Melt strain hardening of polypropylenes and its relation to molecular structure: a review

Strain hardening of polymer melts is a rheological feature that improves properties of items processed by operations with pronounced extensional deformations. Thus, developments have been undertaken over the years to modify linear polypropylenes not showing strain hardening. The different ways to generate strain hardening polypropylenes are reviewed, and it is shown how rheological relations can be used for their characterization. Chemical reactions are shortly addressed, but the main focus lies on the generation of long-chain branches by electron-beams or γ-irradiation. Blends of a commercial long-chain branched and a linear isotactic polypropylene exhibit strain hardening and branching features determined in the highly diluted state by usual techniques that are comparable to the electron-beam irradiated samples. But measurements of the zero-shear viscosity as a function of molar mass provide a hint to different branching structures: Tree-like components for the blends and star-like molecules for the weakly electron-beam irradiated species. Using the results of these rheological methods, the influence of various experimental parameters like type of irradiation source and irradiation dose or temperature and molar mass of the irradiated polypropylene on the branching structures is elucidated. Furthermore, the different dependencies of strain hardening on elongation rate for samples with various branching structures are discussed, and a qualitative interpretation is presented.