DDN networks based on metallo-supramolecular interactions: An example of DDNs studied within DoDyNet are metal-supramolecular polymer networks. By playing with the nature of the metal ions used to create reversible junctions between the polymer star-like building blocks, the solid-to-liquid transition of the networks can be tuned. In particular, Yanzhao Li (ESR12) showed that blending different types of ions leads to samples with intermediate properties between the two single ions systems. However, as highlighted by Christina Pyromali (ESR5), despite of their fast exchange dynamics, systems with a longer relaxation time are also able to deform more and resist a large shear deformation before partially breaking and reaching a steady viscosity. Similar results were found by Bruno Di Dio (ESR4) on a completely different system, composed of sticky particles. In this case also, the more the particles are sticky, the more they can be brought out of equilibrium before their dissociation.
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The viscoelastic response of metallo-supramolecular networks is also very sensitive to the position and density of the reversible bonds, as shown by Rowanne Lyons (ESR2), who investigated the properties of unentangled sticky polymer chains (see Figure). Thanks to these multiple sites of stickers along the chain backbone, this system showed a very long large elastic memory.
But these samples could not resist high temperatures, at which they behave as viscoelastic liquid materials! To overcome this limitation, two strategies were developed. First, Clement Coutouly (ESR1) introduced polymer blocks of another nature at the extremities of the chain. In such a way, the chain ends phase separate, which strongly enhances the elasticity of the samples. Another strategy was proposed by Larissa Hammer (ESR3) who introduced a loosely crosslinked reversible covalent network within the metallo-supramolecular matrix, able to resist much higher temperature. This sample showed high elasticity and very good shape memory, while keeping its re-processing ability.
In fact, this behavior is very interesting for designing new pressure sensitive adhesive. Our ESR 8, Consiglia Carillo, studied the properties of such adhesives from our associated partner tesa. She developed a model to relate their composition to their elastic and loss moduli.
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Metallo-supramolecular systems are also of interest when diluted in solution. As an example, Paola Nicolella (ESR10) synthesized double networks composed of a tetra-arm Polyethylene glycol that is functionalized at each arm with a terpyridine that can form metal complexes, and with a thermoresponsive Poly (N-isopropylacrylamide), that can undergo phase-separation close to body temperature, making it appealing for biomedical applications. On one side, the thermo-responsiveness of Pnipam makes it possible to be used as a switchable membrane. On the other side, the metal ion complex allows the network to be degradable, since the complex can be broken in several ways.
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How do these double networks break? Tests have been achieved by Jianzhu Ju (ESR13) and PEA multiple networks. He showed that due to the introduction of secondary networks, energy can be dissipated by the breaking of the first network but the material remains intact. As a result, the fracture happens at a much larger strain while at lower strain the damage region can be detected.
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In fracture mechanisms, the architecture of the polymer building blocks plays an important role, as demonstrated by Simone Sbrescia (ESR9), who studied the mechanical properties of thermoplastic elastomers of different length and weight fraction of hard segments, as well as by Wendy Wang (ESR6) who studied the elongation properties of ionomers.
