Shear amplification and re-crystallization of isotactic polypropylene from an oriented melt in presence of oriented clay platelets

Abstract

In this study, we first prepared isotactic Polypropylene (iPP)/organoclay nanocomposite specimens via twin-screw extruder and by adding compatibilizer (maleic anhybride grafted PP). Then PP and the composites were subjected to dynamic packing injection molding, in which the melt was firstly injected into the mold then forced to move repeatedly in a chamber by two pistons that moved reversibly with the same frequency as the solidification progressively occurred from the mold wall to the molding core part. The dispersion and orientation of layered organoclay in the nanocomposite were estimated by transmission electron microscopy (TEM) and 2d-wide angle X-ray scattering (2d-WAXS). A much higher degree of orientation of PP was found in the composites compared with the pure PP. This was explained by so called shear amplification in that a great enhancement of local stress occurred in the small interparticles region of two adjacent layered tactoids with different velocities. Furthermore, re-crystallization of isotactic polypropylene (iPP) by melting the dynamic packing injection molded samples has been investigated by polarizing light microscopy (PLM). A highly oriented threadlike crystallites was observed for the first time when crystallization occurs by melting the dynamic packing injection molded samples at 180°C. However, spherulitic morphology is always obtained once PP crystallizes from an isotropic melt by melting the samples at 200°C. The shear amplification mechanism and the formation mechanism of oriented threadlike crystallites have been discussed in detail.