Viscoelastic response of a vitrimer diluted in different polymer matrices

The development of ”vitrimers” emerged recently as a very promising solution to the problem of mechanical degradation of plastic waste. In parallel, it has been shown that interpenetrated networks can be developed to obtain hydrogels and elastomers with high mechanical performances. Our work is in line with these two concepts and aims at combining networks with different dynamics in order to better understand the influence of their combination on their viscoelastic properties. We investigate the viscoelastic properties of a poly(n-butyl acrylate) (PnBA) network sparsely crosslinked with slow dynamic covalent bonds (imine-aldehyde bonds), and diluted in different polymer matrices. The latter are either composed of unentangled or entangled linear PnBA chains, or of short building blocks highly crosslinked by metal-ligand supramolecular junctions characterized by fast association/dissociation dynamics. We first study the influence of the different matrices on the partial relaxation and equilibration of the dynamic covalent network through Constraint Release Rouse (CRR) mechanisms. In particular, to understand the role played by the topology of the supramolecular building blocks, we compare the viscoelastic response of the dynamic covalent network diluted in two different metallosupramolecular networks, based on the same metal-ligand junctions, but built from different building blocks, i.e. linear chains containing sticky side-groups or from the association of telechelic star polymers. The results are rationalized in a tube-based model, which allows us to discuss how the CRR time depends on the relaxation time of the matrix. In this model, a statistical approach allows us to predict the proportion of dangling chain segments in the sparsely crosslinked dynamic covalent network. Then, based on creep-recovery experiments, we investigate the ability of the dynamic covalent network diluted in linear matrices and of the double dynamic network to resist creep under a constant load and to recover their initial shape when the stress is removed. We show again the important role played by the polymer matrix. The results are analyzed, based on the CRR time determined in the linear regime, in order to provide guidelines to control the modulus, extensibility and shape memory properties of these materials.