Studies on the monoclinic and hexagonal modifications of isotactic polypropylene

Abstract

Pure isotactic polypropylene, which is isothermally crystallized from the melt in the temperature range between 0°C and 140°C, displays only the monoclinic α-modification whereas samples which contain a nucleating agent exhibit the α-modification as well as the hexagonal β-modifications for crystallization temperatures ranging from 0°C to 130°C. It is possible to determine the equilibrium melting points and the surface free energies of both modifications, using combined structural and thermal studies. The β-modification is found to be metastable with respect to the α-modification. The melting and recrystallization behavior of both modifications differs strongly. A method is discussed by which the relative amount of the two modifications can be determined on the basis of wide angle X-ray diffraction.