Stabilization Schemes of PAN and Associated Observations

In 1950, Houtz [143] first observed the color change when a PAN fiber produced by DuPont (Orion) was heated in air for 16 h at 200°C, the product becoming insoluble in typical PAN solvents, such as dimethylsulfone and not undergoing combustion in a naked 

From other IR work, Burlant and Parsons concluded that aromatization of the ring structure occurred at temperatures above 300°C. 

Grassie and co-workers (1958) [146] proposed that the hydrogenated naphthyridine structure was formed by the nitrile polymerization reaction alone. 

This scheme for a so-called ladder polymer became widely accepted and is normally quoted in textbooks. The conjugated —C=N—C=N— structure occurring during stabilization behaves as a chromophore, which would explain the appearance of color. 

Schurz also stated that the UV spectrum was different from that expected of a small molecule heterocyclic compound, which would be similar to substituted pyridines and naphthyridines, resembling six-membered rings arising from cyclization. 

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