Gel immobilization of centimeter-sized and uniform colloidal crystals formed under temperature gradient.

We report the fabrication of large and high-quality charged colloidal crystals that are incorporated in a polymer hydrogel matrix. The colloidal crystals are prepared by the thermally induced unidirectional crystallization of colloidal silica in coexistence with pyridine, whose dissociation degree increases with temperature. Their crystal structures are immobilized in the polymer hydrogel matrix by photoinduced polymerizations. The crystals are large sized (maximum dimensions: 1 x 10 x approximately 30 mm), and their lattice planes are well oriented and parallel to the gel surface. Furthermore, they have excellent spatial uniformity in the Bragg wavelengths (<0.7% in standard deviation). The present gelled colloidal crystals, which are unique in that they have large sizes as well as good optical uniformity, will be useful as photonic materials.

Unidirectional crystallization of charged colloidal silica due to the diffusion of a base.

Dilute aqueous dispersions of charged colloidal silica (particle volume fraction = approximately 0.03-0.04, particle diameter = 110 nm) exhibit unidirectional crystal growth due to the diffusion of a weak base, pyridine (Py). Similar diffusion-crystallization is enabled by a salt of a weak acid and a strong base, sodium hydrogen carbonate (NaHCO3). The resulting crystals consist of columnar (or cubic) crystal grains with a maximum height of a few centimeters and a maximum width of 1 cm. The crystal growth process is attributed to a combination of (i) the diffusion of Py or NaHCO3 accompanied by a charging reaction of the silica particles and (ii) the charge-induced crystallization of the silica colloids. Theoretical growth curves based on the reaction-diffusion model for the case of Py were in good agreement with the observed curves. We also report the immobilization of the resulting large crystals by using a polymer hydrogel matrix.