Structure-property relations in molded, nucleated isotactic polypropylene

The influence of molecular characteristics and nucleating agents on the morphology distribution and properties of injection molded isotactic polypropylene (iPP) is investigated using optical microscopy, X-ray diffraction and mechanical testing. To have better control over the thermo-mechanical history, instead of a reciprocating screw, a capillary rheometer is used to drive the melt into the simple rectangular mold. Molecular weight (MW), molecular weight distribution (MWD) and addition of ethylene via copolymerization all influence the thickness of the oriented shear layer, the crystallinity, the type and amount of crystal phases, and the lamellar thickness. The addition of a nucleating agent (DMDBS), dictates the crystallization process, and resulting morphology, and samples with an oriented morphology over the full thickness are created without changing other morphological features, by applying a thermal treatment to the melt prior to injection, which is based on the specific phase behavior of the iPP-DMDBS system. The thermally treated samples show a considerable improvement in mechanical properties.