Modification change during spherulitic growth of polypropylene

Isothermal crystallization of polypropylene in two temperature steps has been investigated by means of polarizing optical microscopy. On the edges of growing β‐spherulites a β → α modification change is observed when the temperature is raised beyond 140°C. The growth rate of the β‐modification exceeds that of the β‐modification above this temperature. Kinetic conditions are, therefore, attained for the β → α transition. Altering the crystallization conditions (T < 140°C) the α → β transition can also be induced. The modification change gives rise to the formation of characteristic twin‐spherulites. The boundary of the modifications corresponds to the shape of limacon of Pascal. Regions of the modifications developed at two different temperatures in a twin‐spherulite show separate melting.