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Title:Structure-property relationships in polymeric materials via advanced rheological and neutron scattering techniques
Author(s):Lee, Ching-Wei (Johnny)
Director of Research:Rogers, Simon A.
Doctoral Committee Chair(s):Rogers, Simon A.
Doctoral Committee Member(s):Schweizer, Kenneth S.; Schroeder, Charles M.; Sing, Charles E.
Department / Program:Chemical & Biomolecular Engr
Discipline:Chemical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
neutron scattering
Large amplitude oscillatory shear
stress relaxation
polymer physics
fluid dynamics
Abstract:Linking microstructural rearrangements and macroscopic material response is a long-standing challenge in understanding the non-equilibrium behavior of soft materials. Soft viscoelastic materials, when reacting to dynamic flows such as large-amplitude oscillatory shear (LAOS) flows, tend to generate complex material responses. Several analysis approaches have been suggested to interpret the underlying material physics. Despite recent progress in understanding the macroscopic response, the vast majority of the studies has focused on a purely macroscopic level and has no straightforward path linking underlying microstructure. To this end, in this thesis, we aim to unveil the transient nonlinear structure-property correlation in polymeric materials in both macroscopic and microscopic perspectives. We approach the topic by developing an analysis framework that indicates transient viscoelastic parameters. Simultaneously, we exploit in-situ time-resolved small-angle neutron scattering (SANS) techniques to monitor how the structure evolves with flows. We first investigate the sequence of physical processes exhibited during LAOS of two concentrated poly- mer solutions. It is shown that the LAOS response can be understood as a clear transition from linear viscoelasticity to viscoplastic deformation and back again during a period. We further develop a freeware that can automatically perform the analysis and suggest material physics such as stiffening or softening. A detailed protocol of performing a LAOS measurement, analyzing nonlinear responses with the freeware, and interpreting material responses is presented. The sequence-of-physical-processes (SPP) analysis is further applied to a soft colloidal glass system formed by a suspension of multi-arm star polymers. It is demonstrated that cages reform and break twice per period and exhibit maximum elasticity around the maximum strain. With the analysis, we observe that the cages are recoverably strained by a nearly constant amount of 5%, even with the total strain amplitude of 420%. To understand how microscopic rearrangements manifest in time-dependent macroscopic responses, we present a natural 3D structure-rheology space that temporally correlates the structural and nonlinear viscoelastic parameters. Exploiting the rheo-small-angle neutron scattering (rheo-SANS) techniques, we demonstrate the use of the framework with a model system of polymer-like micelles (PLMs), where we unveil a sequence of microscopic events that micelles experience under dynamic shearing across a range of frequencies. The least-aligned state of the PLMs is observed to migrate from the total strain extreme toward zero strain with increasing frequency. Our proposed 3D space is generic, and can be equally applied to other soft materials under any sort of deformation, such as startup shear or uniaxial extension. Motivated by the experimental phenomenology and interpretation allowed in the SPP analysis, we follow the idea of recovery rheology originated from the work of Weissenberg [K. Weissenberg, Nature, 1947] and Laun [H.M. Laun, J. Rheol., 1986] to measure the time-resolved recoverable strain under oscillatory shearing and investigate its implication in the response and correlation to microstructure. Employing rheo-SANS techniques, it is shown that the recoverable strain is linearly correlated to the temporal alignment of micellar segments. Investigating two distinct polymeric materials of wormlike micelles and fibrin network, we further demonstrate that the shear and normal stress evolution is dictated by the recoverable strain. A distinct sequence of physical events under LAOS is identified that clearly contains information regarding both the steady-state 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. Having observed the benefits of performing recovery rheology in oscillatory shearing, we further apply the technique to understand a non-equilibrium step strain test, where a material is not fully relaxed prior to the imposition of step strain. We use recovery rheology and rheo-SANS to probe the nonlinear dynamics of an entangled wormlike micellar solution by applying step strains after complex shear histories enforced by 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 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. Finally, we exploit the recovery rheology to explain an experimental phenomenology and propose an alternative and efficient approach to measure the zero-shear rate viscosity. Defining zero strain as the state prior to external shearing, it is shown that strain responses to small-amplitude oscillatory stressing are naturally shifted from the starting point by an amount proportional to the phase of the applied stress. The phenomenology is experimentally observed with entangled polymer-like micelles and polyethylene oxide solutions. A theory of strain shifting in the steady alternating state is provided based on recovery rheology, where differences between total strain and recoverable strains are acknowledged. User-controlled variables, such as the amplitude of the stress, the angular frequency, and the phase of the stress, as well as a material parameter, the zero-shear rate viscosity, are shown to dictate the amount of shifting. A rapid and efficient approach of determining the zero-shear viscosity is, therefore, presented. We investigate the microstructural evolution via in-situ SANS when strain shifting appears. The microscopic orientation is shown to correlate to the recoverable strain independent of the shifting. Additional measurements are carried out on collagen, pluronic-hyaluronic acid, alginate gels, and polystyrene melts to show the generic nature of the strain shift phenomenon. In addition, we demonstrate that the strain-shift knowledge can be applied to determine the horizontal shift factor in time-temperature superposition, free of any numerical fitting procedures. Overall, this thesis tackles the most fundamental structure-property relationships in soft polymeric materials. Combining bulk rheological and neutron scattering techniques, we identify the importance of the fundamental recoverable strain, that is closely linked to microstructural evolution. Broadly speaking, this work provides a starting point to further relate bulk rheological response to molecular-level orientation, therefore proving a new perspective to advance the design of soft materials.
Issue Date:2020-04-07
Rights Information:Copyright 2020 Ching-Wei Lee
Date Available in IDEALS:2020-08-27
Date Deposited:2020-05

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