A current account of chain extension, fibrous crystallization, and fiber formation

A broad, rather personal survey is presented on chain alignment and fibrous crystallization. First, attention is drawn to the fact that fibrous crystal entities can be of fundamentally different origin and certain methods for their identification are proposed. Following this, the largest part of the survey is concerned with fibrous crystallization from pre‐extended molecules. As a first step, methods for extending chains and for observing their extension are outlined. Elongational flow is receiving special attention: conditions by which it can lead to ultrahigh chain extension are briefly defined as high‐lighted both by a priori considerations and by most recent experiments, methods of realizing these conditions being indicated both in solutions and melts. Following this, attention is given to the mode of crystallization of extended chains and to the structures which arise. Amongst the many factors involved, the part played by the local flow field arising from the presence of the growing crystal entity within the general flow field is outlined, leading up to the basic mechanism by which fibrous crystals grow. It is shown how the recognition of these principles can be utilized for producing crystalline fibers. The unusual thermal and mechanical properties of the products with special emphasis on melting behavior and modulus are indicated. A short section is devoted to the achievement of very high chain extensions by means of deforming material crystalline to begin with and the resulting products are contrasted with those obtained by means of crystallizing pre‐extended chains as regards thermal and mechanical behavior.