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Inorganic Polymers 431

many crucibles for melting metals are graphite-lined. Graphite has good stability to even strong
acids, thus it is employed to coat acid tanks. It is also effective at slowing down neutrons, and thus
composite bricks and rods (often called carbon rods) are used in some nuclear generators to regu-
late the progress of the nuclear reaction. Its slipperiness allows its use as a lubricant for clocks, door
locks, and handheld tools. Graphite is also the major starting material for the synthesis of synthetic
diamonds. Graphite is sometimes used as a component of industrial coatings. Dry cells and some
types of alkali-storage batteries also employ graphite. Graphite fibers are used for the reinforcement

of certain composites.

12.18 INTERNAL CYCLIZATION—CARBON FIBERS AND RELATED MATERIALS
There are a number of important polymers that are formed through internal cyclization. In almost

all cases, these are five- and six-membered rings with the vast majority being six membered. The
tendency to form six-membered rings is related to a statistical feature. In studying the most proba-
ble distances from the beginning point using random statistics for units with a bond angle of about
o
109.5 (for a tetrahedral), the most probable distance for a chain of six units long is back at the
starting point. The number of units required before the most probable distance is the starting point,
which is dependant upon the bond angle and is called the Kuhn element. As note before, the Kuhn
2
element for connected methylenes is six. The Kuhn element for sp geometry is not six, but in this
case, the driving force is the formation of six-membered rings with three alternating pi bonds—that
is, the formation of the aromatic structure.
Often these internal cyclizations are incomplete giving products with mixed moieties. Even so,
such internal cyclization is the source of a number of interesting and important polymers. A number
of ladder-like structures have been synthesized from the internal cyclization of polymers. Following
are several examples that illustrate this. The most important commercial products are those utilized
to form the so-called carbon fi bers.
Carbon fibers, and associated composite materials, are the result of internal cyclization.

Polyacrylonitrile, when heated, undergoes internal addition forming condensed polycyclic material
o
called “black orlon.” Further heating to about 1,000 C removes the hydrogen atoms and most of
the nitrogens giving a polyaromatic structure containing about 95% carbon (Figure 12.8). Further
o
heating to about 2,800 C gives a product with almost 99% carbon. These products can be forced

through tiny holes to form tiny fiber-like materials, fibrals, that are combined to give fi brous-like

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FIGURE12.8 Idealized structure of “black orlon.”

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