THE SWELLING PRESSURE OF WEAKLY IONIC ACR YLAMIDE GELS

The swelling pressure of some weakly cross-linked and partly hydrolyze d (2.1-7.8 mol % of units) polyacrylamide hydrogels was measured as a function of the fraction of units with ionic carboxyl groups, the degr ee of swelling, and salt concentration in the external solution. To an alyze the experimental data, a model was used the considered the swell ing pressure as a sum of the contribution from the uncharged network a nd the contribution caused by the difference of pressures of the gas o f mobile ions inside and outside the gel. It was shown that the former contribution can be approximated by the Flory-Rehner equation. The io nic contribution is determined by the charge density in the network an d the salt concentration in the solution; it can be described by the i deal Donnan equilibrium. This results suggest that the common theoreti cal model using these approximations can adequately describe the behav ior of weakly ionic hydrogels on moderate swelling.