Hierarchical structure in nanoscale thin films of a poly(styrene-b- Methacrylate grafted with POSS) (PS₂₁₄-b-PMAPOSS₂₇)

An interesting organic-inorganic diblock copolymer, poly(styrene-b- methacrylate grafted with polyhedral oligomeric silsesquioxane) (PS-b-PMAPOSS), with PS (an average degree of polymerization DP of 214) and PMAPOSS (DP = 27) blocks was prepared: PS₂₁₄-b-PMAPOSS₂₇. The morphological structures in nanoscale thin films of PS₂₁₄-b-PMAPOSS₂₇ and their temperature-dependent changes were investigated in detail by performing grazing incidence X-ray scattering (GIXS) with a synchrotron radiation source. GIXS formulas based on structural models were derived for the quantitative analysis of the two-dimensional (2D) scattering data. In addition, the thermal properties of the diblock copolymer were characterized: it was found to be stable up to 330 °C, and to undergo a glass transition at 96 °C (PS block) and a melting transition at 173 °C (PMAPOSS block). The quantitative 2D GIXS analysis found that the diblock copolymer molecules in the thin films undergo phase separation into PS and PMAPOSS domains, which generates a lamellar structure. Surprisingly, in the solvent-annealed films with and without subsequent thermal annealing the lamellar structure is completely oriented in the out-of-plane of the film and is stable up to the degradation temperature. The PMAPOSS layers in the lamellar structure were found to consist of two sublayers, namely a densely ordered layer composed of vertically oriented orthorhombic crystals and a less ordered layer that consists of molecularly stacked layers that are oriented vertically. In both the sublayers, the PMAPOSS block chains have helical conformations and thus are present as molecular cylinders. These molecular cylinders are always oriented in the film plane. On heating, the orthorhombic crystals and the molecularly stacked layers melt over the range 170-190 °C. On subsequent cooling, the orthorhombic crystals and molecularly stacked layers are not fully recovered, which indicates that the formation of orthorhombic crystals and stacked layers in the PMAPOSS domains requires thermal annealing.