The Effects of the Size of Crystal Domains to the Polymorphism of Close-Packed Micelles

Polymorphism is technologically important, but its mechanistic origin is not well understood. We recently investigated the transitional structures of strongly-segregated block copolymer micelles in water in the liquid-to-solid and solid-to-solid transitions. In both transition processes, we found that the polytypes of close-packed block copolymer micelles are regulated by the size of crystal domains. In the liquid-to-solid phase transition, the crystallization of micelles proceeds by forming metastable hexagonal close-packed (HCP) structures that transform into stable face-centered cubic (FCC) structures via intermediate randomly stacked two-dimensional hexagonal close-packed (RHCP) structures. In the solid-to-solid transformation, the martensitic shear transformation of metastable FCC structures could be initiated by heating that reduces the size of metastable FCC crystal domains. These observations suggest that the polymorphism of crystalline solids is likely regulated by the size of crystal domains.