Rate of swelling of sodium polyacrylate

The kinetics of the swelling of sodium polyacrylate (NaPA), a hydrophilic polymer, was studied gravimetrically, by microscopic observation, and calorimetrically. The swelling process followed first‐order kinetics and the rate constant was of the order of 10 −2 s −1 . The gravimetric method, however, was not useful for kinetic studies. The rate constant was depressed by the addition of sodium chloride. The activation energy of the swelling was 46.0 ± 6.2 (kJ mol −1 ) and decreased with the increase in sodium chloride concentration. In acid solution, the activation energy was almost the same as that in water. The heat of the swelling was 196 ± 17 J g −1 and did not vary with the addition of sodium chloride. © 1993 John Wiley & Sons, Inc.