Structure-Property Relationships via Recovery Rheology in Viscoelastic Materials

Abstract

The recoverable strain is shown to correlate to the temporal evolution of microstructure via time-resolved small-angle neutron scattering (SANS) and dynamic shear rheology. Investigating two distinct polymeric materials of wormlike micelles and fibrin network, we demonstrate that, in addition to the nonlinear structure-property relationships, the shear and normal stress evolution is dictated by the recoverable strain. A distinct sequence of physical processes under large amplitude oscillatory shear (LAOS) is identified that clearly contains information regarding both the steadystate flow curve and the linear-regime frequency sweep, contrary to most interpretations that LAOS responses are either distinct from, or somehow intermediate between the two cases. This work provides a physically-motivated and straightforward path to further explore the structure-property relationships of viscoelastic materials under dynamic flow conditions.A long-standing challenge in understanding the out-ofequilibrium behavior of soft matter is to link microstructural rearrangements and the macroscopic flow properties. We show, via time-resolved rheo-small-angle neutron scattering (rheo-SANS), that such a link can be made, forming nonlinear rheological structure-property relations, by considering the evolution of the recoverable component of the strain.The simultaneous collection of macroscopic rheological information and particle-or molecular-level data has been used understand the interplay between material constituents that cover a wide range of length scales. Recent examples include the applications of scattering techniques in polymeric materials [1][2][3][4][5][6][7], confocal microscopy in colloidal systems [8][9][10][11][12][13][14][15][16][17][18] and simulation methods [19][20][21][22][23][24]. Despite these efforts, rheological structure-property relations remain incompletely understood. Oscillatory shearing provides a reproducible way to probe viscoelastic behaviors and has been used to study a wide array of soft materials [25][26][27][28][29][30][31]. Many processing conditions and practical applications of soft materials can be approximated by large amplitude oscillatory shear (LAOS) because it offers independent control of the length and time scales of structural rearrangements. As such, LAOS has been widely adopted as a model transient flow protocol capable of eliciting nonlinear responses [2,19,24,[32][33][34][35][36][37][38][39][40].Typical mathematically-based descriptions of experimental responses to LAOS are based on Fourier transformation (FT), which represents the complex sequence of processes exhibited by soft materials in the time domain as a sum of harmonic contributions in the frequency domain [41], performing an averaging of sorts. Physical processes that take place sequentially over intervals of time shorter than a period of oscillation are therefore not easily discerned via such analysis methods. The lack of a generic understanding of the higher harmonics [42] has limited the widespread adoption of these met...

Nonlinear shear of entangled polymers from nonequilibrium molecular dynamics

“…There has been recent experimental and theoretical interest in recoverable strain in polymers. Experiments by Lee et al have linked recoverable strain to microscopic structure and normal stress in two very different polymeric systems.…”

“…There has been recent experimental and theoretical interest in recoverable strain in polymers. Experiments by Lee et al have linked recoverable strain to microscopic structure and normal stress in two very different polymeric systems. In entangled polymers, the bulk of the strain is typically recovered in a very fast, time‐dependent, reversing shear flow, as the slow‐relaxing polymeric stress far exceeds the viscous stresses .…”

“…Our reversing flow protocol requires only shear stress data from a rapid reversing flow and the linear viscoelastic spectrum and so should be achievable in experiments, to provide an experimental method to probe the polymer stress in the velocity gradient direction from shear stress measurements alone. Indeed, recent experiments by Lee et al also linked recoverable strain to normal stress measurements.…”

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

“…Recently, in searching for structure–property relationships, we have reported, via in situ rheo‐small‐angle neutron scattering (rheo‐SANS), a direct correlation between the macroscopic rheological response and structural evolution . It was demonstrated that under LAOS, the nonlinear viscoelastic properties, including the shear and normal stresses, shear modulus and viscosity, and the underlying segmental alignment, are all dictated by the recoverable strain , and not the total strain.…”

“…To this end, the way one perceives and analyzes particular responses may be influenced by concepts central to the prevailing theory in the field, and emerging experimental studies that challenge the theory have been reported . Motivated by recent advances in understanding out‐of‐equilibrium responses to LAOS via recoverable strain, the disagreement between the prevailing polymer dynamics theory and experiments, and experimental insights obtained from the recovery rheology, we aim to study the step strain response of a polymeric system with a complex shear history within the framework of recoverable strain. We concatenate two often‐performed tests, using an integer number of periods of LAOS to induce a complex nonlinear flow which is followed after some time by a single‐step strain.…”