The autoorientation mechanism of crystallization

Abstract

AbstractAttempts have been made to explain a few details of the sometimes misunderstood autoorientation mechanism of crystallization. The autoorientation mechanism is not a model for a particular type of spherulite, but one of the fundamental factors which influence the structure of substances with linear molecules or linear aggregates of molecules or atoms. It is possible to explain on this basis why spherulitic structures are formed or not formed, depending on circumstances. Another argument for autoorientation is that under certain conditions the molecules in adjacent regions are perpendicular to each other. From that point of view the structure of “herringbone” spherulites and ringed spherulities is pertinent to autoorientation. These structures have been investigated and models for these spherulites are proposed. Herringbone spherulites are easily reproducible in gutta‐percha. The ringed spherulites have a rhythmic layer structure. The molecules in adjacent layers are perpendicular to the radius and perpendicular to each other. They grow probably by a process of twinning at regular intervals. The visibility of the rings, especially when the rings are out of focus in unpolarized light, is caused by Becke lines. Clear evidence for the layer structure is the fact that borderlines between adjacent spherulites sometimes follow a zigzag path. The structure of the center of spherulites is also in agreement with autoorientation. Popoff's double leaves in gutta‐percha spherulites are formed if regions at both sides of the sheaflike structure, in the center of spherulites, crystallize in a different manner from that of the sheaf itself. Spherulites formed at low temperatures usually have only an apparent fibrosity, whereas spherulites formed at high temperatures can consist of real needle‐shaped crystals, sometimes separated by less crystalline interstitial material. These and other aspects of the crystallization of polyolefins and other polymers are shown to be in general agreement with the conception of autoorientation.