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Title:Tribological properties of gradient-density hydrogel surfaces
Author(s):Johnson, Christopher Lin
Director of Research:Dunn, Alison C
Doctoral Committee Chair(s):Dunn, Alison C
Doctoral Committee Member(s):Wagoner Johnson, Amy J; Espinosa-Marzal, Rosa M; Juarez, Gabriel
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Abstract:Hydrogels are soft hydrated polymer networks that are widely used in research and industry due to their similarity to biological tissues. As one of the leading materials used for tissue surrogates or biological interfaces, hydrogels exhibit useful properties such as biocompatibility, high elasticity, and low friction. While there is a continuously expanding body of research investigating hydrogels, some tribological studies are seemingly contradictory, even showing vastly different frictional behavior for the same hydrogel composition. One explanation for this comes from the ‘mold effect’, first mentioned in 2001, whereby the mold material alters the polymerization process of the hydrogel, imparting different properties to the surface in contact with it. The ‘mold effect’ was not considered or studied in detail until recent work published in 2019, where researchers found that the effect drastically reduced the stiffness of hydrogels polymerized against polymer molds compared to glass-molded hydrogels. Several studies in subsequent years finally determined that the ‘mold effect’ is due to absorbed oxygen within the mold surface interfering with the polymerization, inducing a dilute gradient-density surface layer exhibiting altered properties. However, the precise structure of the gradient surface layer, its contact response, and its effect on lubrication have not yet been characterized. Such knowledge would prove useful for designs of composite hydrogels utilizing the gradient surface layer for its special frictional properties. In order to fully characterize the hydrogel gradient surface layer, we first developed a method to view contact area of a probe on a hydrogel substrate in real time using particle exclusion microscopy. We then utilized this technique during indentation, creep, and sliding experiments on a standard hydrogel composition. Indentation experiments confirmed the dilute gradient nature of the surface layer, showing that it follows an evolving contact response with depth. This contact response could be approximated through piecewise modeling starting with a polymer brush contact model, then a Winkler foundation model, and finally the classic Hertzian contact model. Calculation of the contact area using these contact models verified that the indentation data was aligned with the piecewise model. This provided information regarding the polymer structure of the gradient layer: that it is composed of brush-like polymer segments swollen in water, where the polymer density and degree of crosslinking increase further into the depth. Creep experiments revealed that the gradient layer allows poroelastic relaxation even at pressures far below the reported osmotic pressure of the bulk crosslinked structure, which further supports the less-dense, ‘brushy’ nature of the gradient layer. Sliding experiments showed enhanced lubrication and strong speed dependence on the friction, but no dependence on the contact area. The lack of transient friction behavior also demonstrated the quick rehydration of the gradient layer during the phases out of contact. Indentation, creep, and sliding experiments were also conducted on four other hydrogels of varying monomer and crosslinker percent in order to determine the effect of composition on the gradient layer. All compositions exhibited a poroelastic gradient layer that followed the evolving contact model previously formulated. Stiffer compositions had thinner gradient layers whose response under shear was less consistent with changes to load and speed. Finally, we found that changing the monomer-to-crosslinker ratio was more effective at changing the consistency of the shear response and the gradient layer thickness. These findings prove that the dilute structure of the gradient layer provides enhanced lubrication under sliding conditions by improving hydrodynamic lubrication and reducing polymer-probe interactions. This knowledge can potentially be used to create hydrogels with a stiff load-bearing bulk that retains optimal lubrication across a wide range of operating conditions.
Issue Date:2021-10-08
Rights Information:Copyright 2021 Christopher Lin Johnson
Date Available in IDEALS:2022-04-29
Date Deposited:2021-12

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