Kinetics and dynamics of wormlike micelles under shear

Mimicking giant cylindrical micelles in solution, self-assembled linear polymers are studied by Langevin dynamics in shear flow. The kinetics of chain scission (with rate k_s per bond) and chain end recombination (with rate k_r) is modelled by associating to each monomer two arms for binding. A switch of pair potentials (from bounded to unbounded or vice versa) is tried with a fixed attempt frequency per arm ω modelling the effect of a potential barrier along a reaction path. The opening of a selected bond or the bonding between two selected free ends are accepted or not according to a Monte Carlo algorithm. We are interested in the coupled effects of the shear flow and the scission-recombination kinetics on the structural and rheological properties of this micellar system. Our study is performed in semi-dilute and dynamically unentangled regime conditions with an average chain size L₀. The explored ω range is chosen sufficiently high for the lifetime of the average size chain to remain shorter than its intrinsic (Rouse) longest relaxation time τ_R=τ₀L₀². Central to our analysis is the concept of dynamical unit of size Λ=(τ₀k_s)^{- 1/3}<L₀, the chain fragment for which the lifetime τ_Λ and the Rouse time are equal. Shear thinning, chain orientation and bond stretching are found to depend upon the reduced shear rate , while the average micelle size is found to decrease with increasing shear rate, independently of the height of the barrier of the scission-recombination process.