Kinetics and Mechanisms for the Cylinder-to-Gyroid Transition in a Block Copolymer Solution

The cylinder-to-gyroid transition in a block copolymer solution has been studied using a combination of rheology and small-angle X-ray scattering (SAXS). A poly(styrene-b-isoprene) diblock copolymer with block molecular weights of 11 000 and 32 000 g/mol, respectively, dissolved in the styrene-selective solvent di-n-butyl phthalate (DBP) at a volume fraction of 0.67 exhibits an order−order transition between hexagonally packed cylinders (C) and the cubic gyroid phase (G) at 74 ± 3 °C. A shear-oriented C phase transforms to G epitaxially, as previously established in melts. For shallow quenches into G, the transition proceeds directly by a nucleation and growth process. For deeper quenches, a metastable intermediate structure appears, with scattering and rheological features consistent with the hexagonally perforated layer (HPL) state. The appearance of the HPL state beyond a certain quench depth is reconciled with previous experiments and theory for the lamellar (L)-to-gyroid transition. The C → G transit...