Solvent Processing and Ionic Liquid-Enabled Long-Range Vertical Ordering in Block Copolymer Films with Enhanced Film Stability

Rapid and reliable processing methods for forming ordered block copolymer (BCP) materials with low defect density in a thin film geometry are required for many nanotechnology applications. Vertically aligned BCP structures, in particular, have applications ranging from nanolithography for electronics and photonics applications to nanoporous membranes for water remediation and novel batteries for flexible electronics. However, the attainment of a nearly complete vertical orientational order of the BCP-ordered phase remains challenging. Solvent-based techniques, such as direct immersion annealing (DIA) and solvent vapor annealing (SVA), have immense potential for these applications of BCP films, as it allows for a ‘tuning’ of their thermodynamic, structural, and chain mobility-driven kinetic properties. We first demonstrate that DIA, using a judicious choice of binary solvent mixtures, along with a relatively hydrophobic ionic liquid (IL), induces the rapid vertical ordering in polystyrene-b-poly­(methyl methacrylate) (PS-PMMA) in lamellae-forming block copolymer films. The IL in a binary solvent mixture with toluene and heptane apparently gives rise to a near-neutral solvent for the BCP for itself and the boundary that is useful for attaining the vertical microstructure within the BCP film. Next, we show that an IL can moreover suppress the dewetting of the PS-PMMA films to achieve long-range order using SVA in cylindrical films after long annealing times. Both vertical and horizontal morphologies are attained in these films by selecting different solvent conditions. Attaining enhanced vertical and horizontal BCP structures with long-range defect-free order by tuning solvent quality and using additives like ILs can render them useful for many nanotech applications.