Topographic Analysis of Isotactic Polypropylene Spherulites by Atomic Force Microscopy

Spherulites are the main crystalline morphology of polymers crystallized from melt. Their size, distribution and shape affect the mechanical resistance of thermoplastic materials and therefore a deeply knowledge of their geometry is needed to properly correlate properties and microstructure of polymers. Polymer spherulites obtained by crystallization of thin films in a hot stage device for optical observation are considered to be planar, bi-dimensional object. On a lower scale, atomic force microscopy (AFM) of a thin film of polypropylene has revealed that spherulites are instead conics with heights of few m and that the conical shape is due to a progressive reduction of the thickness of the growth front during crystallization. Moreover, relatively large and sudden reduction of the thickness causes the development of concentric short walls in the spherulites that are observable in AFM images and even, in the last stage of crystallization, by optical polarized microscopy as rings at the boundaries of the spherulites.