Strain Shifts under Stress-controlled Oscillatory Shearing in Theoretical, Experimental and Structural Perspectives: Application to Probing Zero-Shear Viscosity
Johnny Ching-Wei Lee, Yu-Tong Hong, Kathleen M. Weigandt, Elizabeth G. Kelley, Hyunjoon Kong, Simon A. Rogers
Strain response under stress-driven oscillatory shearing could shift from the origin. The phenomenology is experimentally confirmed with entangled polymer-like micelles and polyethylene oxide solutions. A theory of strain shifting is provided based on the recovery rheology where the zero-shear viscosity is shown to dictate the amount of shifting. We further investigate the microstructural evolution via in-situ small-angle neutron scattering when strain shifting appears. The recoverable strain is observed to correlate to the microscopic orientation albeit the total strain contains distinct shifting. An approach of determining the zero-shear viscosity is proposed and successfully demonstrated with numerous stiff materials including collagen, pluronic-hyaluronic acid, alginate gels and polystrene melts. In addition, the strain-shift knowledge can be further applied to determine the horizontal shift factor in time-temperature superposition principle in an algorithm-free manner. The work provides theoretical, experimental and structural understanding for strain shifting, and paths a rigorous way to generically quantify the flow activation energy of viscoelastic materials.