The Order of the Molecular Chains in Isotactic Polypropylene Crystals

When isotactic polypropylene α form was annealed at high temperatures, a structural transition from a disordered state to an ordered state was observed and studied by X-ray diffraction. The space groups of the two forms were proved to be C2/c and P21/c respectively. We proposed an “order domain eduction model” for the transition, from an analysis of reflected intensity and line width. The structure factor of the intermediate state was then written Fint=nFC2/c+(1−n)Fp21/c where n is the volume fraction of the C2/c form; this gave a good value of the R-factor, 0.152. From structural considerations, the transition was interpreted as the arrangement of CH—CH3 bonds (upward or downward) changing into complete order, where the helical arrangement was already in complete order in both forms; reasons for this model were discussed using energy considerations. The origins of streak-like diffuse scattering and of various disorders in the crystal were also discussed.