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Double Dynamics for design of new responsive polymer networks and gels

Periodic Reporting for period 2 - DoDyNet (Double Dynamics for design of new responsive polymer networks and gels)

Reporting period: 2019-11-01 to 2021-10-31

Polymer gels and networks are fascinating versatile soft materials which are ubiquitous in daily life (foodstuff, cosmetics) and high-added value applications (tissue engineering, sensors, drug storage and delivery, portable batteries), and can exhibit both solid-like and liquid-like properties. Classical methods of crosslinking these networks fall into two categories, chemical and physical, and the properties of the respective networks generally encompass two extreme cases: (i) Permanent networks, covalently crosslinked, able to resist flow and creep, and to swell without losing their coherence, but with limitations in terms of their processing and recycling. (ii) Physical networks, which are reversible, easily processable and recyclable, but creep at long times and show residual deformations in loading/unloading cycles. Today, the biggest challenge with this important class of responsive materials is to efficiently combine and control within the same material all distinct features that make them ideal for applications, such as high mechanical strength, large reversible deformability in shear and extension, substantial reversible swelling, and self-healing properties.

The objective of DoDyNet was to address this challenge, inspired by the recent progress made in this field, reached thanks to the development of new types of polymer networks. While different, these new materials share a common concept, which is the combination of at least two distinct dynamic modes within the same material in order to display multiscale viscoelastic responses under the influence of an external stress or deformation field. Consequently, they are characterized by a ‘double’ mechanical behavior, which offers a better balance between the desired properties. To fulfill these objectives, we joined our expertise to design, synthesized, characterize and model the properties of new double polymer networks.

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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.
The strength of our project is the combination of diverse expertise's for the same objective: make the link between sample composition and its macroscopic properties, to develop new, smart, materials, for new applications. The added value of DDNs is potentially very important. Today, they are already used in several fields, but there is still large room for new materials and new applications. To this end, understanding their fundamental structure-dynamics relationships by altering material parameters is of prime importance, as for the development of sustainable energy-saving materials, high-productivity processes or high-throughput responsive materials. The re-usability offered by the DDNs is also a key property that we would like to exploit.
In terms of cost and process, morphological separation along with hydrogen bonded supramolecular associations offers an economically sound approach to introduce DDNs starting from commercial compounds requiring no or little chemical modification.

Beside the industrial impact, a strong collaborative network of researchers in the field of polymer gels and networks has been developed with DoDyNet. Thanks to the different activities, to the presentations of our results at international conferences, or to the active role of the visiting experts, many interactions and new collaborations have started, that we would like to enhance even more in the future!

More news about our projects and about our activities? Please, visit our website, or have a look to our Special Issue on DDNs (Journal of Rheology, June 2022)!
Pressure Sensitive Adhesives
Organization of DoDyNet
Switchable membrane
Breaking mechanisms
Metallo-supramolecular networks