Synchrotron grazing incidence x-ray scattering study of the morphological structures in thin films of a polymethacrylate diblock copolymer bearing POSS moieties

The morphological structures in thin films of a diblock copolymer of methyl methacrylate and polyhedral oligomeric silsesquioxane (POSS) functionalized methacrylate (PMMA-b-PMAPOSS) with a volume ratio of 13/87 were investigated in detail by using synchrotron grazing incidence small and wide-angle X-ray scattering (GISAXS and GIWAXS). In addition, its thermal properties were studied. Thin films of this diblock copolymer were found to undergo phase-separation during solvent-annealing with carbon disulfide and post thermal annealing. To quantitatively analyze the scattering data, GISAXS and GIWAXS formulas were derived and applied. Our detailed analysis found that cylinders of PMMA blocks are induced to form in the diblock copolymer films by solvent-annealing and are hexagonally packed in the PMAPOSS matrix, in which the cylinders are oriented vertically with respect to the film plane. In the solvent-annealed films, both the PMMA cylinders and the PMAPOSS matrix are featureless, i.e., amorphous. However, the post thermal annealing process induces aggregation of the POSS moieties, which results in the formation of crystals with an orthorhombic lattice unit cell. These crystals were found to consist of PMAPOSS block chains in a helical conformation in which the molecular PMAPOSS cylinders are aligned in the film plane. The formation of these crystals is induced by the ordering ability of the POSS moieties. The crystals were found to melt above 190 °C during heating and subsequent cooling. In contrast, the hexagonally packed structure of the PMMA cylinders in the solvent-annealed and post thermally annealed films was found to be retained during the heating and the subsequent cooling. In addition, the scattering analysis provides detailed structural parameters. The 2D GISAXS and GIWAXS patterns were reconstructed from the determined structural parameters by using the derived scattering formulas, and found to be in good agreement with the experimental patterns. Moreover, a model for the structure of the films of the diblock copolymer is proposed.