Polymer Nanocomposites: Status and Opportunities
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Polymer
Nanocomposites: Status and Opportunities
Richard A.Vaia and Emmanuel P. Giannelis Introduction Reinforcement of polymers with a second phase, whether inorganic or organic, to produce a polymer composite is common in the production of modern plastics. Polymer nanocomposites (PNCs) represent a radical alternative to these conventional polymer composites.1–5 Efforts within the last 10 years have demonstrated a doubling in tensile modulus and strength without sacrificing impact resistance for nylon-layer silicate nanocomposites containing as little as 2 vol% inorganics. In addition, the heat-distortion temperature of the nanocomposites can be increased by up to 100C, extending the use of the composite to higher temperature environments, such as under-the-hood parts in automobiles. Besides their improved properties, PNCs are also easily extruded or molded to nearfinal shape, simplifying the manufacturing process. Since high degrees of stiffness and strength can be realized using far less high-density inorganic material, PNCs are much lighter compared to conventional polymer composites. This weight advantage could have a significant impact on environmental concerns. For example, it has been reported that widespread use of PNCs by U.S. vehicle manufacturers could save 1.5 billion liters of gasoline over one year of vehicle production and reduce related carbon dioxide emissions by more than 10 billion pounds.6 In addition, the outstanding combination of barrier and mechanical properties of PNCs may eliminate the need for a multipolymer layer design in packaging materials, enabling greater recycling of food and beverage packaging. Even though significant progress has been made in developing different PNCs,
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a general understanding has yet to emerge. For example, the combination of enhanced modulus, strength, and toughness is a unique feature of only a fraction of the PNCs fabricated to date. A major challenge in developing nanocomposites for systems ranging from high-performance to commodity polymers is the lack of even simple structure–property models. Without such models, progress in nanocomposites has remained largely empirical. Similarly, predicting the ultimate material limits or maximum performances for different classes of nanocomposites is almost impossible at present. In this article, we briefly discuss PNCs and illustrate some successes and remaining challenges from the perspective of recent efforts, mainly in our groups, in layered silicate (clay) polymer nanocomposites.
The Nanocomposite Concept Uniform dispersion of nanoscopically sized particles (nanoelements) can lead to an ultralarge interfacial area between the constituents per volume of material, approaching 700 m2/cm3 in dispersions of layered silicates in polymers. This is comparable to a football field within a raindrop. Figure 1 summarizes the effect and implication of decreasing the thickness of a plate—and thus increasing the number of plates per volume for a given volume fraction of plates—on the mean distance between plates. For a
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