Structural Changes of Water in Carboxymethyl Cellulose Nanofiber Hydrogels during Vapor Swelling and Drying

Carboxymethyl cellulose nanofiber (CMCF) forms mechanically strong hydrogels via freeze cross-linking. We investigated the vapor swelling and drying processes of the freeze cross-linked CMCF hydrogels using infrared spectroscopy and X-ray diffraction. From the shifts of the O-H and C=O stretching modes, the structural changes of water and carboxymethyl celluloses (CMC) were analyzed. The results show that two types of bound water exist in CMCF hydrogels due to a difference in hydrophilicity between the amorphous and crystalline regions of CMCF. Bound water adsorbed on the amorphous region forms a strong hydrogen bond with dangling O-H or C=O bonds of CMC, whereas that adsorbed on the crystalline region has a weak hydrogen bond with the localized hydrophilic groups on the hydrophobic surface. Due to the difference in the hydrogen bonding strength of the two types of bound water, the vapor swelling process of water in CMCF hydrogels is classified into four stages. For the drying process, the residual water, which formed a strong hydrogen bond with the hydrophilic groups of the CMC, has effects on the CMCF structure. The present result suggests that the adsorption and desorption of water are important factors governing the physical and chemical properties of the CMCF hydrogels.