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

Abstract

Four EBE copolymers (E ) oxyethylene repeat unit, B ) oxybutylene repeat unit) based on a B20 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 (óy) for aqueous gels formed at
copolymer concentrations below 20 wt %. The phase diagrams contained a region of hard gel (G¢ > G¢¢,
high G¢ and óy), an extensive region of soft gel ((G¢ > G¢¢, low G¢ and óy), 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, Im3hm) structure. The
structure of dilute gels near to the hard gel boundary was not defined, although the Im3hmand Pm3hn cubic
structures were eliminated. Depending on concentration, the soft gels were assigned either to percolationinduced
fractal structures (soft gel 1) or to defective cubic structures (soft gels 1 and 2).