Molecular Weight Effect on the Transition Processes of a Symmetric PS-b-P2VP during Spin-Coating

We studied the morphological transition behaviors in symmetric poly(styrene-b-2-vinyl pyridine) (PS-b-P2VP) with different molecular weights during spin-coating from a toluene solution using grazing-incidence small-angle X-ray scattering (GISAXS). For the lowest molecular weight condition used here (the number-average molecular weight, Mn, was 16,500), the disordered micelles transitioned into disordered structures during evaporation. When Mn = 50,000, the micelles transitioned into hexagonally packed cylindrical structures parallel to the substrate and then into lamellar structures. The effect of selective solvent evaporation on the transition process became significant when Mn was larger than 84,000. For Mn = 84,000 and 199,000, the micelles first transitioned into spheres on the body-centered cubic lattice (BCC), where the BCC (110) plane was aligned perpendicular to the substrate. Then, the spheres coalesced into loosely packed cylindrical structures. The convection effect of the solvent caused this coalescence, and vitrification prohibited continued transitioning into the lamellar structures from the cylindrical structures. PS-b-P2VP with Mn = 428,000 self-assembled into loosely packed cylindrical structures perpendicular to the substrate; thin films were formed after the transition from micelles into BCC structures. However, the BCC (110) plane did not become aligned perpendicular to the substrate because a large amount of solvent contained in the PS component prevented alignment due to convection during the selective solvent evaporation.