Systematic Study on the Effect of Solvent Removal Rate on the Morphology of Solvent Vapor Annealed ABA Triblock Copolymer Thin Films

Nanoscale self-assembly of block copolymer thin films has garnered significant research interest for nanotemplate design and membrane applications. To fulfill these roles, control of thin film morphology and orientation is critical. Solvent vapor annealing (SVA) treatments can be used to kinetically trap morphologies in thin films not achievable by traditional thermal treatments, but many variables affect the outcome of SVA, including solvent choice, total solvent concentration/swollen film thickness, and solvent removal rate. In this work, we systematically examined the effect of solvent removal rate on the final thin film morphology of a cylinder-forming ABA triblock copolymer. By kinetically trapping the film morphologies at key points during the solvent removal process and then using successive ultraviolet ozone (UVO) etching steps followed by atomic force microscopy (AFM) imaging to examine the through-film morphologies of the films, we determined that the mechanism for cylinder reorientation from substrate-parallel to substrate-perpendicular involved the propagation of changes at the free surface through the film toward the substrate as a front. The degree of reorientation increased with successively slower solvent removal rates. Furthermore, the AFM/UVO etching scheme permitted facile real-space analysis of the thin film internal structure in comparison to cross-sectional transmission electron microscopy.