Doctor of Philosophy
School of Mechanical, Materials and Mechatronic Engineering
Xin, Hai, Mechanism of Toughening in Hydrogels, Doctor of Philosophy thesis, School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, 2014. https://ro.uow.edu.au/theses/4231
Tough hydrogel materials have recently attracted broad attentions and interests. Due to their high fracture energies, they are promising candidates for a wide range of biological and biomechanical applications. Two important categories of tough hydrogels are Double Network hydrogel (DN) and ionic-covalent PAAm-alginate hybrid gel (abbreviated as the “hybrid gel”). Both gel systems exhibit interesting network structures and remarkably enhanced mechanical toughness with respect to the conventional covalently-crosslinked hydrogels. The objective of the present PhD study is to provide a quantitative analysis on the toughening mechanism and damage process for DN and the hybrid hydrogels. The investigation for each of the two hydrogel systems consisted of two sections. The first section was to study the effect of hydrogel network topologies on the mechanical properties. Hydrogel samples were synthesized with various chemical formulations to alter their network topologies. Tensile and tearing tests were then performed to characterize the mechanical strength and toughness of the resultant gels. In the second section, load/unload tests comprising a series of stretch/retract cycles were carried out, and some widely-used fracture models and theories were applied on the test results to determine the damage process for the tested hydrogel samples.
Chapter 1 briefly introduces the research background, scope, objective, and methodology, and gives a literature review on the recent progress on the tough hydrogels. Chapter 2 investigates the influence of the 1st network topology on the mechanical properties of DN gels. With an increase in the crosslinker or monomer concentration to prepare the 1st network, mechanical strength and fracture energies of the DN gels increased. Lake-Thomas theory was applied to estimate the 1st network toughness at relevant DN equilibrium, and a linear relation between the 1st network and DN toughness was found. Chapter 3 presents the load/unload tests performed on DN gels prepared with various crosslinker or monomer concentration of the 1st network. Gent and Wang-Hong model were fitted to the test results and a fracture process of DN gels was determined. The model parameters of Wang-Hong model, which are related to the probability distribution of strand fracture, were considered to describe the molecular weight distribution of the 1st network strands. The model fit parameters were used to calculate the strand length distribution of the DN gels prepared with various crosslinker concentrations of the 1st network.
For the hybrid gel system, Chapter 4 elucidates the effect of tight (alginate) and loose (PAAm) network on the mechanical properties of PAAm-alginate hybrid gels. It was found that the hybrid gels were toughened by increasing the concentration of ionic crosslinker or decreasing the concentration of covalent crosslinker. More interestingly, the fracture toughness of the hybrid gel was found to be rate-dependent, and the gel also exhibited a very rapid and substantial stress relaxation which is distinctive from the conventional hydrogels. In Chapter 5, a three series of load/unload testes were sequentially conducted on the virgin PAAm-alginate hybrid gel and the damaged gel after a 1st and 2nd recovery process. The damage process of the hybrid gel was clearly demonstrated by the Gent and Wang-Hong model fits. For the tested gel, a partial recovery of the mechanical properties was evident between the 1st and 2nd series while the mechanical performances became fully reversible from the 2nd to 3rd series. All the research findings were summarized in Chapter 6 and some suggestions on the future work were also given.