FLUORESCENCE STUDIES OF THE VOLUME PHASE-TRANSITION OF POLY(ACRYLAMIDE) GELS WITH A DANSYL GROUP

The changes in microenvironments during a pH-induced volume transition of poly(acrylamide) (PAAm) gels with a dansyl group as a fluorescent probe in an acetone/water (9/11) mixed solvent have been studied by the fluorescence spectrum, anisotropy, and lifetime measurements. Irrespective of the presence or absence of fluorescence probe groups labeled on the network of PAAm gels, the gels are in the collapsed state at pH = 1.0-5.0. They show a discontinuous volume phase transition (DVPT) at pH = 5.1 and remain swollen up to pH = 9.6. The fluorescence anisotropy ratio, r, also changes at pH = 5.1 for the dansyl group, but the peak shift of the fluorescence spectra of the dansyl group takes place at pH = 3.8, i.e., before the DVPT. The fluorescence decay measurements demonstrated two emitting states for the excited dansyl group depending on pH values. The microscopic changes in hydrophobicity and mobility around polymer segments are demonstrated at pH = 3.8 by the fluorescent dansyl probe, although the macroscopic DVPT occurs at pH = 5.1.