Tuning the Mechanical Properties of Metallopolymers via Ligand Interactions: A Combined Experimental and Theoretical Study

Metal–ligand interactions provide a means for modulating the mechanical properties of metallopolymers as well as an avenue toward understanding the connection between cross-link interaction strength and macroscale mechanical properties. In this work, we used nickel carboxylate as the tunable cross-linking interaction in a metallopolymer. Different numbers and types of neutral ligands that coordinate to the metal center are introduced as an easy approach to adjust the strength of the ionic interactions in the nickel carboxylate cross-links, thus allowing macroscale mechanical properties to be tuned. Density functional theory (DFT) calculations, with the external forces explicitly included (EFEI) approach, were used to quantify how the number and type of ligands affect the stiffness, strength, and thermodynamic stability of the nickel carboxylate cross-linking interactions. Interpreting the bulk material properties in the context of these DFT results suggests that the stiffness and strength of the cross-linking interactions primarily control the initial stiffness and yield strength of the metallopolymer, while the mechanical behavior at higher strain is controlled by dynamical bond re-formation and interactions with the polymer environment. The physicochemical insight gained from this work can be used in the rational design of metallopolymers with a wide scope of targeted mechanical properties.