Effect of in-situ formed core–shell inclusions on the mechanical properties and impact fracture of polypropylene

Abstract

To obviate the reduction of stiffness in rubber toughened and yield strength in rigid particle toughened iPP, various strategies were adopted. Here, we report on the static and impact properties of isotactic polypropylene (iPP) with in situ formed (rigid thermotropic liquid crystalline core)–(soft copolymer shell) inclusions (RCSS inclusions). We found that, at constant soft shell thickness (ts) and stiffness (Es), the elastic modulus of iPP/RCSS (Ec) increased with the total RCSS volume fraction (vRCSS), and, at constant vRCSS, the Ec increased with Es. At constant ts and Es, the yield strength of iPP/RCSS (σyc) decreased with the vRCSS and, at constant vRCSS and ts, the σyc decreased with Es in an agreement with the simple micromechanics models of Matonis and Small. At small vRCSS, the changes in Ec and σyc with vRCSS contributed simultaneously to the significant enlargement of the crack tip plastic zone size, enhancing the impact fracture toughness in terms of the apparent critical strain energy release rate (Gc′), substantially, without significantly reducing stiffness and yield strength. The SEM fractography revealed extension of the RCSS inclusions into fibrous shape providing an additional means for dissipation of mechanical energy stored in the fracturing solid. Experimental data agreed reasonably well with the prediction based on a simple mixed mode fracture model proposed earlier.