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 °C, 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 °C 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.
Sequential surface chemical reactions of poly(acrylic acid-co-N-isopropylacrylamide) [poly(AAc-co-NIPAAm)] with AAc contents of 5, 10, and 15 mol %, of which carboxyl groups were previously activated by 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide hydrochloride (EDC), plus poly(vinylamine hydrochloride) through amide linkages produced ultrathin films on a solid substrate. Assembly processes were quantitatively monitored by a quartz crystal microbalance as substrates. Assembled amounts increased with decreasing both AAc and EDC amounts. Subsequent immersion of ultrathin films into aqueous media resulted in the thickness increases, producing ultrathin hydrogels. Swelling ratios were estimated by percent increases in the thickness and increased with increasing AAc and EDC amounts. Swelling ratios were regularly changed by varying the ionic strength and pH of aqueous media. Swelling properties were interpreted on the basis of structural information on ultrathin hydrogels. Cyclic voltammetries using potassium ferricyanide revealed that ions permeated ultrathin hydrogels, and permeabilities were clearly suppressed above a lower critical solution temperature (LCST) of polyNIPAAm. Reversible on−off changes in permeabilities below and above a LCST were potentially observed. Not only structural control but also stimuli responsive functions of ultrathin hydrogels were realized within the present study.
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