Fluorescence label studies of thermo-responsive poly(N-isopropylacrylamide) hydrogels

Temperature-induced phase transitions and microenvironment of poly(N-isopropylacrylamide) (PNIPAM) hydrogels were studied in water using 9-(4-N,N-dimethylaminophenyl)phenanthrene (DP) as an intramolecular fluorescence pro be. Fluorescence behavior of the DP-labeled PNIPAM gels depended on the conditions of the gel preparation such as concentrations of monomer and crosslinker. Thermo-responsive behavior of the PNIPAM gel was affected by copolymerization of NIPAM with a hydrophilic monomer N,N-dimethylacrylamide (DMAM) and a hydrophobic monomer methyl methacrylate (MMA). Incorporation of DMAM raised the lower critical solution temperature (LCST) of the PNIPAM gel, and that of MMA lowered it. Obtained results indicate that the NIPAM-DMAM copolymer gels with higher LCST are of a more open, water-swollen nature above their LCST and the NIPAM-MMA copolymer gels with lower LCST are of a less open, water-shrunken nature below their LCST than that of the NIPAM homopolymer gel. (C) 2000 Elsevier Science B.V. All rights reserved.