Page 160 - Carbon Nanotube Fibres and Yarns

Carbon nanotube yarn structures and properties 151

Fig. 7.12 Tortuosity and misalignment of CNTs in forest and drawn web. (A) SEM image
of CNTs in vertically aligned CNT array (forest) and (B) SEM image of CNTs in a drawn
web [14]. (C) Schematic of misaligned straight fibers (longitudinal view) and (D) sche-
matic of a void formed between misaligned straight fibers (end view of schematic C).
(Panels (A and B) reprinted with permission from M. Miao, J. McDonnell, L. Vuckovic, S.C.
Hawkins, Poisson’s ratio and porosity of carbon nanotube dry-spun yarns, Carbon 48 (10)
(2010) 2802–2811.)

applied to the CNTs during web drawing removed some of the crimps
present in the CNT. The van der Waals force and the entanglement between
the CNTs in the drawn web give the web its strength for the dry-spinning
process.
Unlike conventional staple or short fiber textile webs such as those of
wool or cotton, the CNT web does not draft or stretch easily once formed.
Under sufficient tensile load, the CNT web will break sharply instead of
drafting apart through slippage between individual CNTs. This is because
the constituent CNTs are strongly connected to each other by van der Waals
and other interactions (e.g., amorphous carbon bridging) and thus do not
slip readily. The tortuosities (the hooks, reversals, and crimps) and misalign-
ment of the CNTs observed in the web will thus persist into the yarn while
the straight CNTs will take up the tension applied to the yarn. The presence
of crimps, hooks, and misalignment keep the CNTs apart from each other,
resulting in voids between them, as illustrated in Fig. 7.12C and D.
Enabling drafting of CNT strands (webs, yarns) has been considered an
effective strategy for improving CNT alignment and thus the strength of the
strands. This usually requires the use of a lubricating liquid or an uncured