Tuning Dynamic Mechanical Response in Metallopolymer Networks through Simultaneous Control of Structural and Temporal Properties of the Networks

Tunable mechanical response under dynamic and static loading is desirable for many technological applications. Traditionally, mechanical performance of polymeric materials is controlled by modulating structural (i.e., molecular weight, chain packing, or cross-link density) or temporal parameters (such as kinetics of the exchange of dynamic cross-linkers). Metal-ligand interactions are uniquely suited to control both structural and temporal parameters as the thermodynamics and kinetics of mechanically active cross-linkers can be varied by careful selection of metal without significant synthetic modification of the polymer backbone. Here, we have demonstrated that it is possible to engineer desired mechanical properties in a metallopolymer with a high degree of tunability by simply changing the type and amount of added metal. Specifically, we cross-linked an imidazole-containing brush copolymer system with the divalent cations of zinc, copper, and cobalt. Using rheology and tensile experiments, we have correlated the emergent mechanical properties to the stoichiometric ratio of ligand to metal as well as the coordination number and ligand exchange mechanism of the imidazole-metal cross-links. In contrary to the general view that unbound free ligands are normally regarded as mechanically inactive dangling chains in metallopolymer networks, this study clearly shows that they can play a critical role in stress distribution and chain relaxation. Importantly, this work shows for the first time that it is possible to simultaneously control both the structure of networks and the temporal response of bulk materials using dynamic association of weak and monodentate ligands with transition metals.