Lamellar-to-gyroid transition in a poly(oxyethylene)-poly(oxybutylene) diblock copolymer melt

The transition from the lamellar (lam) to the gyroid (gyr) phase in the mel t of a poly(oxyethylene)-poly(oxybutylene) (EB) diblock copolymer was shown to proceed via an intermediate perforated lamellar (pl) structure. The tra nsition was studied starting from a shear-oriented lamellar phase using syn chrotron small-angle X-ray scattering (SAXS) with simultaneous rheology. Th e transition occurred with a continuous variation of q*, the position of th e principal peak position, although the slope of q* as a function of temper ature increased in the region of the transition (pl state). In the intermed iate state, the variation of q* with T was shown to be much stronger than t hat of the radius of gyration of EB diblocks. It was observed that the gyr phase developed from an oriented pi phase precursor as an isotropic distrib ution of grains. This indicates that growth of this structure did not occur epitaxially on a macroscopic scale. Finally, density images corresponding to lam, hexagonal perforated lamellar and gyr structures were generated usi ng a single wavenumber approximation, based on the SAXS evidence that diffr action patterns are dominated by the first Fourier component.