A Quantitative Description of the Effects of Molecular Weight and Atactic Level on the Spherulite Growth Rate of Ziegler-Natta Isotactic Polypropylene

Publisher Summary There has been extensive investigation on crystallization kinetics of isotactic polypropylene (iPP). Another reason for the numerous references on iPP spherulite radial growth rates might be the straightforward determination by conventional optical microscopy techniques for a large gamut of isothermal crystallization temperatures. In this chapter, the effects of MW, MWD (molecular weight distribution), and tacticity are quantified on spherulite growth rates G of iPP for isothermal crystallization temperatures within regime 3. Spherulite growth rates, as opposed to overall growth rates, are expected to be independent of metastable nuclei concentration. The chapter reports a quantitative description of iPP spherulite growth rate G as a function of isothermal crystallization temperature Tc, number average molecular weight Mn, and concentration of atactic PP. The effects of the molecular parameters are independent of Tc. While the dependency of G on Mn is small, it is highly significant. To decouple effects of molecular weight from effects of atactic concentration on G, one has to work with a wide gamut of iPP grades. The overall crystallization kinetics cannot be used to infer molecular information (on atactic content), because they are function of the previous (usually uncontrolled) thermo–mechanical history in the melt.