Characterization of Ionic Water Absorbent Polymers: Determination of Ionic Content and Effective Crosslink Density

Abstract

SUMMARY The most important property of an absorbent polymer is its sorption capacity. The key variables which control this property are the polymer-solvent interaction parameter $cH1, the crosslink density, and the concentration of the ionic groups in the polymer matrix. For a polymer of given hydrophilicity (quantified by $cH1), crosslink density ($rH1) and ionic content (i) are the variables which can be most readily manipulated to adjust sorption capacity. Both $rHx and i must be measured experimentally, since they cannot be predicted reliably from synthesis data. Here, different methods for determining these parameters are critically evaluated. For samples of well defined geometry, compression experiments can be used to determine crosslink density; for particulate gels, equilibrium swelling measurements can be used to estimate $rHx. Ionic content can be determined in several ways, most directly by titration. Because titration is slow and tedious, a technique exploiting Donnan ion exclusion can be much more effective for routine assays.