Effect of Initial N,N'-Methylene Bisacrylamide Concentration on the Swelling Behaviour of Acrylic-based Superabsorbent Polymers

Aseries of superabsorbent polymers based on acrylic acid has been synthesized by a solution polymerization technique at 60°C. The effect of the N,N_-methylene bisacrylamide concentration on the swelling behaviour of these hydrogels in deionized water and saline solution was investigated. Experimental results indicate that the absorbance in deionized water or saline solution decreases on increasing the concentration of crosslinker. Moreover, the amount of swelling in 0.2 wt% NaCl(aq) is smaller than that in deionized water, a result that is due to the decrease in the osmotic pressure (ionic pressure) difference between polymer and external solution at high ionic strength. High swelling and appropriate absorption kinetics can be obtained by altering parameters such as initial monomer concentration, range of neutralization, temperature, initiating system and range of crosslinker concentration. These changes were studied but are not reviewed here. Rates of swelling for these absorbents were found to fit the Voigt expression. Relationships were also established between the ultimate degree of swelling and the ratio of crosslinker to monomer. The results of the investigation involving water or saline solution diffusion in hydrogels approximate to Case II sorption in the early stages of the swelling process and to Case I in the later stages.