Building lamellae from blocks: The pathway followed in the formation of crystallites of syndiotactic polypropylene

AFM images of isothermally crystallized samples of syndiotactic polypropylene and poly(propene-co-octene) show that the lamellar crystallites are composed of blocks. Upon heating the granular structure vanishes. Observations support the view recently established by SAXS experiments that polymer crystallization is a two-step process, with the block formation near the growth front as a first and their fusion into a lamella as a second step. It is proposed that the blocks have the minimum size necessary to remain intrinsically stable. This could explain the observed invariability of the crystal thickness with changing co-unit content.