Résumé
We survey the linear viscoelasticity of a new type of transient network:
bridged wormlike micelles, whose structure has been characterized recently
[Ramos and Ligoure, (2007)]. This composite material is obtained by adding
telechelic copolymers (water-soluble chains with hydrophobic stickers at each
extremity) to a solution of entangled wormlike micelles (WM). For comparison,
naked WM and WM decorated by amphiphilic copolymers are also investigated.
While these latter systems exhibit almost a same single ideal Maxwell behavior,
solutions of bridged WM can be described as two Maxwell fluids components
blends, characterized by two markedly different characteristic times, t_fast
and t_slow, and two elastic moduli, G_fast and G_slow, with G_fast >> G_slow.
We show that the slow mode is related to the viscoelasticity of the transient
network of entangled WM, and the fast mode to the network of telechelic active
chains (i.e. chains that do not form loops but bridge two micelles). The
dependence of the viscoelasticity with the surfactant concentration, phi, and
the sticker-to-surfactant molar ratio, beta, is discussed. In particular, we
show that G_fast is proportional to the number of active chains in the
material, phi beta. Simple theoretical expectations allow then to evaluate the
bridges/loops ratio for the telechelic polymers.