Electrostatic crosslinking-enabled highly asymmetric lamellar nanostructures of polyzwitterionic block copolymers for lithography

Abstract

For the bulk self-assembly of traditional diblock copolymers (di-BCPs), lamellar structures only occur when two constituents have similar volume fractions (f) and two alternating layers tend to have similar thicknesses. Highly asymmetric lamellar (A-LAM) structures, in which the thickness of one layer is several times higher than the other, are hardly formed in di-BCPs, while they have potential applications in nanolithography. In this work, A-LAM structures with different dimensions were constructed using a type of simple linear di-BCP, polystyrene-b-poly(4-vinylpyridine)propane-1-sulfonate (PS-b-PVPS) with the polyzwitterionic block PVPS in minority. The origin of the A-LAM structure was ascribed to the electrostatic crosslinking and confirmed by doping PS-b-PVPS block copolymers (BCPs) with N-butyl pyridinium methane sulfonate (BPMS). The morphology of compositionally asymmetric PS-b-PVPS BCPs changed from A-LAM to cylindrical structures upon salt-doping, i.e. the phase behavior of common BCPs was recovered. In addition, the morphologies of PS-b-PVPS BCPs with similar molecular weights but varied compositions were also studied, and only two kinds of structures (lamellar or ill-defined spherical structure) were observed when the volume fraction of PVPS (f_PVPS) was less than 0.5, and the composition range for the formation of the lamellar structure was found to be f_PVPS ≥ 0.188.