Polymer as Structure Director in the Self-Assembly of Block Copolymer Colloids

The close-packed structures of block copolymer materials have been puzzling states. Depending on the stacking order of the 2D hexagonal close-packed (2D-HCP) layers, spheres self-assemble into face-centered cubic (FCC), HCP, or random stacking of 2D-HCP layers (RHCP). However, the fundamental origin of those polytypes has not been well understood. We investigated the close-packed structures of strongly segregated block copolymer micellar colloids in water and observed that the close-packed structures gradually change from FCC to HCP through a stable RHCP state as the number density of micellar colloids increases. Two competing entropic contributions are attributed as the origin of this continuous change. At low concentrations, FCC-like polytypes are preferred due to the universal configurational entropy of FCC. At high concentrations, HCP-like polytypes become dominant because the long corona chains in the interstitial space prefer HCP polytypes forming larger interstitial space groups which offer higher chain conformational entropy. At the equal point of those entropies, thermal fluctuation stabilizes RHCP states. This result reveals the unrealized potential of polymer chains in the interstitial space as a new structure director in self-assembly.