Flory-Huggins parameters for weakly crosslinked hydrogels of poly(acrylic acid) and poly(methacrylic acid) with various degrees of ionization

Abstract

The interaction of poly(acrylic acid) and poly(methacrylic acid) polyelectrolyte networks with water in relation to their degrees of ionization is studied via the methods of isothermal equilibrium sorption and microcalorimetry. On the basis of these data, the integral Flory-Huggins parameters and their enthalpy and entropy components are calculated. It is shown that, at all degrees of ionization, water is a good solvent for poly(acrylic acid) gels and a poor solvent for poly(methacrylic acid) gels. At low degrees of ionization of networks, the thermodynamic quality of the swelling medium worsens for both gel series relative to that of the uncharged network. As the degree of ionization of the poly(acrylic acid) gel is increased above 10%, the quality of the solvent improves, and, at high degrees of ionization, the Flory-Huggins parameter becomes close to zero. For PMAA gels, the quality of the solvent improves when the degree of ionization is above 40%. An analysis of the enthalpy and entropy components of the binary interaction parameter shows that the observed tendencies are primarily related to the loss of entropy as a result of structuring processes in poly(acrylic acid) and poly(methacrylic acid) hydrogels. For poly(methacrylic acid) gels, an increase in the degree of ionization leads to increases in the negative values of the noncombinatorial entropy and the positive values of the entropy interaction parameter. This phenomenon is probably associated with the development of hydrophobic association processes. For hydrophilic poly(acrylic acid) gels, the entropy interaction parameter monotonically declines to zero with an increase in the degree of ionization, thereby ensuring improvement in the thermodynamic quality of the medium.