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

• Published in: AIChE journal Vol. 65; no. 12
• Main Authors: Lee, Johnny C.‐W., Porcar, Lionel, Rogers, Simon A.
• Format: Journal Article
• Language: English
• Published: Hoboken, USA John Wiley & Sons, Inc 01.12.2019; American Institute of Chemical Engineers
• Subjects: Chain entanglement; Entanglement; Equilibrium conditions; LAOS; Micelles; Neutron scattering; Nonlinear dynamics; nonlinear rheology; Nonlinear systems; polymers rheology; Recoverable strain; Recovery; Rheological properties; Rheology; rheo‐small‐angle neutron scattering; Shear; step strain; Strain; Stress relaxation
• DOI: 10.1002/aic.16797
• ISSN: 0001-1541

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.