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

Abstract

The effect of steady shear on the alignment of a gel formed by block copolymer micelles in a body-centered-cubic (bcc) phase has been investigated using small-angle x-ray scattering (SAXS) for samples in a Couette cell. The gels were formed by an amphiphilic poly(oxyethylene)-poly(oxybutylene) diblock 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, and it was found that flow occurs with a [111] direction of the unit cell coincident with the shear direction, with twinning of the crystal about this axis. The SAXS experiments indicated a critical shear rate, $\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{\ensuremath{\gamma}}\ensuremath{\sim}50{\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ for macroscopic alignment of the gel. This value is lower than that reported in earlier publications from our group, where large-amplitude oscillatory shear was used to prepare highly oriented samples, possibly reflecting a dependence on strain amplitude. The viscoelastic behavior of the gel was probed using oscillatory shear and highly nonlinear rheology was observed at large strain amplitudes, which was assigned to a stick-slip mechanism of flow. The magnitude of the mutual diffusion coefficient, compared to the relaxation time obtained from the critical shear rate, also indicated a cooperative flow mechanism in the gel.