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From bond breaking to material failure in soft polymer networks

Periodic Reporting for period 3 - SOFTBREAK (From bond breaking to material failure in soft polymer networks)

Reporting period: 2019-06-01 to 2020-11-30

The microscopic mechanisms that lead to mechanical failure of soft polymer materials are still poorly understood. The main reason for this is a lack of experimental tools to prepare well-controlled model systems and to observe the failure process in real time at the microscopic scale. Here, I propose to fill this gap by taking a multidisciplinary approach that combines innovative chemical tools with state-of-the art physical experiments and modelling.

To visualize how the failure process proceeds, we will make use of recently developed mechanosensors, molecules that change colour in response to a force or that emit light when they break. These chemical tools will allow us to map in real time the spatial distribution of both strains and bond rupture events. Together with computer simulations carried out in parallel, this will give us unprecedented insight in the microscopic processes that occur during failure of the material, from the very first bonds that rupture, to the gradual accumulation of damage, all the way to macroscopic failure. We will use this to address the following unresolved questions about failure of polymer networks:
1. What is the microscopic mechanism that leads to delayed failure of polymer networks at subcritical loads?
2. How does the initiation of failure depend on the material's heterogeneity?
3. How does failure occur in a network with transient (viscoelastic) bonds?
We developed two methods to visualize spatially varying strains in materials : (i) Polymeric strain sensors based on FRET pairs that act as single molecule force sensors, and (ii) a multiple light scattering-based technique called laser speckle imaging. With the latter technique, we have visualized the cascade of events in a polymer material undergoing delayed failure.

We also developed network-based developed simulation methods to study fracture in diluted networks and in double networks. These simulations were compared to experiments on collagen/hyaluronic acid mixtures.

Finally, we studied failure in colloidal gels, using bothg experiments and simulations and discovered a novel mechanism leading to yielding in these gels.
New methods to study the microscopic patterns leading to failure in polymer materials; novel simulation methods; insights in the non-linear mechanics of doubl enetworks; discovery of a new failure mechanism in colloidal gels.