Swelling Behavior of Poly(2‐hydroxyethyl Methacrylate‐co‐acrylic Acid‐co‐ammonium Acrylate) Hydrogels

The purpose of this study was to prepare a series of pH‐sensitive hydrogels from 2‐hydroxyethyl methacrylate (HEMA), acrylic acid (AA), and ammonium acrylate, NH₄Ac, which was obtained by neutralization of AA with ammonium hydroxide. Hydrogels were prepared by free radical copolymerization in aqueous solution in the presence of redox initiators, Na₂S₂O₈/Na₂S₂O₅, and crosslinker, ethylene glycol dimethacrylate, EGDMA. The copolymers were synthesized by varying AA, EGDMA, and NH₄OH concentrations. The effect of pH, temperature, ionic strength, concentration of crosslinker, and AA content on the swelling behavior of the copolymeric gels were investigated. Equilibrium swelling studies, in the pH range of 2–8, were performed to determine the polymer mesh size (ξ), 14.17–127.6 Å, molecular weight between crosslinks (M _c ), 324–10229 g/mol, and crosslinking density (q), 0.012–0.366, by using the Flory‐Rehner equation. Copolymeric gels exhibited reversible change in their swelling behaviors in response to cycling pH. The diffusional exponent values (n) of the synthesized hydrogels were found in the range of 0.77–0.80, indicating a non‐Fickian diffusion mechanism. FT‐IR spectral analysis was also performed in order to confirm the formation of copolymer from the bands that appeared as a result of functional groups. It was concluded that these hydrogels demonstrated a sharp change in their water absorbency and mesh size of the networks with a change in the pH of the swelling media, suggesting their strong candidature for being used as oral drug delivery systems and ion‐exchangers for removal of metal ions from aqueous media, owing to the carboxylate groups within the polymeric network.

Keywords:

• hydrogel
• pH‐sensitive hydrogels
• acrylic acid
• 2‐hydroxyethyl methacrylate
• molecular weigth between crosslinks
• mesh size