A new X-ray study of the quenched isotactic polypropylene transition by annealing

Isotactic polypropylene (iPP) samples, quenched and then annealed at 70 degrees C, 80 degrees C, 110 degrees C, 140 degrees C, and 150 degrees C, were investigated by X-ray diffraction. The transition from the quenched form (smectic) to the well-crystallized one is analyzed to explain better the morphological-structural changes that occur by annealing. For all the samples, both the quenched and annealed ones, the alpha-monoclinic structure was assumed, with different degrees of order and crystallite dimensions. The same alpha-monoclinic structure was also assumed for the so-called amorphous phase of the polymer, described here in terms of highly disordered nanocrystals. The high degree of disorder is due, in our interpretation, mainly to the disordered fraction inside the nanocrystals, which is considered wholly amorphous. The changes induced by annealing are interpreted as a rearrangement of the alpha-monoclinic structure, which presents both I and II types of disorder. The paracrystalline parameter alpha*, in its more detailed formulation, is introduced to obtain a more reliable estimation of the crystallite dimensions. The analysis of the transition process evidences two annealing temperature ranges: 70 degrees C-80 degrees C, involving a rearrangement in the chain axis direction mainly, and 80 degrees C-150 degrees C, with a prevalent rearrangement first in the directions normal to the chain axis and then in the chain direction again. During the whole process, the crystallites (crystalline phase of the polymer) undergo a remarkable increase of their dimensions, while the nanocrystals (microcrystalline phase) undergo significant changes in their degree of order only. The method adopted allows a more quantitative interpretation of the transition and provides a more structural criterion to define the amorphous phase and the crystallinity in the polymer samples.