Reversible gelation in semidilute aqueous solutions of associative polymers: A small-angle neutron scattering study

Thermally reversible gelation in 1% aqueous solutions of poly(ethylene oxid e)-b-poly(propylene oxide)-b-poly(ethylene oxide)-g-poly(sodium acrylate) h as been observed. The transition that occurs as the temperature is increase d through the gelation threshold (T-gel) is clearly observable by rheologic al and small-angle neutron scattering (SANS) measurements. SANS measurement s suggest that within the hydrogel there exists an unusually uniform matrix of scattering centers, attributed to close packing of micelle-like aggrega tes surrounded by highly hydrated poly(sodium acrylate) and unassociated et hylene oxide/propylene oxide copolymer. The physical cross-linking caused b y these scattering centers induces gelation. Direct model fits to the SANS data suggest that the scattering centers are similar in structure, but some what smaller than micelles of the poly(ethylene oxide)-b-poly(propylene oxi de)-b-poly(ethylene oxide) component, with a dehydrated poly(propylene oxid e) core of radius 30 Angstrom surrounded by a hydrated PEO corona of outer radius 60 Angstrom.