Behavior of Poly electrolyte Gels in Concentrated Solutions of Highly Soluble Salts
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MRS Advances © 2020 Materials Research Society DOI: 10.1557/adv.2020.10
Behavior of Polyelectrolyte Gels in Concentrated Solutions of Highly Soluble Salts Jessica L. Sargent,1 Xunkai Chen,1,2 Mitchell C. Brezina,1 Sebastian Aldwin,1 John A. Howarter1,3 and Kendra A. Erk1 1
School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907
2
Department of Chemical and Biomolecular Engineering, University of California, Berkeley, Berkeley, California 94720
3
Environmental and Ecological Engineering, Purdue University, West Lafayette, Indiana 47907
ABSTRACT
Ionic hydrogels are an abundant class of materials with applications ranging from drug delivery devices to high performance concrete to baby diapers. A more thorough understanding of interactions between polyelectrolyte networks and ionic solutes is critical as these materials are further tailored for performance applications in highly targeted ionic environments. In this work, we seek to develop structure-property relationships between polyelectrolyte gels and environments containing high concentrations of multivalent ions. Specifically, this work seeks to elucidate the causes behind differences in hydrogel response to divalent ions of main group metals versus transition metals. PANa-co-PAM hydrogels containing low and high fractions of ionic groups are investigated in solutions of DI water, NaCl, CaCl2, and CuSO4 at concentrations ranging from 5 to 100 mM in order to understand 1) the transient or permanent nature of crosslinks produced in these networks by divalent counter-ions, 2) the role of polymer ionic content in these interactions, and 3) how these interactions scale with salt concentration. Gravimetric swelling and mechanical compression testing are employed to characterize water and salt-swollen hydrogels in order to develop guiding principles to control and manipulate material properties through polymer-counterion interactions. The work presented here confirms the formation of permanent crosslinks by transition metal ions, offers explanation for the behavioral discrepancy observed between ionic hydrogels and main group versus transition metal ions, and illustrates how such hydrogel properties scale with counter-ion concentration.
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INTRODUCTION Ionic hydrogels are an important class of stimuli-responsive superabsorbent polymers with a rapidly growing breadth of applications [1-5]. These lightly crosslinked networks can contain fractions of or be composed entirely of polyelectrolytes, which imparts the networks with the ability to respond to changes in pH and ionic composition of their environments. These responses include conformational changes in the polymer chains, which leads to changes in permeability, chemical selectivity, and mechanical properties. As such, a thorough understandin
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