Fluctuations with Cubic Symmetry in a Hexagonal Copolymer Microstructure

A triblock copolymer that undergoes a thermotropic order-order transition between hexagonally packed cylinders ( $C$) and body-centered-cubic spheres ( $S$) is examined. Transmission electron microscopy provides real space images, and small-angle x-ray scattering the reciprocal space signature, of fluctuations with cubic symmetry in the stable (shear-aligned) hexagonal phase. These results support recent theoretical predictions. There is evidence of an interesting induction time for the onset of fluctuations that is attributed to a slight mismatch in principal lattice spacings between the $C$ and $S$ microstructures.