Structural and mechanical properties of single-wall carbon nanotube fibers

We report quantitative experimental study correlating the structure and mechanical properties of fibers made from single-walled carbon nanotubes (SWNTs) and polyvinyl alcohol (PVA). A post-synthesis solvent drawing treatment is used to vary nanotube alignment, whose detailed understanding is a prerequisite for fiber development. Quantitative analysis of nanotube alignment within the fibers with different draw ratios is performed using x-ray scattering. The method is described in detail, and we also show that the improvement of nanotube alignment with draw ratio can be understood within a model of induced orientation at constant volume. Young’s modulus and tensile strength increase with nanotube alignment. This is modeled using continuum mechanics in qualitative agreement with experiment, however quantitative differences show that nanotube alignment is not the only parameter controlling the fiber mechanical properties. We suggest that interaction between the SWNTs and PVA chains should also play a significant role.

Figure 1

Diffraction pattern obtained from a HiPCO fiber. The white and black arrows point the signal coming from the nanotubes and the PVA chains, respectively.

Figure 2

Azimuthal intensities distribution at wave vectors below $1{\mathrm{\AA }}^{-1}$ (NT signal) and at $1.4{\AA }^{-1}$ (PVA signal).

Figure 3

Half width at half maximum of a Gaussian distribution of orientations in direct space $w_d$ as a function of the one of the corresponding Gaussian distribution in intensity on a planar detector in reciprocal space $w_r$, for ${\theta }_B=5.9°$ (corresponding to $Q\sim 0.7{\mathrm{\AA }}^{-1}$). The straight line has equation $w_d=w_r\cos \left({\theta }_B\right)$.

Figure 4

Half width at half maximum $w_d$ of the Gaussian distribution function describing the alignment of the oriented fraction of nanotubes (PVA chains) and fraction of nonoriented nanotubes (PVA chains) with respect to the oriented one, as functions of fiber draw ratios.

Figure 5

Stress versus strain curve for fiber Hi5 (Young modulus $E=22\mathrm{GPa}$, tensile strength ${\sigma }_R=170\mathrm{MPa}$).

Figure 6

Young modulus and tensile strength for fibers with different draw ratios.

Figure 7

Scheme of an affine extension (left: as-cast fiber, right: stretched fiber).

Figure 8

Evolution of $w_d$ and $f$ as a function of the draw ratio of the nanotubes fibers, open circles are experimental data and filled triangles correspond to calculations within the scope of an affine model of induced ordering.

Figure 9

Fibers Hi1–Hi5: measured Young modulus (open circles) and calculations (cross) as a function of the draw ratio of the fibers.

Figure 10

Definition of angles used in the text.

Figure 11

Schematic drawing of the experimental configuration using the Ewald sphere representation. Since one is only interested here in angular quantities, to simplify the drawing, the detector is not drawn at distance $D$ from the sample but tangent to the Ewald sphere.