AbstractAn examination by X‐ray, birefringence, and infrared methods was carried out on partially oriented polyester yarns subjected to dry and wet heat under varying tensions that allowed shrinkage and extension processes to occur. The results can be interpreted in terms of a two‐phase morphology in which one phase is a network and the other crystalline. On this basis the evidence suggests that the crystalline phase is composed of extended chains and the network of randomly folded chains with cohesion points randomly distributed between t
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