Dynamic and structural studies on synergetic energy dissipation mechanisms of single-, double-, and triple-network hydrogels sequentially crosslinked by multiple non-covalent interactions

A combination of multiple non-covalent interactions has been used to fabricate strong and tough hydrogels. However, mechanisms behind toughening remain unclear. In this study, a series of single-network (SN), double-network (DN), and triple-network (TN) hydrogels sequentially crosslinked by chemical bonds, freeze-thawed polyvinyl alcohol (PVA) crystallites, and ion coordination of alginate chains are systematically prepared. The contributions of hydrogen bonding, entanglements, crystallite crosslinking, and ion coordination to the dynamics of the hydrogels are comparatively studied. Dynamic mechanical analysis (DMA) reveals the apparent activation energy of hydrogen bonding from 41.1 kJ/mol to 50.6, 56.8, and 65.5 kJ/mol for SN gels with increasing PVA concentration (from 0 to 2, 6, 10 wt/vol%, respectively), whereas the corresponding apparent activation energy of chain entanglements increases from 148.3 to 160.2, 163.5, and 167.0 kJ/mol. PVA crystallites formed upon freeze-thawing act as physical crosslinks to further improve the strength and toughness of gels, as well as the activation energy (251.7 kJ/mol). Subsequent ion coordination with alginate further enhances the activation energy to 304.7 kJ/mol (Cu2+), 309.3 kJ/mol (Fe3+). Structural evolution studies on pre-cracked hydrogels reveal that PVA crystallites render crack blunting upon stretching. Polarized optical microscopy and scanning electron microscopy reveal the rearrangement and reconstruction of internal structures during crack propagation, including orientation and alignment of PVA crystallites. This study provides first insights to synergetic energy dissipation by non-covalent interactions of multi-network hydrogels.