Abstract
Previous studies have shown that the degree of ordering and alignment in block copolymer (BCP) films can be enhanced by increasing the thermodynamic driving force for microphase separation, chi N, where chi is the Flory-Huggins interaction parameter between the polymer components and N is the number of statistical segments in the BCP. In practice, this strategy for controlling the microstructure of any BCP film normally involves reducing the temperature T and/or increasing N. However, both of these methods have the drawback of leading to a corresponding slowing down of the rate of ordering and dynamic-heterogeneity-associated defect formation in the material, related to both glass formation and entanglement. In the present work, we explore the use of an ionic liquid (IL) having a high cohesive interaction strength with a relatively low volatility to increase the cohesive interaction parameter chi, while at the same time keeping the molecular mobility high. In particular, we show that IL-driven enhancement of chi and higher molecular mobility, coupled with the poly(methyl methacrylate) (PMMA) surface wetting interaction strength, induces enhanced substrate-driven stratification of parallel lamellae in polystyrene-b-poly(methyl methacrylate) BCP (PS-PMMA) films over much larger distances than without IL. We anticipate that this method can be used to prepare relatively defect-free multilayer films wherein the IL is mostly removed under vacuum annealing during the short processing time while preserving the intrinsic lamellar morphology despite the initial high-IL mass fraction. This approach should be extremely useful in applications like barrier materials and batteries, solid-state dielectric capacitors, optical waveguides, and other applications where substrate-parallel multilayer films of controlled thickness are required.