Report
Teaser, summary, work performed and final results
Periodic Reporting for period 2 - chemech (From Chemical Bond Forces and Breakage to Macroscopic Fracture of Soft Materials)
Teaser
Soft materials are irreplaceable in engineering applications where large reversible deformations are needed, and in life sciences to mimic ever more closely or replace a variety of living tissues. While mechanical strength may not be essential for all applications, excessive...
Summary
Soft materials are irreplaceable in engineering applications where large reversible deformations are needed, and in life sciences to mimic ever more closely or 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. Taking advantage of the recent developments and discoveries, we have developed a ground-breaking investigation of the mechanisms of fracture of tough soft materials. To achieve this objective we have developed a series of model materials containing a variable population of internal sacrificial bonds, breaking before the material fails macroscopically, and use a combination of advanced characterization techniques and molecular probes never used before to map, stress, strain, bond breakage and structure in a region ~100 µm in size ahead of the propagating crack. By using mechanoluminescent and mechanophore molecules incorporated in these model material in selected positions, laser scanning confocal laser microscopy and small-angle X-ray scattering we have gained an unprecedented molecular understanding of where and when bonds break as the material fails and before the end of teh project we hope to establish a direct relation between the architecture of soft polymer networks and their fracture energy, leading for the first time in 50 years to a new molecular and multi-scale vision of macroscopic fracture of soft materials. Such advances will be invaluable to guide materials chemists 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.
Work performed
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 in elastomers with confocal laser scanning microscopy
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This has taken a very large effort due to the irreproducible nature fo 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 advancew wit 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
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 udnerstanding 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 models systems for soft tough materials such as filled elastomers.
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 reduction of the stress concentration near broken bonds. In other words, in tough materials, bonds break more randomly in the material up to much larger levels of applied stress.
Final results
We are targeting several additional advances before the end of the project.
- We would like to transfer the use of mechanophores from hydrophobic elastomers to a hydrophilic environment and into hydrogels through the modification of the chemistry.
This achievement was much more challenging than we expected in particular because the mechanophores that have been developed are all hydrophobic and insoluble in water. We succeeded in synthesising a water-soluble mechanophore but the emission of light in water was much weaker than in an elastomer and the energy transfer did not work in the same way. Using fluorescent mechanophores we are still confident that the methodology will be extendable to water-based hydrogels and open paths to biomedical applications.
- We are hoping to consolidate our early data on bond scission with a vatiery of loading situations and different materials to build a real molecularly based models that could be used to design more robust and fracture resistant soft materials. We hope to validate our results with molecular simulations (throuhg collaborations).
- We are now extending the use of mechanophores to heterogeneous materials such as elastomers filled with nanoparticles (soft and hard) and soft weakly crosslinked adhesives and polymer melts.
- We hope to extend our methodology to the detection of damage in situations where repeated cycles of loading are applied.