Effect of Ring Mobility on the Dynamics of the Slide-Ring Gels

Random cross-linking in polymeric gels leads to heterogeneities in the obtained network, resulting in poor mechanical properties and limitations of practical applications. To overcome this issue, networks with movable cross-links have been developed. Slide-ring gels (SRGs), which are polymer networks with cross-links that can slide along the network strands, belong to this category. These gels are softer, more stretchable, tougher, and with good recovery properties due to the sliding of cross-links, which equalizes the applied stress on polymer strands. Although the origin of the excellent mechanical properties of these gels has been well-studied, the specific viscoelastic properties of slide-ring gels remain ambiguous due to the complexity of controlling and detecting the ring mobility. In this study, the development of a palladium-based slide-ring gel from a metal-coordinated pseudorotaxane motif enables us, for the first time, to control the ring's mobility without changing any other parameters and keeping the overall network connectivity. The rheology data of fixed-ring versus free-ring gels, analyzed with a tube-based model, reveal that the softness and lower elasticity of the slide-ring gels originate from the sliding of cross-links, which increases the effective mesh size of the network well above the length of a single polyrotaxane chain. Moreover, we demonstrate the key role played by the entanglements, which limit the sliding distance of the rings. This study thus provides new insights for understanding the peculiar viscoelastic behavior and huge stretchability of slide-ring gels, toward the rational design of SRGs with controlled properties.