Two-dimensional self-assembly of diblock copolymers into nanoscopic aggregates: from dots to disks, then rings, and finally short and long rods

Abstract

This paper presents a simple strategy that enables the two-dimensional self-assembly of an amphiphilic polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer into a variety of nanoscopic aggregates to proceed on a solid substrate. After being made by dripping several drops of PS-b-PMMA dilute solution in toluene on mica, the samples are immediately put into a closed Petri dish under a relative humidity of 100%. These wet samples are then periodically taken out to freeze intermediate self-assembled structures by drying under ambient conditions, and assessed by atomic force microscopy (AFM). It is found that the self-assembly of PS-b-PMMA diblock copolymers on mica can be such a multi-scale process that the molecules of block copolymers self-assemble into dot-like aggregates at first, and then the nanosized aggregates self-assemble step-by-step from dots into disks, then rings, and finally into short rods and long rods. A precise AFM tip relocation technique is employed to confirm the mechanism in detail by repeatedly taking images at the same marked area on a sample surface.