Fracture mechanism of high impact strength polypropylene containing carbon nanotubes

Abstract

In our earlier contribution, it was shown that with the addition of 1 wt% functionalized multiwall carbon nanotube (f-MWNT), an unprecedented 152% increase in PP impact strength without a significant loss in stiffness and ductility can be achieved. In the context of these observations, this work examined the impact fracture mechanism of such PP/f-MWNT nanocomposite, in which the PP interphase was tailored by the same co-solvent solution process. The nanotube-matrix stress transfer efficacy in PP/f-MWNT as compared to that of PP/pristine MWNT (p-MWNT) was determined by Raman spectroscopy. The calculated interfacial shear strength (τi) is 17.8 MPa in PP/f-MWNT and 2.2 MPa in PP/p-MWNT, suggesting improved matrix-CNT adhesion in the former. This strong interfacial adhesion allows CNTs to bridge the opening crack, absorb fracture energy and promote local plastic deformation of the polymer matrix. Such process was demonstrated using SEM fractography where breakage/pull out of the CNTs, microcracks, and the intensive fibril formation accompanied with the extension of the PP matrix were captured at the impact fracture surface. Both the interfacial shear strength and the SEM fractography supported the hypothesis that better matrix-CNT adhesion can be achieved through interphase engineering.