Crystallization from a strained melt or solution

Abstract

AbstractThe elongation and orientation of randomly coiled macromolecules in a strained melt or solution reduces their entropy and thus increases the crystallization or melting temperature of the ideal lattice. At any given temperature of experiment this enhances nucleation and crystal growth rate. As a rule, linear primary nuclei are formed. They contain more or less extended chains. The existence of row nuclei reduces the local gradient in the liquid to such an extent that further crystallization proceeds by epitaxial overgrowth of folded chain lamellae. Densely packed cylindrites are formed with the ribbon‐like lamellae radiating from the central row nucleus. The irregular shish‐kebab structure observed in stirred or sonicated solutions seems to be formed by subsequent exial deformation of cylindrites in the flow field. It displaces the lamellae irregularly and thus produces a great many microfibrillar elements parallel to the original row nuclei. The almost completely extended chains in the shish yield a high elastic modulus and tensile strength for exial loading. The shish‐kebab morphology in fibers as spun does not affect to a great extent the mechanical properties obtainable by subsequent drawing. The lamellae are transformed into microfibrils in very much the same manner as in spherulitic samples. But the highly regular orientation of lamellae seems to result in a more uniform drawing and hence a stronger fiber. In an extremely high temperature and pressure gradient, the melt extrusion produces hard elastomers where the lamellae of the cylindrites seem to be locally stapled. Upon application of tensile load in the extrusion direction, the intervening sections bend like beams, thus forming thin holes extending in the direction perpendicular to the load. The holes enormously enhance the permeability for gases and liquids. The elastic bending of lamellae yields the high recoverable strain and low tensile modulus.