Chapter 8 Polymer Nanocomposites

Chapter 8 Polymer Nanocomposites - 8 Polymer Nanocomposites...

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8 Polymer Nanocomposites Polymer nanocomposites are polymer matrix composites in which the reinforcement has at least one of its dimensions in the nanometer range (1 nanometer (nm) ¼ 10 ± 3 m m (micron) ¼ 10 ± 9 m). These composites show great promise not only in terms of superior mechanical properties, but also in terms of superior thermal, electrical, optical, and other properties, and, in general, at relatively low-reinforcement volume fractions. The principal reasons for such highly improved properties are (1) the properties of nano-reinforce- ments are considerably higher than the reinforcing fibers in use and (2) the ratio of their surface area to volume is very high, which provides a greater interfacial interaction with the matrix. In this chapter, we discuss three types of nanoreinforcements, namely nanoclay, carbon nanofibers, and carbon nanotubes. The emphasis here will be on the improvement in the mechanical properties of the polymer matrix. The improvement in other properties is not discussed in this chapter and can be found in the references listed at the end of this chapter. 8.1 NANOCLAY The reinforcement used in nanoclay composites is a layered silicate clay min- eral, such as smectite clay, that belongs to a family of silicates known as 2:1 phyllosilicates [1]. In the natural form, the layered smectite clay particles are 6–10 m m thick and contain > 3000 planar layers. Unlike the common clay minerals, such as talc and mica, smectite clay can be exfoliated or delaminated and dispersed as individual layers, each ~1 nm thick. In the exfoliated form, the surface area of each nanoclay particle is ~750 m 2 = g and the aspect ratio is > 50. The crystal structure of each layer of smectite clays contains two outer tetrahedral sheets, filled mainly with Si, and a central octahedral sheet of alumina or magnesia ( Figure 8.1 ). The thickness of each layer is ~1 nm, but the lateral dimensions of these layers may range from 200 to 2000 nm. The layers are separated by a very small gap, called the interlayer or the gallery. The negative charge, generated by isomorphic substitution of Al 3 þ with Mg 2 þ or Mg 2 þ with Li þ within the layers, is counterbalanced by the presence of hydrated alkaline cations, such as Na or Ca, in the interlayer. Since the forces that hold the layers together are relatively weak, it is possible to intercalate small organic molecules between the layers. ß 2007 by Taylor & Francis Group, LLC.
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One of the common smectite clays used for nanocomposite applications is called montmorillonite that has the following chemical formula M x (Al 4 ± x Mg x )Si 8 O 20 (OH) 4 , where M represents a monovalent cation, such as a sodium ion, and x is the degree of isomorphic substitution (between 0.5 and 1.3). Montmorillonite is hydrophilic which makes its exfoliation in conventional polymers difficult. For exfoliation, montmorillonite is chemically modified to exchange the cations with alkyl ammonium ions. Since the majority of the cations are located inside the galleries and the alkyl ammonium ions are bulkier than the cations, the
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This note was uploaded on 01/16/2012 for the course MAE 582 taught by Professor Rw during the Fall '11 term at SUNY Buffalo.

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Chapter 8 Polymer Nanocomposites - 8 Polymer Nanocomposites...

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