The persistence of birefringence of polyethylene above its melting temperature

It has long been known that polyethylene shish‐kebob crystals show birefringence which can persist up to 150°C or higher, while the equilibrium melting temperature is 141,4°C. Birefringence in macromolecules may be due to crystallinity or to orientation in the amorphous state. By carrying out parallel birefringence and heat of fusion measurements it is shown that polyethylene shish‐kebob crystals are metastable and have a zero entropy production melting temperature of 134–135°C. Their melting region is shifted to higher temperature due to effects of orientation in the amorphous portions of partially crystallized molecules.