Visualization of structural rearrangements during annealing of solvent-crazed isotactic polypropylene

Structural rearrangements taking place upon the annealing of solvent-crazed isotactic PP are studied by the direct microscopic method. Independently of the type of its crystalline structure, solvent-crazed PP undergoes shrinkage in a wide temperature interval, starting even from room temperature and up to its melting temperature. This shrinkage is a result of the structural processes in crazes and proceeds via shutting down of the walls of individual crazes. This low-temperature shrinkage of solvent-crazed PP is assumed to have an entropy nature. This process involves the contraction of extended polymer chains and their transition into thermodynamically favorable conformations. This contraction is allowed because, upon annealing, the entropy contracting force increases. As a result, the crystalline framework of oriented PP melts down (amorphization), extended chains appear contracted, stored stresses relax, and subsequent recrystallization in the unstressed state takes place.