THERMODYNAMICS OF THE INTERACTION OF POLYACRYLIC-ACID WITH WATER

The Gibbs energies, enthalpies, and entropies of mixing water with polyacrylic acid (PAA) having a molecular mass (MM) in the range from 4 x 10(3) to 2 x 10(6) were determined over the entire range of compositions. The plots of the enthalpy and entropy of mixing versus the solution composition have shapes that have not been reported earlier for polymer solutions: the curves for all MMs exhibit additional minima and maxima at a PAA content of 65 - 75% in the solution. The results were analyzed taking into account the glassy state of PAA, interactions between AA units and water molecules, UCST and LCST criteria, and conformational changes in PAA. It is suggested that the observed additional extrema in the concentration dependence of enthalpy and entropy of mixing are due to the conformational transitions in PAA macromolecules.