Annealing-induced high impact toughness of polypropylene/CaCO3 nanocomposites

Our previous study (Macromolecule 2008, 41, 9204-9213) reported that annealing significantly increased the impact toughness of polypropylene (PP)/calcium carbonate (CaCO3) nanocomposites. We further investigated the underlying mechanism and report the results in this article. The impact strength of a high-molecular-weight PP filled with 20 wt % CaCO3 nanoparticles increased to 890 J/m upon 155 degrees C-annealing, about 20 times that of neat unannealed PP. This exceptionally high impact toughness is partially attributed to the high-molecular-weight PP, which provided strong ligaments. Moreover, this high-molecular-weight PP has a low concentration of cross-hatched structure upon annealing, indicating that the cross-hatched structure, which was suspected to be responsible for the annealing-induced high impact toughness in the previous study, is in fact irrelevant to the annealing-promoted impact toughness. A large number of cavities were observed in the impact-fractured annealed nanocomposites because the difference in the stiffness between the crystalline and amorphous regions was enlarged upon annealing. These cavities, formed in the early stage of deformation, may have contributed to the annealing-induced high impact toughness because these numerous cavities, in addition to the debonding of the CaCO3 nanoparticles, further released the plastic constraint of the PP matrix. Massive plastic deformation, therefore, became operative, leading to large energy dissipation. In addition, we found that the tensile toughness of the annealed nanocomposites was considerably reduced due to a significant reduction in the strain-at-break because the numerous cavities caused an earlier development of macro-cracks, leading to smaller strain-at-breaks. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 124: 77-86, 2012