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Carbon Nanotubes Processing for Polymer Composites by Preeti Prakas
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Carbon Nanotubes Processing for Polymer Composites |
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Science & Technology,Business
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For all the following methods it must be noticed that during the composite processing, metastable dispersions of Carbon Nanotubes are obtained and it is very critical to prevent reaggregation after the initial dispersion has been achieved. This is less of an issue when surface modified tubes are employed, since the dispersion is stabilized by short-range repulsion potentials due to either charge or satiric effects. However, as it is well known from colloidal science, that long-term stability is a real challenge. When dispersion is achieved using pristine tubes, no specific stabilization mechanisms are present and reaggregation rate depends on the viscosity of the suspending medium. The only practical method to prevent re-aggregation is by “freezing” the tubes in their position by either chemical cross linking of the matrix or by bringing the polymer below its glass transition temperature. Perhaps the most common method for preparing CNT/polymer composites has been from solution, since it facilitates Nanotubes dispersion. Both organic solvents and water have been used to produce CNT/polymer composites. First, the Nanotubes are dispersed in the solvent via energetic agitation (magnetic or mechanic stirring, reflux or sonication). This step requires the employment of surface modified Nanotubes (either covalent or non-covalent) in order to achieve a metastable dispersion. Once the Nanotubes are dispersed in the solvent, the polymer, which was previously dissolved in the same solvent, is added to the dispersion so that the polymer chains intercalate between the tubes. The last step consists in removing the solvent by evaporation. While solution processing is a valuable technique for both Nanotubes dispersion and composite formation, it is less suitable for industrial scale processes, where melt processing is preferred because of its speed and simplicity. In this case, the strategy consists in blending a molten thermoplastic with the Carbon Nanotubes in order to optimize the polymer-Nanotubes interactions and form a Nano composite. The polymer chains experience a dramatic loss of conformational entropy during the process. The proposed driving force for this mechanism is the important enthalpy contribution of the polymernanotube interactions during the blending step.
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