Effect of Molecular Weight Selection and Relative Humidity on the Moisture Absorption and Desorption Behavior of Poly(N-isopropylacrylamide)-based Hydrogels

Temperature-sensitive random copolymerized nanohydrogels were prepared via a one-pot polymerization method using N-isopropylacrylamide (NIPAM) and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) as raw materials. Transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) analyses revealed the partially crystallized porous nanostructure of the gels, which is consistent with the characteristics of porous nanohydrogel materials. The low-molecular-weight polymers exhibited enhancement and sharpening of the end group peaks in Fourier-transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), and nuclear magnetic resonance hydrogen (¹H-NMR) spectra due to the high proportion of small molecules or low-molecular-weight chain segments. In turn, the high-molecular-weight polymers showed pronounced peaks in the main chain segments because of the long-chain effect. Hygroscopicity increased with the molecular weight of the selected polymers, but was inhibited by temperatures below the lower critical solution temperature (LCST). Meanwhile, moisture desorption was faster in low-molecular-weight samples, and the overall moisture desorption rate rose above the LCST value. According to the kinetic analysis, the moisture absorption process conformed to the quasi-primary or quasi-secondary kinetic model, whereas the moisture desorption followed the quasi-secondary model. Moisture cycling experiments showed that the material maintained stable moisture absorption and desorption performance after several cycles, which is essential for long-term cycling.