Macromolecules, Vol.51, No.23, 10040-10051, 2018
Order-Order Transition Processes of Thin-Film Symmetric and Asymmetric PS-b-P2VP during Spin Coating
Order-order transitions during spin-coating processes in symmetric and asymmetric poly(styrene-block-2-vinylpyridine) (PS-b-P2VP) were investigated by using grazing incidence small-angle X-ray scattering (GISAXS). When the composition was symmetric (e.g., the copolymer P2VP volume fraction, f(P2VP), was 0.50), we found that loose-packed nanocylindrical structures formed after the transition from micelles to a body-centered cubic (BCC) lattice structure during evaporation. After the BCC (110) plane aligned perpendicular to the substrate, microdomains coalesced into a cylindrical structure. This coalescence was caused by the convection effect of the solvent, and vitrification of PS and P2VP further prohibited transitions from the cylindrical structure, although generally, the equilibrium morphology in the bulk state is lamellae. For an asymmetric composition with f(P2VP) = 0.70, lamellar structures formed perpendicular to the substrate. During evaporation, the micelles transformed to hexagonal close-packed (HCP) structures, and then BCC structures appeared. The microdomains of the BCC structures merged parallel and perpendicular to the film surface before evaporation was complete, and hence, the lamellar structures were formed. Furthermore, well-packed hexagonal-packed nanocylindrical structures with long-range order perpendicular to the substrate were formed in the PS-b-P2VP thin film with an f(P2VP) = 0.30 and corresponded to the equilibrium morphology in the bulk state. During evaporation, the micelle structures directly transitioned to nanocylindrical structures. The micelles grew perpendicular to the film surface before evaporation was complete.