Triple Stimuli-Responsive N-Isopropylacrylamide Copolymer toward Metal Ion Recognition and Adsorption via a Thermally Induced Sol–Gel Transition

Abstract

Poly­(N-isopropylacrylamide-co-maleic acid-co-1-vinylimidazole), P­(NIPAM-MA-VI), has been prepared in water by free radical polymerization. The aqueous solution of low-molecular-weight P­(NIPAM-MA-VI) has four distinct phases above a critical concentration. Effects of monomer ratio, copolymer concentration, and pH on the phase transition temperatures of P­(NIPAM-MA-VI) were studied. The addition of divalent metal ions promoted the gelation process of P­(NIPAM-MA-VI). Therefore, the adsorption behavior of P­(NIPAM-MA-VI) to Cu2+ ions was further investigated; this adsorption behavior was pH-dependent, and the adsorption isotherm could be well fitted by the Freundlich model. The maximum adsorption capacity reaches 21.1 mg g–1 dried copolymer at 60 °C and pH 5. The adsorbed P­(NIPAM-MA-VI) can be regenerated by the treatment of hydrochloric acid and reused in the following adsorption process. X-ray photoelectron spectroscopy (XPS) and synchrotron radiation small-angle X-ray scattering (SAXS) were used to investigate the effect of Cu2+ ions on the phase transition behavior of P­(NIPAM-MA-VI). O 1s and N 1s XPS spectra and SAXS results analyzed by the Ornstein–Zernike and generalized Ornstein–Zernike models are in agreement with the proposed mechanisms and adsorption behavior. As a thermo-, pH-, and metal ion-responsive copolymer, P­(NIPAM-MA-VI) could be used for the ion recognition, water purification, and enrichment of heavy metal ions.

Triple‐Responsive Polyampholytic Graft Copolymers as Smart Sensors with Varying Output

“…[ 1 ] Among others, promising materials for corresponding sensors are smart polymers, which can translate external stimuli such as light, temperature, pH, or the presence of metal ions into a physical or chemical readout. [ 2–4 ] If such materials contain specific binding sites for different metal ions, the optical properties such as absorbance, fluorescence, and luminescence [ 5–9 ] or the solution behavior, e.g., the lower critical solution temperature (LCST) [ 10,11 ] or simply the hydrodynamic size, [ 12 ] of a responsive polymer will significantly change upon metal chelation. Hereby, recent progress in the polymerization of functional monomers allows us to incorporate strong ligands such as bipyridine units, [ 9 ] crown ethers, [ 10 ] carboxylates, or imidazoles.…”

“…Hereby, recent progress in the polymerization of functional monomers allows us to incorporate strong ligands such as bipyridine units, [ 9 ] crown ethers, [ 10 ] carboxylates, or imidazoles. [ 11 ] However, as recently shown for materials featuring both carboxylate and hydroxyl moieties, complexation also strongly depends on coordination behavior and concentration of the respective metal ion, affecting inter‐ and intramolecular interactions within polymer chains. [ 13,14 ]…”

“…However, high NIPAAm contents (>90 mol%) in poly( N ‐isopropylacrylamide‐ co ‐maleic acid‐ co ‐1‐vinylimidazole) terpolymers were shown to be of interest for metal ion recognition. [ 11 ] In our case, we combine thermoresponse of NIPAAm grafts with pH‐dependent charge and metal ion complexation of the PDha backbone, resulting in triple‐responsive graft copolymers. These materials are highly sensitive sensors, where even the output can be tuned.…”

Polyvinyl Alcohol‐Based Thermogel with Tunable Gelation and Self‐Healing Property

“…Besides, the thermally driven sol–gel transition further extends its unique applications to several other engineering fields that require more sophisticated sol–gel transitions. Impressive recent examples include the self‐protection of electrochemical systems enabled by thermogelated electrolyte, metal scavenging through reversible sol–gel transition to immobilize metal ions in a recyclable manner, and 3D printing that can construct complex 3D configuration by kinetically controlling the thermogelation process …”

“…The reported self‐catalyzed self‐healing takes 24 h to complete. More recently, Zhang et al developed a type of thermogelated and self‐healing ABA triblock copolymer . Its randomly appendant ureido pyrimidinone (UPy) moieties offers automatous self‐healing properties via dynamic di‐hydrogen bonding.…”