Mechanical enhancement of melt-stretched β-nucleated isotactic polypropylene: The role of lamellar branching of β-crystal

Melt-stretched isotactic polypropylene (iPP) filled with different weight fractions (0, 0.1, 0.2 and 0.4 wt %) of β-nucleating agent (β-NA) was extruded via a single screw extruder with a slit die and immediately melt-stretched at the die exit. The microstructure and mechanical properties (i.e., monotonic tensile, step-cycle and stress relaxation behavior) of these samples were analyzed. Unexpectedly, the lamellar branching of β-crystal, which is similar to lamellar branching of α-crystal, is observed in melt-stretched β-nucleated iPP. More interestingly, it is found that, compared with other samples, iPP containing 0.2 wt % of β-NA has the best mechanical properties. Through thorough analysis of various structural characterizations, the improved tensile strength obtained from step-cycle tests and viscous force obtained from stress relaxation measurements for iPP with 0.2 wt% of β-NA is ascribed to lamellar branching of β-crystal. The study is an example of preparing high performance polymers via practical polymer processing methods.