Synthesis and characterization of a pH- and ionic strength-responsive hydrogel

Novel polyelectrolyte hydrogels based on methacrylic acid (MAA) and acrylonitrile (AN) monomers were prepared in the presence of a cross-linker, N,N- methylenebisacrylamide. The effect of pH and ionic strength of buffer solutions on swelling behavior of the hydrogels was examined. In particular, a reversible swelling behavior at pH 1.4 and 7.4 solutions was investigated. The structure and morphologies of the sensitive hydrogels were characterized by a Fourier transformation infrared spectroscopy (FTIR) and a scanning electron microscope (SEM). A differential scanning calorimeter (DSC) was used to analyze the state of water and to quantitively determine the amount of free water, non-freezing bound water, and so on. The results indicated that the hydrogels were strikingly sensitive to pH value and ionic strength of buffer solutions. The swelling ratios of gels increased with the increase in pH values, but decreased with ionic strength increased. The gels presented reversible phase transition characteristics with the change in environmental pH. It is concerned with the formation and dissociation of the carboxyl groups and is expected to be adopted as an excellent material in oral drug controlled release.