State of water in chitosan-PVA hydrogel

The bound water fraction (XBW) of a newly-developed pH-sensitive, biodegradable chitosan-polyvinyl alcohol (PVA) hydrogel crosslinked with glutaraldehyde (GA) was investigated as a function of the chitosan/PVA molar ratio, GA concentration (CGA), and ionization state. Differential scanning calorimetry (DSC) was used to determine the XBW of the initial hydrogel, and of the hydrogel equilibrated in pH 3 and pH 7 buffers. Changes in XBW during swelling and shrinking of hydrogel were also investigated. In the initial state of hydrogel, XBW increased with increasing PVA concentration (CPVA), without being significantly affected by CGA. In the buffer-equilibrated hydrogels, XBW decreased with increasing CPVA and decreasing CGA. The amount of bound water based on dry mass (CBW) was substantially higher when the hydrogel was in the ionized (swollen) state compared to its unionized counterpart. This may be due to the association of a large quantity of water molecules with -NH3+ groups of chitosan when the gel swelled in the acidic environment.