This study reports the manipulation of order-order transition (OOT) from hexagonally packed cylinders (HEX) into a lamellar structure (LAM) in the nanocomposite film formed by a polystyrene (PS) homopolymer, a polystyrene-block polyethylene oxide (PS-b-PEO) diblock copolymer, and a small number (=0.07 vol % with respect to PEO) of amino group-tethered Fe3O4 magnetic nanoparticles (NH2-Fe3O4), primarily mediated by an applied external magnetic field with a quite low magnetic flux intensity (=30 mT). Upon annealing the HEX-forming nanocomposite film under this magnetic field applied parallel to the film surface at 150 degrees C for less than 6 h followed by cooling to 30 degrees C, the nanocomposite film with the NH2-Fe3O4 nanoparticles being localized in the PEO cylindrical microdomains via hydrogen bonding was found to exhibit the perpendicular orientation of PEO cylinders with respect to the film surface. Meanwhile, if the time period of magnetic thermal annealing process at 30 mT and 150 degrees C lasted for more than 6 h, the morphology of the nanocomposite film thus obtained at 30 degrees C completely transformed into a LAM structure with the stacking orientation of lamellar microdomains perpendicular to the film surface, revealing the first instance of the magnetic field-induced OOT in the block copolymer system. We propose a new kinetic mechanism where the HEX-to-LAM OOT observed here was a thermodynamically favored process driven by a significant increase in the interfacial tension at the junction points between PEO and PS blocks in a unique intermediate domain (i.e., a small-grain local domain that could be formed upon magnetic thermal annealing and could act as a nucleus), where the PEO block chains that were intimately mixed with the magnetic field-assisted uniformly arranged NH2-Fe3O4 nanoparticles showed a strong segregation against the PS block chains to be able to induce the flat domain interface. Upon subsequent cooling, the growth of the LAM structure could take place throughout the nanocomposite film from the nucleus for attaining a lower interfacial energy, thereby leading to the large-scale orientation; meanwhile, the PS homopolymer chains could migrate to the middle region of the PS lamellar microdomains to gain translational entropy, which thus expanded the interdomain spacing across the OOT. This work represents a significant advance in the magnetic field directed self-assembly of block copolymers through identifying not only large-scale alignment of the microdomains but also an OOT induced by quite a low-intensity magnetic field.
