ELASTIC FRACTURE OF POLYMERIC FLUIDS

The elastic fracture of polystyrene solutions and melts was investigated using a plunger driven viscometer. The polymeric extrudate emerging from a circular capillary was photographed to determine the onset of fracture, and an effort was made to relate this information to the shear stress at the capillary wall, the recoverable shear strain, the entanglement density and the geometrical parameters characterizing the system. It was found that narrow molecular weight distribution polystyrenes dissolved in benzene clearly fractured as reported in the literature, but the onset of fracture could not be predicted by available criteria. As opposed to this, highly concentrated (polymer volume fraction up to 0.57) and elastic solutions of a wide molecular weight distribution polystyrene in benzene showed no fracture when judged using similar criteria. Nonetheless, this latter polymer fractured as a melt at a value of the wall shear stress less than that achieved in the solution runs. From an examination of the polymer rheology, it is concluded that the recoverable shear strain is the key quantity influencing the onset of elastic fracture. Also, the recoverable shear-shear rate behavior is different for the melt and the solution and it depends not only on the polymer molecular weight and its distribution but also on the solvent used. The surprising absence of elastic fracture for highly viscoelastic solutions can be understood if one realizes that a critical value of the recoverable shear strain is needed for fracture to occur.