Explanation for Slip‐Stick Melt Fracture in Terms of Molecular Dynamics in Polymer Melts

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Abstract
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. In the case of polystyrene samples, the linear viscoelastic data were analyzed in terms of the proposed general linear viscoelastictheory. The analyzed results were then used in the constitutive equation to calculate their flow curves. In the reptational region, the agreement between theory and experiment is very good. Despite some quantitative difference between theory and experiment in the high rate region, the present theory correctly show the effect of the relaxation rates of the reptational motion and chain tension relaxation relative to each other. The extrudates of the polyisoprene, polystyrene and polyethylene samples were examined and compared. We have also shown from numerical calculations that the dip can be eliminated by the broadening of the molecular weight distribution (MWD). We conclude 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.

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