Abstract
Bulk moduli, densities, and heat capacities of aqueous solutions of a triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) copolymer with average composition (EO) 28 (PO) 48 (EO) 28 (where EO is ethylene oxide, PO is propylene oxide) and homopolymers with average compositions (EO) 133 and (PO) 37 have been measured as a function of temperature and concentration. Block copolymers form micelles with increasing temperature. Micellization is shown to be an endothermic transition with a ΔH of 50 ± 4 J g -1 of copolymer and with an increase of the apparent copolymer volume of 0.029±0.003 ml g -1 . The measured storage bulk moduli show that the dissolved unimer structures are nearly incompressible. The modulus decreases during micellization for copolymers or phase separation for poly(propylene oxide) (PPO). A simple two-phase model of micelles with a core of pure PPO and a mantle of water-swollen poly(ethylene oxide) chains describes both the measured bulk moduli and volume properties well. The relaxation time for unimer to micelle dynamics has been estimated from the loss bulk modulus maximum with temperature to be approximately 1 μs at 20° C.
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