EFFECT OF SHEAR ON CUBIC PHASES IN GELS OF A DIBLOCK COPOLYMER

The effect of shear on the orientation of cubic micellar phases formed by a poly(oxyethylene)poly(oxybutylene) diblock copolymer in aqueous solution has been investigated using small-angle x-ray scattering (SAX S) and small-angle neutron scattering (SANS). SAXS was performed on sa mples oriented in a Couette cell using steady shear, and SANS was perf ormed on samples subject to oscillatory shear in situ in a rheometer w ith a shear sandwich configuration. A body-centered-cubic (bcc) phase observed for gels with concentrations greater than 30 wt % copolymer w as found to orient into a polydomain structure, with the close-packed {110} planes both parallel and perpendicular to the shear plane. For g els with 30 wt % copolymer or less, a face-centered-cubic (fcc) phase was observed, and this was also observed on heating the more concentra ted gels that formed a bcc phase at room temperature. The fcc phase co uld be oriented to form a highly twinned structure, with a significant deviation from the ABCABC... stacking sequence of the ideal structure due to random sequences resulting from slip of {111} hexagonal-close- packed planes. For the lower concentration gels, a transition from har d to soft gel on increasing temperature was found to be characterized by a change in the susceptibility of the sample to macroscopic shear o rientation, as probed using SAXS. The hard gel could be oriented by sh ear into a twinned fee structure, whereas the soft gel comprised a fee phase with a small grain size, which could not be sheared to form a m acroscopically oriented domain. Shear only homogenized the sample, pro ducing a powder SAXS pattern from a fcc structure. (C) 1998 American I nstitute of Physics.