We report a flow phenomenon in entangled polymer solutions that has never been described in the literature. A large-amplitude oscillatory shear was imposed on the polymer sample at a frequency higher than the overall chain relaxation rate. The resulting chain orientation led to a new environment in which the initially well-entangled chains managed to disentangle inhomogeneously in space. A layer lacking chain entanglement developed to take the load of the imposed strain. As a result of this nonlinearity, the rest of the sample avoided significant deformation and its chain entanglement remained intact.

We describe an unexpected constitutive transition in entangled polymer solutions. At and beyond a critical stress, the initial spatially homogeneous and well-entangled sample transforms from its entangled (coiled) state into a fully disentangled (stretched) state over a period during which the resulting shear rate increases in a spatially inhomogeneous fashion. In the mode of controlled shear rate, the sample exhibits a stress plateau over three decades. Flow birefringence and normal stress observations unravel additional features of these flow phenomena.

We have carried out controlled-stress experiments in addition to the conventional controlledrate measurements to probe the nature of nonlinear flow behavior of entangled 1,4-polybutadiene solutions. The flow responses are found to be drastically different depending on whether the shear flow is imposed by applying a constant torque or a constant velocity on one of the two surfaces in a cone-plate flow cell. When the applied shear stress is of a comparable magnitude to the elastic plateau modulus of the entangled solutions, a sharp yieldlike constitutive transition is observed, revealing a discontinuous relationship between the shear rate and the shear stress. Following such an entanglement-disentanglement transition (EDT), the chain orientation appears to further increase as a function of time as evidenced by the rising normal stress N 1, reflecting a plausible coil-uncoil transition (C-UCT). The relaxation of N1 consists of an initial rapid decay, likely due to chain recoil from the C-UCT and a subsequent slow decrease characteristic of its relaxation in the presence of chain entanglement below the EDT. The controlled-rate measurements reveal familiar stress plateau behavior in a range of over 3 decades in the apparent shear rate, much of which is inaccessible by the controlled-stress experiment in steady state.

Using a particle tracking velocimetric technique, we show direct evidence of nonlinear velocity profiles during simple-shear flow of an entangled polymer solution, offering new insight into the origins of such characteristics as stress overshoot. Upon a startup shear by imposing a constant velocity on one of the two surfaces that confine the sample, the velocity field evolves from the initial linearity across the gap to a final state with a shear rate gradient. The unexpected deviation from the widely assumed linear variation of the velocity along the gap direction is most plausibly due to the entangled polymer's ability to disentangle in the presence of high shear that can orient the polymer chains leading to anisotropy in their mutual constraint.

This article describes a systematic investigation of a discontinuous interfacial stick-slip transition ͑SST͒ in simple shear of monodisperse entangled 1,4-polybutadiene ͑PBD͒ and polyisoprene ͑PIP͒ melts with different molecular weights and architecture, using a specially designed controlled-force shear rheometer. The magnitude of the transition is found to be determined by the level of chain entanglement. Specifically, the dependence of extrapolation length b on molecular weight as b ϳ M w 3.4 and of the melt viscosity as b ϳ is consistent with the observations based on capillary rheometric studies ͓X. Yang et al., Rheol. Acta 37, 415-423 ͑1998͔͒. The interfacial nature of the flow behavior is explicitly demonstrated by a surface treatment of the shearing plates and dependence of the abrupt increase of the apparent shear rate on the gap distance as well as by particle tracking velocimetry. The critical stress for different molecular weights of PBD and PIP is about 0.2 and 0.1 MPa, respectively, independent of molecular weight and architecture. These results are consistent with the previous conclusion of an interfacial SST as the origin of the discontinuous spurt flow behavior observed with pressure-driven capillary rheometry. The critical stress for the SST is found to be lower in simple shear flow. Finally, chain architecture is observed to also influence the magnitude of the SST apart from the level of chain entanglement.