Modulating metallopolymer mechanical properties by controlling metal ligand crosslinking

Abstract

INTRODUCTION Noncovalent interactions such as hydrogen bonds, hydrophobic interaction, and metal-ligand interactions (neutral and ionic) enable networks in both natural systems 1,2 and synthetic materials. [3][4][5][6] These noncovalent networks can potentially mimic classical covalent network behaviors such as elastic recovery from large deformation and swelling in the presence of good solvents, 7,8 while boasting appealing properties such as self-healing, 9,10 processability, 11,12 energy absorption, 13 self-assembly, 14 and stimuli responsiveness. 15,16 This combination of covalent-like and dynamic properties is accomplished by tuning the dynamics of the network connections. Metallopolymers in particular have attracted much attention in the past two decades due to their possible mechanical properties, synthetic methodologies, and processing. 17,18 Metallopolymers are highly customizable since in addition to normal polymer control knobs such as monomer selection, molecular weight and architecture, metallopolymers can be differentiated by where the metal complexes are bound in the polymer architecture (i.e., as crosslinkers, side chain substituent, or part of a linear backbone), and by the type of metal cation and ligand chemistry. 17The strength of the metal complex to resist breaking under mechanical force has been shown to strongly influence the mechanical behavior of metallopolymers. [19][20][21] It was demonstrated that by clever design of supramolecular polymers, metallopolymers synthesized with different metals 19,20,22 or different ligands 21,23 presented diverse organometallic interactions that lead to different stiffness, toughness, and viscoelastic dissipation. These distinct mechanical behaviors can also be controlled by external stimuli. The groups of Holten-Andersen and Meyer showed how changing metal-ligand interactions in metallo-hydrogels by changing the pH, 8 the oxidation state of the metals with UV exposure, 24 or electrochemically 25,26 allowed control over mechanical properties. Recently, Manners et al. showed that polarity and coordination ability of a solvent dictate the dynamic nature of metal-ligand interactions in a poly-nickelocene solution. 27 Here we present an innovative material system in which the addition of copper(II) to a linear elastomer increases strength and decreases ductility through the formation of metal-ligand bonds that behave in a covalent-like manner. Unlike covalent crosslinks, however, the copper-based crosslinks can be weakened and even removed with proper choice of ligands, enabling full thermoplastic-like processing. Furthermore, we show in our final portion that ligands can be incorporated within the solid-state material, essentially removing or making the crosslinks mechanically dynamic, increasing toughness, and enabling self-healing. In contrast to much of the previous work in this field, our polymers are solid even without the metal-ligand bonds, making the dynamic behavior regime, and its applications distinct.Our design strategy for creating an elastomer...

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

“…21,22 Studies that unveil how each of these factors influences the viscoelasticity have been performed both theoretically and experimentally. [23][24][25][26] Various models have been proposed for reversible networks with different architectures. For example, Tanaka and Edwards established a transient network model to describe the dynamics of telechelic polymer gels end-functionalized with associating groups, in which the reversible junctions connecting the chains can contain an arbitrary number of stickers.…”

Highly Stretchable Polymers: Mechanical Properties Improvement by Balancing Intra‐ and Intermolecular Interactions

“…For instance, the elongation at break of highly branched star polymers was found to be greater than that of the linear architecture. Another approach is to create networks by the incorporation of non‐covalent CL such as hydrogen or coordinative bonds between chains . A promising new strategy that is explored in this manuscript, is balancing the inter‐ and intramolecular interactions through the use of chemical bonds to fold polymer chains, forming an architecture commonly known as single chain polymer nanoparticles (SCPNs) .…”

Supramolekulare Metallopolymere: Von linearen Materialien zu infiniten Netzwerken

“…These results bode well for the development of polyMOFs as a unique class of new porous materials. Other carboxylatecontaining polymers have shown the ability to form MOF-like SBUs in presence of Cu 2+ , [43] but did not show evidence of formation of polyMOFs (i.e., lack of crystallinity, porosity). Nonetheless, other carboxylate-containing and metalcoordinating polymers may serve as useful precursors for polyMOF materials.…”