Page 470 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Inorganic Polymers 433

Carbon nanotubes are carbon allotropes that have attracted much attention. They have a diam-
eter of about 1/50,000 that of a human hair. Some have suggested that CNTs will be one of the
most important twenty-first century materials because of the exceptional properties and ready abun-

dance of the feedstock, carbon. CNTs are generally classified into two groups. Multiwalled CNTs

(MWCNTs) are composed of 2–30 concentric graphitic layers with diameters ranging from 10 to
50 nm with lengths that can exceed 10 μm. Single-walled CNTs (SWCNTs) have diameters ranging
from 1.0 to 1.4 nm with lengths that can reach several micrometers.
An ideal CNT can be envisioned as a single sheet of fused hexagonal rings, that is, graphite, that
has been rolled up forming a seamless cylinder with each end “capped” with half of a fullerene mol-
ecule. SWCNTs can be thought of as the fundamental cylindrical structure, with MWCNTs simply
being concentric tubes. They can also be conceived of as being the fundamental building block of
ordered arrays of single-walled nanotubes called ropes.

12.19.1 STRUCTURES
2
Carbon nanotubes are composed of carbon sp bonded structures, similar to those of graphite.
3
These bonds are stronger than typical carbon sp bonds resulting in the strength of the CNTs. Unlike
two-dimensional sheets of graphite, the CNTs align themselves into rope-like structures. Because
of bonding similarity to graphite, these materials are often referred to as graphene.
2
Carbon nanotubes are composed of sp bonds similar to those in graphite. They naturally form
rope-like structures where the ropes are held together by van der Waals secondary forces.
Geometrically, CNTs can be described in terms of a two-dimensional graphene (graphite) sheet.

A chiral vector is defined on the hexagonal lattice as

C = nx + my (12.30)
h
where x and y are unit vectors, and n and m are integers, also tube indices. The chiral angle is mea-
sured relative to the direction defi ned by nx.
When the graphene sheet is rolled up forming a nanotube, the two ends of the chiral vector meet one
another. The chiral vector thus forms the circumference of the CNTs circular cross section. Different
values of n and m give different nanotube structures with different diameters (Figure 12.10).
There are three general types of CNT structure (Figure 12.11). The zigzag nanotubes correspond
o
to (n, 0) or (0, m) and have a chiral angle of 0 . The carbon–carbon position is parallel to the tube
o
axis. Armchair nanotubes have (n, n) with a chiral angle of 30 . The carbon–carbon positions are
perpendicular to the tube axis. Chiral nanotubes have general (n, m) values and a chiral angle of
o
o
between 0 and 30 and as the name implies, they are chiral.
In real life, nothing is perfect. As is the case with CNTs the defects are mainly inclusion of
wrong-membered rings. Pentagonal defects, that is, the replacement of a hexagonal with a fi ve-
membered ring, results in a positive curvature causing the tube to curve inward like a horse shoe.
The closure of an open cyclindrical surface necessarily involves topological defects—often for-
mation of pentagons. Heptagonal defects result in a negative curvature with the lattice looking
expanded around the defect.
The tendency to include pentagonal units can be seen by comparing the presence of pentagonal
units in fullerene structures. The C structure contains 12 pentagons and 20 hexagons. The larger
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the fullerenes the smaller the ratio of pentagons to hexagons. This is consistent with the use of pen-
tagons to “cause” sharper bends and greater curvature in comparison to hexagons. Interestingly,
fullerene C is one of the most strained molecules known but it exhibits good kinetic stability.
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o
It begins to decompose at about 750 C. There are a number of higher numbered (carbon number)
fullerenes, including C , C , C (two geometric isomers), C , . . . . Fullerenes can act as a source
70 76 78 80
of the CNTs with the different-sized fullerenes producing different nanotubes. The three gen-
eral structures of nanotubes can be produced using different fullerenes with C giving armchair
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