Chemistry Reference
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Fig. 11 Simulation of torsion (a) and collapse (b) as functions of the torsion angle. Reprinted with
kind permission of Springer Science
รพ
Business Media from [
29
]
0.5 TPa was also found by Wong et al., who took
advantage of an AFM to pin MWCNTs at one end of the tip and measured the
bending force vs. displacement along the unpinned ends [
40
].
Other approaches have given Young's modulus in the range of 10-50 GPa,
reflecting the dependence upon the degree of defects within the tubes network [
41
].
Concerning the resilience, both theory-simulations and experimental evidence
suggested the capacity of nanotubes to change their shape and absorb energy
without creating a permanent distortion.
Although resilience is unexpected for a graphite-like material, carbon nanotubes
under compression or bending form ripples and under torsion flatten into a helix,
without irreversible atomic rearrangements. This property has been attributed to the
small dimension of the tubes and to the competition between van der Waals and
elastic energies [
31
].
Iijima et al. reported high-resolution electron microscopy observations of
the occurrence of kinks on the sidewalls of SWCNTs under high bending angles,
which were quantified by means of atomistic simulations [
42
]. The remarkable
flexibility of the hexagonal grid, up to very high strain values, could be connected to
the ability of the sp
2
network to rehybridize into sp
3
proportionally to the local
curvature out of the plane (Fig.
11
).
Concerning tensile strength, which is the maximum amount of tensile stress that
a material can support without fracture, CNTs have been estimated to come close to
values of 300 GPa, gaining the title of materials with the highest tensile strength.
By virtue of their outstanding mechanical properties combined with low density,
CNTs are promising materials for the strengthening of fibers, replacing carbon and
glass fibers, and for the development of high-performing composites with polymers,
ceramics, or epoxides [
43
].
The similar value of 1.28
