Structure and dynamics of cylindrical micelles at equilibrium and under shear flow

The dynamics and rheology of semidilute unentangled micellar solutions are investigated by Langevin dynamics mesoscopic simulations coupled to a microreversible kinetic model for scissions and recombinations. Two equilibrium state points, differing by the scission energy and therefore by the corresponding average micelle length, have been examined. The kinetic rates are tuned by an independent parameter of the model, whose range is chosen in such a way that the kinetics always strongly couple to the chain dynamics. Our results confirm, as predicted by Faivre and Gardissat, that the stress relaxation, as well as the monomer diffusion, is characterized by a time tauLambda, defined by the lifetime of a segment Lambda, whose Rouse relaxation time is equal to its lifetime. Moreover, the power-law dependence of the zero-shear viscosity versus tauLambda was evidenced. Under stationary shear, the chains are deformed and their average bond length is increased, which enhances the overall scission frequency. In turn, this induces an overall shortening of the chains in order to increase the overall corresponding chain-end recombination frequency, as required by the stationary conditions. Nonequilibrium simulations show that the chain deformation and orientation, as well as the rheology of the system, can be expressed as universal functions of a single reduced shear rate betaLambda=gammatauLambda (with gamma the bare shear rate). Furthermore, local analysis of the kinetics under stationary shear gives insights on the variation of the average length with shear rate.