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Context
Soft materials find widespread applications in engineering and life sciences due to a combination of attractive properties. Elastomers used in engineering are ideal when large deformations are needed without damage such as tyres, seals, vibration dampers or soft adhesives. In the life sciences the most common soft materials are hydrogels, which can mimic living tissues and be used in vivo in addition to many biomedical applications such as controlled drug release. As societal demand for better performance with lighter weight uptake in engineering, and new biomedical applications develop, it is desirable to have a high level of mechanical strength and fully reversible deformation without permanent damage, while maintaining the other functional properties such as biocompatibility, swelling and deswelling capability for drug release, chemical or UV resistance.
As recently reviewed by Creton and Ciccotti, these attractive properties of reversible deformability of many soft materials are due to their structure, based on a chemically or physically connected network of flexible and mobile polymeric chains that change entropy upon deformation, creating weak and long range restoring forces. However, reversible deformability is limited by the occurrence of fracture whereby a macroscopic crack propagates through the material. Fracture of soft materials, with elastic moduli in the kPa to MPa range, is still a poorly understood complex process involving localized large viscoelastic deformations, molecular damage and very heterogeneous stresses all dissipating energy during the propagation of a crack. The dissipative mechanisms are of three general types: friction between monomers (active only in elastomers), relaxation of pendant polymer chains (active in gels and elastomers) and fracture of stretched chains by bond breakage. Our project applies state of the art methods to understand how these dissipative mechanisms are active and couple in front of crack tips and how they use the molecular structure of soft materials to control fracture toughness.
Summary
The ground breaking objective of this proposal is to develop a comprehensive picture of the mechanisms of dissipation in front of the tip of cracks propagating in soft materials. To achieve this ambitious goal, we will rely on a combination of newly developed soft materials[1,2] (elastomers and hydrogels) containing a well controlled and tunable fraction of breakable internal bonds and combine mechanical tests in homogeneous samples with a detailed analysis of the zone in front of the crack tip. The stress on the polymer backbone will be directly mapped with mechanophore molecules[3] and the breakage of bonds will be mapped with mechanoluminescent molecules[1]. These spatially resolved molecular insights, never used systematically on cracks in soft materials before, will be complemented with the mapping of local strain by digital image correlation[4] and of the local average molecular orientation by small angle X-ray scattering with the same spatial resolution.
While many new tough materials have been reported, their development has been for the most part due to chemistry groups and empirically guided. Our project will specifically focus on the molecular mechanisms by which energy is dissipated during polymer fracture in soft materials. We will rely on my own experience on the molecular interpretation of the mechanical properties of polymers and on an interdisciplinary team of senior staff spanning an exceptionally wide breadth of competences in mechanics, physics and chemistry. The outcome of the measurements will be used to build multi-scale physically based models in collaboration with solid mechanics groups to relate in a quantitative way the molecular and microscopic structure of soft materials to their fracture toughness. These models will provide invaluable guidance to the synthetic chemists in the development of novel soft and high strength materials for biomedical, and high-tech applications and strengthen Europe’s leadership in the field
References
(1) Ducrot, E.; Chen, Y.; Bulters, M.; Sijbesma, R. P.; Creton, C. Science 2014, 344, 186.
(2) Zhao, X. Soft Matter 2014, 10, 672.
(3) Zhang, H.; Chen, Y. J.; Lin, Y. J.; Fang, X. L.; Xu, Y. Z.; Ruan, Y. H.; Weng, W. G. Macromolecules 2014, 47, 6783.
(4) Mzabi, S.; Berghezan, D.; Roux, S.; Hild, F.; Creton, C. Journal of Polymer Science: Polymer Physics 2011, 49, 1518.
Further Reading
Creton, C. and M. Ciccotti, Fracture and Adhesion of Soft Materials. Reports On Progress In Physics, 2016. 79(4): p. 046601.