Hexagonal Close-Packed Sphere Phase of Conformationally Symmetric Block Copolymer

The self-consistent field theory (SCFT) predicted the existence of a close-packed sphere phase over a narrow window in the phase diagram of a block copolymer (bcp). It however remains unclear whether the face-centered cubic (FCC) or hexagonal close-packed (HCP) lattice represents the more stable close-packed lattice of the spherical micelles formed by the neat bcp in the quiescent melt. Here, we revisited this problem by exploring the stable close-packed lattice of conformationally symmetric poly(ethylene oxide)-block-poly(1,2-butadiene) (PEOb-PB). We disclosed that an HCP structure eventually formed in the ordered phase upon cooling from the micellar liquid phase. The micelle ordering was found to follow the Ostwald's step rule of the Alexander-McTague type, where a metastable BCC phase first developed followed by a transformation into the stable HCP structure. The higher thermodynamic stability of HCP relative to that of FCC was consistent with the prediction of a later SCFT calculation by Matsen and also demonstrated the generic difference between soft colloids and hard colloids in selecting their stable close-packed lattices.