Vacuum-driven orientation of nanostructured diblock copolymer thin films for nanopatterning.

Block copolymer (BCP) self-assembly has gained greater attention due to its ability to form well-ordered nanostructures that can be utilized as a bottom-up approach for next-generation lithographic applications, flash memory devices, magnetic storage, templated synthesis, nanofiltration, and so forth. For nanopatterning applications, one of the notorious problems is the high surface tension discrepancy between the constituents of BCP that results in the formation of a wetting layer on the air surface obstructing the perpendicular orientation. Herein, we demonstrate a facile method for controlled orientation of nanostructured BCP thin films. A simple diblock copolymer system, polystyrene-block-polydimethylsiloxane (PS-b-PDMS), is chosen to demonstrate vacuum-driven orientation for solving the notorious surface tension discrepancy problem between the constituents of PS-b-PDMS for nanopatterning. Owing to the pressure dependence of the surface tension of polymeric materials, a neutral air surface for the PS-b-PDMS thin film can be formed under high vacuum degree (~10^-4 Pa), allowing the formation of perpendicular cylinders and lamellae at the air surface upon thermal annealing. Accordingly, this approach is also implemented on the polystyrene-block-poly(L-lactide) (PS-b-PLLA) diblock copolymer system. The experimental results are in line with the PS-b-PDMS system indicating generalization of this approach.