Solvato-morphologically controlled, reversible NIPAAm hydrogel photoactuators

Abstract

Photo-actuator hydrogels were generated using a N-isopropylacrylamide-co-acrylated spiropyran-co-acrylic acid (p(NIPAAm-co-SP-co-AA)) copolymer, in 100–1–5 mole ratio. Different ratios of deionised water : organic solvent (tetrahydrofuran, dioxane and acetone) were used as the polymerisation solvent. By changing the polymerisation solvent, the pore size and density of the hydrogels were altered, which in turn had an impact on the diffusion path-length of water molecules, thus influencing the swelling and photo-induced shrinking kinetics of the hydrogel. We successfully demonstrated that the polymerisation solvent has a significant effect on the curing time, the elasticity and morphology of the resulting hydrogel. The highest shrinking ratio was obtained for hydrogels produced using 4 : 1 acetone : deionised water (CI) as the polymerisation solvent, with the hydrogel reaching 39.56% (±2.37% (n = 3)) of its hydrated area after 4 min of white light irradiation followed by reswelling in the dark to 61.95% (±5.76% (n = 3)) after 11 min. Conversely, the best reswelling capabilities were obtained for the hydrogels produced using 1 : 1 tetrahydrofuran : deionised water (AIII), when the shrunk hydrogel (61.78 ± 0.26% (n = 3)) regained 91.31% (±0.22% (n = 3)) of its original size after 11 min in the dark. To our knowledge, this is the largest reported photo-induced area change for self-protonated spiropyran containing hydrogels. The shrinking/reswelling process was completely reversible in DI water with no detectable hysteresis over three repeat irradiation cycles.