Melt fracture of entangled polymers.

We construct a model for a slippage plane in a sheared melt, based on a balance between reptation bridging and shear debonding. The resulting state could show up at rather low shear rates and be locally stable. But it is not easy to nucleate: the conventional entangled state is also locally stable. We propose that slippage occurs on solid walls: either at the container surface, or on dust particles floating in the melt. Slippage at solid/melt interfaces was studied (experimentally and theoretically) long ago. There is a critical stress sigma* for slippage: our estimate (for strong adsorption of melt chains on the solid) gives sigma* approximately 1/3 (plateau modulus) for typical cases. Thus, melt fracture is expected at moderate stresses, in agreement with observations by S.Q. Wang and coworkers.