Effect of ethanol on the gelation of aqueous solutions of Pluronic F127

Abstract

In dilute aqueous solution unimers of copolymer F127 (E 98P 67E 98) associate to form micelles, and in more concentrated solution micelles pack to form high-modulus gels. Cosolvents are known to affect these processes, and ethanol/water mixtures have been of particular interest. Dynamic light scattering from dilute solutions was used to confirm micellization, but major attention was directed towards the gels. Visual observation of mobility (tube inversion) was used to detect gel formation, oscillatory rheometry to confirm gel formation and provide values of the elastic moduli over a wide temperature range, and small-angle X-ray scattering to determine gel structure. The solvents were limited to 10, 20 and 30 wt.% ethanol/water. Critical concentrations for gel formation were similar for 10 and 20 wt.% ethanol/water but were significantly increased for 30 wt.% ethanol/water, e.g. at T = 45 °C from c ≈ 15 wt.% to c ≈ 28 wt.%. The elastic moduli reached maximum values at T ≈ 50 °C: e.g. G′ ≈ 25 kPa for 25 wt.% F127 in 10 and 20 wt.% ethanol/water and a similar value for 30 wt.% F127 in 30 wt.% ethanol/water. Hard gels of 30 and 35 wt.% F127 in ethanol/water at 25 and 40 °C had the body-centered cubic (bcc) structure.