Elastic “hard” fibers. I.

Elastic “hard” fibers may be prepared from a number of semicrystalline polymers, notably polypropylene, poly-3-methylbutene-1, and polyoxymethylene. These materials show typically a high degree of length recovery from large extensions, a marked, more-or-less recoverable reduction of apparent density on stretching, and the generation of very large amounts of accessible volume and surface area on stretching. Wide- and low-angle X-ray and electron-microscopic observations indicate that the morphological basis for these properties lies in an array of closely packed lamellae of which the normals lie predominantly parallel to the fiber extrusion direction. It is the tilting and splaying apart of the lamellar network which creates the internal volume and surface area on stretching. The long-range elasticity is believed to be distinctly nonrubberlike, as reflected in an insensitivity of mechanical properties to low temperatures, and to arise from bending of the lamellae. This unusual class of materials provides a significant link between macroscopic properties and a particular morphological structure.