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Carbon Nanotubes 101
Although both probe style and bath style ultrasonic systems can be used for dispersing CNTs, it is widely believed that the probe style ultrasonic systems work better for dispersing CNTs. It is also widely known that adding a dispersing reagent (surfactant) into the solution will accelerate the dispersion effect. The reagent Polyvinyl Pyrrolidone (PVP) is a good dispersion agent. Some people like to use the reagent Sodium Dodecyl Benzene Sulfonate (SDBS) or Poly Vinyl Alcohol (PVA) as well. The dispersing reagent and proportions listed above do change when using different solvents. Typically, it is a question of chemistry to achieve a stable dispersion. A stable dispersion will last for days, weeks, or months with little to no settling.
In some applications, achieving a stable dispersion can require other agents in the solution to prevent the CNTs from falling out of solution over time. Emulsifier T-60 (also known as Tween 60) is commonly used with Di water or Isopropyl Alcohol. Organic titanates can be used with Acetone and Xylene. The specific application determines whether these agents remain in the solution when further processing, or if they need to be removed. Some organic titanates can be removed by heating the solution above 250°C. The addition of the OH and COOH functional groups assists the CNTs dispersing in DI water and other solvents as well as the chemical bonding to other materials during further processing.
Pristine nanotubes are unfortunately insoluble in many liquids such as water, polymer resins, and most solvents. Thus they are difficult to evenly disperse in a liquid matrix such as epoxies and other polymers. This complicates efforts to utilize the nanotubesí outstanding physical properties in the manufacture of composite materials, as well as in other practical applications which require preparation of uniform mixtures of CNTs with many different organic, inorganic, and polymeric materials.
To make nanotubes more easily dispersible in liquids, it is necessary to physically or chemically attach certain molecules, or functional groups, to their smooth sidewalls without significantly changing the nanotubesí desirable properties. This process is called functionalization. The production of robust composite materials requires strong covalent chemical bonding between the filler particles and the polymer matrix, rather than the much weaker van der Waals physical bonds which occur if the CNTs are not properly functionalized.
Functionalization methods such as chopping, oxidation, and "wrapping" of the CNTs in certain polymers can create more active bonding sites on the surface of the nanotubes. For biological uses, CNTs can be functionalized by attaching biological molecules, such as lipids, proteins, biotins, etc. to them. Then they can usefully mimic certain biological functions, such as protein adsorption, and bind to DNA and drug molecules. This would enable medially and commercially significant applications such as gene therapy and drug delivery. In biochemical and chemical applications such as the development of very specific biosensors, molecules such as carboxylic acid (COOH), poly m-aminobenzoic sulfonic acid (PABS), polyimide, and polyvinyl alcohol (PVA) have been used to functionalize CNTs, as have amino acid derivatives, halogens, and compounds. Some types of functionalized CNTs are soluble in water and other highly polar, aqueous solvents.