The formation of crystalline hydrogel films by self-crosslinking microgels

Monodisperse spheres composed of a copolymer of poly(N-isopropylacrylamide) (PNIPAM) and N-hydroxymethylacrylamide (NMA) were prepared and used as building blocks for a thin hydrogel film. A dilute dispersion of these microgels was allowed to dry in air, resulting in the formation of crystalline microgel structures at the air/dispersion interface. Taking advantage of self-crosslinking property of NMA, the neighboring PNIPAM-NMA microgels reacted to form covalent bonds to stabilize the crystalline structure after the water was completely evaporated from the particle dispersion at ambient temperature. After immersion in water, this film not only absorbed a large amount of water like a conventional hydrogel but also exhibited iridescent colors due to long range ordered microgel arrays. The thermally responsive properties and mechanical properties of this film were studied.