Manipulating Nanoscale Morphologies in Cylinder-Forming Poly(styrene‑b‑isoprene‑b‑styrene) Thin Films Using Film Thickness and Substrate Surface Chemistry Gradients

Controlling the nanostructure of self-assembled block copolymer thin films is critical for applications in nanotemplate design, nanoporous membranes, and organic optoelectronics. In this study, we employed a gradient approach to examine the effects of substrate surface chemistry and film thickness on the self-assembly of cylinder-forming poly(styrene-b-isoprene-b-styrene) (SIS) thin films. Using gradients in film thickness from 85 to 120 nm (3.1d to 4.4d), we found that the thin films contained parallel cylinders on both bare silicon substrates and benzyldimethylchlorosilane (benzyl silane)-modified substrates regardless of film thickness, while thin films contained surface patterns of hexagonally arranged dots on n-butyldimethylchlorosilane (n-butyl silane)-modified substrates. These surface patterns were further investigated using film etching, cross-sectional transmission electron microscopy (TEM), and grazing-incidence small-angle X-ray scattering (GISAXS) techniques. We determined that the nanostructures represented a hexagonally perforated lamellar (HPL) morphology in which the parallel cylinder layering was preserved during the phase transformation to HPL. Additionally, controlled vapor deposition was used to generate a nearly linear substrate surface chemistry gradient from benzyl silane to n-butyl silane. Examination of SIS thin films on this surface gradient revealed a morphological transformation from parallel cylinders to HPL with changing substrate surface composition. Thus, we demonstrated the combined usage of film thickness and monolayer substrate surface chemistry gradients to manipulate the nanostructure of block copolymer films, such as SIS, that possess moderate differences in surface energy between individual blocks. Our gradients represent a high-throughput and versatile screening tool that facilitates the examination of new materials and furthers the understanding of block copolymer thin film self-assembly.