Fracture toughness and deformation mechanism of un-vulcanized and dynamically vulcanized polypropylene/ethylene propylene diene monomer/graphene nanocomposites

Polypropylene (PP)/Ethylene propylene diene monomer (EPDM)/graphene nanocomposites were prepared by melt mixing process via an internal mixer (Brabender plasti-corder). The effect of multi-layer graphene (MLG), few-layer graphene (FLG) and dynamic vulcanization on microstructure and fracture toughness of the multicomponent system were investigated. The morphology of the samples were characterized by wide X-ray diffraction (WAX), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) which revealed that graphene mostly was dispersed into the PP phase and the size of the dispersed phase (EPDM) was decreased with incorporation of graphene, more specifically FLG, in both un-vulcanized and vulcanized systems. The concept of essential work of fracture (EWF) was used to analyze the fracture toughening behavior and deformation mechanism of PP/EPDM/graphene nanocomposites. In the case of un-vulcanized polymer, the total work of fracture improvements (+23.5%) was noted for PP/EPDM/FLG in which FLG platelets with higher aspect ratio were used, showed the better dispersion and higher ductility fracture behavior compared with MLG. The fracture mechanism of un-vulcanized samples occurs due to debonding and/or cavitation of the dispersed phase (EPDM), while for vulcanized samples the fracture mechanism proceeds through creation of nanovoids and cavitation in the dispersed phase which lead to the shear yielding of PP matrix. The graphene platelets acted as crack initiation sites as well as obstacles against crack propagation. The results of microstructure properties along with the fracture analysis data showed that the aggregated MLG platelets act as stress concentration and facilitate the initiate cracks while the FLG platelets hindered the crack path. Improvement in toughness behavior and matrix resistance were attributed to higher voiding stresses and finer dispersion of graphene.