Structural Parameters and Swelling Behavior of pH Sensitive Poly(acrylamide-co-acrylic acid) Hydrogels

In the present work, hydrogels based on acrylamide (AAm) and acrylic acid (AAc), crosslinked with N,N'-methylenebisacrylamide (MBAAm) were prepared by free radical polymerization in solution. The effect of initial AAm/AAc mole ratio and nominal crosslinking ratio (moles of crosslinking agent/moles of polymer repeat unit) on the dynamic and equilibrium swelling behaviour of hydrogels was investigated. Hydrogels were characterized by the polymer volume fraction in the swollen state (nu(2,s)), the number average molecular mass between crosslinks ((M) over bar (c)) and the mesh size (xi). The swelling capacity of hydrogels was found to decrease with increasing nominal crosslinking ratio. The results show a significant influence of AAc monomer concentration on swelling behavior of poly(acrylamide-co-acrylic acid) hydrogels. Hydrogels containing higher acrylic acid content had a higher equilibrium mass swelling at pH 7.4 and in distilled water than at pH 3.0 where the acrylic acid is present in nonionized state. Further, swelling data was fitted to various models and model parameters were evaluated using regression technique. Model analysis indicated that the swelling transport followed non-Fickian mechanism. Scanning electron microscopy (SEM) revealed the macroporous surface morphology of the matrix with pore size varying between 2-64 microns depending on the amounts of AAc in the hydrogel.