Continuous Transition of Close-packed Structures of Block Copolymer Micellar Colloid

We investigated close-packed structures of strongly segregated block copolymer micellar colloids using X-ray scattering and simulation-based scattering pattern analysis. Our experimental phase diagram reveals a phase domain of continuously varying polytypic close-packing structures from FCC to HCP in the concentration parameter space. The stability of the polytypic close-packed crystals was tested with crystal growth, and the crystals with nearly perfect random stacking of 2D-HCP layers (RHCP) were found stable. We attribute the stability of the RHCP crystals to the equal thermodynamic preference for the FCC and HCP lattices. Because close-packed structures of equal spheres are the archetypes of all polytypic crystal systems with permutable 2D-HCP layer stacking choices, the stable RHCP crystals of micellar colloids may indicate the universal stability of RHCP orders in the crystal systems with competing preference for the FCC and HCP lattices. Indeed, RHCP orders have been widely observed from other polytypic systems, such as cobalt, ice, and hard-sphere colloids, which are known to form FCC and HCP orders in the phase space.