Inducing Lamellar Morphology in Intrinsically Non-Lamellar Block Copolymer Films using Ionic Liquids

Asymmetric morphological patterns are of importance to lithographic techniques and are not trivial to achieve by tuning the asymmetry of block copolymers (BCP) that controls the relative volume fractions of the constituent blocks. For instance, an asymmetric architecture on a linear diblock will yield a cylindrical or spherical morphology while a symmetric BCP would normally produce a symmetric lamellar morphology. An asymmetric lamellar morphology has not been observed in the phase diagram of diblock copolymers. Herein, we order an intrinsiccylinder forming PS-b-PMMA thin film system into an asymmetric lamellar structure which is a deeply quenched non-equilibrium state for this diblock. We use a simple combination of selective solvents (acetone) and a plasticizing ionic liquid (IL, EMIM-TFSI) both selective to the PMMA block to cast a micellar film. This transforms into a lamellar structure on high temperature vacuum thermal annealing that extracts the ionic liquid and shrinks the IL-swollen PMMA domain producing asymmetric layer sizes for PS and PMMA (PMMA forms the thinner layer). This simple strategy has potential to be universally applied to dial-in alternate morphologies that defy traditional thermodynamically driven ones, for many pristine BCP systems, important for many nanotechnology applications.