Multi-tool investigation of multiple elastomer networks – MultiNet

Network-forming materials are ubiquitous, from industrial products (tires, foodstuff, cosmetics…) to living organisms (e.g. in the cytoskeleton). The reason behind their extraordinary success is that network-based materials often possess a remarkable combination of desirable properties, such as high reversible deformability, lightweight, optical transparency. These materials find new applications in flexible electronics, biomedical devices and soft actuators and sensors. Empirical design strategies have revealed that the combination of multiple gel networks into multi-networks can yield exceptional mechanical properties, reconciling reversible elasticity with minimal dissipative losses and high strength. These findings are critical regarding the downscaling of devices for various applications.

Understanding the mechanical properties of multi-network gels on the molecular scale is essential for improving the quality of these novel macromolecular architectures. Our project is at the forefront of the research in this field, both with respect to the systems to be investigated (multiple elastomer networks) and in terms of the methods envisioned. In addition to the usual mechanical characterization, two distinct optical methods have emerged in the last years to investigate experimentally the response of networks to stress or strain, to a significant extent thanks to the work by partners of this consortium. First, inserting in the network mechanophores, i.e. mechanically sensitive chemical groups with an optical response to the force acting on them or upon bond breaking. Second, measuring by advanced light scattering techniques that are both space- and time-resolved the local microscopic dynamics.

The first method reveals the spatial distribution of the bond breaking with a spatial resolution on the of the order of 10 µm, by imaging the fluorescent light emission of the mechanophores. The second method quantifies the network rearrangement in response to a damage event. It allows displacements on the nano to micron scale to be detected and mapped spatially with a resolution of 50-200 µm. Our project brings the two methods together. This will allow us to bridge the gap between fast and localized bond scission and the resulting network rearrangement.

We will combine these techniques with molecular dynamics simulations of a multi-network polymer model, to unravel the mechanisms behind the unusual properties that can be achieved in these type of networks. So far, most theoretical approaches are based on strongly coarse-grained finite element or mean-field approaches. These methods cannot shed light on the microscopic mechanisms leading to stress and strain localization by which cracks are nucleated. Molecular-level simulations are thus necessary to access the microscopic level and to describe the effect of the coupling between network strands. The resulting understanding of the local dissipation and stress propagation mechanisms will then be used to implement stochastic finite-element models on the scale of the local rupture events (mesh size), thereby providing new tools for the modelling of multi-networks.

In conclusion, we will use novel experimental techniques and numerical simulations to probe at the micro and mesoscale the fundamental mechanisms that govern the macroscopic mechanical behavior of simple and multiple elastomer networks. An unprecedented combination of modern light-scattering techniques, the insertion of network mechanophores sensitive to bond breaking, and coarse-grained particle-based simulations will allow for a detailed study of the microscopic processes that ultimately lead to the macroscopic behavior of the network, including its failure. This novel approach has the potential to lead to new proposals for the architectural design of these materials.