Macroporous composite IPN hydrogels based on poly(acrylamide) and chitosan with tuned swelling and sorption of cationic dyes

The synthesis of macroporous ionic composite cryogels consisting of two independently cross-linked and oppositely charged networks is reported in the paper. Semi-interpenetrating network cryogels were prepared first by cross-linking polymerization of acrylamide (AAm) with N,N′-methylenebisacrylamide (BAAm) in the presence of chitosan (CS), under freezing conditions, the main parameters varied being cross-linker ratio (X), pH of the CS solution, and CS molar mass. The fraction of CS trapped in the semi-IPN cryogels increased with the increase of both X and pH of CS solution. The CS chains trapped in the semi-IPN cryogels have been cross-linked with epichlorohydrin under alkaline conditions, to generate the second network, when a partial hydrolysis of the amide groups in the PAAm matrix simultaneously occurred. Examination of scanning electron microscope images showed inter-connected macropores, in both semi-IPN and full-IPN cryogels. The generation of an anionic matrix during the formation of the second network led to a high sorption capacity of a model cationic dye, methylene blue (MB), by the full-IPN cryogel, around 750 mg dye/g gel, at 25 °C. The benefits of the full-IPN cryogels consist of their high reusability and in the high selectivity in separation of MB from the mixture with methyl orange.

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► Macroporous hydrogels by cryopolymerization of acrylamide in presence of chitosan.
► Anionic full-IPN cryogels by cross-linking of chitosan chains in alkaline medium.
► Cryogels with high sorption capacity of cationic dyes and high reusability.
► High selectivity in separation of cationic dyes from the mixture with anionic dyes.