Close-packed Structures of Block Copolymer Micelles Induced by the Size of Crystallites

Block copolymer micelles are versatile model spherical particles in the investigation of self-assembling structures of spherical particles and associated phase transformation phenomena. We investigated the close-packed structures of spherical poly(butadiene-b-ethylene oxide) (PB-PEO) diblock copolymer micelles dispersed in water using synchrotron X-ray scattering measurements. Remarkably, rapid thermal quenching of disordered PB-PEO micelle solutions to different temperatures induced three representative close-packed structures: face-centered cubic (fcc), random stacking of 2-dimensional hexagonal close-packing (rhcp), and hexagonal close-packing (hcp). Careful examination of the 2-dimensional scattering patterns revealed that the clear correlation between the type of close-packed structures and the size of crystallites controlled by the depth of thermal quenching: the smallest crystallites stabilize hcp, and as the size of crystallites increases, the hcp transforms to rhcp and eventually settled to fcc. This observation shows the interfacial tension effect is crucial for the selection of the metastable crystal structures confined in small crystallites.