Crystallization of hollow microgels assisted by regular ones*

Microgels are colloidal polymer networks swollen by a good solvent. Due to their compressibility, they represent an excellent model system to study phase behavior and flow properties of soft spheres. However, hollow microgels with a solvent-filled cavity in the center, unlike regular ones, do not form crystals at any volume fractions even when the polydispersity is below the limit for crystallization of hard spheres (12%). Here, we show that hollow microgels can be successfully co-crystallized with regular ones of the same size. Both the regular and hollow microgel we use are relatively monodisperse and virtually of the same size, as probed using light and small-angle neutron scattering. The phase diagram of the binary mixture is obtained as a function of number of hollow microgels in the suspension. Surprisingly, we found that crystals can still form at fractions of hollow microgels as high as 50%. The evolution of the crystalline structures, obtained using both small-angle X-ray scattering and UV-Vis spectroscopy, suggests that hollow microgels gradually inhibit crystallization. Therefore, they can be considered a new type of "defect" that has no counterpart in the atomic crystals since they suppress crystallization not due to size mismatch but a different internal architecture