Synthesis and characterization of poly(4-acetyl acryloyl ethyl acetate-co-acrylic acid) novel temperature/pH-sensitive hydrogels

A novel monomer called 4-acetyl acryloyl ethyl acetate (AAEA) was synthesized from acetoacetic ester and acryloyl chloride. Copolymerization was carried out between AAEA and acrylic acid (AA) using azoisobutyronitrile as initiator and N,N′-methylene bis(acrylamide) as crosslinking agent. The novel monomer and the hydrogel were characterized using 1H NMR and FT-IR. A peak at 17.78 ppm in the 1H NMR spectra and an absorption band at 1584 cm−1 in the FT-IR spectra of AAEA indicated that AAEA mainly existed as enol forms. In the IR spectra of P(AAEA-co-AA), no peak was observed at around 1600 cm−1 confirmed that copolymerization between AAEA and AA was carried out successfully. The volume phase transition temperature (VPTT) of the hydrogels was determined by their respective DSC thermograms of swollen hydrogel specimens. The results showed that the VPTT of the hydrogels ranged from 50.3 to 61.8 °C, and increased significantly with the increasing dosage of AA. These series of hydrogels exhibited good thermo-sensitivity. When the temperature was higher than the critical temperature, the hydrogels deswelled significantly. Moreover, the critical temperature of the hydrogels increased significantly with the increasing dosage of AA. The pH sensitivity of the hydrogels was also studied. The results indicated that the hydrogels exhibited a discontinuous volume change at around pH = 9.5.