Self-Organization of Soft Hydrogel Microspheres during the Evaporation of Aqueous Droplets.

The unique drying behavior of aqueous droplets that contain soft hydrogel microspheres (microgels) upon evaporation was systematically investigated. Compared to the ring-shaped deposits that are obtained from drying solid microsphere dispersions, we have previously reported that uniformly ordered thin films are obtained from drying ∼1.2 µm-sized poly( N-isopropyl acrylamide) microgel dispersions. In the present study, we thoroughly investigated several hitherto unexplored aspects of this self-organization, such as the effect of the size, chemical structure, and "softness" of the microgels (or rigid microspheres). For the macro- and microscopic observation of the drying behavior of various microsphere dispersions, an optical microscope and a digital camera were employed. The results suggested that the convection in the aqueous droplets plays an important role for the transportation of the microgels to the air/water interface, where the softness and surface activity of the microgels strongly affects the adsorption of the microgels. On the basis of these discoveries, a design concept for the rapid formation of uniform thin films of soft microgels was proposed.

Assembly of oppositely charged microgels at the air/water interface.

We present an assembly of cationic and anionic microgels at the air/water interface. Monodispersed, micrometer-sized, and oppositely charged microgels were synthesized by aqueous free-radical precipitation polymerization by using N-isopropylmethacrylamide, comonomers, and cross-linkers. The oppositely charged microgels were prepared by using different types of water-soluble initiators. These microgels were mixed in a solution to assemble them at the air/water interface. In particular, linear and branched chainlike structures of the microgels were spontaneously formed at the air/water interface. Effects of salt concentration, microgel concentration, and number ratio of cationic and anionic microgels on the structures of microgel assembly were investigated by optical microscopy and scanning electron microscopy.

Drying mechanism of poly(N-isopropylacrylamide) microgel dispersions.

The drying mechanism of poly(N-isopropylacrylamide) (pNIPAm) microgel dispersions was investigated. The microgels were synthesized by temperature-programmed aqueous free radical precipitation polymerization using NIPAm, N,N'-methylenebis(acrylamide), and water-soluble initiator. Drying processes of the microgel dispersions were observed with a digital camera and an optical microscope, and the resultant dried structures were observed by scanning electron microscopy. We found that the presence of the microgels changed the behavior of the drying process of water. In particular, the microgels were adsorbed at the air/water interface selectively within a few minutes irrespective of the microgel concentration. The relationship between the drying mechanism and structure of the resultant microgel thin film has been clarified by changing the microgel concentration of the dispersions.

Binary mixtures of cationic and anionic microgels.

Colloidal behaviors of binary mixtures composed of cationic and anionic microgels are reported. Both microgels were synthesized by aqueous free radical precipitation polymerization using N-isopropylacrylamide and N,N'-methylenebisacrylamide but using different types of water-soluble initiators and comonomer. Effects of temperature and salt concentration on phase behaviors of binary mixtures of cationic and anionic microgels were investigated as well as single-species microgels by UV-vis spectroscopy. We found that the presence of a small amount of NaCl altered the dispersing behavior of the binary mixtures of cationic and anionic microgels when they were in hydrated and swollen states. In particular, scanning electron microscope observation clarified that the binary mixtures containing a small amount of NaCl were not flocculated, and microgels showed non-close-packed structures on a planar substrate in the dry state. Furthermore, flocculations formed when both microgels were in the swollen states could be redispersed by adding a small amount of NaCl and gently stirring. These tunable properties have not been observed in mixtures of hard particles, and are due to the coexistence of electrostatic interactions and steric hindrance of highly hydrated soft particles.