Anomalous behavior of acoustic phonon in dense thermo-responsive microgel suspension studied using Brillouin scattering

We report the propagation of sound waves in a suspension of dense thermo-responsive poly(N-isopropyl acrylamide) (PNIPAM) microgels as a function of temperature using Brillouin light scattering. In the dense PNIPAM microgel glass, a single longitudinal acoustic (LA) mode is found to propagate at the velocity intermediate between that of polymer and water phases, and is attributed to the sound wave propagating through the water-microgel composite media. The LA mode shows two interesting features with respect to temperature: i) frequency of LA mode increases upon increase in temperature with anomalous behavior in the vicinity of glass transition and volume phase transition temperatures, ii) the width of LA mode shows minimum near volume phase transition temperature. These observations are attributed to the variation of size and elasticity of PNIPAM microgel particles with temperature. At room temperature the static and dynamic light scattering (DLS) studies on dense PNIPAM microgel suspension confirm a glassy state. In addition, DLS is used to identify the volume phase transition of microgels and glass transition of dense suspension. These temperatures corresponding to glass transition and volume phase transition are found to be consistent with that obtained from Brillouin light scattering results.