Mechanistic Study on the Shape Transition of Block Copolymer Particles Driven by Length-Controlled Nanorod Surfactants

Abstract

Interface engineering of evaporative emulsion droplets containing block copolymers (BCPs) provides an effective route to generate nonspherical particles. Here, we demonstrate the impact of length-controlled nanorods (NRs) on the interfacial properties of BCP emulsions to produce anisotropic BCP particles. A series of lamellae- and cylinder-forming polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) and a series of NRs with different lengths (l) are coassembled, and selective arrangement of the NRs on the P4VP domain at the particle surface enables the production of striped football (prolate) and convex lens-shaped (oblate) particles. In particular, the ratio of the NR length to the size of the NR-hosting domain (l/L), which is varied from 0.07 to 3.60, is the key parameter in determining the location of the NRs in the BCP particles as well as the final particle shape. The oblate particles are generated only in the range of 0.36 ≤ l/L ≤ 0.96, whereas the prolate particles are produced for much wider range of l/L ≥ 0.83 without upper limit. This difference is attributed to larger entropic penalty for the NRs confined within the P4VP cylinders than the entropic penalty for those within the lamellae. To better understand and support our experimental observations, we performed dissipative particle dynamics simulation and calculated the free energy for the NR/BCP assembly within the emulsion droplets.