Shape-Anisotropic Diblock Copolymer Particles from Evaporative Emulsions: Experiment and Theory

Abstract

Self-assembly of block copolymers (BCPs) in evaporative emulsion provides a simple and effective route for the preparation of anisotropic particles with controlled shape and size. Understanding of thermodynamic phenomena associated with the bending/stretching of the BCP chains confined within the particles is necessary to enable precise control of the shape and microstructure of the particles. Herein, we report the systematic design of shape-anisotropic diblock copolymer (dBCP) particles based on a new theoretical model that includes entropic penalty associated with bending of dBCP chains upon deformation of the particles. First, we produced convex lens-shaped (oblate) and football-shaped (prolate) polystyrene-b-polydimethylsiloxane (PS-b-PDMS) particles, where the aspect ratios (AR, defined as the major axis length divided by minor axis length) were varied. Of note, the AR of the oblate particles increased almost linearly up to 10 as the particle size increased, whereas the increase of AR for the prolate particles was limited to 2.0. For oblate particles, the high bending energy of the curved cylinders at the periphery of a particle can be released by increasing the AR of particle. However, the relatively low bending energy of curved lamellae of prolate particles prevents the particles from having a high AR. Furthermore, our theoretical model that considers these bending energies successfully explains the experimental observations on the variation of particle shape depending on the particle size and the dBCP molecular weight.