Non-equilibrium thermal annealing of a polymer blend in bilayer settings for complex micro/nano-patterning.

Micro phase separation in a thin film of a polymer blend leads to interesting patterns on different substrates. A plethora of studies in this field discussed the effect of the surface energy of the underlying tethered polymer brush or substrate on the final morphology of the polymer blend. The conventional process toward the final morphology is rather slow. Here, aiming fast lithography, we induce the kinetically driven morphological evolution by rapid thermal annealing (RTA) of the polymer blend of polystyrene (PS) and polymethylmethacrylate (PMMA) in bilayer settings at a very high temperature. The underlying film consists of untethered constituent homopolymers or their blend or random-co-polymer (RCP). Apart from the phase inversion of the blend on the PS homopolymer, a rich morphology of the blend on the RCP underlayer is uncovered with systematic investigation of the film using sequential washing with selective solvents. The dissolution characteristics and the thermal stability of the constituent polymers corroborated the observation. Based on the understanding of the morphological evolution, fabrication of a complex shaped micro/nano-pattern with multiple length scales is demonstrated using this blend/RCP system. This study shows a novel methodology for easy fabrication of hierarchical small length scale complex structures.