The effect of processing design and the stabilizing effect of nanostructures on the swelling and adsorption properties of polyvinylpyrrolidone macroporous ferrogels obtained through a cryogenic route

Abstract

This paper reports the synthesis of macroporous polyvinylpyrrolidone (PVP) hydrogels modified with in-situ generated magnetic nanoparticles (NPs). The hydrogels were formed by an optimized thermally induced crosslinking route, initiated by the presence of ammonium persulfate, whereas the macroporosity was created through a cryogenic technique. Crosslinking stabilizes the three dimensional (3D) gel structure, allowing the replacement of expensive and time-consuming lyophilization steps in favor of simpler drying procedures at atmospheric pressure. The in-situ formation of iron oxide NPs (IONPs) was achieved through oxide precursors impregnation followed by precipitation of the IONPs in the highly swellable hydrogels. This reaction was performed by addressing two strategies, namely, before or after cryogenic processing, to evaluate its effect on the final swelling and adsorption properties of the gel matrices. It is demonstrated that the inclusion of the magnetic NPs before cryo-structuring allows the stabilization of the gel against collapse, favoring the conservation of a highly swellable environment. The synergy between the adsorption capacity of the IONPs and the high swelling of the macroporous structure allows obtaining materials with potential in adsorption processes. Preliminary analyses showed that macroporous samples obtained with 32 wt% IONPs led to methylene blue (MB) adsorption capacities as high as 400 mg dye/g ferrogel, for dye concentrations as low as 10 mg/L. These results show that the use of an adequate strategy for the processing of macroporous ferrogels can be of high importance in the design of new adsorbents and superabsorbent hydrogels.