The kinetics analyses of isothermal dehydration of PDMAA hydrogel by application of dispersive kinetics model

The poly-N,N-dimethyl acrylamide (PDMAA) hydrogel was synthesized and its physicochemical properties were determined. The isothermal thermogravimetric curves of PDMAA hydrogel dehydration were measured at the five temperatures in the temperature range from 313 to 353 K. The apparent activation energy dependence on the dehydration degree was determined by Vyazovkin's isoconversional method. The conversion curves were fitted by the commonly-used equations for the processes with nucleation-limited steps. It was found that all conversion curves can be completely described by the acceleratory model equation of the dispersive kinetics model based on the Maxwell-Boltzmann distribution of activation energies. The dependencies of model equation parameters (a and b), the rate constant, and the activation energy on dehydration time were determined for all investigated temperatures. During the dehydration, the state of absorbed water within the hydrogel is changed continuously, relaxation processes within the hydrogel become faster and thus, dehydration nuclei with a bigger critical radius are formed, resulting in a decrease in the activation energy.