The self-assembled transient networks constitute a class of complex materials forming spontaneously 3D networks at thermodynamical equilibrium, that can transmit transiently elastic stresses over macroscopic distances. These physical gels are generally formed of a network of polymer chains reversibly linked in a solvent. The transient nature of the junctions allow them, contrary to cross-linked networks, to relax the constraints by dissociation an reformation of the junctions: so they are able for instance to self-repair after dammage. Because of their spectacular viscoelastic properties, these materials have many applications . From the experimental results obtained with an experimental model this lecture will consider successively some general interesting physical properties for this class of of systems:(i) the pair potential induced by two beads reversibly linked by telechelic polymers, (ii), the phase behavior of the gels, (iii) the linear viscoelastic properties, and (iv) the nucleation and propagation of a fracture in this complex fuid.

Pendant drop experiments performed with a model physical gel (made from oil in water microemulsion droplets reversibly linked together by triblock copolymers), exhibit a very peculiar filament rupture corresponding to highly brittle failure of a viscoelastic fluid. Indeed, we show that a large class of viscoelastic fluids, i.e., transient networks, are brittle according to the Griffith’s theory of solid fracture. However, contrary to solids, cracks are intrinsic to the material arising from the equilibrium nature of the fluid microstructure. The brittleness of these fluids comes from thermal fluctuations of bonds distribution. In this approach, the rupture stress is predicted to be on the order of the Young modulus, in very good agreement with experimental values. The fracture propagation has been tracked by high speed videomicroscopy. Analysis of the time evolution of the fracture profile shows that the fracture is purely elastic and reversible without any significant bulk and interfacial viscous disspation . This behavior is well explained by the viscoelastic trumpet model of de Gennes. The velocity of the fracture’s propagation is measured and compared to the predictions of a simple microscopic model.

We report on a new class of self-assembled transient networks made of surfactant micelles of tunable morphology (from spheres, to rodlike to wormlike) reversibly linked by telechelic polymers. Linear rheological measurements show that three distinct domains can be defined depending on the morphologies of the micelles: a domain where the micelles are isolated and not entangled, an intermediate domain where the micelles are partially entangled and a domain where the micelles are fully entangled. Flow curves of the transient networks of tunable morphology suggest that one can associate to the three domains distinct failures modes: a brittle mode, an intermediate mode and finally a ductile/shear banding mode, as the micelles grow. Thanks to this unique class of self-assembled networks, a continuous failure mode transition from brittle to ductile has been evidenced.

Résumé ( à complèter)

Résumé (à complèter)