Investigation of non-linear rheological behavior of polymeric liquids.

摘要

Entangled polymeric liquids necessarily show significant nonlinear responses to fast and large external deformation. Recent particle-tracking velocimetric measurements showed that the nonlinear behavior may involve inhomogeneous shear. But even in absence of shear banding, it remains a challenge to characterize and establish a connection between various nonlinear rheological characteristics and the molecular rearrangements in the entanglement network.;In this dissertation, the relationship between the dynamics of chain entanglement and the rheological behavior was studied under different deformation conditions. We emphasized with the present entangled polymeric liquids that nonlinearity in large amplitude oscillatory shear could arise due to rearrangement of their microstructures over time in response to large amplitude oscillatory shear. In this case, no correlation is obvious between strain dependence of the steady-state stress response and deviation of the steady-state stress from the sinusoidal wave. We investigated the nature of steady shear flow of entangled polymeric liquids by superimposing either small amplitude oscillatory shear or small step strain and analyzing the resultant mechanic responses. Our results showed that a) polymer dynamics (in terms of stress relaxation) were accelerated relative to the quiescent dynamics in direct proportion to the underlying shear rate, and b) the steady shear was a viscous state where chains were displaced past one another on a time scale comparable to the reciprocal rate, consistent with the idea of convective constraint release (CCR). We carried out rate-switching tests to further elucidate the changes of the transient strength of chain entanglement in response to various forms of shearing. Our results showed that a) elastic yielding could occur during quiescent relaxation after a large step strain, b) the new state of chain entanglement was stable for a significant period after shear cessation from steady state, and c) the strength of chain entanglement could achieve a higher level than that of the equilibrium state during re-entanglement.