EXPLANATION FOR SLIP-STICK MELT FRACTURE IN TERMS OF MOLECULAR-DYNAMICS IN POLYMER MELTS

A general constitutive equation, which includes both reptational motion and chain tension relaxation, has been obtained from modifying Doi's theory. Based on the constitutive equation, a dip on the flow curve can occur as the two relaxation processes separate apart with increasing molecular weight (MW). Theoretically, the maximum on the flow curve is an instability point. Experimentally, we have measured the flow curves of a series of nearly monodisperse polyisoprene and polystyrene samples and a commercial polyethylene sample and shown that slip-stick melt fracture occurs as the shear rate reaches the dip, where the stress-vs-rate curve has a negative slope. The author concludes that slip-stick melt fracture is a general phenomenon of linear flexible polymers and that it will occur when MW is high enough and MWD is narrow enough.