Analysis of the edge fracture process in oscillation for polystyrene melts

The occurrence and development of the edge fracture phenomenon in oscillatory flow was investigated. Large-amplitude oscillatory shear experiments were performed with polystyrene melts of different molecular weights at 170 degrees C in parallel-plate geometry. Based on Tanner and Keentok's criterion for the formation of the edge fracture in steady-state rotational flow, an equation was derived for the oscillatory flow. This equation can be used to predict the onset conditions in terms of the critical deformation amplitude gamma (c) and the critical frequency omega (c). A very good agreement with the experimental observation was found. Within the critical parameter range, the kinetics of the fracture front propagation was studied by detailed visual observations and by analysing the effect of the fracture process on the measured rheological data. An empirical equation was found that allows the description of the time development of the fracture front, to be characterised by a single parameter b. This finding was also theoretically confirmed, by assuming a linear relationship between the driving force for the fracture formation and the fracture front propagation rate.