Rheological Behavior of Aqueous Micellar Solutions of a Triblock Copolymer of Ethylene Oxide and 1,2-Butylene Oxide: B10E410B10

Abstract

A triblock copolymer of ethylene oxide and 1,2-butylene oxide, denoted B10E410B10, was prepared by sequential oxyanionic polymerization and characterized by 13C NMR spectroscopy and gel permeation chromatography. Micellization and the formation of micelle clusters in dilute aqueous solution, the latter a consequence of micelle bridging, was confirmed by dynamic light scattering, and average association numbers of the micelles were determined by static light scattering for T = 20-40 degrees C. The frequency dependence of the dynamic storage and loss moduli was investigated for solutions in the range of 5-20 wt %. Comparison with results for poly(oxyethylene) dialkyl ethers (10 wt %, T = 25 degrees C) indicated that the viscoelasticity of a copolymer with terminal B10 hydrophobic blocks was roughly equivalent to one with terminal C14 alkyl chains. The temperature dependence of the modulus was investigated for 15 wt % solutions at T = 5-40 degrees C. Superposition of the data led, via an Arrhenius plot, to an activation energy for the relaxation process of -40 kJ mol(-1). The negative value contrasts with the positive values found for poly(oxyethylene) dialkyl ethers and related HEUR copolymers with urethane-linked terminal alkyl chains. This difference is attributed to the block-length distribution in copolymer B10E410B10, whereby the activation energy of the relaxation process has a positive contribution from the disengagement of B blocks from micelles but a negative contribution from micellization. The negative value of the activation energy for solutions of B10E410B10 was confirmed by determining the temperature dependence of the zero-shear viscosity of its 15 wt % solution.