Light-scattering study on the association behavior of triblock polymers of ethylene oxide and propylene oxide in aqueous solution

Abstract

Static and dynamic light-scattering studies on the temperature-induced micellization behavior in aqueous solutions of poly (oxyethylene-oxypropylene-oxyethylene) block copolymer, poloxamer 188, are presented. A wide temperature range from 21.5 to 78°C was covered in this work. Three temperature regions exist, namely, unimer, transition, and micelle. At room temperatures, i.e., below the critical micellization temperature, molecularly dispersed particles of an average radius of ∼2.3 nm with a broad polydispersity have been detected. In the micelle region, the micellar weights measured are of the order of 10 5 g mol −1 and increase linearly with increasing temperature, while the hydrodynamic radius of micelles remains nearly constant (∼8.0 nm). Such a dual effect of temperature could be interpreted in terms of enhanced dehydration with temperature. The polydispersity of polymolecular micelles becomes much narrower than that of unimers. In the intermediate transition region, the equilibrium unimers ⇌ micelles shifts toward micelles with rising temperature and increasing concentration. Thus a reasonably complete picture about the association process in the aqueous poloxamer system has been given in accordance with the closed association model.