Thermoreversible order-order transition between spherical and cylindrical microdomain structures of block copolymer

The mechanism and dynamics of the order-order transition (OOT) in a block copolymer were investigated by means of small-angle X-ray scattering (SAXS), ultra-small-angle X-ray scattering (USAXS) and polarized-light optical microscopy. At high temperatures the spherical microdomains in bcc lattice (bcc-sphere) exist, while at low temperatures the cylindrical microdomains in hexagonal lattice (hex-cylinder) exist. The transition between these ordered phases thermoreversibly occurred with conservation of the grain structures and with conservation of a memory of the lattice orientation. By examining the SAXS pattern from a single grain, it was found that the cylinders are transformed into a series of spheres with their cylindrical axes corresponding to the [111] direction of the bcc-spheres: orientation of cylinders and orientation of the [111] direction of the bcc lattice are conserved in the OOT process from the hex-cylinder phase to the bcc-sphere phase and its reverse transition process, or in a repeated OOT process between these two phases (the conservation of a memory of the lattice orientation). The process of the OOT studied here was found to be spinodal-like, which was contrary to the nucleation and growth process of the order-disorder transition in asymmetric block copolymers.