Radiation synthesis of low swelling acrylamide based hydrogels and determination of average molecular weight between cross-links

A comparative analysis of determination of cross-link density (νe) of hydrogels by using swelling tests and mechanical measurements has been made. Poly(acrylamide/methacrylamide) P(AAm/MAAm) and poly(acrylamide/hydroxyethyl methacrylate) P(AAm/HEMA) hydrogels were prepared by using gamma rays and used as model hydrogel systems. The uniaxial compression test was applied to cylindrical gel samples in the swollen state at pH 7. Stress–strain curves of hydrogels were evaluated to calculate the shear modulus values. The average molecular weight between cross-links (M¯c) and νe obtained from mechanical measurements were significantly different than the values obtained from swelling experiments. Large differences were attributed to the uncertainty on the value of the χ   parameter used in the Flory–Rehner equation. ±1% change in this parameter doubled or reduced the M¯c value of hydrogel to half value.