Viscoelastic Properties of Metallo-Supramolecular Networks: Relationship Between Sticker/Entanglement Dynamics and Terminal Relaxation

We investigate the linear viscoelastic properties of poly(n-butyl acrylate) (PnBA) metallo-supramolecular networks in the melt state, based on bis-complexes formed by terpyridine (tpy) ligands and divalent metal ions. These networks are constructed from telechelic stars, entangled or not, bearing tpy chain-ends. The nature of the metal ions is varied (Zn(II) and Cu(II) ions, blended or not) as well as the used amount (stoichiometric amount or excess). By systematically varying the network composition (building block architecture and ion content), we elucidate the influence of stickers on terminal relaxation with a focus on the interplay between sticker and entanglement dynamics. The delayed terminal relaxation of the transient networks is rationalized by the means of modified Rouse and time-marching algorithm (TMA) tube models. Our theoretical analysis supports a multistep relaxation mechanism involving repeated dissociation/reassociation events. It also allows us to extract an intrinsic value of approximately 40 kJ/mol for the activation energy for the dissociation of Zn²⁺/tpy bis-complexes, regardless of the building block topology or the ion content. In the presence of entanglements, this activation energy cannot be found by only looking at the temperature dependence of the terminal relaxation time, which demonstrates the importance of separating the sticker and entanglement dynamics to unravel the viscoelastic response of these transient networks.