Preparation of γ-PGA hydrogels and swelling behaviors in salt solutions with different ionic valence numbers

In this paper, a novel poly γ-glutamic acid (γ-PGA) hydrogel was successfully synthesized by solution polymerization and ethylene glycol diglycidyl ether (EGDE) was used as crosslinker. The structure and morphology of the γ-PGA was investigated using scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) techniques. The influence of ionic (Na+, Ca2+ and Fe3+) concentrations, temperature and pH on the swelling degree of γ-PGA hydrogels was studied. The results revealed that the swelling capacity is enhanced with increasing pH, but decreased with increasing ionic concentration. The swelling kinetics study indicated that γ-PGA hydrogels were ion-sensitive and presented more limited swelling kinetics in metal ion solutions with higher ionic valence numbers. The results showed that the pseudo-second-order adsorption kinetics were predominant. Maximum swelling degrees were obtained for γ-PGA hydrogels with 21.03 g g−1 in 0.154 mol L−1 NaCl solution, 8.5 g g−1 in 0.154 mol L−1 CaCl2 solution and 12.8 g g−1 in 0.154 mol L−1 FeCl3 solution. Considering the simple preparation process, the ionic- and pH-sensitive properties, this γ-PGA hydrogel might have high potential to be used in medical and other fields.