Colloidal crystallization of thermosensitive gel spheres of poly(N-isopropylacrylamide) with low degree of cross-linking

Morphology, phase diagram, and reflection spectroscopy of the colloidal crystals of thermosensitive gel spheres, poly(N-isopropylacrylamide) ((200–0.5), 318 and 116 nm in the hydrodynamic diameter at 25°C and 45°C, and 0.5% in the degree of cross-linking) were studied. Giant colloidal single crystals formed at very low gel concentrations. Densities of the gel spheres were 0.030 and 0.61 at 25°C and 45°C, respectively. Critical concentration of melting of gel spheres (0.8 wt.% without ion-exchange resins) decreased sharply to 0.015 wt.% at 25°C as the gel suspension was deionized exhaustively with coexistence of the mixtures of cation and anion exchange resins. These results demonstrate that the colloidal crystallization takes place by the extended electrical double layers formed around the gel spheres in addition of the excluded volume effect of the gels. Extent of the contribution of the electrical double layers on the crystallization increased sharply when the degree of cross-linking increased, the gel spheres shrank, and/or the density of the gel spheres increased.