Tough-to-brittle transition in multiwalled carbon nanotube (MWNT)/polycarbonate nanocomposites

Morphology and fracture behaviour of polycarbonate (PC)/multiwalled carbon nanotubes (MWNT) composites have been studied by AFM and essential work of fracture approach (EWF). A maximum in the non-essential work of fracture was observed at 2 wt.% MWNT demonstrating enhanced resistance to crack propagation compared to pure PC. DMA analysis shows an increase in the Tg and broadening of the tan δ peaks with the increase in the MWNT content suggesting confinement of the PC chain mobility. Fracture kinetics studies reveal that the crack growth mechanism is broadly dominated by crack propagation rather than crack blunting. Crack resistance curves (R-curves) demonstrate a transition in the crack growth mechanism (depending on crack extension) from pure crack-tip blunting/crack initiation mode to mixed mode of crack blunting and crack initiation/propagation. Above 2 wt.% MWNT a tough-to-brittle transition has been observed which is also in agreement with the substantially higher (almost 10 times) speed of crack growth without any fracture instability before the failure. The time-resolved in situ strain field analysis revealed that the onset of crack initiation has been shifted to shorter time for nanocomposites with 4 wt.% MWNT than that of 2 wt.% MWNT and thus explaining the existence of a tough-to-brittle transition in these nanocomposites. Fracture surface investigation by SEM shows bridging of the nanotubes across the radial and transverse cracks imparting higher toughness at 2 wt.% of MWNT.The main conclusion of our study is that the resistance against crack initiation (J0.5, EWF) and the resistance against crack propagation (TJ, βwp) in these nanocomposites are structurally correlated with the matrix behaviour (state of percolation) and morphology (distance between MWNT and their dispersion) respectively.