Cooperative self-assembly and crystallization into fractal patterns by PNIPAM-based nonlinear multihydrophilic block copolymers under alkaline conditions

Abstract

Hierarchical two-dimensional patterned nanomaterials fabricated by bottom-up self-assembly are of great interests in practical applications. A series of nonlinear multihydrophilic block copolymers (MHBCs), consisting of poly(N-isopropylacrylamide) (PNIPAM), poly(acrylic acid) (PAA) and/or poly(vinyl pyrrolidone) (PVP) segments, were employed to fabricate well-defined fractal patterns under alkaline conditions by a simple solvent evaporation process. The formation of fractal patterns was discovered to obey a diffusion-limited aggregation process by needle-shaped intermediate nanostructures, which are actually nanohybrids formed by the cooperative self-assembly and crystallization of polymers and alkalis. The ability of MHBCs to form fractal patterns is closely related to their structural architectures, and is also significantly influenced by temperature, alkaline concentration, alkali type and substrate. Upon evaporation, PAA segments serve as the growing scaffolds of alkali crystallization along with the occurrence of self-contraction and intermolecular association of PNIPAM segments. PVP segments are not necessary for fractal patterning, which, however, can prompt the self-assembly into more perfect dendritic patterns in the case of a proper structural linking sequence. The observations reported here can provide a better understanding of molecular self-assembly toward complex patterns at multiple length scales, which may gain potential applications in advanced devices, sensors and microprinting.