Spray Deposited Block Copolymer Thin Films with Solvent Evaporation Controlled Domain Orientation

With their ability to self-assemble spontaneously into well-defined nanoscale morphologies, block copolymer (BCP) thin films are a versatile platform to fabricate functional nanomaterials such as lithographic nanopatterns, nanoporous membranes and optical metasurfaces. An important yet unsolved challenge to wider deployment of BCPs in nanofabrication, however, is combining the desired precise control of BCP assembly with scalable deposition techniques that are applicable to large-area, curved and flexible substrates for high-volume manufacturing. Ultrasonic spraying is a scalable, non-batch casting method with minimal wasted polymer, yet its use in thin film BCP assembly has not been investigated.<br/><br/>Here, we show that the latent orientation (vertical <i>vs.</i> horizontal) of well-ordered cylindrical domains in spray-deposited, optically-smooth thin films of polystyrene-block-poly(4-vinylpyridine) (PS-<i>b</i>-P4VP) can be controlled by regulating solvent evaporation kinetics. Faster evaporation during deposition facilitates assembly of vertically oriented cylinders spanning the entire film thickness (100-300 nm) upon subsequent solvent vapor annealing. In comparison, slow solvent evaporation induces assembly of horizontal cylinders, imposing an additional energy barrier for reorientation to vertical cylinders during solvent vapor annealing. Using in-situ characterization by synchrotron grazing-incidence small-angle X-ray scattering (GISAXS), we identify the relevant timescale for polymer vitrification which determines latent cylinder orientation. This dependency on solvent evaporation kinetics is exploited to control cylinder orientation in multiple ways, including substrate heating, forced convection, and solvent formulation. To highlight the scalability and utility of the spray casting technique, an exemplar application using spray deposited self-assembled patterns on curved surfaces will also be described.<br/><br/><i>Funding: This research was conducted at the Center for Functional Nanomaterials (CFN) and the National Synchrotron Light Source II (NSLS-II), which are U.S. Department of Energy (DOE) Office of Science User Facilities, at Brookhaven National Laboratory under Contract No. DE-SC0012704. S.C., B.B., and G.S.D. were supported by a DOE Early Career Research Program grant.</i>