Page 475 - Carrahers_Polymer_Chemistry,_Eighth_Edition

438 Carraher’s Polymer Chemistry

linear or rotating bearings. This behavior allowed the construction of the world’s smallest rotational
motor and nanorheostat.
Along with their high tensile strengths they also offer a high elastic modulus on the order of
1 TPa. It has a relatively low density of 1.3–1.4 g/mL. While they offer extremely high strengths
when tension is applied, they are not as strong under compression because of their hollow structure.
Another problem is defects. While we can synthesize almost defect-free tubes many of the tubes
contain defects. Because these nanotubes are essentially one-dimensional structures, the defect acts
as the weak link and can lower the tensile strength by 85%. The presence of defects also lowers the
thermal and electronic conductivity properties. Most of the defects are of the Stone Wales variety,
where a pentagon and heptagon pair is formed by rearrangement of the bonds.
As we are studying CNTs other interesting properties are being found. For instance, recently it
was found that CNTs are like hot wheels tubes allowing water and gas to flow 100–10,000 times

faster than as classical models predict. It is believed that just as Teflon sheds water because of its
inert surface so also CNTs are slick because of its atomically smooth surface and lack of polarity
differences within the tube allowing almost a zero friction situation. Under high pressure, CNTs
2
3
can be forced together exchanging some of the sp bonding for sp bonding, giving the possibility
for forming strong wiring. Electrically and thermally conductive carbon nanopaper can be made
that is 250 times stronger than steel and much lighter that can be used as a heat sink for chipboards,
blacklight for LCD screens, and to protect electronic devices.
Carbon nanotubes have been used as templates producing other nanotubes such as gold and zinc
oxide nanotubes. These nanotubes are hydrophilic while CNTs are hydrophobic allowing alterna-
tive uses where the behavior of the tube can be varied according to its environment.
A major concern involves their possible toxicity. Research involving their potential and real
toxic effects is just beginning. This is particularly important since these materials may become part
of our everyday lives in many ways. Current results indicate that nanotubes can cross membrane
barriers. If this is true, then access to our organs will occur. There is building biological evidence
suggesting that under certain conditions nanotubes, of any origin, pose a real risk to our health.
Compounding the question of toxicity is the large variety of nanotubes potentially and really
available each with its own toxicity profile. Further, the description of these various materials by

name or other convention has yet to be established.
12.20 BITUMENS

The petroleum industry, including the commercial bitumen industry, was born in the United States in
August 27, 1859 when Colonel Drake drilled a hole about 70 feet deep near Titusville, Pennsylvania
to “bring in” the first producing well. By 1908, Henry Ford began to mass produce his Model “T”

Ford creating an additional need for this petroleum in the form of gasoline. The distillation residue
became more plentiful and a need for large-scale usage of bitumens increased.
Even so, the bitumens are a very old material. They were used in the water-proofing of the cradle

that baby Moses was floated in. It was used by the ancient Egyptians in their mummifi cation pro-

cess. Bitumens were used in sand stabilization and for lighting the naval base by the Second Muslim
Caliph, Omar ben Khattab, at Basra on Shattul-Arab on the West Coast of what is now Saudi Arabia
around 640 AD.
Bitumens occur naturally or are formed as the residue in the distillation of coal tar, petroleum,
and so on. Industrially, the two most important bitumens are asphalt and coal tar. Asphalt is a brown
to black tar-like variety of bitumen that again occurs naturally or is the residue of distillation. Coal
tar is the black, thick liquid obtained as the residue from the distillation of bituminous coal.
Bitumens are examples of materials that have only an approximate structure. Bitumens are car-
bon-intense small polymers with molecular weights from about 200 to 1,000 Da for coal tar with
a calculated average number of carbons in a chain of about 15–70. Asphalt has a molecular weight
averaging about 400–5,000 Da with a calculated average number of carbons in a chain of about

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