Recovery rheology via rheo‐SANS: Application to step strains under out‐of‐equilibrium conditions

Abstract

Stress relaxation from a step strain test provides important information about constituent dynamics, but if a material has experienced a complex shear history, the underlying physics is not straightforward to access. We use recovery rheology and rheo‐small‐angle neutron scattering to probe the nonlinear dynamics of an entangled wormlike micelle solution by applying step strains after complex shear histories enforced by large‐amplitude oscillatory shear (LAOS) flow. We show that a universal relaxation modulus can be obtained from step strain tests with complex shear histories, as long as the modulus is defined in terms of the recoverable strain. The shear and normal stresses, as well as the alignment of micellar Kuhn segments, are shown to be positively correlated with the recoverable strain. We identify re‐entanglement of polymeric chains after cessation of LAOS and show that this process occurs over the same timescales as linear‐regime stress relaxation. This work, therefore, lays the foundation of how to accurately probe out‐of‐equilibrium rheology in a consistent manner.

Molecular Characterization of Polymer Networks

“…Recently, scattering coupled with in situ rheological measurements has become more popular to aid in the development of structure–property relationships. − Rheo-small-angle-neutron scattering (rheo-SANS, Figure ) is most common in polymer networks due to the contrast selectivity as mentioned for static SANS, and it provides a direct correlation between the structure and the macroscopic rheological response. There are a number of flow devices compatible with in situ neutron scattering measurements (flow-SANS or rheo-SANS), most notably, Couette cells, capillary flow cells, 1–2 shear cells, and even entire oscillatory rheometers .…”

“…Capillary and microfluidic flow devices, , for low sample volume experiments under arbitrary deformation fields, are also available. Such rheo-SANS experiments have given insights into the interplay of chain branching and alignment with the formation/breakage of reversible cross-links under different shear conditions. − Further, rheo-SAXS has shown that the glassy styrene domains (within a microphase-separated poly­(styrene- b -ethylene- co -butylene- b -styrene) (SEBS) triblock thermoplastic elastomer) change shape, spacing, and ordering while undergoing uniaxial or biaxial stretching …”

“…(b) 2D SANS pattern overlaid with the azimuthal angle φ = 0–360°. Figure reproduced with permission from ref . Copyright 2019 American Institute of Chemical Engineers.…”

Materials Design of Highly Branched Bottlebrush Polymers at the Intersection of Modeling, Synthesis, Processing, and Characterization

“…However, there are weaknesses in this approach due to the challenge of interpreting the physical meaning of these higher-order harmonics in the context of molecular deformations. Recently, a class of methods known as “recovery rheology” approaches have sought to specifically overcome this limitation. − Here, oscillatory shear is abruptly “stopped” mid-deformation, and the relaxation of the material to the applied shear is also measured. This enables the extraction of molecular relaxation processes and has been applied to a number of material classes including linear polymer solutions and melts , and colloidal yield-stress fluids .…”