Deciphering how the viscoelastic properties of mussel-inspired metal-coordinate hydrogels dictate their adhesive and interfacial mechanics
Author(s)Lai, Erica L.
Massachusetts Institute of Technology. Department of Materials Science and Engineering.
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In the world of adhesives, tunable viscoelasticity and adhesion to wet surfaces are two highly desirable properties. Mussels have already mastered both of these properties within the threads they create to anchor themselves in harsh intertidal conditions (collectively called the byssus). The key to both the mussel's ability to stick to a wide variety of surfaces and the highly energy-dissipative viscoelastic behavior of its byssal threads is a type of reversible bonding called metal-ligand coordination, which is comprised of amino acid functional groups binding to metal ions. Recently, researchers have incorporated metal-coordinate cross-links into various types of polymeric networks to improve their mechanical properties, particularly toughness, self-healing, and adhesion. However, there is not as much fundamental understanding of how the linear viscoelastic properties of these networks dictate adhesive behavior, both cohesively and at an interface.In this thesis, we use shear rheology, tack tests, and spherical probe indentation tests to explore correlations between linear viscoelastic properties (i.e., plateau modulus, G[subscript p], and characteristic relaxation time, [tau][subscript c]) and adhesive behavior (e.g., peak stress, energy dissipation per volume or work of debonding per area) of transiently cross-linked hydrogels comprised of histidine-functionalized 4-arm PEG coordinated with Ni². It is important to note that this fully transient model system is technically a viscoelastic fluid even if it has gel-like behavior on the timescales studied. To control the viscoelastic properties of the transient networks, we varied the Ni²+-histidine ratio, the polymer wt %, or the choice of buffer; in a case study, we also added Co²+ for a second relaxation timescale. The experimental conditions of pull rate and substrate choice were also varied.From our tack results, a strong dependence of peak stress on G[subscript p] and [tau][subscript c] was observed, and this correlation between network dynamics and mechanics under tensile load is in good quantitative agreement with our theoretical framework for peak stress, which includes the linear viscoelastic properties as parameters. Energy dissipation per volume is also influenced by G[subscript p] and [tau][subscript c], with an additional dependence on the polymer wt % at higher strains when the network is remodeling. These findings are consistent with previously proposed molecular mechanics of reversible His[subscript x]Ni²+ cross-links. From our ongoing spherical probe indentation tests, we have demonstrated that metal-ligand coordination at the interface can be a dominant contributor to adhesion, and we are starting to provide quantitative information about how that contribution is modulated by probe material choice and buffer-influenced timescales.In addition to the adhesive studies, we also replicated the effect of the macroscopic byssal thread structure - a stiff metal-coordinate coating surrounding a compliant core - on its mechanical behavior. To do so, we mimicked the thread structure by coating PDMS fibers with dried 4-arm PEG that was end-functionalized with Dopa or nitroDopa and coordinated with Fe³⁺, and performed tensile tests on these coated fibers. From these studies, we demonstrated that the coating allowed for improved toughness, with the magnitude dependent on the coating composition (i.e. pH and covalent cross-linking content). Collectively, these findings provide us with new insights into the correlations between bulk mechanics and adhesive dynamics of gels with transient metal-coordinate cross-links, as well as ways to tune the toughness of mussel-inspired materials during larger extensions under tensile load.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2020Cataloged from student-submitted PDF of thesis.Includes bibliographical references (pages 75-79).
DepartmentMassachusetts Institute of Technology. Department of Materials Science and Engineering
Massachusetts Institute of Technology
Materials Science and Engineering.