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 11000 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 degreesC. 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 transition follows the same pathways, and at approximately the same rates, even when the initial C phase is not shear-oriented. The transition rates are quantified as a function of quench depth. The reverse G --> C transition demonstrates a memory of the initial cylinder orientation that persists even after annealing the G phase for 48 h. The results are discussed and compared with related work in the literature.