Inducing asymmetric lamellar morphology in cylinder-forming block copolymer films using an extractable ionic liquid.

Abstract

The morphology of block copolymers (BCPs) is dictated by the relative volume fractions of the constituent blocks. Thus, in an equilibrated state, BCPs exhibit lamellar and non-lamellar structures, such as rods and spheres, for symmetric and asymmetric chain lengths, respectively. Transforming a cylinder-forming BCP with an inherent asymmetric composition into its equivalent asymmetric lamellar morphology is a challenging proposition, but could enable unique applications, such as harmonic frequency selective waveguides. Herein, we transform such an intrinsic PMMA cylinder forming PS-b-PMMA thin film system into its equivalent asymmetric lamellar structure, using the PMMA-philic ionic liquid (IL) EMIM-TFSI. We observed that the system formed nano-micelles consisting of a spherical PS core block and a PMMA/IL corona, upon casting films from a PMMA selective solvent, acetone. The IL migrates to the PMMA domains due to a combination of the preferential solvent and the high cohesive energy density (CED) of the IL promoting connectivity of the corona phase. Subsequent vacuum annealing of the films at elevated temperature (10 h at 180 °C) extracts the IL, lowering the CED barrier and allowing for progressive coalescence of the nano-micelles into continuous thin PS and PMMA layers, thus producing substrate parallel asymmetric PS-b-PMMA lamellae as confirmed by neutron reflectometry and surface island and hole formation phenomena. Stabilized by the substrate and air interfacial wetting interactions, these PS-b-PMMA lamellae with asymmetric composition in a deep meta-stable state, maintain their film structure with elevated temperature vacuum thermal annealing. This simple strategy has potential to be universally applied to dial-in alternate non-equilibrium morphologies for many pristine BCP systems, in defiance of thermodynamically controlled equilibrium morphologies, making them attractive for many nanotechnology applications, from membranes to photonic crystals.