Page 473 - Carrahers_Polymer_Chemistry,_Eighth_Edition

436 Carraher’s Polymer Chemistry

CNT bundles have been used to construct elementary computing circuits known as logic gates.
Applications include composites and coatings for enclosures and gaskets and use as stealth mate-
rial. They perform better than tungsten filaments in incandescent lamps. Metallic nanotubes can

have electrical current densities on the order of one thousand times that of silver and copper.
Conductive Adhesives and Connectors—Because of their molecular-level characteristics, they
have been used to connect electronic circuits and to bind together electrical units acting as a molec-
ular solder.

Mechanical—CNTs have superior resilience and tensile strength. They can be bent and pressed over
a large angle before they begin to ripple or buckle, finally developing kinks. Until the elastic limit is

exceeded, the deformations are elastic with the deformation disappearing when the stress is removed.
It is envisioned that buildings and bridges built from them may sway during an earthquake rather than
fracturing and crumbling. Multiwalled nanotubes can easily slide past one another without friction
resulting in the construction of rotating bearings used in rotational motors based on this observation.

Field Emission—When stood on end electrified CNTs act as a lightning rod concentrating the elec-

trical field at their tips. While a lightning rod conducts an arc of electricity to a ground, a nanotube
emits electrons from its tip at a rapid rate. Because the ends are so sharp, the nanotube emits electrons
at lower voltages than do electrodes made from other materials and their strength allows nanotubes
to operate for longer periods without damage. Field emission is important in several industrial areas,

including lighting and displays. Commercial use of CNTs as field emitters has begun. Vacuum-tube
lamps in six colors have been developed that are twice as bright as conventional light bulbs, longer-

lived, and at least ten times more energy-efficient as conventional light bulbs.
Hydrogen and Ion Storage—While we can picture CNTs as being composed of hexagonal car-
bon atoms with lots of empty space between the carbons, atoms “thrown” against them generally
just bounce off. Even helium atoms at an energy up to 5 eV do not readily penetrate the nanotube.
Thus, the graphene sheet and CNTs are really membranes or fabrics that are one atom thick made
of strong material that is also impenetrable (to a limit). Thus, CNTs can be used for hydrogen stor-
age in their hollow centers with release being controlled allowing them to act as inexpensive and
effective fuel cells.
Chemical and Genetic Probes—Nanotube-tipped atomic force microscopes can trace a strand of
DNA and identify chemical markers that reveal DNA fine structure. A miniaturized sensor has

been constructed based on coupling the electronic properties of nanotubes with the specifi c recog-
nition properties of immobilized biomolecules through attaching organic molecules—handles—to
these tubular nanostructures. In one study, the pi-electron network on the CNT is used to anchor
a molecule that irreversibly adsorbs to the surface of the SWNT. The anchored molecules have a
“tail” to which proteins, or a variety of other molecules, can be covalently attached. The result is
that these molecules are immobilized on the sidewall of the nanotube with high specifi city and

efficiency. The molecule’s tail is tipped with a succinimidyl ester group, which is readily displaced
when an amine group attacks the ester function, forming an amide bond. Thus, the CNTs are used
as both highly sensitive probes and highly selective immobilizing sites that allow specifi c reactions
to occur.

Analytical Tools—SWCNTs are being used as tips of scanning probe microscopes. Because of
their strength, stability, and controllable and reproducible size, the tup probes allow better image
fidelity and longer tip lifetimes.

Solar Cells—Some nanotubes exhibit a photovoltaic effect and can replace material in solar cells
acting as a transparent conductive film allowing light to pass to the active layers and generate pho-

tocurrent. It has also been shown that nanotubes can be used in conjunction with other materials to
act as an ignition device that is triggered with a simple camera fl ash.

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