Transformations to and from the Gyroid Phase in a Diblock Copolymer

Simultaneous small-angle scattering and in situ dynamic mechanical measurements offer an excellent opportunity to relate the macroscopic dynamical mechanical response of block copolymers and their mesoscopic structural behavior. We use small-angle neutron scattering and rheology to examine the ordered phases and the order−order transitions exhibited by a poly(ethylene- alt -propylene)−poly(dimethylsiloxane) diblock copolymer. An intermediate structure very similar to the hexagonal perforated layer (HPL) phase reported in other diblock systems proves to be metastable, and we study the kinetics and epitaxy of its relaxation to the “gyroid” phase of Ia 3̄ d symmetry. Likewise we study the relaxation of a supercooled hexagonal phase to the gyroid structure and also observe that the gyroid phase is bypassed in a slow cool from the hexagonal phase to the HPL-like structure. The origin of a typical scattering pattern obtained from a highly oriented crystal structure of a sample in the gyroid phase is investigated and related to real space projections of the gyroid morphology. Synchrotron small-angle X-ray scattering is used for high-resolution studies of the ordered phases. Reversibility of transitions between different mesoscopic structures is studied, especially to and from the gyroid phase, and puzzling patterns of the HPL-like structure are observed.