Gelation of copolymers of acrylamide and sodium acrylate upon coordination with Cu²⁺ ions in concentrated solutions

Abstract

The phase state of Cu²⁺ complexes with acrylamide and sodium acrylate copolymers at different copolymer concentrations and components molar ratios was studied. The change in complex stoichiometry from 2 to 3 carboxylate groups per Cu²⁺ ion was detected with an increase of copolymer concentration. In the concentrated solutions of copolymers, gelation is observed upon addition of Cu²⁺ ions, with the lowest concentration of copper required for gelation decreasing with the increase in copolymer concentration. These hydrogels are attributed to the formation of meta-stable inter-polymer metal complexes, which are able to persist due to a low segment mobility. The phase separation of complexes is shown to be associated with the bidentate coordination of carboxylate groups to Cu²⁺ ions, while the hydrogels formed in concentrated solutions mostly consist of bridging structures. Gelation threshold corresponds to the second critical concentration of poly(acrylamide-co-sodium acrylate) and is lower for copolymers with the higher molecular weight. Low acrylate content in the copolymers prevents collapse of the hydrogels caused by the salting-out effect. A higher gel content can be reached by increasing the copolymer concentration or the molar ratio of copper ions to carboxylate groups, with the latter reducing the swelling degree.

Graphical abstract

Keywords:

• Hydrogels
• polymer-metal complexes
• phase diagram
• gelation
• second critical concentration
• copolymers of acrylamide and sodium acrylate

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The research was supported by the Belarusian Republican Foundation for Fundamental Research with a grant No. X23В-012.