Melting behaviour of isotactic polypropylene isothermally crystallized from the melt

Abstract

A detailed study using d.s.c. of the melting behaviour of isotactic polypropylene isothermally crystallized from the melt has been carried out. The origin of multiple fusion endotherms was sought. It is shown that at lower crystallization temperatures ( T c ⩽ 400 K), this phenomenon is dominated by crystal imperfections. On heating, the crystallized fraction undergoes reorganization or recrystallization resulting in a fraction consisting of crystals of greater degree of perfection, which melt at a higher temperature. As the crystallization temperature is increased, the crystallization proceeds more slowly accompanied by simultaneous annealing. Therefore, the extent of reorganization or recrystallization goes on decreasing with T c. At higher crystallization temperatures ( T c > 400 K), the inbuilt defects in the molecular chain begin to dominate. At these temperatures, the multiple endotherms are caused by the presence of crystalline species having different degrees of crystal disorder and stereoblock character. It is further shown that the observed multiple values for the equilibrium melting temperature are also due to the presence of such crystalline species.