Introduction

Polymeric nanocomposites can be considered an important category of organic-inorganic hybrid materials, in which inorganic nanoscale building blocks (e.g., nanoparticles, nan-otubes, or nanometer-thick sheets) are dispersed in an organic polymer matrix [1, 2]. They represent the current trend in developing novel nanostructured materials. When compared with conventional composites based on micrometer-sized fillers, the interface between the filler particles and the matrix in a polymer nanocomposite constitutes a much greater area within the bulk material (Fig. 1) and hence influences the composite's properties to a much greater extent, even at a rather low filler loading [3].

From the literature, it is somewhat ambiguous who proposed the term "nanocomposites" for the first time. Different authors gave different opinions [4-6]. Nevertheless, the definition of "nanocomposites" is generally accepted without any argument. That is, it refers to materials consisting of

'This chapter first appeared in Handbook of Organic-Inorganic Hybrid Materials and Nanocompsites, Volume 2: Nanocomposites, Edited by H. S. Nalwa. ©2003, American Scientific Publishers.

various (two or more) solid phases with different compositions or structures, where at least one dimension is in the nanometer range. It has become familiar worldwide nowadays, especially because of some successful commercial products based on these materials, such as air intake covers and bathroom deodorizers. As far as polymer nano-composites are concerned, not so many results have been published (Fig. 2), despite a large interest in them. However, polymer nanocomposites have been investigated for a long time, even though the technical term has only recently become popular. For example, extensive studies of carbon black-filled rubbers began before the mid-twentieth century [7]. Semicrystalline polymers can also be considered in some sense another example of nanocomposite materials. They consist of crystalline lamellae (typically 10 nm in width and thickness) dispersed in an amorphous matrix [8].

The recent interest in polymer nanocomposites is based on new achievements expected from their nanostructure, such as unusual mechanical, optical, and magnetic properties. A new degree of freedom for the development of advanced materials with enhanced performance becomes available in this way. In addition, the push for polymer nanocomposites arises from the facts that (1) polymers are still easily processible in many different ways, even with nanoscale solid fillers and (2) nanoparticles are provided by a polymer matrix with a high resistance to any chemical attack. In this way, the unique properties of nanosized objects can be combined with the qualities of a polymer matrix, to fit the requirements of specific applications in diverse areas (Table 1). Examples of polymer nanocomposites include biomimetic ceramic/polymer composites [33, 34], nonlinear optical metal colloid/polymer nanocomposites [35], and intercalated clay/polymer nanocomposites [36, 37].

Numerous procedures for the preparation of polymer nanocomposites have been proposed [38, 39]. They can

Copyright © 2004 by American Scientific Publishers All rights of reproduction in any form reserved.

Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 7: Pages (125-160)

Figure 1. Schematic drawings of the microstructural appearance of typical particulate vs. fine particulate vs. nanoparticulate composites based on electronic microscopic observations. (a) 3 vol.% of particles with 10-pm diameter (2.86 particles within a volume of 50,000 pm3). (b) 3 vol.% of particles with 1-pm diameter (2860 particles within a volume of 50,000 pm3). (c) 3 vol.% of particles with 0.1 pm = 100-nm diameter (2.86 million particles within a volume of 50,000 pm3). Reprinted with permission from [3], M. Z. Rong et al., Polymer 42, 3301 (2001). © 2001, Elsevier Science.

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Table 1. Potential applications of polymer nanocomposites based on their specific properties.

Figure 1. Schematic drawings of the microstructural appearance of typical particulate vs. fine particulate vs. nanoparticulate composites based on electronic microscopic observations. (a) 3 vol.% of particles with 10-pm diameter (2.86 particles within a volume of 50,000 pm3). (b) 3 vol.% of particles with 1-pm diameter (2860 particles within a volume of 50,000 pm3). (c) 3 vol.% of particles with 0.1 pm = 100-nm diameter (2.86 million particles within a volume of 50,000 pm3). Reprinted with permission from [3], M. Z. Rong et al., Polymer 42, 3301 (2001). © 2001, Elsevier Science.

be basically classified in form of the following approaches: (i) direct incorporation of nanoscale building blocks into a polymer melt or solution [22, 40-42]; (ii) in-situ generation of nanoscale building blocks in a polymer matrix (e.g., vacuum evaporation of metals, thermal decomposition of precursors, reduction of metal ions through electrochemical procedures, etc.) [15, 43, 44]; (iii) polymerization of monomers in the presence of nanoscale building blocks [45, 46]; and (iv) a combination of polymerization and formation of nanoscale building blocks (e.g., sol-gel method, intercalation of monomers into a layered structure followed by polymerization, etc.) [47-50]. The key issue of these techniques lies in the fact that the geometry, spatial distribution, and volume content of the nanofillers must be effectively controlled through adjustment of the preparation conditions so as to ensure the structural requirements of nano-

I I Nanoparticles V//A Nanocomposites ■ Polymer nanocomposites

I I Nanoparticles V//A Nanocomposites ■ Polymer nanocomposites rH-l"0-l

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 Year

Figure 2. Number of papers published in the period 1991-2000 related to nanostructured materials. Data taken from EI Compendex.

Properties

Examples of probable applications

Ref.

Catalytic Long-life nanocatalysts Mechanical Strengthened and toughened polymers

Magnetic High-density storage elements, electromagnetic interference shielding materials Electric Low-cost conductive coatings, nonlinear resistors, electrostatic discharging filaments, electromagnetic interference shielding materials Optic Nonlinear optic materials, optical waveguides, photoelectric materials Thermal High thermal conductive materials, low-temperature sintered materials Others Sensors, biomimetic materials, wear-resisting materials, etc.

composites stated above. An ideal nanocomposite should be free of any agglomerates, should contain an optimum filler content, and should maintain at the same time the physic-ochemical characteristics of its individual components [27]. It is believed that the development of a mixing process for a stable, homogeneous dispersion of the nanophases is crucial for producing a uniform and void-free microstructure in polymer nanocomposites.

It is the aim of this chapter to provide a review of the main results achieved in the preparation and the mechanical behavior of nonlayered, nanoparticle-filled thermoplastic nanocomposites, as manufactured according to the first way (i) cited in the last paragraph. Thermosetting polymer-based composites, like nano-TiO2/epoxy [51], carbon nano-tube/epoxy [52], nano-SiO2/unsaturated polyester [53], and nano-SiO2/cycloaliphatic bisepoxide, with the use of an aqueous sodium silicate solution as a starting material [54], are excluded from the main concerns here. Instead, large-scale and low-cost production routes as well as a broad applicability of thermoplastic nanocomposites, intended to be used as structural materials, shall be considered. This can be achieved best by the employment of commercially available, nonlayered nanoparticles and by the use of blending techniques already widely used in the plastics industry.

From an examination of the literature, however, it was surprisingly found that works in this respect are relatively less documented. No global study of such systems has been carried out so far. Most of the papers dealing with thermoplastic systems as candidates for mechanical applications discuss intercalated and exfoliated (i.e., layered) nanocomposites. This phenomenon might be related to the shortcoming of the mixing dispersion method as characterized by the difficulties in (a) breaking up premade nanoparticle agglomerates, and (b) homogeneously arranging individual particles in a polymer matrix. Although great efforts have been made to solve these problems, techniques with sufficiently wide applicability are not yet available. As implied by the statistics in Figure 2, the development of nanocomposites and polymer nanocomposites is growing moderately, presenting a striking contrast to the case of nanoparticles. This means there is a real need for a revolutionary breakthrough in making and utilizing polymeric nanocomposites.

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