BRILLOUIN-SCATTERING STUDY OF THE VOLUME PHASE-TRANSITION IN POLY-N-ISOPROPYLACRYLAMIDE GELS

Brillouin scattering spectra of poly-N-isopropylacrylamide (NIPA) gels swollen to equilibrium in water were measured as a function of temperature both in the swollen and the shrunken phases. This is the brst Brillouin scattering experiment on a gel undergoing a volume phase transition. At the discontinuous volume phase transition occuring at 33.6 degrees C, large changes were observed in both of the Brillouin shift and the spectral width. From the measured spectra and the temperature-dependent refractive index of the gel, the hypersonic velocity and the attenuation were determined as a function of temperature. The results are analyzed in terms of the nature of sound propagation in gels, structure of the shrunken phase, relaxation processes of the network and solvent, and a role played by water molecules at the phase transition.