Crosslink Polymerization Kinetics and Mechanism of Hydrogels Composed of Acrylic Acid and 2-Acrylamido-2-methylpropane Sulfonic Acid

Crosslink polymerization kinetics of poly(acrylic acid-co-2-acrylamido-2-methylpropane sulfonic acid), AA/AMPS hydrogels, was investigated by using dilatometry in the presence of sodium persulfate as initiator and N,N′-methylene bis(acrylamide) as crosslinker. It was found that the reaction for the crosslink polymerization of AA/AMPS hydrogels had orders of 0.58, 1.14, and 0.86 with respect to the initiator, AMPS, and AA, respectively. From the Arrhenius plots, the activation energy of the crosslink polymerization was found to be about 140 and 89 kJ·mol−1 in the presence and absence of the crosslinker, respectively, in the temperature range from 45 to 65 °C. It was noted that the crosslinker had effects on the reaction order of the initiator and the activation energy due to the formation of cross-linked networks, which was verified by Fourier transfer infrared (FTIR) spectrum. To further confirm the influences of the cross-linked network structure on kinetic parameters of the crosslink polymerization, a mechanism was proposed, which highlights the different termination routes between free radical polymerization and crosslink polymerization. These results suggest that dilatometry provides a convenient tool for crosslink polymerization study, and confirm that the cross-linked networks are formed in the crosslink polymerization.