Step Shear Dynamics of Entangled Polymer Liquids

Abstract

Nonlinear step shear dynamics of entangled solutions of an ultrahigh molecular weight polystyrene (PS20M, = 20.06 × 106 g/mol) in diethyl phthalate (DEP) are investigated. PS20M/DEP solutions with concentrations φ spanning the range from marginally entangled to highly entangled liquids are used to quantify the effect of entanglement density on dynamics. Step shear measurements are performed in a setting where errors due to interfacial slip can be determined and minimized. Two characteristic “separability” times λk1 = (16.5 ± 4.7)τRouse and λk2 ≫ τRouse are identified, beyond which nonlinear shear relaxation moduli G(γ,t) = σ(γ,t)/γ can be factorized into separate time-dependent G(t) and strain-dependent h(γ) functions. Contrary to expectations from theory, both separability times are stronger functions of polymer concentration (λk1 ∼ φ 0.7, λk2 ∼ φ3.2) than expected for a pure Rouse stretch relaxation route to factorability, λk ∝ τRouse ∼ φ 0. On the other hand, we find that a single shear damping function, h(γ), close to the universal damping function hDE-IA predicted by Doi−Edwards theory accurately describes the strain dependence of step shear material response, irrespective of PS20M/DEP entanglement density.