Studies of the fast synthesis and swelling behavior of thermo-sensitive poly (N-isopropylacrylamide) hydrogels synthesized by microwave irradiation

N-isopropylacrylamide (PNIPAAm) hydrogels was synthesized in acetone solution by microwave irradiation. Chemical groups, morphologies and textural properties of the PNIPAAm hydrogels were measured separately by means of Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and accelerated surface area and porosimetry apparatus (ASAP-2010). The swelling kinetics and equilibrium swelling ratio of the PN (synthesized by traditional water-bath heating) and PM (synthesized by microwave irradiation) hydrogels were measured and their swelling kinetic parameters were compared. Results showed that using microwave irradiation method could greatly shorten the reaction time from 24 h by traditional water-bath heating to 20 min under microwave irradiation. SEM micrographs revealed that the PM hydrogels obtained a rough surface, while the surfaces of the PN hydrogels were rather smooth. In addition, the swollen hydrogels synthesized by microwave irradiation had much more interconnected and uniform pores within the matrixes than those of the hydrogels synthesized by normal water-bath heating, which enabled the hydrogels to absorb large amounts of water and led the improvement of their temperature sensitivity. According to the swelling kinetic data, the diffusion of the PM hydrogels at 14°C belonged to non-Fick diffusion, while it changed into Fick diffusion when the solution temperature arrived at 20°C; the diffusion of the PN hydrogels at 14°C belonged to Case II diffusion, while it changed into non-Fick diffusion when the solution temperature reached 20°C.