Strategies for Dispersing Nanoparticles in Polymers
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Dispersing Nanoparticles in Polymers Ramanan Krishnamoorti
Abstract Controlling the dispersion of nanoparticles in polymeric matrices is the most significant impediment in the development of high-performance polymer nanocomposite materials and results primarily from the strong interparticle interactions between the nanoparticles. This review examines the theoretical and experimental strategies employed in developing appropriate chemical and physical methods to achieve controlled dispersion of nanoparticles. Methods to characterize the state of dispersion, including force and electron microscopy, and scattering, electrical, and mechanical spectroscopy, are considered with special emphasis on achieving quantitative measures of the dispersion. Some of the outstanding issues, such as longterm aging and the implication for the dispersion of nanoparticles, development of high-throughput methods for rapid screening, and methods for in-line monitoring, are also discussed.
Introduction The dispersion of nanoparticles in polymeric matrices is the fundamental challenge surrounding the development of polymer nanocomposites. In addition to the development of methods to characterize the dispersions of the nanoparticles (challenged by the multitude of hierarchical length scales), techniques for dispersing aggregated or ordered nanoparticles while not compromising the inherent advantages of the nanoparticles (such as their unique mechanical, thermal, and optical properties and extremely large internal surface area) remain a significant challenge. The interparticle interactions are clearly a function of the chemical nature of the nanoparticles, with the shape of the nanoparticles, the distance between the nanoparticles, and the polydispersity of particle sizes being the most prominent secondary contributors. Israelachvili has summarized the interaction laws for different particles in vacuum (Table I), and these reveal the strong dependence of the interaction energies on the shape and aspect ratio of the nanoparticles.1 Further, many nanoparticles form low-dimensional crystals or tight agglomerates that render
their dispersion even more difficult. For instance, single-wall carbon nanotubes (SWNTs) have strong van der Waals interactions, resulting in parallel alignment and formation of a triangular lattice with an interaction energy of 500 eV/μm of nanotube length,2,3 which is partially responsible for the lack of dispersability of pristine SWNTs. The lack of dispersability of carbon nanotubes is illustrated by considering the concentrations of the nanotubes dispersible in a range of solvents, as shown in Table II.4 A second example of the strong interactions between nanoparticles is observed in layered silicates, where it has recently been estimated that the cleavage energy for an unmodified montmorillonite is 133 mJ/m2, whereas for a similar octadecyl ammoniummodified montmorillonite (where the layers are an additional two nm apart), it is 40 mJ/m2, in excellent agreement with experiments.5 Finally, for chemically identical m
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