Ribbon-to-Fiber Transformation in the Process of Spinning of Carbon-Nanotube Dispersion

We describe a phenomenon of ribbon-to-fiber transformation observed in the process of spinning of single wall carbon nanotubes dispersed in polymer solutions. In the process of spinning, a gel-like ribbon comprised of nanotube bundles bound by polymer is withdrawn from a solvent bath. We show that upon crossing the liquid-air interface, the ribbon may either retain its flat shape or fold into a compact hairlike fiber. The ribbon-to-fiber transformation is caused by the capillary action of the liquid meniscus embracing the ribbon. Only sufficiently stiff ribbons can withhold the capillary compression. The critical conditions of folding, as well as the number of folds in the contractive ribbon, depend on the ribbon width, its flexural rigidity, and the solvent surface tension. We show that the ribbon rigidity can be efficiently modulated by varying the solvent composition, allowing us to control the pore structure of carbon-nanotube fibers.

Figure 1

Morphological variety of fibers produced from SWNT ribbons extracted from solvents of different composition: (a) $\sim 60\mathrm{cm}$ long gel-like ribbon in the extraction bath; (b) withdrawal of a ribbon from pure methanol: ribbon is stiff and maintains its flat shape; (c) withdrawal of a ribbon from pure water: ribbon is soft and transforms into a hairlike fiber. (d) SEM micrograph of the edge of a flat fiber extracted from 80% ethanol aqueous solution. (e) SEM micrograph of the edge of a hairlike fiber extracted from pure water. SEM images were obtained with field emission scanning electron microscope (FESEM) Hitachi S-4500.

Figure 2

Capillary contraction of SWNT ribbons in the process of withdrawal from the extraction bath with pure water: (a) $\sim 15\mathrm{mm}$ wide ($\sim 15\mathrm{mm}$) and $\sim 5\mu \mathrm{m}$ thin ($\sim 5\mu \mathrm{m}$) ribbon is squashed into a multifolded fiber (insets show schematics of the ribbon cross section during folding); (b) narrow ($\sim 4\mathrm{mm}$) and thick ($\sim 20\mu \mathrm{m}$) ribbon either rolls up or buckles. According to Eq. (2), the number of folds in case (b) exceeds the numbers of folds in case (c) by $\sim 30$ times. The images presented were obtained with a high speed/high resolution digital camera FastCam-X 1280 PCI (Photron USA Inc., San Diego, CA) mounted on an optical system.

Figure 3

Hierarchal pore structure morphology of SWNT fibers: (a) edge of a hairlike fiber extracted from pure water: multifolded structure is composed by nanofelt layers with longitudinal transport pores; (b) edge of a flat fiber extracted from 80% ethanol-water solution: laminated structure is comprised of nanofelt layers. Note the similarity in the nanofelt structure and the difference in the shape and size of transport channels. Nanofelt consists of SWCN bundles of size $\sim 10–30\mathrm{nm}$. Transport pores range from $\sim 0.5$ to $\sim 2\mu \mathrm{m}$. The cuts were done upon immersion of fibers in liquid nitrogen to avoid melting of the nanofelt structure. SEM images were obtained with FESEM Hitachi S-4500.