Periodic Reporting for period 4 - chemech (From Chemical Bond Forces and Breakage to Macroscopic Fracture of Soft Materials)
Reporting period: 2021-03-01 to 2022-02-28
Soft materials are irreplaceable in engineering applications where large reversible deformations are needed, and in life sciences to replace a variety of living tissues. While mechanical strength may not be essential for all applications, excessive brittleness is a strong limitation. Yet predicting if a soft material will be tough or brittle from its molecular composition or structure relies on empirical concepts for the lack of proper tools to detect the damage occurring to the material before it breaks. Using model materials containing a variable population of internal sacrificial bonds, breaking before the material fails macroscopically, we have developed quantitative methods to detect chemical bond scission associated with macroscopic fracture of tough soft materials. We have used a combination of scattering and optical techniques such as small angle X-ray scattering, diffusive wave spectroscopy and fluorescent molecular probes to map, stress, strain, bond breakage and structure in a region ~100 µm in size ahead of the propagating crack. We have thus gained an unprecedented molecular understanding of where and when bonds break near the fracture plane as the material fails, establishing some key concepts between the architecture of soft polymer networks and their fracture energy, reistance to cavitation or to crack propagation under cyclic low amplitude loading. These results lead to a new molecular and multi-scale vision of macroscopic fracture of soft materials, that will be invaluable to design and develop better and more finely tuned soft but tough and sometimes self-healing materials to replace living tissues (in bio engineering) and make lightweight tough and flexible parts for energy efficient transport.
The core objective of the ERC CHMECH is the development and implementation of mechanosensitive molecules as sensitive force and damage probe to study the fracture of soft materials including elastomers and hydrogels. Current models of the fracture of soft materials are either fully continuum mechanics based or very simplistic molecular models with very little experimental evidence to compare it with. Direct detection of molecular bond scission has only recently been made available but one key aspect that is still missing from current bond scission detection approaches was the quantification of the data and to a lesser extent the high resolution 3D-mapping. So far we achieved three important successes and several more minor ones.
- We have developed a high resolution quantitative method to detect molecular damage by bond scission in elastomers with confocal laser scanning microscopy (one paper in Science Advances, one in soft matter, one in PNAS and a PRX)
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This has taken a very large effort due to the irreproducible nature of fracture events, even in the presence of a notch the fracture process of a soft elastomer is a complex 3D process and the bond scission is also a spatially very complex process. Yet we have developed methods to compare differential bond scission under load of the same crack and to analyse bond scission post-mortem for different elastomers. The combination of these advances with the development of a reliable calibration method has really now made the method of detection of bond scission useful, and usable in transparent materials for a variety of situations.
- We demonstrated the effect of strain rate and temperature on molecular fracture (a main paper in PRX and two in preparation)
Our direct visualizations have demonstrated that the current molecular models of network fracture completely ignore the stochastic nature of polymer networks and grossly simplify the molecular bond scission process. We have shown that temperature and rate dependent viscoelastic dissipation is closely coupled with bond scission and have now made major advances in understanding the way molecular fracture occurs when cracks grow.
- We made significant progress in understanding the details of the fracture mechanisms of elastomers with interpenetrated networks as model systems for soft tough materials such as filled elastomers. (main paper in PNAS with another in preparation)
Many commercial elastomers are designed with a stiff and a soft phase in order to increase the toughness of the material. We have shown, thanks to the mechanochemical probes that the stiff phase breaks first and progressively transfers the stress to the soft phase by creating a heterogeneous structure with less deformed and more deformed regions in the material. We have also shown that the key mechanisms of toughening is the delocalization of molecular forces upon bond scission. In other words, in tough materials, bonds break more randomly in the material up to much larger levels of applied stress bore they break in a localized and correlated way. We have also shown that multiple networks have a remarkable toughness at high temperature and fatigue threshold.
In the last period we also have achieved two successes in the application of the method to specific fracture problems.
- We have shown that mechanophores can be used to detect molecular damage under cyclic fatigue (up to 400 000 cycles) a key result to extend the method to the non-destructive characterization of materials in use. We have also shown that the molecular structure needed for toughness is not that needed for fatigue (paper in Science Advances)
- We have shown how cavities nucleate and grow as cracks under sudden decompression of elastomers in a high pressure hydrogen atmosphere, an important result for the future use of hydrogen as a fuel source. Paper in Soft Matter.
- We have been able to use mechanophores in hydrogels and in nanocomposites. Although this was a difficult achievement we hope to extend it to different systems in the future. Post-mortem observations were crucial for this particular Project.
- We have developed a high resolution quantitative method to detect molecular damage by bond scission in elastomers with confocal laser scanning microscopy (one paper in Science Advances, one in soft matter, one in PNAS and a PRX)
-
This has taken a very large effort due to the irreproducible nature of fracture events, even in the presence of a notch the fracture process of a soft elastomer is a complex 3D process and the bond scission is also a spatially very complex process. Yet we have developed methods to compare differential bond scission under load of the same crack and to analyse bond scission post-mortem for different elastomers. The combination of these advances with the development of a reliable calibration method has really now made the method of detection of bond scission useful, and usable in transparent materials for a variety of situations.
- We demonstrated the effect of strain rate and temperature on molecular fracture (a main paper in PRX and two in preparation)
Our direct visualizations have demonstrated that the current molecular models of network fracture completely ignore the stochastic nature of polymer networks and grossly simplify the molecular bond scission process. We have shown that temperature and rate dependent viscoelastic dissipation is closely coupled with bond scission and have now made major advances in understanding the way molecular fracture occurs when cracks grow.
- We made significant progress in understanding the details of the fracture mechanisms of elastomers with interpenetrated networks as model systems for soft tough materials such as filled elastomers. (main paper in PNAS with another in preparation)
Many commercial elastomers are designed with a stiff and a soft phase in order to increase the toughness of the material. We have shown, thanks to the mechanochemical probes that the stiff phase breaks first and progressively transfers the stress to the soft phase by creating a heterogeneous structure with less deformed and more deformed regions in the material. We have also shown that the key mechanisms of toughening is the delocalization of molecular forces upon bond scission. In other words, in tough materials, bonds break more randomly in the material up to much larger levels of applied stress bore they break in a localized and correlated way. We have also shown that multiple networks have a remarkable toughness at high temperature and fatigue threshold.
In the last period we also have achieved two successes in the application of the method to specific fracture problems.
- We have shown that mechanophores can be used to detect molecular damage under cyclic fatigue (up to 400 000 cycles) a key result to extend the method to the non-destructive characterization of materials in use. We have also shown that the molecular structure needed for toughness is not that needed for fatigue (paper in Science Advances)
- We have shown how cavities nucleate and grow as cracks under sudden decompression of elastomers in a high pressure hydrogen atmosphere, an important result for the future use of hydrogen as a fuel source. Paper in Soft Matter.
- We have been able to use mechanophores in hydrogels and in nanocomposites. Although this was a difficult achievement we hope to extend it to different systems in the future. Post-mortem observations were crucial for this particular Project.
Although the project is now finished we still have an ongoing ERC-POC on applying mechanophores as a non-destructive technique to detect early damage and we have two ongoing collaborations with EU companies for the application of mechanophores to specific problems. One on the subject of detecting molecualr damage during low intensity frictional stress resulting in wear and the other to detect molecular bond scission damage occurring over time in soft optical fiber coatings. Both collaborations are a direct consequence of the ERC project and the know-how developed.