Superfast and Reversible Thermoresponse of Poly(N-isopropylacrylamide) Hydrogels Grafted on Macroporous Poly(vinyl alcohol) Formaldehyde Sponges

Poly(N-isopropylacrylamide) (PNIPAAm), a typical thermoresponsive polymer, exhibits potential application in smart materials. However, bulk PNIPAAm hydrogel monoliths undergo slow volume phase transition at least tens of minutes to hours as determined by the shape and size of polymers due to the formation of the skin layer. In this regard, novel macroporous sponges with rapid thermoresponse are prepared via grafting polymerization of N-isopropylacrylamide (NIPAAm) onto the macroporous poly(vinyl alcohol) formaldehyde (PVF) network as confirmed by attenuated total reflection-infrared (ATR IR) and ¹H NMR spectra. As prepared PVF-g-PNIPAAm sponges display interconnected open-cell structures, and their average pore sizes and porosities are ∼90 μm and >85%, respectively. The equilibrium swelling ratio of PVF-g-PNIPAAm sponges varies from 11 to 50 with temperature. The volume phase transition temperature is at 30–34 °C, as detected in the DSC curves of swollen samples. These features indicate that the existence of the original PVF network exerts almost no influence on the PNIPAAm temperature responsibility. As prepared samples can reach the swelling equilibrium in less than 80 s, and their rapid swelling kinetics can be fitted using the pseudo-first-order rate kinetic equation. Notably, the samples also display rapid deswelling rate in less than 40 s at relative high temperature (48 °C), thereby indicating a superfast responsive behavior to temperature change. The PVF-g-PNIPAAm sponges exhibit rapid and reversible thermoresponse in repeatable swelling–deswelling cycles, which can satisfy the need of special smart materials. In particular, combined with iodine solution (i.e., PVF-g-PNIPAAm/I₂), these sponges can serve as a novel temperature indicator and exhibit excellent antibacterial performances.