Understanding the nonlinear flow behavior of entangled polymer liquids has immense significance because of its direct relevance and usefulness in predicting behavior of these liquids during processing. In startup shear, it is well known that shear stress overshoot emerges when the rate of shearing is higher than the inverse of overall chain relaxation time τ. For solutions with the number of entanglements per chain Z greater or equal to 25, we have revealed using PTV that the shear field becomes inhomogeneous across the gap after the stress overshoot. For solutions with Z greater or equal to 40, the shear stratification persists even in steady state, indicating that different states of chain entanglement are possible corresponding to the same shear stress. The number of entanglements per chain, Weissenberg number, the shear history, the type of solvent used, and polydispersity of the system are some of the control parameters that can strongly affect the observed nonlinearity.
PTV observations of step strain experiments revealed a great deal of information about the physics of polymer flow. Contrary to the common perception that the entanglement network would be strong enough so that it would not collapse after a large step strain, macroscopic motions in the sample interior were observed after shear cessation at strains greater or equal to 135%. The collapse of the entanglement network after shear cessation suggested that the entanglement network is a fragile system of finite cohesive strength and cannot escape structural failure. In other words, the network disintegrates when an elastic retractive force greater than the cohesive strength of the network is built due to deformation. PTV technique also revealed the coexistence of multiple shear rates under large amplitude oscillatory shear (LAOS) across the sample thickness in entangled solutions with Z greater or equal to 25. In the Lissajous plots (shear stress vs. shear strain), distortion of an ellipse begins to appear when the nonlinear velocity profile is first noticed. These observations are again contrary to the conventional perception that the system undergoing LAOS will experience homogeneous shear in each cycle so that material functions can be introduced to analyze the nonlinear dependence of these functions on the amplitude and frequency.
The above mentioned experimental observations in simple shear as well as in uniaxial elongation of entangled polymers has helped us to recognize that three forces (intermolecular locking force, retractive force and entanglement cohesive force) play important roles during response of a deformed entanglement network. The new theoretical considerations have further helped us to discover the striking scaling features associated with stress overshoot in well entangled polymer solutions and melts.