Approximately 70% of the commercial output of acrylonitrile is polymerized
(with minor amounts of comonomers) to give polymers which are used for
textile fibres:
The most important methods for the preparation of polyacrylonitrile are
solution polymerization and suspension polymerization. The former method
is particularly convenient, since when a solvent for the polymer is used, the resulting solution may be utilized directly for fibre spinning. Concentrated
aqueous solutions of inorganic salts such as calcium thiocyanate, sodium
perchlorate and zinc chloride make suitable solvents; suitable organic solvents include dimethylacetamide, dimethylformamide and dimethylsulphoxide. Emulsion polymerization suffers from the disadvantage that the monomer has appreciable water-solubility and the formation of polymer in the
aqueous phase can lead to coagulation of the latex. This tendency is reduced
by the addition of ethylene dichloride to the system.
Fibres prepared from straight polyacrylonitrile are difficult to dye and, in
order to improve dyeability, commercial fibres invariably contain a minor
proportion (about 10%) of one or two comonomers such as methylmethacrylate, vinyl acetate and 2-vinylpyridine.
The average molecular weight (Mw) of commercial polyacrylonitrile is
generally in the range 80000-170000.
In polyacrylonitrile appreciable electrostatic forces occur between the
dipoles of adjacent nitrile groups on the same polymer molecule. This
intramolecular interaction restricts bond rotation and leads to a stiff chain.
As a result, polyacrylonitrile has a very high crystalline melting point (317??C)
and is soluble in only a few solvents such as dimethylacetamide and dimethylformamide and in aqueous solutions of inorganic salts. Polyacrylonitrile
cannot be melt processed since extensive decomposition occurs before any
appreciable flow occurs and fibres are therefore spun from solution. In one
process, for example, a solution of the polymer in dimethylformamide is
extruded into a coagulating bath of glycerol and the fibre formed is drawn
and wound.
Polyacrylonitrile is unstable at elevated temperatures. On heating above about 200??C, polyacrylonitrile yields a red solid with
very little formation of volatile products. When the red residue is heated at
about 350??C there is produced a brittle black material of high thermal
stability. The first step in these changes consists of a nitrile polymerization
reaction whilst the second step involves aromatization to form a condensed
polypyridine ladder polymer:
Continued heating at high temperatures (1500-3000??C) results in the elimination of all elements other than carbon to leave a carbon fibre with graphitic crystalline structure of great strength. Polyacrylonitrile fibres have become
the most important source for carbon fibres.
Polyacrylonitrile is hydrolysed by heating with concentrated aqueous
sodium hydroxide to poly(sodium acrylate).
