pH-sensitive release from poly(Acrylamide-CO-N,N'-methylene bisacrylamide) microspheres.

Poly(Acrylamide-co-N,N′-methylene bisacrylamide) microspheres were prepared and the sustained release of Fe3+ via ion exchange was investigated in response to the hydrochloric acid concentration. The experimental results revealed that the adsorption/desorption of Fe3+ with microspheres proceeds via ion exchange between Fe3+ and proton. The release rates of Fe3+ from microspheres were found to be moderate under proton concentrations below 10–1 mol·dm–3. However, an abrupt increase in the release rate was observed above 10–1 mol·dm–3. The permeability coefficient was proportional to the hydrochloric acid concentration and had a discontinuity at around 10–1 mol·dm–3. A on-off release mechanism from microspheres was demonstrated in response to a small change in the hydrochloric acid concentration between 10–1 mol·dm–3 and 10–3 mol·dm–3. The permeability coefficients were independent of the stirring speed, and the activation energy for sustained release was 63.9 kJ·mol–1, indicating that sustained release was controlled by ion exchange of Fe3+ and proton.