Water Uptake, Thin-Film\nCharacterization, and Gravimetric\npH-Sensing of Poly(vinylphosphonate)-Based Hydrogels

Herein, novel, superabsorbent, and pH-responsive hydrogels\nobtained\nby the photochemical cross-linking of hydrophilic poly(vinylphosphonates)\nare introduced. First, statistical copolymers of diethyl vinylphosphonate\n(DEVP) and diallyl vinylphosphonate (DAlVP) are synthesized via rare\nearth metal-mediated group-transfer polymerization (REM-GTP) yielding\nsimilar molecular weights (<i>M</i><sub>n,NMR</sub> = 127–142\nkg/mol) and narrow polydispersities (<i>Đ</i> <\n1.12). Subsequently, polymer analogous transformations of P(DEVP-<i>stat</i>-DAlVP) introduced vinylphosphonic acid (VPA) units\ninto the polymers. In this context, the partial dealkylation of the\npolymers revealed a preference for DAlVP hydrolysis, which was observed\nvia <sup>1</sup>H NMR spectroscopy and explained mechanistically.\nFurthermore, the P(DEVP-<i>stat</i>-DAlVP-<i>stat</i>-VPA) polymers were cross-linked under photochemical reaction conditions\n(λ = 365 nm) via thiol–ene click chemistry, yielding\nsuperabsorbent hydrogels with water uptakes up to 150 ± 27 g\n(H<sub>2</sub>O)/g (hydrogel). Regarding water absorption, evident\nstructure–property relationships between cross-linking density,\npolarity, and swelling behavior were found. Finally, the pH-responsiveness\nof thin films of these hydrogels was investigated. In this regard,\nfilms with a thickness of 39.4 ± 2.33 nm determined via profilometry\nwere spin-coated on sensors of a quartz crystal microbalance with\ndissipation monitoring (QCM-D) and thoroughly characterized by atomic\nforce microscopy (AFM). QCM-D measurements exposing the hydrogel films\nto different aqueous media revealed different swelling states of the\nhydrogels depending on the pH values (1, 6, 10, and 13) of the surrounding\nenvironment, as reflected by corresponding frequency and dissipation\nvalues. The hydrogels exhibited fully reversible swelling and deswelling\nupon switching between pH 1 and 13 (three cycles), sustaining the\nharsh conditions without erosion from the gold surface and thus acting\nas a gravimetric sensor discriminating between the two pH values.\nThe high stability of the films on the gold surfaces of QCM-D sensors\nwas explained by anchoring of the P(DEVP-<i>stat</i>-DAlVP-<i>stat</i>-VPA) networks through the dithiol cross-linker as confirmed\nby detailed X-ray photoelectron spectroscopy (XPS) and time-of-flight\nsecondary ion mass spectrometry (ToF-SIMS) studies.