Reversible Gelation in Semidilute Aqueous Solutions of Associative Polymers: A Small-Angle Neutron Scattering Study

Abstract

Thermally reversible gelation in 1% aqueous solutions of poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide)-g-poly(sodium acrylate) has been observed. The transition that occurs as the temperature is increased through the gelation threshold (Tgel) is clearly observable by rheological and small-angle neutron scattering (SANS) measurements. SANS measurements suggest that within the hydrogel there exists an unusually uniform matrix of scattering centers, attributed to close packing of micelle-like aggregates surrounded by highly hydrated poly(sodium acrylate) and unassociated ethylene oxide/propylene oxide copolymer. The physical cross-linking caused by these scattering centers induces gelation. Direct model fits to the SANS data suggest that the scattering centers are similar in structure, but somewhat smaller than micelles of the poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) component, with a dehydrated poly(propylene oxide) core of radius 30 Å surrounded by a hydrated PEO corona of outer radius 60 Å.