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.