Rheology and Structures of Aqueous Gels of Triblock(oxyethylene/oxybutylene/oxyethylene) Copolymers with Lengthy Oxyethylene Blocks

Four EBE copolymers (E = oxyethylene repeat unit, B = oxybutylene repeat unit) based on a B 20 central block and end blocks ranging from 58 to 260 E units were studied. Rheological measurements (oscillatory and steady shear) were used to confirm gel boundaries (previously explored by a tube inversion method) and to determine storage and loss moduli ( G ‘ and G ‘ ‘) and yield stresses ( σ y ) for aqueous gels formed at copolymer concentrations below 20 wt %. The phase diagrams contained a region of hard gel ( G ‘ > G ‘ ‘, high G ‘ and σ y ), an extensive region of soft gel (( G ‘ > G ‘ ‘, low G ‘ and σ y ), denoted 1, and a second region of soft gel at low temperatures, denoted 2. Small-angle X-ray scattering was used to probe the hard gel structure. Concentrated aqueous gels (20 wt %) had a body-centered cubic (bcc, Im 3̄ m ) structure. The structure of dilute gels near to the hard gel boundary was not defined, although the Im 3̄ m and Pm 3̄ n cubic structures were eliminated. Depending on concentration, the soft gels were assigned either to percolation-induced fractal structures (soft gel 1) or to defective cubic structures (soft gels 1 and 2).