handbook of carbon, graphite, diamond and fullerenes

174 Carbon, Graphite, Diamond, and Fullerenes
Precursor Requirements. To produce the optimum carbon fiber, a
precursor fiber should perform the following functions: [5 l
• Maximize the preferred orientation of the polycarbon
layers in the direction of the fiber axis to achieve highest
stiffness.
• Preserve the highest degree of defects in order to hinder
the formation of crystalline graphite with its low shear and
modulus.
• Preserve the fibrous shape of the polymer precursor
during pyrolysis.
• Provide as high a carbon yield as possible.
PAN essentially meets the requirements. It was originally developed
as an organic synthetic textile fiber and this is still by far its major use. It has
also shown to bean ideal precursor for the production of carbon fibers, but
only when produced by the wet-spinning process (as opposed to the more
common and cheaper dry-spinning).
PAN is usually co-polymerized with a small amount of another
monomer such as methyl acrylate or vinyl acetate. This co-polymerization
lowers the glass-transition temperature from 120°C to