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Title: Linear and nonlinear rheology and structural relaxation in dense glassy and jammed soft repulsive pNIPAM microgel suspensions

Abstract

We present an integrated experimental and quantitative theoretical study of the mechanics of self-crosslinked, slightly charged, repulsive pNIPAM microgel suspensions over a very wide range of concentrations (c) that span the fluid, glassy and putative “soft jammed” regimes. In the glassy regime we measure a linear elastic dynamic shear modulus over 3 decades which follows an apparent power law concentration dependence G' ~ c5.64, a variation that appears distinct from prior studies of crosslinked ionic microgel suspensions. At very high concentrations there is a sharp crossover to a nearly linear growth of the modulus. To theoretically understand these observations, we formulate an approach to address all three regimes within a single conceptual Brownian dynamics framework. A minimalist single particle description is constructed that allows microgel size to vary with concentration due to steric de-swelling effects. Using a Hertzian repulsion interparticle potential and a suite of statistical mechanical theories, quantitative predictions under quiescent conditions of microgel collective structure, dynamic localization length, elastic modulus, and the structural relaxation time are made. Based on a constant inter-particle repulsion strength parameter which is determined by requiring the theory to reproduce the linear elastic shear modulus over the entire concentration regime, we demonstrate good agreementmore » between theory and experiment. Testable predictions are then made. We also measured nonlinear rheological properties with a focus on the yield stress and strain. A theoretical analysis with no adjustable parameters predicts how the quiescent structural relaxation time changes under deformation, and how the yield stress and strain change as a function of concentration. Reasonable agreement with our observations is obtained. To the best of our knowledge, this is the first attempt to quantitatively understand structure, quiescent relaxation and shear elasticity, and nonlinear yielding of dense microgel suspensions using microscopic force based theoretical methods that include activated hopping processes. Here, we expect our approach will be useful for other soft polymeric particle suspensions in the core–shell family.« less

Authors:
ORCiD logo [1]; ORCiD logo [1];  [1]; ORCiD logo [1]; ORCiD logo [1];  [1]
  1. Univ. of Illinois at Urbana-Champaign (United States)
Publication Date:
Research Org.:
Univ. of Illinois at Urbana-Champaign, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES)
OSTI Identifier:
1634191
Alternate Identifier(s):
OSTI ID: 1491277
Grant/Contract Number:  
FG02-07ER46471
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Soft Matter
Additional Journal Information:
Journal Volume: 15; Journal Issue: 5; Journal ID: ISSN 1744-683X
Publisher:
Royal Society of Chemistry
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE

Citation Formats

Ghosh, Ashesh, Chaudhary, Gaurav, Kang, Jin Gu, Braun, Paul V., Ewoldt, Randy H., and Schweizer, Kenneth S. Linear and nonlinear rheology and structural relaxation in dense glassy and jammed soft repulsive pNIPAM microgel suspensions. United States: N. p., 2018. Web. doi:10.1039/C8SM02014K.
Ghosh, Ashesh, Chaudhary, Gaurav, Kang, Jin Gu, Braun, Paul V., Ewoldt, Randy H., & Schweizer, Kenneth S. Linear and nonlinear rheology and structural relaxation in dense glassy and jammed soft repulsive pNIPAM microgel suspensions. United States. https://doi.org/10.1039/C8SM02014K
Ghosh, Ashesh, Chaudhary, Gaurav, Kang, Jin Gu, Braun, Paul V., Ewoldt, Randy H., and Schweizer, Kenneth S. 2018. "Linear and nonlinear rheology and structural relaxation in dense glassy and jammed soft repulsive pNIPAM microgel suspensions". United States. https://doi.org/10.1039/C8SM02014K. https://www.osti.gov/servlets/purl/1634191.
@article{osti_1634191,
title = {Linear and nonlinear rheology and structural relaxation in dense glassy and jammed soft repulsive pNIPAM microgel suspensions},
author = {Ghosh, Ashesh and Chaudhary, Gaurav and Kang, Jin Gu and Braun, Paul V. and Ewoldt, Randy H. and Schweizer, Kenneth S.},
abstractNote = {We present an integrated experimental and quantitative theoretical study of the mechanics of self-crosslinked, slightly charged, repulsive pNIPAM microgel suspensions over a very wide range of concentrations (c) that span the fluid, glassy and putative “soft jammed” regimes. In the glassy regime we measure a linear elastic dynamic shear modulus over 3 decades which follows an apparent power law concentration dependence G' ~ c5.64, a variation that appears distinct from prior studies of crosslinked ionic microgel suspensions. At very high concentrations there is a sharp crossover to a nearly linear growth of the modulus. To theoretically understand these observations, we formulate an approach to address all three regimes within a single conceptual Brownian dynamics framework. A minimalist single particle description is constructed that allows microgel size to vary with concentration due to steric de-swelling effects. Using a Hertzian repulsion interparticle potential and a suite of statistical mechanical theories, quantitative predictions under quiescent conditions of microgel collective structure, dynamic localization length, elastic modulus, and the structural relaxation time are made. Based on a constant inter-particle repulsion strength parameter which is determined by requiring the theory to reproduce the linear elastic shear modulus over the entire concentration regime, we demonstrate good agreement between theory and experiment. Testable predictions are then made. We also measured nonlinear rheological properties with a focus on the yield stress and strain. A theoretical analysis with no adjustable parameters predicts how the quiescent structural relaxation time changes under deformation, and how the yield stress and strain change as a function of concentration. Reasonable agreement with our observations is obtained. To the best of our knowledge, this is the first attempt to quantitatively understand structure, quiescent relaxation and shear elasticity, and nonlinear yielding of dense microgel suspensions using microscopic force based theoretical methods that include activated hopping processes. Here, we expect our approach will be useful for other soft polymeric particle suspensions in the core–shell family.},
doi = {10.1039/C8SM02014K},
url = {https://www.osti.gov/biblio/1634191}, journal = {Soft Matter},
issn = {1744-683X},
number = 5,
volume = 15,
place = {United States},
year = {2018},
month = {12}
}

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Works referencing / citing this record:

Directed Printing and Reconfiguration of Thermoresponsive Silica‐pNIPAM Nanocomposites
journal, June 2019


Flow properties reveal the particle-to-polymer transition of ultra-low crosslinked microgels
journal, January 2020


Directed Printing and Reconfiguration of Thermoresponsive Silica-pNIPAM Nanocomposites
journal, July 2019