A Transition from Cylindrical to Spherical Morphology in Diblock Copolymer Thin Films

Microphase-separated structures in poly(styrene-<i>block</i>-isoprene) (SI) block copolymer thin films were investigated by transmission electron microtomography (TEMT). The SI block copolymer showed <i>cylindrical microdomains in the bulk state</i>. Several block copolymer thin films with different thicknesses were prepared by spin-coating and were extensively annealed before the TEMT experiments. Intriguingly, although the cylindrical morphology orienting parallel to the substrate was observed in most of the cases, <i>spherical microdomains</i> were found at certain film thicknesses. The thickness dependence was investigated using a computer simulation based on the self-consistent-field theory, producing a morphological phase diagram based on minimizing free energy. We find that the distortion of the hexagonal lattice of the cylindrical microdomains caused the morphological transition to the spherical microdomains.