Mechanics of Tough Chemically Crosslinked Hydrogels

摘要

Double-network (DN) hydrogels developed by Gong et al. (Advanced Materials 2003, 15, 1155) are interesting polymeric materials that despite their large water content (ca. 90 wt%) possess excellent strength and toughness. Those gels can undergo large deformations and exhibit intriguing mechanical behavior such as necking in tensile loading and idealized Mullins effect.;DN hydrogels are the product of free radical polymerization of a water-soluble monomer like acrylamide (AAm) inside a highly crosslinked polyelectrolyte network like poly(2-acrylamido-2-methylpropsnesulfonic acid) [poly(AMPS)]. That polymerization process can be done with or without using a cross-linking monomer. Therefore, DN hydrogels were first thought to be interpenetrating polymer networks (IPNs) or semi-IPNs (SIPNs).;The main objective of this dissertation was to understand the structure-property relationships in DN hydrogels and develop a model to capture their mechanical behavior.;The experimental part of this study involves synthesis and characterization of tough chemically crosslinked hydrogels based on the DN concept and performing mechanical tests on them.;A physical picture was developed to describe necking phenomenon in DN hydrogels. It was found that the necking phenomenon is triggered by the damage of the first network and necking occurs at the onset of load transfer from the first network to the second one. By providing experimental evidence, it was discovered that in DN hydrogels there is a covalent grafting between first and second networks and more importantly that grafting is necessary for achieving toughness. Therefore, DN hydrogels are not true IPN or SIPN structures and depending on whether crosslinking agent is used in the second polymerization step or not, the actual microstructure of a tough DN hydrogel is either a pseudo-IPN or pseudo-SIPN, respectively, where the prefix pseudo denotes connectivity of the two networks.;Crack propagation and finite tensile deformation of DN hydrogels with pseudo-SIPNs and pseudo-IPNs architectures were compared. Moreover, the effect of polymerization of a third loosely crosslinked network inside a DN hydrogel was studied and discussed.;In the theoretical part, a continuum damage model was developed to describe the large strain damage elasto-plastic behavior of DN hydrogels under tensile loading. The model was formulated by developing a physical picture of fracture process and incorporating a damage variable to a strain energy density function. The model is consistent with the experimental data and can capture the elasto-plastic behavior of the material without using a yield function. It was shown that a dimensionless parameter which is a ratio of two material parameters controls the behavior of the material. Those material parameters can be related to the elastic moduli of the first and second networks and in a fundamental level can be attributed to the crosslink densities of the first and second networks. The model can capture the stable branch of material response during necking when the engineering stress becomes constant during neck propagation.