Page 204 - Carbon Nanotube Fibres and Yarns

194 Carbon Nanotube Fibers and Yarns

or cross-linked polymers is believed to induce an enhancement in the in-
terfacial shear property between CNTs. There have been many MD and
MM investigations on the pull-out of CNT dispersed in a polymer matrix
[101–103]. However, these studies cannot be directly used to analyze the
mechanical behaviors of polymer-strengthened CNT yarns. In the former,
polymer molecules are entangled or cross-linked with each other to form
the matrix and the CNTs are not assembled together, while in the latter,
the polymer molecule is introduced into CNT assemblies, especially CNT
bundles, to change the interfacial characteristics between the CNTs.
To understand how a polymer molecule affects the sliding between
CNTs, an MD simulation was performed on PVA-modified CNT contacts,
as exhibited in Fig. 8.6 [104]. When a PVA chain is introduced into a CNT
bundle, most segments of the polymer chain lay in the grooves formed be-
tween adjacent CNTs, while some short segments of the molecule chain
span over the contact lines between the CNTs, making an indentation (de-
formation) on the tubular structure (Fig. 8.6A). When the tubes slide rel-
ative to each other, the position of this indentation moves along the tubes.
This continuous deformation is accompanied by energy dissipation which
is manifested as intertube friction. This new mechanism can increase the
friction force by up to eightfold, from 0.0032 to 0.0081–0.0255 meV/Å
per carbon atom (Fig. 8.6B and C). The highest friction 0.0255 meV/Å
was found when PVA molecule chain transverses the contact line between
CNT-3 and CNT-16, followed by 0.0250 meV/Å between CNT-6 and
CNT-16, and 0.0205 meV/Å between CNT-5 and CNT-16 (see Fig. 8.6A).
The simulation results suggest that a network of polymer chains traversing
between CNTs could lead to composites with high mechanical properties.

8.4 Microstructural evolution of CNT yarns

While a lot of effort has been spent on the fabrication of CNT yarns and
characterization of their physical and mechanical properties, there remains a
lack of detailed analysis of the yarn microstructural evolution. Since a CNT
yarn is composed of a large number of CNTs and complex pairwise intertube
vdW interactions exist in the yarn, it is computationally prohibitive to simulate
the structural evolution using full atomistic MD simulations. Thus CGMD has
become an efficient mesoscopic modeling tool for mechanics studies of large-
scale CNT assemblies. In addition, multi-scale modeling can be used to deal
with even more complicated CNT assemblies. These simulation methods are
discussed in the order according to their computational capacity.