Crystallization Under Extreme Temperature and Pressure Gradients

Crystallization under high temperature or pressure gradients is observed in extrusion-orientation processes such as fiber formation during spin-orientation. Thermodynamic considerations indicate that crystallization temperatures and rates of crystallization in such processes are tremendously increased. For materials crystallizing fast and to a high degree (for example polypropylene), the increased crystallization rates at high spin stresses lead to well oriented lamellar structures which form epitaxially on the originally produced fibrillar nuclei and fibrillar crystallites. For slow crystallizer (such as polyethylene terephthalate) high spin-stress extrusion leads to an oriented, crystalline fibrillar system. The lamellar structures show highly elastic properties to high extension (50–100%) and over an extensive temperature range (–180 to 140°C for polypropylene) especially after perfecting the lamellar morphology by annealing. The elastic mechanism of these “hard” elastic materials is elucidated on the basis of X-ray diffraction, electron microscopy, and other studies. A major part of the reversible extension seems to be energy elastic due to the deformation of lamellae and reversible void formation of the interlamellar regions. This special lamellar morphology is intermediate between the unoriented spherulitic morphology obtained at low extrusion stresses and a largely fibrillar morphology obtainable by subsequent drawing or by extrusion of exceptionally stiff polymers. While the hard elastic materials at present have not found technical applications, a microporous derivative has been commercialized.