Formation of chemical networks of acrylamide and acrylic acid hydrogels initiated by ammonium persulfate

The process of crosslinking in hydrogels of polyacrylamide, poly(acrylic acid), and their copolymers that is initiated by ammonium persulfate is studied. It is shown that, when the threshold concentration of ammonium persulfate, which amounts to 3 mmol/L for PAcA and 18 mol/L for poly(acrylic acid), is exceeded, the chemical network of the gel is formed. A mechanism is advanced to explain the formation of junctions in the chemical networks of polyacrylamide and poly(acrylic acid) gels. It is based on the reaction of chain transfer involving the hydrogen atom in the alpha position relative to the functional groups of a chain. With the use of the Flory-Rehner equation, the degrees of polymerization of network subchains are calculated. Effects of the temperature of synthesis, the concentration of ammonium persulfate, and the composition of the monomer mixture on the degrees of swelling of the resulting gels are examined. The degrees of swelling of polyacrylamide gels filled with nanodispersed zirconia powder are measured. It is found that in the presence of the filler, the formation of the chemical network initiated by ammonium persulfate may be one of the important factors affecting gelation.