Toughening mechanism of polypropylene/calcium carbonate nanocomposites

The toughening mechanism of isotactic-polypropylene (iPP) filled with calcium carbonate (CaCO3) nanoparticles is studied. Good dispersion of the nanoparticles in the iPP matrix was achieved with a monolayer coating of stearic acid on the nanoparticles surface. Two iPPs with different molecular weights (MW) were used in this study. The impact strength of the high-MW-iPP/20 wt% CaCO3 nanocomposite was increased from 50 to 890 J/m after annealing, which is the highest impact strength of iPP/CaCO3 nanocomposites ever reported in the literature so far. The micro-morphology on an arrested crack tip of the annealed nanocomposite revealed that the collective collapse of the strained ligaments at the crack tip was responsible for the formation of macro-cracks and the consequent catastrophic crack propagation. Strong ligaments, which have high fracture stress and were achieved by using the high MW iPP, stabilized the crack initiation process and hence promoted energy dissipation. Moreover, a large number of cavities formed in the annealed nanocomposites after deformation facilitated the shear yielding of the ligaments, leading to massive plastic deformation and remarkable energy dissipation. In addition, a critical nanoparticle concentration, which was corresponding to a brittle-to-ductile transition, was observed in the low-MW-iPP/CaCO3 nanocomposites. The massive plastic deformation of the ligaments of iPP matrix enabled by the plane-strain to plane-stress transition is concluded to be the toughening mechanism of the PP/CaCO3 nanocomposites. The scale and the extent of the plastic deformation are determined by the competition between the fracture stress and yield stress of ligaments. A high fracture stress of the ligaments is favorable to stabilize the crack initiation stage and to allow for the expansion of the plastic deformation zone. A lower stress level for yielding is accomplished when the plastic constraint of the ligaments is significantly released. Both the numerous cavities and the nanoparticle concentrations above the critical point significantly release the plastic constraint