Morphological changes during annealing of doubly oriented polycaprolactam

Doubly oriented polycaprolactam films were annealed with the ends free at temperatures between 145 and 200°C for 30 min. As the annealing temperature rose the long period increased, the transverse dimensions of fibrils grew anisotropically, and the low‐angle diffraction intensity and density also increased. On the other hand, there was a decrease in the breadth of crystalline reflections. During annealing the film shrank in the direction of the fibril axis (M) and expanded in the direction (N) in which the pressure was acting during the production process. No changes could be observed in the direction normal to the MN plane. The supermolecular structure was also investigated by means of electron microscopy. The phenomena observed are explained in terms of the Peterlin two‐stage model of plastic deformation of the crystalline polymers, which assumes the formation of strongly extended fibrils, connected by interfibrillar “tie” molecules.