  Macroscopic fibres of carbon nanotubes are hierarchical structures combining
long building blocks preferentially oriented along the fibre axis and a large
porosity arising from the imperfect packing of bundles. Synchrotron small-angle
X-ray scattering SAXS measurements show that such structure is a surface
fractal with fractal dimension (Ds) of 2.5 for MWCNT fibres and 2.8 for SWCNT
fibres. N2 adsorption measurements give similar values of 2.54 and 2.50,
respectively. The fractal dimension and deviation from Porods law are related
to density fluctuations associated with the wide distribution of separations
between CNTs. These fluctuations are also evident as diffuse wide-angle X-ray
scattering (WAXS) from CNTs at distances above turbostratic separation. The
structure of CNT fibres produced at different draw ratios is compared in terms
of degree of orientation and characteristic lengths parallel and perpendicular
to the fibre. Drawing not only increases alignment but also the fraction of
graphitic planes forming coherent domains capable of taking part in stress
transfer by shear; thus increasing both tensile modulus and strength. The
invariant-normalized intensity of the (002) equatorial reflection thus takes
the form of a degree of crystallinity closely related to tensile properties.
