A very interesting article:

Reinforcement of single-walled carbon nanotube bundles by intertube bridging.
A. Kis1, G. Csányi2, J.-P. Salvetat3, Thien-Nga Lee1, E. Couteau1, A. J. Kulik1, W. Benoit1, J. Brugger4 & L.Forró1
Nature Materials 3, 153 - 157 (2004)
Published online: 15 February 2004 | doi:10.1038/nmat1076
"During their production, single-walled carbon nanotubes form bundles. Owing to the weak van der Waals interaction that holds them together in the bundle, the tubes can easily slide on each other, resulting in a shear modulus comparable to that of graphite. This low shear modulus is also a major obstacle in the fabrication of macroscopic fibres composed of carbon nanotubes. Here, we have introduced stable links between neighbouring carbon nanotubes within bundles, using moderate electron-beam irradiation inside a transmission electron microscope.
Concurrent measurements of the mechanical properties using an atomic force microscope show a 30-fold increase of the bending modulus, due to the formation of stable crosslinks that effectively eliminate sliding between the nanotubes. Crosslinks were modelled using first-principles calculations, showing that interstitial carbon atoms formed during irradiation in addition to carboxyl groups, can independently lead to bridge formation between neighbouring nanotubes."
http://intl.emboj.org/nmat/journal/v...EF1287838C1#f2

News and Views
Nanotubes: Strong bundles.
Pulickel M. Ajayan1 & Florian Banhart2
Nature Materials 3, 135 - 136 (2004) doi:10.1038/nmat1078
"The mechanical properties of nanotube bundles are limited by the sliding of individual nanotubes across each other. Introducing crosslinks between the nanotubes by electron irradiation prevents sliding, and leads to dramatic improvements in strength."
http://npg.nature.com/nmat/journal/v.../nmat1078.html
[free full text]

The research team was able to give bundles of nanotubes nearly the same strength in bending modulus as individual nanotubes. Individual nanotubes have a bending modulus of about 1 terapascal. However, nanotubes merely bundled together don't come anywhere near this because the nanotubes slide along each other. The researchers were
able to get about 70% of the bending strength of individual nanotubes in their bundles by inducing bonds between the tubes by electron irradiation.
Could this work to produce nanotubes of arbitrarily long lengths at nearly the same tensile strength of individual tubes by using the electron radiation method to induce bonds between the ends of nanotubes?

Individual nanotubes have been made at centimeter lengths both in multi-wall and single wall forms:

UC Researchers Shatter World Records with Length of Carbon Nanotube Arrays.
Date: 4/27/2007
"UC engineering researchers have developed a novel composite catalyst and optimal synthesis conditions for oriented growth of multi-wall CNT arrays. And right now they lead the world in synthesis of extremely long aligned carbon nanotube arrays."
http://www.uc.edu/news/NR.asp?id=5700

Reports
Direct Synthesis of Long Single-Walled Carbon Nanotube Strands.
Science 3 May 2002:
Vol. 296. no. 5569, pp. 884 - 886
"In the processes that are used to produce single-walled nanotubes (electric arc, laser ablation, and chemical vapor deposition), the typical lengths of tangled nanotube bundles reach several tens of micrometers. We report that long nanotube strands, up to several centimeters in length, consisting of aligned single-walled nanotubes can be synthesized by the catalytic pyrolysis of n-hexane with an enhanced vertical floating technique. The long strands of nanotubes assemble continuously from arrays of nanotubes, which are intrinsically long."
http://www.sciencemag.org/cgi/content/full/296/5569/884 [free full text]

Ultralong single-wall carbon nanotubes.
Nature Materials 3, 673 - 676 (2004)
"...Here we report the synthesis of 4-cm-long individual single-wall carbon nanotubes (SWNTs) at a high growth rate of 11 m s-1 by catalytic chemical vapour deposition. Our results suggest the possibility of growing SWNTs continuously without any apparent length limitation."
http://www.nature.com/nmat/journal/v.../nmat1216.html [abstract only]

The authors of this second report said they believed their process could be scaled up to produce arbitrarily long nanotubes, but I haven't seen any further reports that suggest they've actually done so.
Despite the fact that centimeter long nanotubes have been made, there still have been no experiments to determine if these centimeter long tubes have the same tensile strength as has been shown for the micron scale nanotubes.
The same thing is true in regards to the strongly bonded nanotube bundles. This work was published 3 years ago and there still have been no experiments to determine if they also have the individual nanotubes tensile strength, as well as their bending strength.


Bob Clark