Nucleation and growth of crystallites in high polymers. Formation of spherulites

Essential features of the crystallization of unoriented high polymers from the molten and supercooled liquid conditions are outlined to provide a background for a proposed structure and mechanism of growth of spherulites. Homogeneous and heterogeneous nucleation mechanisms of crystallite growth are discussed. Spherulites are recognized as crudely oriented spheroidal aggregates of crystallites and their attached amorphous regions. It is proposed that a spherulite originates from a single nucleus and that growth proceeds thence in a statistically radical fashion until all crystallizable domains are utilized or until growth is arrested due to increasing viscosity of the medium. It is assumed that crystallite growth occurs by lateral accretion of suitable oriented molecular segments and that nucleation proceeds from one crystallite to another by means of fine streamers of crystalline order (protofibrils) which may grow either by longitudinal extension along fringes or by lateral aggregation of very short segments. The predominant shapes of the crystallites are believed to determine the structure and optical properties of the spherulite. A roughly conical arrangement of crystallites around a spherulite radius is preferred to a helical one.