Isothermal Dehydration of the Poly(acrylic -co-methacrylic acid) Hydrogel

The isothermal thermogravimetric dehydration curves of an equilibrium swollen hydrogel of a poly(acrylic-co-methacrylic acid) (PAM) hydrogel at temperatures in the range of 293-313 K were determined. By applying a model-fitting method, it was established that the isothermal dehydration of equilibrium swollen PAM hydrogel was a phase-boundary-controlled process with kinetics limited with the rate of the decrease of the area of hydrogel pores. Based on the results of a differential isoconversional method, it was concluded that, in the degree of dehydration range of 0.1 <= alpha <= 0.8, the activation energy is independent of the degree of dehydration, whereas, for alpha > 0.8, it decreases with increasing degree of dehydration. It is assumed that the kinetics of the isothermal dehydration of hydrogel is determined with the changes in the fluctuation structure, because of dehydration. It is established that the dominant effect on the kinetics of dehydration of PAM hydrogel in the range of 0.1 <= alpha <= 0.8 is referred to as a so-called "lambda-relaxation process", whereas in the range of alpha >= 0.8, it is the so-called "alpha-relaxation process" in hydrogels.