Brillouin-scattering study of hyaluronic acid: Dynamic coupling with the water of hydration and phase transitions

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Abstract
Brillouin spectra have been measured from wet-spun films of Li and Na hyaluronate (NaHA) between 0% and 93% relative humidity (RH). The speed of sound is very high (about 5 km/s) in both Li and NaHA films at low values of RH. The Brillouin spectra show substantial coupling between the longitudinal-acoustic phonons and a relaxation mode of the water of hydration. A coupled-modes model is used to interpret these data. This analysis shows that the relaxation time of the water of hydration (about 50 ps) is constant over the range of hydration. The microscopic coupling constant also appears to be independent of hydration. Between 84% and 88% RH, the uncoupled phonon frequency for both Li and NaHA is found to drop by ∼40%, and ∼25% for phonons propagating in the parallel and perpendicular directions, respectively, indicative of a phase transition.