Page 467 - Carrahers_Polymer_Chemistry,_Eighth_Edition

430 Carraher’s Polymer Chemistry

(a)

(b)

FIGURE 12.7 Representations of graphite emphasizing the layered (a) and sheet (b) nature of graphite.

are traditional covalent bonds. The bonds holding the sheets together are weaker than the bonding
within the sheets consisting of a weak overlapping of pi-electron orbitals. Thus, graphite exhibits
many properties that are dependent on the angle at which they are measured. They show some
strength when measured along the sheet, but very little strength if the layers are allowed to slide

past one another. This sliding allows the graphite its flexibility, much like the bending of bundles of

proteins sliding past one another, allowing our hair flexibility. The fused hexagons are situated such
that the atoms in each layer lie opposite to the centers of the six-membered rings in the next layer.
This arrangement further weakens the overlapping of the pi electrons between layers such that the
magnitude of layer-to-layer attraction is on the order of ordinary secondary van der Waals forces.
The “slipperiness” of the layers accounts for graphite’s ability to be a good lubricant.
The variance of property with angle of applied force, light, magnetism, and so on, is called
anisotropic behavior. Calcite is anisotropic in its crystal structure, resulting in a dependency of its
interaction with light with the angle of incidence of the light.
As with diamond, graphite’s discovery and initial usage is lost in antiquity. It was long confused
with other minerals such as molybdenite (MoS ). At one time it was known as plumbago (like lead),
2

crayon noir, silver lead, black lead, and carbo mineralis. Werner in 1789 first named it graphit,
meaning (in Greek) “to write.”
The Acheson process for graphite production begins by heating a mixture of charcoal, or coke,
and sand. The silica is believed to be reduced to silicon that combines with carbon forming silicon
carbide, which subsequently dissociates into carbon and silicon. The silicon vaporizes and the car-
bon condenses forming graphite. Graphite is also produced using other techniques.
Today, graphite is mixed with clay to form the “lead” in pencils. Graphite conducts electricity
and is not easily burned so many industrial electrical contact points (electrodes) are made of graph-
ite. Graphite is a good conductor of heat and is chemically inert, even at high temperatures. Thus,

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