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.
