modulus of MWNT is higher than that of SWNT due to contributions from the van der Waals forces between the concentric SWNTs in MWNT [14]. Experimental determination of the mechanical properties of carbon nano- tubes is extremely difficult and has produced a variety of results.Several investigators have used atomic force microscope (AFM)to determine the Young's modulus and strength.The Young's modulus of MWNT determined on AFM has ranged from 0.27 to 1.8 TPa and that of SWNT ranges from 0.32 to 1.47 TPa.Similarly,the strength of MWNT ranges from 11 to 63 GPa and that of SWNT from 10 to 52 GPa.In the TPM experiments,the carbon nanotubes have also shown high tensile strain (up to 15%)before fracture.It has also been observed that carbon nanotubes exhibit nonlinear elastic deform- ation under tensile,bending,as well as twisting loads.At high strains,they tend to buckle of the wall. The large variation in Young's modulus and strength is attributed to the fact that nanotubes produced by the current production methods may vary in length, diameter,number of walls,chirality,and even atomic structure [21].Further- more,nanotubes produced by different methods contain different levels of defects and impurities,which influence their mechanical properties.For example,the Young's modulus of CVD-produced MWNTs is found to be an order of magnitude lower than that of the arc-discharged MWNTs,which is due to the presence of higher amount of defects in the CVD-produced MWNTs. 8.3.5 CARBON NANOTUBE-POLYMER COMPOSITES The three principal processing methods for combining carbon nanotubes with polymer matrix [22,23]are(1)in situ polymerization,(2)solution processing, and (3)melt processing. 1.In Situ Polymerization:In this process,the nanotubes are first dispersed in the monomer and then the polymerization reaction is initiated to transform the monomer to polymer.Depending on the polymer formed and the surface functionality of the nanotubes,the polymer molecules are either covalently bonded to the nanotubes or wrapped around the nanotubes at the completion of the polymerization reaction. 2.Solution Processing:In this process,the nanotubes are mixed with a polymer solution,which is prepared by dissolving the polymer in a suitable solvent.The mixing is done using magnetic stirring,high shear mixing,or sonication.The dispersion of the nanotubes in the solution can be improved by treating them with a surfactant,such as derivatives of sodium dodecylsulfate.The solution is poured in a casting mold and the solvent is allowed to evaporate.The resulting material is a cast film or sheet of carbon nanotube-reinforced polymer. 3.Melt Processing:Melt processing is the preferred method for incorpor- ating carbon nanotubes in thermoplastics,particularly for high volume 2007 by Taylor Francis Group.LLC.modulus of MWNT is higher than that of SWNT due to contributions from the van der Waals forces between the concentric SWNTs in MWNT [14]. Experimental determination of the mechanical properties of carbon nano￾tubes is extremely difficult and has produced a variety of results. Several investigators have used atomic force microscope (AFM) to determine the Young’s modulus and strength. The Young’s modulus of MWNT determined on AFM has ranged from 0.27 to 1.8 TPa and that of SWNT ranges from 0.32 to 1.47 TPa. Similarly, the strength of MWNT ranges from 11 to 63 GPa and that of SWNT from 10 to 52 GPa. In the TPM experiments, the carbon nanotubes have also shown high tensile strain (up to 15%) before fracture. It has also been observed that carbon nanotubes exhibit nonlinear elastic deform￾ation under tensile, bending, as well as twisting loads. At high strains, they tend to buckle of the wall. The large variation in Young’s modulus and strength is attributed to the fact that nanotubes produced by the current production methods may vary in length, diameter, number of walls, chirality, and even atomic structure [21]. Further￾more, nanotubes produced by different methods contain different levels of defects and impurities, which influence their mechanical properties. For example, the Young’s modulus of CVD-produced MWNTs is found to be an order of magnitude lower than that of the arc-discharged MWNTs, which is due to the presence of higher amount of defects in the CVD-produced MWNTs. 8.3.5 CARBON NANOTUBE–POLYMER COMPOSITES The three principal processing methods for combining carbon nanotubes with polymer matrix [22,23] are (1) in situ polymerization, (2) solution processing, and (3) melt processing. 1. In Situ Polymerization: In this process, the nanotubes are first dispersed in the monomer and then the polymerization reaction is initiated to transform the monomer to polymer. Depending on the polymer formed and the surface functionality of the nanotubes, the polymer molecules are either covalently bonded to the nanotubes or wrapped around the nanotubes at the completion of the polymerization reaction. 2. Solution Processing: In this process, the nanotubes are mixed with a polymer solution, which is prepared by dissolving the polymer in a suitable solvent. The mixing is done using magnetic stirring, high shear mixing, or sonication. The dispersion of the nanotubes in the solution can be improved by treating them with a surfactant, such as derivatives of sodium dodecylsulfate. The solution is poured in a casting mold and the solvent is allowed to evaporate. The resulting material is a cast film or sheet of carbon nanotube-reinforced polymer. 3. Melt Processing: Melt processing is the preferred method for incorpor￾ating carbon nanotubes in thermoplastics, particularly for high volume
2007 by Taylor & Francis Group, LLC