Kinetic of non-isothermal dehydration of equilibrium swollen poly(acrylic acid-co-methacrylic acid) hydrogel

The kinetic of non-isothermal dehydration of equilibrium swollen poly(acrylic acid-co-methacrylic acid) hydrogel (PAM) has been investigated. Thermogravimetric and conversion dehydration curves were recorded at various heating rates 5–30 K min−1. The conversion dehydration curves at all investigated temperatures can be mathematically described using the logistic regression function in entire. It was found that activation energy complexly changes with the increasing dehydration degree. Physical meaning of the parameters of logistic function (b and w) is given. It was established that, during the dehydration, changes in the fluctuating hydrogel structure occur, and that limiting step on the kinetics of hydrogel dehydration have rate of structural rearrangement of hydrogel (actual relaxation mechanism). A procedure for determining the dependence of effective activation energy on temperature and dehydration degree, based on logistic function, is exposed. Possible explanation for the existence of negative values of activation energy in the certain range of temperature, is given.