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 un it) based on a B-20 central block and end blocks ranging from 58 to 260 E u nits were studied. Rheological measurements (oscillatory and steady shear) were used to confirm gel boundaries (previously explored by a tube inversio n method) and to determine storage and loss moduli (G ' and G ") and yield stresses (sigma (y)) for aqueous gels formed at copolymer concentrations be low 20 wt %. The phase diagrams contained a region of hard gel (G ' > G ", high G ' and sigma (y)), an extensive region of soft gel ((G ' > G ", low G ' and sigma (y)), denoted 1, and a second region of soft gel at low temper atures, denoted 2. Small-angle X-ray scattering was used to probe the hard gel structure. Concentrated aqueous gels (20 wt %) had a body-centered cubi c (bcc, Im (3) over barm) structure. The structure of dilute gels near to t he hard gel boundary was not defined, although the Im (3) over barm and Pm (3) over barm cubic structures were eliminated. Depending on concentration, the soft gels were assigned either to percolation-induced fractal structur es (soft gel 1) or to defective cubic structures (soft gels 1 and 2).