Porous self-protonating spiropyran-based NIPAAm gels with improved reswelling kinetics

[EN] This study focuses on improving the speed ofactuation of poly(N-isopropylacrylamide)-based photoresponsive gels containing copolymerised spiropyran as the photoswitch and acrylic acid as the proton source. The improvement is realised by introducing pores into the material. For this purpose, polyethylene glycol (PEG) of two molecular weights (2000 and 20000 g mol-1) has been used as the porophore in the polymerisation mixture. Upon removal of the PEG porophore post polymerisation, hydrogels of different pore sizes were obtained. This impacts the diffusion of water molecules moving in/out of the hydrogel matrix, thus improving the swelling and shrinking kinetics of the hydrogel due to reduction of the average diffusion pathlength. Most significant improvement was observed in the reswelling step and is demonstrated with optical microscopy combined with kinetic ultraviolet–visible spectroscopy (UV–Vis) analysis. Scanning electron microscopy reveals the PEG-induced pores to be in the range of 0.1–2 lm. Moreover, the mechanical stability of the gels is confirmed with rheometry. Lastly, the presented photoresponsive porous gels exhibit an order of magnitude faster reswelling rate compared to the non-PEG-produced control sample.