Drying structures of micrometer-sized cationic gel spheres of lightly cross-linked poly(2-vinyl pyridine) in the deionized aqueous suspension

Drying dissipative patterns of cationic gel crystals of lightly cross-linked poly(2-vinyl pyridine) spheres stabilized with poly(ethylene glycol) (μPEGMA-P2VP (0.1), μPEGMA-P2VP (0.5), and μPEGMA-P2VP (1) (1.12 ~ 1.20 μm in diameter and 0.1, 0.5, and 1 in degrees of cross-linking) were studied on a cover glass, a watch glass and a Petri glass dish. Iridescent colors and hexagonal spots were observed with the naked eyes in the dried film. Two kinds of the broad rings were observed at the outside edge and inner region in the macroscopic drying patterns. The size of the inner rings increased with gel concentration. Formation of the similar sized aggregates (or agglomerates) and their ordered arrays ((a) ordered ring, (b) spoke-lines, (c) net, and (d) lattice structures) have been observed in common for cationic and anionic gel sphere suspensions examined hitherto. However, the ordering of the agglomerates was not observed for μPEGMA-P2VP spheres. Coexistence of the similar-sized face-centered cubic (fcc) and body-centered cubic (bcc) single crystals was observed instead. Interestingly, fcc crystals looked like white clouds and distributed roughly broad ring-like. This morphological effect on the drying patterns of μPEGMA-P2VP cationic gel spheres is considered to be one of the dissipative crystallization phenomena of colloidal gel spheres during drying processes accompanied with the release of the strain during the course of solidification. Importance of the dissipative convectional flow, formation of the electrical double layers around the spheres, and the interaction of the large gel spheres with the substrates was clear during dryness on the drying pattern formation.