Spherulitic crystallization and cracking during heat aging of polypropylene

The spherulitic crystallizations of polypropylene were investigated by a polarizing microscope. At high temperature, crystallization proceeds through the process of dendritic mechanism. Long needles grow rapidly at first from the nucleus to the supercooled melt, and then they fan out around each needle by branching. The rates of these two processes depend on the polymer and the condition of crystallization. In large well‐grown spherulites, cracks are formed both between radial fibers and at boundaries during heat aging. The cracks are void and single spherulites are separated. The boundary cracks are also caused by thermal decomposition of molecules and the radial cracks by stresses due to the rearrangement of molecules or the perfection of spherulites.