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

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.