Aspect Ratio-ControlledSynthesis of Uniform Colloidal Block Copolymer Ellipsoids from Evaporative Emulsions
Block copolymers (BCPs)confined in evaporative emulsions can assemble into ellipsoidal particles whensolvent 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 1.0to 2.2, and particle sizes ranging from 0.1 to 5 μm by membrane emulsification and subsequent solvent evaporation. Theratio of the propagation distance (lp) of ordered BCP domainsperpendicular to the particle surface to the particle size (rBCP),or lp/rBCP, was found to be a criticalparameter dictating the particle shape, where lp/rBCP>1 yielded ellipsoids. We show that the AR of colloidal BCP ellipsoids canbe precisely controlled by varying particle size (i.e., membrane poresize) and BCP molecular weight, as predicted by theoretical calculations of thefree energy of particle elongation including three terms: (1) the interfacialenergy between the two blocks of the BCP; (2) the entropic penalty associatedwith stretching of the BCP chains upon elongation of the particles; and (3)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 ofcolloidal coatings obtained by drop-casting the ellipsoids into films.