Humidity responsiveness of a poly(N-isopropylacrylamide) gel with a PEG/water mixed solvent

This manuscript presents the humidity responsiveness of a poly(N-isopropylacrylamide) (PNIPAAm) gel, which was capable of exhibiting volume changes. The strategy for inducing volume changes in the gel capitalized on the differences in PNIPAAm solubility, which stemmed from variations in the polyethylene glycol (PEG) proportions of PEG/water solvent mixtures. Due to the hygroscopic nature of PEG, PNIPAAm gels using PEG as the solvent (weight fraction of PEG, φ_PEG = 1.00) absorbed ambient moisture, resulting in a decrease in φ_PEG over time. In concert with this moisture absorption process, the PNIPAAm gel contracted owing to phase separation at a consistent room temperature, which was attributed to the diminished solubility of PNIPAAm in the solvent. During this investigation, we scrutinized the temperature dependency of the optical transmittance and conducted differential scanning calorimetry (DSC) assessments on free PNIPAAm in water across various φ_PEG values to substantiate phase separation of the system. We then demonstrated that a PNIPAAm gel with a PEG/water mixed solvent underwent volume changes in response to humidity changes. Additionally, humidity-induced shape deformations were realized with a PNIPAAm and poly(N,N-dimethylacrylamide) (PDMAAm) composite gel. This study introduces, for the first time, a humidity-responsive PNIPAAm gel with a PEG/water mixed solvent.