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

Gels of a poly(oxyethylene)-poly(oxybutylene) diblock copolymer in aqueous solution forming a bcc phase have been examined using small-angle X-ray scattering (SAXS) with simultaneous rheology. Application of large amplitude oscillatory shear is observed to induce macroscopic alignment of twinned bcc crystals. Alignment at relatively low frequency leads to a structure in which {200} planes are oriented normal to the shear direction. At high frequency, large amplitude shearing leads to a transition to flow with the [111] plane parallel to the shear direction. Flow occurs in the {110} and {211} planes which intersect along the same [111] direction, leading to a high-symmetry SAXS pattern with numerous sharp reflections. These results highlight the similarity between the defect mechanics of cubic phases in block copolymer gels and those in metals.