Chemistry of the Degradation of Vinyl Polymers to give Carbon Fibre Precursors

Abstract

An investigation of the oxidative degradation of polyacrylonitrile, a precursor for carbon fibre manufacture, has been made by examining the oxidation of 'model* compounds of the polymer. During the attempted preparation of one of these 'models', 2,)-dicyanopentane, it became apparent that this compound was unstable in the presence of sodium cyanide. Polyacrylonitrile was found to be unstable under similar conditions; refluxing a solution of the polymer in dimethylformamide with sodium cyanide produced a black nonflammable material with very similar properties to oxidatively degraded polyacrylonitrile, The initial product from the cyanide, polyacrylonitrile reaction is extremely sensitive to oxidation, absorbing oxygen at room temperature. This behaviour can not be explained by the formation of a hydrogenated naphthyridine structure by cyclisation of the nitrile groups in the polymer. An alternative mechanism for the depradetion under these conditions is proposed. Before cyclisation the propogating ketimine groups (>C =H) undergo tautomeric rearrangement to the enamine form ( >C-=C—NH2,), Subsequent cyclisation results in a condensed 1 ,4.-dihydropyridine structure which, by analogy with the known chemistry of 1,4-dihydropyridines, will oxidise on exposure to air to a fully aromatic condensed pyrinoid system. This theory has been extended to cover the thermal and oxidative degradations of polyacrylonitrile in the solid state, It is concluded that the condensed 1 ,-dihydropyridine is formed first and, under oxidative conditions, is transformed to a mixture of condensed pyrinoid and 4«pyridone sequences. Chain scission also occurs but hydrogen bonding through the 4=pyridone species prevents loss of orientation and damage to the fibre structure.