Page 206 - Carbon Nanotube Fibres and Yarns

196 Carbon Nanotube Fibers and Yarns

yarn. To understand the toughening mechanism, Beese et al. [105] performed
MD simulations of shearing adjacent CNTs to calculate the interface stiffness
as a function of the added PVA. The CNTs were initially attached covalently
with 4-mer poly(methyl methacrylate) (PMMA) chains. The simulations re-
vealed that the introduced PVA interacts strongly with the polymeric coatings
on the CNTs and serves as links with adjacent CNTs.
To uncover the deformation mechanisms present in the nanoscale, MD
simulations of shearing adjacent CNTs were performed to calculate the in-
terface stiffness as a function of added PVA. As shown in Fig. 8.7A, with
increasing PVA content up to 20 wt%, the shear strength reached the highest
value owing to the formation of a monolayer of PVA on the CNT surface.
Upon tensioning, the PVA molecules were stretched (Fig. 8.7B). However,
if overloaded with PVA (60 wt%), a thick PVA layer was formed to allow for
deformation without stretching the individual PVA molecules, correspond-
ing to a remarkable reduction in shear strength. Obviously, this study is quite
different from the simulation of PVA-impregnated CNT bundles [104], as
the interfacial shear properties have changed from CNT-CNT contacts to
the linking between polymer chains. Nevertheless, it reveals that a thin poly-
mer layer between CNTs can bring about certain plasticity to CNT bundles.
Covalent bonding is the most efficient way to improve the intertube
load transfer. Kis et al. reported a 30-fold increase in the bending modulus
for a CNT bundle via intertube cross-links [106]. Cornwell et al. [107]
reported that interstitial carbon atom cross-links between CNTs in a large

Fig. 8.7 Effect of PVA infiltration on the interfacial stiffness between CNTs. (A) Force-
displacement relations for different PVA contents. (B) Snapshots of MD simulations for
10 wt% PVA (top) and 60 wt% PVA (bottom) at an applied shear displacement of 9 Å.
(Reproduced with permission from A.M. Beese, S. Sarkar, A. Nair, M. Naraghi, Z. An, A. Moravsky,
R.O. Loutfy, M.J. Buehler, S.T. Nguyen, H.D. Espinosa, Bio-inspired carbon nanotube-polymer
composite yarns with hydrogen bond-mediated lateral interactions. ACS Nano 7 (4) (2013)
3434–3446.)