Properties And Crack Behaviors Of PC/BMPE Blends

Maleic anhydride grafted bimodal polyethylene (BMPE-g-MAH) was prepared by melt blending , and it's structure was characterized by IR spectrum. polycarbonate (PC)/bimodal polyethylene (BMPE) compatibilized blends were prepared by introducing BMPE-g-MAH as a compatibilizer. The organic solvent resistance and static/dynamic mechanical properties of PC/BMPE blends were tested in this study. The fracture surface and crack tip structure of PC/BMPE blends were observed by scanning electron microscopy (SEM). The process and mechanism of crack evolution were studied intensively.The organic solvent resistance testing suggests that in N, N-Dimethylformamide (DMF), the crack and inflation level of PC/BMPE blends decrease as BMPE concentration increase. Compared with the uncompatibilized blends, the crack and inflation level of BMPE-g-MAH compatibilized blends is lower. The results imply that the addition of BMPE-g-MAH improved the organic solvent resistance of PC/BMPE blends.The mechanical properties testing suggests that the notch impact strength of PC/BMPE blends increases with BMPE-g-MAH concentration: when BMPE-g-MAH concentration is 5wt%, the impact strength is 45% higher than uncompatibilized PC/BMPE blends. The storage modulus, maximum of loss modulus and loss tangent at 140℃of PC/BMPE compatibilized blends are lower as compared with uncompatibilized blends. The results may due to the enhancement of cohesion at PC/BMPE interface by BMPE-g-MAH which brings better disperse of BMPE in PC, and thus relaxed the internal stress stems from orientation of rigid PC molecular chain.The SEM photographs of cryogenic fracture surfaces of the blends suggest that BMPE-g-MAH improved the compatibility of PC and BMPE. Fibrils were formed on the impact fracture surface of the blends. Compared with the uncompatibilized blends, there are more fibrils on the fracture surface of the compatibilized blends. The fracture surface can be divided into two regions: crack initiation region which has cavitation and crack propagation region which has a lot of fibrils.The SEM photograph of crack tip of PC/BMPE blends suggests that the crack structure involves a so-called'cohesive zone'where the two surfaces of the crack are bridged by thin fibrils. These fibrils again are of two types: primitive fibrils close to the craze tip and much thinner fibrils spanning the rest of the crack. The fibril formation mechanism are of two types too: surface-drawing and fibril-stretching, respectively. The cohesive stress at the boundary of the process zone can be described by the hydrostatic traction of fracture criteria. The formation of the fibrils has a micronecking process during crack evolution, and a micronecking model was presented using necking theories. The fibrils blunt the crack through energy dissipation during the fibril formation process.