Stepwise preparation and characterization of ultrathin hydrogels composed of thermoresponsive 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 (LOST) of polyNIPAAm. Reversible on-off changes in permeabilities below and above a LOST were potentially observed. Not only structural control but also stimuli responsive functions of ultrathin hydrogels were realized within the present study.