Rheology of thermoreversible hydrogels from multiblock associating copolymers

Multiblock copolymers of poly(ethylene oxide)99-poly(propylene oxide)67-poly(ethylene oxide)₉₉ (F127) were synthesized by chain extending with hexamethylene diisocyanate (HDI). The resulting multiblock copolymer poly-F127 maintained the thermoreversible properties of the original F127 triblock copolymer. The rheological and structural properties of the gels were characterized as a function of temperature, composition and degree of polymerization. Neutron scattering reveals that a large degree of alignment can be induced in the F127 gel, but no long-range order could be found in the multiblocks or the mixtures of F127 with multiblocks. The shear strain at yield in polymers having an average of 3.2 or more F127 repeats was nearly an order of magnitude higher than in the F127 gel. For F127 solutions just below their gel point, substitution of F127 with as little as 1% multiblock succeeded in forming a physical gel. Percolation theory was used to understand the modulus growth when multiblock was added to F127 solutions just below their gel point, assuming the multiblocks form bridges between adjacent micelles.