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

Abstract

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.

Bridging experiments and theory: isolating the effects of metal–ligand interactions on viscoelasticity of reversible polymer networks

“…55 In Ni(II), besides acetoacetate ligands, water molecules can also participate in the coordination to stabilize the octahedral geometry. 56 To gain insight into how Ni(II) interacts with ligands in the polymer, we grew single crystals of Ni(II) coordinated with the ligand contributing monomer 2-(acetoacetoxy)ethyl methacrylate (NiAAEMA), and characterized the crystal structure by X-ray diffraction (Fig. S4, ESI †).…”

Interfacial Adhesion of Fully Transient, Mussel‐Inspired Hydrogels with Different Network Crosslink Modalities

“…[1][2][3][4] In synthetic materials, reversible crosslinks mimicking those in mussels are often introduced into a covalently crosslinked polymer network, offering new handles to tune toughness, adhesion, and self-healing. [5][6][7][8][9][10][11][12][13] For example, incorporation of Fe-DOPA complexes into elastomers increases their bulk strength and toughness, [14] where enhancements in toughness arise from the breaking and reforming of reversible crosslinks. [5,6,14,15] However, the permanent structure of covalent hydrogels can limit their use for certain applications like injectable medical treatments, [16,17] (bio)adhesives, [18] or mechanically Specifically, by employing macroscale adhesion tests with glass and nickel probes and 4-arm poly(ethylene glycol) polymers (4PEG, Figure 1b) functionalized with histidine or nitrodopamine (nDOPA), we find a clear correlation between the percentage of tris-crosslinking and the normalized peak adhesive stress of a metal-coordinated, transient hydrogel network.…”

“…[ 1–4 ] In synthetic materials, reversible crosslinks mimicking those in mussels are often introduced into a covalently crosslinked polymer network, offering new handles to tune toughness, adhesion, and self‐healing. [ 5–13 ] For example, incorporation of Fe‐DOPA complexes into elastomers increases their bulk strength and toughness, [ 14 ] where enhancements in toughness arise from the breaking and reforming of reversible crosslinks. [ 5,6,14,15 ] However, the permanent structure of covalent hydrogels can limit their use for certain applications like injectable medical treatments, [ 16,17 ] (bio)adhesives, [ 18 ] or mechanically reversible materials.…”

Enabling Tunable Hydrophilicity of PDMS via Metal-ligand Coordinated Dynamic Networks

“…Furthermore, when increasing the strain, PI-Zn(BF 4 ) 2 shows pronounced softening after yield, implying chains slide past each other easily, resulting in a continuous flow of the material to release the stress. [49] On the contrary, DIP-Zn(BF 4 ) 2 breaks at quite a low strain, ϵ ≈ 0.2, without softening, implying the chains are less mobile. The more dynamic behavior of PI-Zn(BF 4 ) 2 compared to DIP-Zn(BF 4 ) 2 can also be seen from the stress response under cyclic loading.…”