nanoparticles doped in

polymer matrix.

Fig. 7. Improved CO2 Barrier properties for clay-polyimide hybrids.

Fig.8. XRD pattern of the nanocomposite film from C12TMS and TMOS Mixture.

Fig.9. SEM image of the edge of the cracked nanocomposite film

(TMOS/C12TMS = 4).

Fig.10. TEM image of the nanocomposite film (TMOS/C12TMS = 4).

Fig.11. TEM image of the 20wt% MMT nanocomposite.

Fig.12. Schematic diagram describing the end tethered polymer layered silicate

nanocomposites.

Fig.13.Effect of filler concentration on density of polyurethane filled with

microsilica and nanosilica.

Fig.14. Swelling behavior in polyurethane filled with microsilica and nanosilica.

Fig.15. Effect of filler concentration on hardness of polyurethane filled with

microsilica and nanosilica.

Fig.16. Dependence of tensile strength on filler concentration of nano-and

microcomposites.

Fig.17. Tensile testing results as a function of MMT weight and volume content.

Fig.18. DSC of the melting region for the low MMT content nanocomposites

(200C/min)

Fig.19. DSC of PVA/MMT nanocomposites with varying fMMT.

Fig.20. Photoluminescence spectra of PPV and PPV/TiO2 nanocomposites.


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