Mechanical Properties of Epoxy Networks with Metal Coordination Bonds: Insights from Temperature and Molar Mass Variation

We investigate the thermal and mechanical properties of poly(ethylene glycol), PEG, networks with either solely covalent epoxy bonds (single networks, SNs) or coexisting epoxy and iron-catecholate bonds (dual networks, DNs). The latter has recently been shown to be a promising material that combines mechanical strength with significant deformability. Here, we address the previously unexplored effects of the temperature and PEG precursor molar mass on the mechanical properties of the networks. We focus on PEG molar masses of 500 g/mol, where crystallization is suppressed, and 1000 g/mol, where some weak crystals are formed. SNs soften with an increasing PEG molar mass. Heating reversibly softens the DN, but it has a minimal effect on SNs. Nonlinear shear deformation of the DN breaks iron-catecholate bonds, and subsequent recovery upon shear cessation occurs to a long-time steady-state modulus whose value is almost triple the original one, likely due to the formation of tris-complexes versus initial sterically or kinetically trapped bis-complexation. The response under elongation indicates that the DN with sacrificial bonds is stiffer and more extensible than the other networks. These results may provide guidelines for designing dual networks with tunable mechanics at the molecular level.