Thermoreversible Growth of Viscosity upon Heating of the Aqueous Solutions of Ethylene Oxide-Propylene Oxide Block Copolymers Unaccompanied by Gelation

The aqueous solutions of ethylene oxide-propylene oxide block copolymers (poloxamers) with a molecular weight of (2.2–15.0) × 10³ and a relative content of propylene oxide units of 19.9–57.0 mol % are studied by rheoviscometry and dynamic light scattering. The temperature and concentration dependences of the properties of solutions are investigated, and the features of their thermoreversible behavior in a wide range of polymer concentrations (4.7–47.2 vol %) and temperatures (15–75°С) are described. A novel type of the thermoreversible behavior previously unknown for these objects is distinguished which consists in the extremum change in the dynamic viscosity with increasing temperature. This behavior is analyzed within the framework of concepts regarding the micellar structure of solutions and is associated with a change in the hydrodynamic size of poloxamer micelles. The effect of molecular weight, ratio of comonomer units, and temperature on critical poloxamer concentrations, limiting manifestation of the novel type of the thermoreversible behavior of solutions, is characterized quantitatively.