Non-Spherical Block Copolymer Particles: Mechanisms of Microstructure Orientation and Particle Elongation

Shape-anisotropic block copolymer (BCP) particles received great attention. Previously, we reported generation of uniform polystyrene-b-polybutadiene (PS-b-PB) BCP ellipsoids by controlling evaporation rate of emulsion solvent (toluene). Here, we extend our study on ellipsoid formation mechanism in two aspects: 1) investigation of lamellae orienting mechanism on particle surface and 2) elongation behavior of ellipsoids. First, the perpendicular orientation of lamellae on ellipsoid results from solvent concentration gradient along the particle. As solvent evaporates, nucleation of BCP occurs at the surface followed by growth into the particle center. To describe this feature, propagation length of perpendicular lamellae (l_per) was estimated using diffusion, partition and permeability coefficient of solvent. Experimentally, particles with length scale smaller than l_per formed well-ordered ellipsoid, reflecting a valid prediction of l_per. For the second case, we analyzed the aspect ratio of ellipsoid experimentally. Increase of aspect ratio was observed by increasing particle size and BCP molecular weight. This feature was captured by theoretical calculation of free energy for particle elongation that predicts the aspect ratio.