Rheology of worm-like micelles composed of tri-block copolymer in the limit of slow dynamics

Abstract

We study the influence of micellar kinetics on the rheological behavior of worm-like micelles composed of tri-block copolymers of ethylene oxide and propylene oxide (EO20PO70EO20) in an aqueous solution containing KCl and ethanol. The kinetics of the micelles are adjusted by changing the ethanol concentration, according to a previous study in which the lifetime of the micelles was shown to decrease exponentially with increasing ethanol concentration. At higher ethanol concentrations (15 vol % EtOH), the worm-like micelles behave like Maxwell fluids at low frequencies, but have an upturn at higher frequencies, probably due to Rouse or breathing relaxation modes. At low ethanol concentrations (5 and 8 vol % EtOH) where the lifetime of the micelles is long, the rheological behavior is clearly non-Maxwellian, revealing a spectrum of relaxation times. The slow, block copolymer dependent growth of the micelles leads to scaling of viscosity with surfactant concentration, which varies with time. In this slow breaking regime, stirring of the solutions causes an increase of the viscosity, which slowly decreases once stirring is stopped. This apparent increase of the viscosity may be induced by the linking of ring-like micelles or by the formation of clusters of worm-like micelles (non-equilibrium structures), which disassemble when stirring is stopped.