Characterizing the mechanical, transport and adhesion properties of hydrogels using indentation method
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Hydrogels are crosslinked polymer networks that imbibe a large amount of water. Various hydrogels exist both in nature and in engineering applications. The mechanical, transport and adhesion properties of hydrogels are all important properties to characterize. The indentation method is practically suitable for hydrogels, but extracting material properties from indentation tests on hydrogels is still challenging, especially at a small length scale. There is a need to establish a set of indentation methods that considers both the current instrument capability and the hydrogel properties. In this study, robust indentation methods are developed to characterize the mechanical, transport and adhesion properties of hydrogels, which can be applied on instruments such as the atomic force microscope and the micro-indenter. In this thesis, two methods, the dynamic indentation method and the indentation adhesion method are developed for characterizing hydrogels’ poroelastic properties and adhesion properties, respectively. The dynamic indentation method proposes to superimpose a fixed indentation depth with a small-amplitude oscillation and to obtain the poroelastic properties from the frequency-dependent force response. The dynamic indentation method is further applied to several hydrogel samples to probe their swelling-dependent properties. With the measurements, the applicability of a widely used nonlinear thermodynamics model for hydrogels, the Flory-Rehner model, is examined, and a modification to the model is suggested. The indentation adhesion method proposes to conduct indentation tests at a wide range of contact radius for the contact time of interest and to extract adhesion parameters from the results obtained at different contact radii. To establish the method, the adhesion behavior of a model hydrogel is examined at a wide range of contact time and length scales, and the existing contact mechanics models are proved to be not adequate to explain the results. Therefore, a modified model is developed in this thesis to properly extract the adhesion parameters. This indentation adhesion method is further applied to hydrogels with different compositions to study the possible relation between the adhesion parameters and the hydrogel composition.