Nanostructures inside colloidal particles of block copolymer/homopolymer blends

Abstract

Colloidal spheres with unique internal morphologies were prepared by blending symmetrical poly(styrene)-block-poly(butadiene) (PS-b-PB) and polystyrene homopolymer (hPS) in emulsion droplets and carrying out subsequent solvent evaporation. We found various unprecedented microphases in these spherically confining geometries that depend on the ratio (r) of the molecular weight of hPS relative to that of the PS block of PS-b-PB and the ratio of the diameter (D) of the emulsion drop to the feature spacing (L(0)) of the phase-separated periodic domains. In particular, the effect of spherical confining geometry was pronounced for D/L(0) < 15, in which directionally oriented structures such as helices and hoops were observed. For r < 1, the PB domains exhibit transitions between morphological microphases of lamellae, perforated lamellae, cylinders, and spheres, in that order, as the weight fraction of hPS phi is increased. The lamellae or perforated lamellae are ordered in concentric shells. Interestingly, cylindrical domains of PB blocks consisting of circular helices or stacked hoops were observed for 0.38 <= phi <= 0.54, in which the particle shape is slightly deformed from spherical to accommodate the ordered regular packing of the hoops and helices. For r similar to 2, concentric spherical lamellae are produced for phi up to 0.30, above which macrophase separation occurs and the spherical lamellae are either nonconcentric or deformed into a domelike structure. For r similar to 4, an onion-like structure is present for the entire range of phi, with a lamellar spacing that is nearly independent of phi due to macrophase separation. In particular, for r similar to 1, spherical lamellar morphology is present up to phi = 0.30, and complex anomalous structures such as multiply jointed hoops are present for phi >= 0.32 because of the combined effects of micro- and macrophase separations.