Rheo-optical Behavior of Polyacrylonitrile: Creep and Creep Recovery

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Abstract
The rheo‐optical behavior of PAN homopolymer film has been investigated by means of creep and creep recovery experiments with simultaneous measurements of birefringence changes. Creep experiments were carried out at temperatures of 70°, 100°, 120°, 140°, and 160°C, at a series of stress levels. This temperature range spanned the 90° and 140°C transition temperatures. A progressive change in shape of the creep curve in a log‐log plot of compliance vs time is seen: from very flat and gradual to more S‐shaped both with increasing temperature and with increasing stress at the higher temperatures. This S‐shaped curve can be identified with the cold‐drawing phenomenon, even though no neck formation was seen. Creep recovery is more complete at lower temperatures and after lower final strains in the creep experiment. Most of the recovery is instantaneous; only a very small amount of further time‐dependent recovery is observed. At higher temperatures and higher strains, creep seems to involve permanent changes in the solid‐state structure; these produce a permanent set which is not recoverable even on heating to higher temperatures. The creep and creep recovery, therefore, cannot be properly described in terms of the laws of simple viscoelasticity. The rheo‐optical behavior of this polymer is clearly more complex than that of PVC, which has been investigated previously. Some speculations regarding the solid‐state structure of PAN are presented, based primarily on the birefringence results.