Water Uptake, Thin-Film Characterization, and Gravimetric pH-Sensing of Poly(vinylphosphonate)-Based Hydrogels

Herein, novel, superabsorbent, and pH-responsive hydrogels obtained by the photochemical cross-linking of hydrophilic poly(vinylphosphonates) are introduced. First, statistical copolymers of diethyl vinylphosphonate (DEVP) and diallyl vinylphosphonate (DAlVP) are synthesized via rare earth metal-mediated group-transfer polymerization (REM-GTP) yielding similar molecular weights (M_n,NMR = 127-142 kg/mol) and narrow polydispersities (Đ < 1.12). Subsequently, polymer analogous transformations of P(DEVP-stat-DAlVP) introduced vinylphosphonic acid (VPA) units into the polymers. In this context, the partial dealkylation of the polymers revealed a preference for DAlVP hydrolysis, which was observed via ¹H NMR spectroscopy and explained mechanistically. Furthermore, the P(DEVP-stat-DAlVP-stat-VPA) polymers were cross-linked under photochemical reaction conditions (λ = 365 nm) via thiol-ene click chemistry, yielding superabsorbent hydrogels with water uptakes up to 150 ± 27 g (H₂O)/g (hydrogel). Regarding water absorption, evident structure-property relationships between cross-linking density, polarity, and swelling behavior were found. Finally, the pH-responsiveness of thin films of these hydrogels was investigated. In this regard, films with a thickness of 39.4 ± 2.33 nm determined via profilometry were spin-coated on sensors of a quartz crystal microbalance with dissipation monitoring (QCM-D) and thoroughly characterized by atomic force microscopy (AFM). QCM-D measurements exposing the hydrogel films to different aqueous media revealed different swelling states of the hydrogels depending on the pH values (1, 6, 10, and 13) of the surrounding environment, as reflected by corresponding frequency and dissipation values. The hydrogels exhibited fully reversible swelling and deswelling upon switching between pH 1 and 13 (three cycles), sustaining the harsh conditions without erosion from the gold surface and thus acting as a gravimetric sensor discriminating between the two pH values. The high stability of the films on the gold surfaces of QCM-D sensors was explained by anchoring of the P(DEVP-stat-DAlVP-stat-VPA) networks through the dithiol cross-linker as confirmed by detailed X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) studies.