2009 Volume 58 Issue 5 Pages 255-260
We examined the effects of temperature on the rheological behavior of worm-like micelles in a nonionic surfactant system consisting of polyoxyethylene (10) phytosterol (PhyEO10)/glyceryl monocaprylate (GFA-C8)/Water. First, the phase diagram of a PhyEO10/GFA-C8/Water system was examined when the weight ratio, R, of GFA-C8 to the total surfactants was changed keeping the total concentration of the surfactants at 5 wt%. The formation of worm-like micelles was confirmed over a wide temperature range. Next, the effect of temperature on the rheological properties of the worm-like micelles was examined. From steady-flow viscosity measurements of the worm-like micelles, it was found that the zero-shear viscosity (η0) gave a maximum value more than 1,000 times greater than the minimum value in the temperature range 20 - 50°C. Further, the temperature at which the maximum η0 was observed decreased with increasing R value. These results indicate that there is an optimal temperature at which the entanglement of worm-like micelles is at its greatest. From dynamic viscoelasticity measurements, it was shown that the viscoelastic behavior observed for the worm-like micelles was consistent with the Maxwell model, which is the basic model for a viscoelastic body. In addition, the plateau modulus (G0), which reflects the volume fraction of entangled worm-like micelles, gradually increased with increasing temperature, while the relaxation time (τ), which reflects the disentanglement time of the worm-like micelles, rapidly decreased with increasing temperature. From these results, it was clarified that, for a nonionic worm-like micelle, τ influences the change in η0 more strongly than G0.
