Properties of bound water in poly(acrylic acid) and its sodium and potassium salts determined by differential scanning calorimetry

The thermal transitions occurring in poly(acrylic acid) and its sodium and potassium salts were investigated over a large range of water content using differential scanning calorimetry at temperatures below the normal melting temperature of water. The bound water was identified as nonfreezing (type I), freezing with a constant melting temperature (type II), and freezing with a melting temperature dependent on the water content (type III). The transition temperatures of the freezing states of the water were determined. Two constant melting temperatures were observed for the type II water in the sodium and potassium polyacrylates, while a single transition of this type was observed for poly(acrylic acid). The sodium polyacrylate absorbed more water in the nonfreezing state than the potassium polyacrylate, and both polyelectrolytes absorbed about three times as much water in this state as the nonionic poly(acrylic acid). The effects of water content on the occurrence of an exotherm at low temperature in the melting scans of the polyelectrolytes are described.