Polyacrylonitrile

Chemical properties

white chalk-like solid

The Uses of Polyacrylonitrile

Polyacrylonitrile (PAN) is used as polymeric carbon precursor to form carbon fibers, electrospun activated carbon materials having meso-macro pores, carbon black additives.These are consecutively used in hydrogen storage, EMI shielding, electrochemistry, separation processes. It may find applications in PAN based single walled carbon nanotube composites.

Definition

ChEBI: A macromolecule composed of repeating cyanoethylene units.

Preparation

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).

General Description

PAN molecule has strong polar nitrile groups.It is relatively insoluble in nature. PAN based carbon fibers possess very high strength compared to other polymeric precursors. When subjected to heat treatment, it can produce high carbon yield giving rise to thermally stable, highly oriented, graphite like molecular structure. Generating carbon-fiber from PAN based fiber is a combination of three processes-- namely stabilization, carbonization and graphitization.

Purification Methods

Precipitate it from dimethylformamide by addition of MeOH.