Triply-responsive (thermo/light/pH) copolymeric hydrogel of N-isopropylacrylamide with an azobenzene-containing monomer

A series of triply-responsive copolymer hydrogel was firstly synthesized via free radical polymerization of N-isopropylacrylamide, 4-[(4-(acryloyloxy)ethoxy) phenylazo]benzoic acid (AEPAZA), a water-soluble azobenzene-containing comonomer, and crosslinker. The properties of the hydrogel were subsequently investigated by UV-Vis absorption spectrometry, differential scanning calorimetry, and gravimetry. UV-Vis spectra showed that the copolymer hydrogel had a reasonable trancis photoisomerization rate upon UV irradiation and a relatively slow recovery rate in dark. The critical solution temperature (CST) of the copolymer hydrogel decreased with increasing of AEPAZA content and subsequently shifted higher value after UV irradiation. Meanwhile, the phase transitions profile became slower and broader with the incorporation of AEPAZA, which could be partly counterbalanced by UV irradiation. The CST could also be affected by the pH value of buffer solution. In addition, equilibrium swelling behavior clearly showed that the swelling ratio could be modulated by temperature, light, and pH. These stimuli-responsive properties should be very important for controlled release delivery system. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012