Thermoresponsive ultrathin hydrogels prepared by sequential chemical reactions

Thermoresponsive ultrathin hydrogels were prepared by the sequential chemical reaction of poly(vinylamine-co-N-vinylisobutyramide) [poly(VAm-co-NVIBA)] and poly(acrylic acid) (polyAAc) on a gold surface. The carboxyl group of polyAAc was activated by 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide hydrochloride (EDC) for the reaction with the amino group of poly(VAm-co-NVIBA) to yield the amide linkage. This stepwise assembly was analyzed quantitatively by a quartz crystal microbalance (QCM), which revealed that the 39 mol % VAm content of poly(VAm-co-NVIBA) was suitable for the preparation of ultrathin hydrogels. A detailed analysis was performed by the assembly of poly(VAm-co-NVIBA), with a 39 mol % VAm content and polyAAc. The reflection - absorption spectra (RAS) showed the presence of both polymers in the assembly. Atomic force microscopic (AFM) observations confirmed the hydrogel structure of the assembly; the thickness of the hydrogel in an aqueous medium (around 320 nm) was 2 times greater than that in air (around 150 nm). The dynamic process probably proceeded by water swelling into the assembly when immersed in an aqueous phase from air. This was also observed by AFM analysis, in which the surface roughness increased from 14 nm in air to 35 nm until 10 min postimmersion, subsequently decreased to 19 nm, and then saturated after 30 min. The thermoresponsive properties were analyzed by the static contact angle measurement in water by using an air bubble. The assembly became more hydrophobic above 30-40 degreesC, suggesting that the phase transition was derived from the polyNVIBA unit. The phase transition was affected by the pH of the water in the present system, and no phase transition was observed from 20 to 60 degreesC at a pH of 2 and 12. Layer-by-layer assembly using chemical reactions will open a new field of research on the suitable selection of functional polymers.