Gelation of concentrated micellar solutions of a triblock copolymer of ethylene oxide and styrene oxide, S5E45S5.

Triblock copolymer S5E45S5 was synthesized by oxyanionic polymerization of styrene oxide initiated by a preformed difunctional polyethylene glycol. Here E denotes OCH2CH2, S denotes OCH2CH(C6H5), and the subscripts denote number-average block lengths in repeat units. Previous work on the closely related copolymer S4E45S4 indicated that micelles would form in aqueous solutions of copolymer S5E45S5, and that they would undergo transient intermicellar bridging. Dynamic light scattering was used to confirm this. Rheometry and small-angle X-ray scattering were used to explore gel boundaries, structures, and properties. At moderate copolymer concentrations (14 and 20 wt %) measurements of the dynamic shear moduli indicated the formation of low-modulus soft gels attributed to spherical micelles forming transient networks. A region of low storage modulus at c approximately 30 wt % preceded a change to hard gel. A 40 wt % hard gel was disordered, while at higher concentrations (49 and 60 wt %) the micelles packed into hexagonal structures with high values of the storage modulus (G' approximately 10 kPa at 25 degrees C and 1 Hz).