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From bond breaking to material failure in soft polymer networks

Periodic Reporting for period 4 - SOFTBREAK (From bond breaking to material failure in soft polymer networks)

Reporting period: 2020-12-01 to 2021-05-31

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. In this proposal, we aim to fill this gap by combining novel imaging tools with state-of-the art physical experiments and modelling.

To visualize how the failure process proceeds, we use several recently developed techniques, including a multiple scattering-based technique for high resolution strain field imaging and molecular mechanosensors that change colour in response to a force or that emit light when they break. These tools allowed 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 used 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 and disorder?

From our experiments and simulations it followed that delayed failure occurs by a gradual, stress-activated accumulation of damage. Our work on fibrous biopolymer gel networks showed that fracture in dilute, disordered networks can occur in a remarkably different way than in synthetic polymer materials made from flexible polymers: rather than stress localization leading to crack propagation, mechanical failure occurs by the accumulation of diffuse damage patterns, which is governed by highly heterogeneous stress patterns in the material. With these findings we have shed new light on the failure of complex ploymer networks.
- We have developed optical 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 this latter technique, we have visualized the cascade of events in a polymer material undergoing delayed failure. (iii) Microscopic visualization of mechanical waves in colloidal materials.
- 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. We found that both the non-linear elastic response and the failure of such networks follow unusual patterns, with very strong strain hardening and diffuse damage patterns.
- Using both experiments and simulations, we discovered a novel mechanism leading to yielding in colloid-based gels.
- We developed an indentation-based method to obtain high-resolution mechanical maps of heterogeneous materials, and applied this to the characterization of artificial meat products.
- Using coarse-grained multi-scale simulations, we provided microscopic insights in the toughening and failure of double network materials.
All results are published in peer review journals.
This project has led to novel microscopic insights in the process of mechanical failure of disordered polymeric networks, in particular fibrous polymer gels, double polymer networks, and colloidal gels. In addition, we have developed novel tools to visualize the failure process in such materials.

New methods to study the microscopic patterns leading to failure in polymer materials; novel simulation methods; insights in the non-linear mechanics of double networks; discovery of a new failure mechanism in colloidal gels.
Visuazation of damage accumulation preceding delayed failure
Heterogeneous stress distribution in a disordered fiber network