The parameter of binary interaction in weakly crosslinked polyelectrolyte gels near the collapse threshold

The behavior of partially ionized weakly crosslinked gels based on polyacrylamide that undergo a discrete phase transition-collapse in water-acetone mixtures and the behavior of gels of polyacrylic and poly(methacrylic acids) for which gradual contraction is observed under the same conditions are studied experimentally. The concentration dependences of the enthalpy of swelling for the gels in the mixtures with volume concentrations of acetone of 0, 20, 30, and 40% are measured via isothermal calorimetry and are used to calculate the parameter of interaction between a polymer network and a medium during the approach toward the threshold concentration of collapse. In sharply collapsing gels based on polyacrylamide at an acetone concentration up to 40%, the enthalpy parameter of interaction does not exceed 0.2, a value that is significantly smaller than the critical value. In contrast, in the gels based on polyacids, for which gradual contraction is observed, the enthalpy parameter is as high as 0.6 at the same content of acetone. The experimental data testify that a reduction in the energetics of interaction between a weakly charged polymer network and a medium is not the decisive cause of the discrete phase transition-collapse.