A New Method for Fabrication of the Oriented Double Gyroid Phase in Diblock Copolymers Hybridized with an Extremely Low Content of Magnetic Nanoparticles by Low-Intensity Magnetic Fields

This study investigates a low-intensity magnetic-field-directed order–order transition (OOT) involving an oriented double gyroid (DG) network phase with unexpected lattice distortions in a model polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymer hybridized with an extremely low content of NH2-tethered Fe3O4 (NH2-Fe3O4) magnetic nanoparticles that initially exhibited a hexagonally packed cylinder (HEX) structure prior to the magnetic manipulation. This HEX-to-DG OOT toward an oriented DG polymer film (despite showing lattice distortions) was driven by sequential generation of 3-fold junctions via the fusion at curved faces of deformed cylindrical segments containing the magnetic-field-guided spatially assembled NH2-Fe3O4 nanoparticles. The DG phase obtained here exhibited high thermal stability, which was retained at least up to 150 °C in the absence of the magnetic field. Interestingly, the distortion of the DG cubic lattice could be completely relaxed at 140 °C but was recovered upon subsequent cooling to temperatures lower than 40 °C without the use of any magnetic field, at which the DG cubic lattice confined by the vitrified PS matrix was surprisingly distorted. The occurrence of lattice distortions was deduced to be governed by the preferred growth direction of PEO crystallites, which promoted distortion of the DG cubic lattice. Overall, this work represents applicability to the low-intensity magnetic manipulation of block copolymer self-assembly for fabricating complex ordered phases mediated by a very small number (= 0.08 vol % with respect to the minority microdomains) of hybridized magnetic nanoparticles.