Correlation of Rheology and Light Scattering in Isotactic Polypropylene during Early Stages of Crystallization

Structure development during crystallization of isotactic polypropylene (iPP) at low undercooling is studied by small-angle light scattering (SALS) and light transmission. The structure development is related to network formation (gelation) as previously measured on the same sample in dynamic mechanical experiments. Close to the rheologically measured gel point, small-angle light scattering (SALS) patterns in both Hv and Vv modes show circular symmetry; density fluctuations (maximum in Vv invariant) as determined by SALS go through a maximum. Orientation fluctuations develop much more slowly and appear at much later stages of the crystallization process. Growing clusters are characterized by very low anisotropy and low internal crystallinity. The characteristic length scale associated with the cluster at early stages is about 1 μm. At later times, evolution of 2-fold and 4-fold symmetry patterns suggests the development of the anisotropic superstructures. The growth of crystalline clusters leads to a strong increase in turbidity as monitored by light transmission in the Vv mode.