Chemical Engineering Seminar

We will review the progress in the field of nonlinear rheology of entangled polymers since 2006 when it was first reported that synthetic polymers could undergo inhomogeneous shear beyond the stress overshoot during startup shear and show non-quiescent relaxation after a large step strain [Phys. Rev. Lett. 96 016001 (2006); Phys. Rev. Lett. 97 187801 (2006)].  The emerging phenomenology, initially derived from the application of particle-tracking velocimetric (PTV) method to visualize simple shear and subsequently enlarged by direct observations of uniaxial extension [Phys. Rev. Lett. 99 237801 (2007)], has forced us to recall the prevailing experimental and theoretical paradigms.  For well entangled polymer solutions and melts, on the experimental side, we found that homogenous shear cannot be guaranteed [Macromolecules 44 183 (2011)] and global steady flow is inaccessible in uniaxial extension [J. Rheol. 57 223 (2013)]; on the theoretical side, it appears oversimplified to represent point-like intermolecular uncrossability by a smooth tube as done in the prevailing tube theory when depicting fast large deformation.  Our research in the past decade focused on answering the central questions in polymer rheology:  (a) how chain deformation takes place, (b) when and why affine deformation ceases [Macromolecules 46 3147 (2013)].  Our PTV observations of shear strain localization prompt us to think [J. Chem. Phys. 127 064903 (2007); J. Rheol. 53 1389 (2009)] that the overshoot is associated with yielding of the entanglement network via chain disentanglement.  The disentanglement occurs when the growing intra-chain elastic retractin force can no longer be sustained by the inter-chain forces.