A time-resolved framework for structure-property-processing relations of polymeric systems using recovery rheology

The rheological properties of complex fluids, such as surfactants, polymers, emulsions, and suspensions, have received significant attention due to their wide applications. The viscoelastic behaviors of complex fluids have stimulated the development of rheological constitutive models that allow for accurate predictions of industrially relevant processes to be made. Conventionally, rheological constitutive models are based on the concept of the displacement gradient tensor, which does not account for differences between recoverable and unrecoverable strains. We are driven to provide a deeper physical understanding of material response under deformations based on the ideas of recovery. In the first stage, the thesis focuses on developing a new rheological formalism based on the ideas of recovery is presented. Our new formalism contains recoverable and unrecoverable contributions to arbitrary deformations. The introduction of the two displacement gradients leads to two distinct measures of strain and strain rates, which highlights the importance of performing recovery experiments. Having established the new formalism, we show the benefits of this way of thinking by performing transient step strain and startup shear recovery measurements in a wide range of shear strains and shear rates on a model viscoelastic solution. With recovery, we show clear similarities in the material behavior between the two test protocols. The resultant recovery material functions – recoverable modulus and flow viscosity – allow the development of a new phenomenological model, which consists of nonlinear elastic and viscous functions, along with a retarded viscous term. The predictions of the model are compared favorably with the experimental data, including responses to extremely large step strains. These observations allow us to revisit the transient entanglement length, relaxation time, and damping function based on recovery rheology. The present findings suggest a clear correlation exists between microstructural evolution and recoverable and unrecoverable components and provide a new direction for the exploration of the relation between recovery material functions and material responses under different dynamic flows. In the second state, the idea of recovery is utilized to understand the gelation. We construct a simple phenomenological model based on recovery rheology to explain and capture the gelation dynamics of various systems that gel via various mechanisms. The model accounts for many qualitative features and is quantitatively accurate for some materials, including smooth growth of storage modulus and overshoot in loss modulus, using only an unrecoverable rate-dependent flow viscosity. We show that the gelation is a transition from unrecoverable to recoverable acquisition of strain throughout the reaction based on the decomposed dynamic moduli. We propose a new direction of defining the gel point based on the instantaneous Deborah number. In the third state, the idea of recovery is applied to propose the structural-property-processing relation for additive manufacturing. Direct ink writing is a 3-D printing technology that has received increasing attention recently due to its low cost, energy efficiency, and design freedom. However, more detailed rheological characterizations are needed to investigate the yielding transition the ink undergoes and the deformation-induced colors that the ink exhibits. In the present work, we use two yield-stress materials formed by bottlebrush block copolymers suspended in toluene and a mixture of toluene and m-xylene as model DIW ink formulations. We perform oscillatory recovery measurements over various deformations, closely resembling the printing conditions, to map out the transient responses of recoverable and unrecoverable contributions. Rheo-optical setups are utilized to monitor the deformation-induced colors of the two systems while undergoing oscillatory tests. The yielding transitions for both systems are captured favorably by the recovery metrics through both experiments and KDR model predictions. The color differences induced by highly nonlinear deformations are correlated with the maximum recoverable strain (or maximum unrecoverable rate) for both systems. Simple equations between structural colors and recovery metrics are proposed. The present findings provide new insights into understanding the rheological phenomena involved in the DIW printing process and tuning ink properties to achieve desirable features of the post-printing product. In the fourth state, the thesis focuses on understanding the linear and nonlinear rheology of complex coacervates with different salt concentrations. The time-salt superposition is applied to not only the frequency sweep determined in the linear region but also the steady shear flow curve that contains nonlinear viscoelastic responses. Thorough recovery analyses are performed at all coacervates with salt concentrations ranging from 0.05M KBr to 1.7M KBr. Decomposed moduli are determined based on the recoverable and unrecoverable contributions. With the recoverable components and the master curve of the steady shear flow curve, a phenomenological model is developed that gives outstanding predictions of the dynamic and decomposed moduli for different coacervates. In the fifth state, the thesis provides ways of relating the recovery bulk rheology to the microscopy rheology using single-molecule fluorescence microscopy. Deoxyribonucleic acid (DNA) solutions in different flows are investigated based on recovery rheology, and material properties are proposed to establish the connection. In the last state, I present the results of the temperature ramp performed on bottlebrush polymer melts using the rheo-Raman setup. Future directions are proposed.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Chemical Engineering

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2024

Author dc:creator

• Shi, Jiachun

Contributors dc:contributor

• Rogers, Simon A
• Schroeder, Charles M
• Schweizer, Kenneth S
• Sing, Charles E

• Rheology
• Material Characterization
• Complex Fluids
• Polymer
• Direct Ink Writing

Statement dc:rights

• Copyright 2024 Jiachun Shi

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/127348

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F127348