Change of Elastic Modulus of Strongly Charged Hydrogels at the Collapse Transition

Abstract

The swelling and mechanical behavior of polyelectrolyte gels with the degree of ionization varying from 7 to 100 mol % were studied in mixtures of a good solvent and a poor solvent. In a good solvent region for weakly charged gels the shear modulus decreases with the gel swelling, while for strongly charged gels swelling leads to the increase of the shear modulus. Also for strongly charged gels the elastic modulus can remain unchanged during the volume phase transition (for a weakly charged gel, a jump in the volume at the collapse transition is usually accompanied by a discontinuous upturn of the elastic modulus). Both of these features of strongly charged gels are attributed to a significant extension of the highly ionized subchains. At the same degree of swelling the modulus of the fully ionized gel was higher in the medium of lower polarity. This may be due to the formation of new effective cross-links between the subchains as a result of dipole−dipole attraction of ion pairs formed between charged groups of network and condensed counterions.