Relaxation Dynamics of Polymer Liquids in Nonlinear Step Shear

Relaxation dynamics of entangled polymer liquids are investigated in nonlinear step shear flow using mechanical rheometry experiments and theory. Entangled solutions of high molar mass polystyrenes (PS), 3 × 10 5 ≤ φ M̄ w ≤ 1.6 × 10 6 g/mol, in diethyl phthalate (DEP) are the main focus of this study. Cone-and-plate rheometer fixtures roughened by attachment of a single layer of 10−30 μm silica glass beads are used to eliminate interfacial slip during step shear measurements. A simple theory for stress relaxation dynamics that accounts for coupled relaxation of molecular orientation, chain stretching, and entanglement density is used to analyze the experimental results. In PS/DEP solutions with φ M̄ w ≥ 5 × 10 5 and in which PS forms an average of eight or more entanglements per chain, we find that the nonlinear relaxation modulus can be factorized into separate strain-dependent and time-dependent functions only after a time λ k 2 ≈ τ d0 ∼ (φ M̄ w ) 3 substantially larger than the longest Rouse relaxation time τ Rouse of the solution. This finding is consistent with results from a previous study of step shear dynamics in solutions of ultrahigh molecular weight polystyrene, M̄ w = 2.06 × 10 7 [Sanchez-Reyes, J.; Archer, L. A. Macromolecules 2002, 35, 5194], but contradicts expectations from current theories for entangled polymer dynamics, which predict λ k 2 ≈ (3 − 5)τ Rouse . The origin of this discrepancy is traced to a greater than expected influence of entanglement loss and recovery processes on polymer relaxation dynamics in nonlinear step shear flow.