Polymorphism in isotactic polypropylene

The conditions under which a given form of any of the three distinct polymorphic crystalline phases of isotactic polypropylene can be obtained have been investigated. The relationship between crystallization conditions and the formation of a given crystalline phase is discussed. Evidence is cited in support of the hypothesis that the β-form of isotactic polypropylene can be produced only when appropriate nucleating materials are present. It is shown that under particular high-pressure crystallization conditions a triclinic γ-phase can be produced that does, not convert to the monoclinic α-phase either by action of heat or mechanical stress. Based on considerations of stability, density, and structure, it is suggested that the equilibrium crystal form of the polymer is the γ-form rather than the α-form which is ordinarily observed.