Glass Transition and Water Dynamics in Hyaluronic Acid Hydrogels

Glass transition and water dynamics in hydrated hyaluronic acid (HA) hydrogels crosslinked by divinyl sulfone (DVS) were studied by differential scanning calorimetry (DSC), dielectric relaxation spectroscopy (DRS) and water sorption—desorption (ESI) measurements. A critical water fraction of about hw = 0.17 (g of water per g of hydrated HA) for a change in the hydration properties of the material was estimated. Water crystallization was recorded by DSC during cooling and heating for water fraction values hw ≥ 0.31. The glass transition of the hydrated system was recorded in the water fraction region 0.06 ≤ hw ≤ 0.59. The Tg was found to decrease with increasing hydration level, starting from Tg = −48 °C down to about Tg = −80 °C and then to stabilize there, for the hydration levels where water crystallization occurs, suggesting that the origin of the glass transition is the combined motion of uncrystallized water molecules attached to primary hydration sites and segments of the HA chains. DRS studies revealed two relaxation peaks, associated with the main secondary relaxation process of uncrystallized water molecules (UCW) triggering the mobility of polar groups and the segmental mobility of HA chains (α relaxation). The α relaxation was in good agreement with the results by DSC. A qualitative change in the dynamics of the α relaxation was found for hw = 0.23 and was attributed to a reorganization of water in the material due to structural changes. Finally, the dielectric strength of the relaxation of UCW was found to decrease in the water fraction region of the structural changes, i.e. for hw ~ 0.23.