Fluorescence study on the swelling behavior of comb-type grafted poly(N-isopropylacrylamide) hydrogels

The shrinking mechanism of comb-type grafted poly(N-isopropylacrylamide) gel due to temperature jump across its volume transition temperature has been investigated. Grafted chains or gel networks were labeled by a dansyl probe, and the temporal change in microenvironment of the dansyl-labeled gel was investigated by means of fluorescence spectroscopy. The comb-type grafted poly(N-isopropylacrylamide) gel exhibited a rapid shrinking compared to normal-type NIPA gel, and the change in its microenvironment was found to become hydrophobic more than 10 times faster than normal-type poly(N-isopropylacrylamide) gel by observation of temporal change in the maximum emission wavelength, λ_em, of the dansyl group. The freely mobile characteristics of grafted chains are expected to show the rapid dehydration to make tightly packed globules with temperature, followed by the subsequent hydrophobic intermolecular aggregation of dehydrated graft chains. The dehydrated grafted chains created the hydrophobic cores, which enhance the hydrophobic aggregation of the networks.