Structural Disorder in the α Form of Isotactic Polypropylene

A detailed analysis of the Bragg as well as of the diffuse X-ray diffraction scattering of isotactic polypropylene (i-PP) fiber samples, suitably crystallized in the α form under different conditions, is reported. A limit-ordered (α 2 form) and a limit-disordered (α 1 form) model structure have been described to account for the features of the experimental X-ray diffraction profiles of i-PP samples prepared in the α form. The disorder consists of the statistical occupation of the lattice sites of isomorphous helices having opposite up/down orientations. Depending on the conditions of crystallization, intermediate-disordered modifications between the limit-ordered and the limit-disordered, can be obtained. The analysis of the Bragg contribution to the X-ray diffraction scattering of isotactic polypropylene samples crystallized in the α form provides direct quantitative information regarding the amount of up/down disorder, whereas the analysis of the diffuse scattering subtending the Bragg reflections in the case of oriented samples may provide detailed information on the development of the up/down disorder within the unit cell. Fiber samples annealed at high temperatures (∼170 °C), hence recrystallized slowly, and thus nearer to thermodynamic equilibrium, show X-ray diffraction patterns very close to that of the ideal limit-ordered model structure for the α form (α 2 form). Almost perfect order is maintained inside the bilayers, as in the limit-ordered structural model, and only 5−10% of defects in the relative orientation (up or down) of the helices belonging to adjacent bilayers along b is probably present. The presence of this kind of defect is consistent with the chain-folding scheme proposed in past literature. In the case of unannealed fiber samples, a large amount of up/down disorder is also present within the bilayers, giving rise, in the case of samples crystallized at lower temperatures, to situations very close to the limit-disordered model structure. We assume that the chain folding might not develop according to the scheme proposed in the literature for kinetic reasons.