Shear-induced orientation of the body-centered-cubic phase in a diblock copolymer gel

The effect of steady shear on the alignment of a gel formed by block copoly mer micelles in a body-centered-cubic (bcc) phase has been investigated usi ng small-angle x-ray scattering (SAXS) for samples in a Couette cell. The g els were formed by an amphiphilic poly(oxyethylene)-poly(oxybutylene) diblo ck copolymer in an aqueous salt solution. The micellar hydrodynamic radius and mutual diffusion coefficient were obtained for solutions (micellar sols and gels) using dynamic light scattering. Static light scattering provided the micellar thermodynamic radius and association number. SAXS was used to probe the orientation of the bcc structure as a function of shear rate, an d it was found that dow occurs with a [111] direction of the unit cell coin cident with the shear direction, with twinning of the crystal about this ax is. The SAXS experiments indicated a critical shear rate, (gamma) over dot similar to 50 s(-1) for macroscopic alignment of the gel. This value is low er than that reported in earlier publications from our group, where large-a mplitude oscillatory shear was used to prepare highly oriented samples, pos sibly reflecting a dependence on strain amplitude. The viscoelastic behavio r of the gel was probed using oscillatory shear and highly nonlinear rheolo gy was observed at large strain amplitudes, which was assigned to a stick-s lip mechanism of flow. The magnitude of the mutual diffusion coefficient, c ompared to the relaxation time obtained from the critical shear rate, also indicated a cooperative dow mechanism in the gel. [S1063-651X(98)07212-2].