Thermoresponsive Ultrathin Hydrogels Prepared by Sequential Chemical Reactions

Abstract

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.

Ultrathin Hydrogels Prepared from Poly(acrylic acid-co-acrylamide) and Poly(vinylamine) on a Solid Substrate

“…These swelling ratios were similar to those reported for ultrathin hydrogels previously prepared using copolymers of N-vinylalkylamides. 5,6 Even in the absence of EDC, the film obtained swelled slightly with water molecules, possibly due to the presence of monomer units that did not related to the noncovalent interactions between the polymers. The percent incremental increase in assembly thickness was dependent on the amount of EDC, and increased with the amount of EDC.…”

“…5,6 It was demonstrated that copolymers of acrylamides are also available as constituents in the formation of ultrathin hydrogels. Thickness analysis in air and water revealed the unique water swelling properties of the resulting ultrathin hydrogels.…”

“…5,6 The crystal (9 mm in diameter) was coated on both sides with gold electrodes 4.5 mm in diameter, the mean roughness of which was 1.7 nm. The frequency was monitored by an Iwatsu frequency counter (Model SC7201).…”

“…In our previous papers, sequential formation of amide linkages in LbL assemblies between primary amine containing copolymers of N-vinylalkylamides such as poly(vinylamine-co-N-vinylisobutyramide) (poly(VAm-co-NVIBA)) 5 or poly(vinylamine-co-Nvinylformamide) (poly(VAm-co-NVF)), 6 plus poly-(acrylic acid) (polyAAc) in the presence of a watersoluble carbodiimide, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) was used in the fabrication of ultrathin films on a solid substrate. Subsequent immersion into aqueous phase resulted in an increase in the film thickness as a result of swelling with water molecules due to the presence of hydrophilic N-vinylalkylamide units, thus fabricating ultrathin hydrogels.…”

“…These findings indicated that ultrathin hydrogels could be prepared by sequential chemical reaction of certain copolymers having hydrophilic neutral units. Although copolymers of Nvinylalkylamides are potentially available for the preparation of ultrathin hydrogels in conjunction with polyAAc, at least approximately 40 mol% 5 and 30 mol% 6 of VAm content in poly(VAm-co-NVIBA) and poly(VAm-co-NVF), respectively, were necessary for stepwise deposition with polyAAc. Otherwise, the films were not deposited in an LbL manner.…”