Self-oscillations in capillary flow of entangled polymers

A well-established "coextrusion" technique is applied to visualize and docu ment a newly found self-oscillation due to an unstable boundary condition i n capillary flow of high-density polyethylene. In contrast to the other typ es of self-oscillations of interfacial origin reported in the literature, t he phenomenon can only occur in a die where the upstream portion of the cap illary die wall has a surface condition different from that of the downstre am portion. The self-oscillation phenomenon takes place under a constant pr essure when one half of the die wall (coated with a layer of fluoropolymer) assumes steady slip due to adhesive breakdown, and the boundary condition of the other half switches between no-slip and slip states through the cohe sive chain disentanglement mechanism. (C) 1999 The Society of Rheology. [S0 148-6055(99)00902-5].