Aspect Ratio-Controlled Synthesis of Uniform Colloidal Block Copolymer Ellipsoids from Evaporative Emulsions

Abstract

Block copolymers (BCPs) confined in evaporative emulsions can assemble into ellipsoidal particles when solvent evaporation is faster than polymer diffusion within the droplet. Here, we report the synthesis of monodisperse, ellipsoidal polystyrene-block-1,4-polybutadiene (PS-b-PB) BCP particles with tunable aspect ratios (ARs) ranging from 1.0 to 2.2 and particle sizes ranging from 0.1 to 5 mu m by membrane emulsification and subsequent solvent evaporation. The ratio of the propagation distance (l(p)) of ordered BCP domains perpendicular to the particle surface to the particle size (r(Bcp)), or l(p)/r(Bcp), was found to be a critical parameter dictating the particle shape, where l(p)/r(Bcp) > 1 yielded ellipsoids. We show that the AR of colloidal BCP ellipsoids can be precisely controlled by varying particle size (i.e., membrane pore size) and BCP molecular weight, as predicted by theoretical calculations of the free energy of particle elongation including three terms: (1) the interfacial energy between the two blocks of the BCP, (2) the entropic penalty associated with stretching of the BCP chains upon elongation of the particles, and (3) the surface energy between the BCP particles and the surrounding medium. Finally, using the resulting AR-controlled and highly monodisperse colloidal ellipsoids, we systematically investigated the effects of AR on the homogeneity of colloidal coatings obtained by drop-casting the ellipsoids into films.