Multiresponsive Poly(N-isopropylacrylamide) Copolymer Toward Metal Ion Recognition and Adsorption Via a Thermally Induced Sol-Gel Transition

Poly(N-isopropylacrylamide-co-maleic acid-co-1-vinylimidazole) (P(NIPAM-MA-VI)) has been prepared in water by free radical polymerization. The composition and molecular weight of copolymers were obtained by ¹H nuclear magnetic resonance (¹H NMR) spectroscopy and gel permeation chromatography (GPC). The aqueous solution of low molecular weight P(NIPAM-MA-VI) above a critical concentration experiences four distinct phases. Effects of monomer ratio, copolymer concentration and pH on phase transition temperatures were studied. The addition of divalent metal ions was found to promote gelation process, therefore the adsorption of P(NIPAM-MA-VI) to Cu²⁺ ions is further investigated. The adsorption behavior is pH-dependent and the adsorption isotherm is well fitted by the Freundlich model. In 3 mL of Cu(Ac)₂ solution containing 5.76 mg Cu²⁺ ions, the adsorption capacity reaches 19.1 mg/g dried polymer at 60 ^oC and pH = 5. The adsorbed P(NIPAM-MA-VI) can be regenerated by hydrochloric acid and reused in the following adsorption process. Synchrotron radiation small-angle X-ray scattering (SAXS) was used to investigate the effect of temperature and Cu²⁺ ions on the microstructure of P(NIPAM-MA-VI). This thermo-, pH-, and metal ion-responsive copolymer can be applied to ion recognition, water purification and enrichment of heavy metal ions.