Structure-Property Relationships of Microphase-Separated Metallosupramolecular Polymers

The structural, thermomechanical, and viscoelastic properties of metallosupramolecular polymers (MSPs) can be controlled through the choice of the multiligand monomer and the nature of the metal salt from which these materials are assembled. This versatility and the dynamic nature of certain metal-ligand (ML) complexes make MSPs very interesting for the design of stimuli-responsive materials. We here report on the investigation of the structure-property relationships of MSPs based on a macromonomer formed by terminating telechelic poly(ethylene-co-butylene) (PEB) with 2,6-bis(1'-methyl-benzimidazolyl)pyridine (Mebip) ligands and transition metal or lanthanoid salts. The nature of the metal ion (Zn2+, Fe2+, Tb3+, La3+, or Gd3+), the counterion (trifluoromethanesulfonate (OTf-), perchlorate (ClO4-), or bis(trifluoromethylsulfonyl)imide (NTf2-)), and the number-average molecular weight (M-n) of the PEB core (2100 or 3100 g mol(-1)) were systematically varied with the aim to provide an improved understanding of how these parameters influence the properties. In all MSPs, the polar ML complexes and the nonpolar PEB were found to microphase separate into lamellar or hexagonal morphologies with a soft PEB phase and a ML hard phase. The microstructure formation and the mechanical properties were significantly influenced by the coordination geometry of the metal-ligand complexes as well as the volume fraction of the ML phase. The nature of the metal and counterions further affected the glass or melting transitions of the hard phase. In general, lower softening temperatures were observed for the MSPs made with lanthanoid salts. Measurements of the frequency-dependent oscillatory shear moduli were used to study the relaxation processes in the different MSPs and allowed determining the activation energy of the ML complexes in lanthanoid-based MSPs.