Parameters of hydrothermal gelation of chitin nanofibers determined using a severity factor

The hydrothermal gelation of α- and β-chitin nanofibers (α- and β-ChNFs) prepared at neutral and acidic pH was conducted by heating them to 120, 160, 180, and 200 °C in a sealed reactor. The optical transmittance and mechanical strength of β-ChNFs gelated at the acidic pH were determined for the first time using a severity factor defined as a function of the integrated heating time and temperature. The width of β-ChNFs increased after the hydrothermal treatment, indicating that these fibers strongly adhered to each other to form a network structure during gelation. Furthermore, the hydrothermal gelation of α- and β-ChNFs with different degrees of disintegration prepared at the neutral and acidic pH was conducted. It was found that the hydrothermal treatment of α-chitin must be performed at the acidic pH to obtain a self-sustaining hydrogel of well-disintegrated NFs. The disintegration of β-chitin into NFs occurred more easily at the acidic pH than under the neutral conditions; however, in the latter case, the same disintegration degree of β-ChNFs could be achieved by increasing the number of disintegration steps. At the same disintegration degree, the strength of the self-sustaining hydrogel obtained at the neutral conditions was greater than that of the gel prepared at the acidic pH, indicating that the electrostatic repulsion caused by acid addition negatively affected the formation of the hydrogel network structure. To maximize the efficiency of the hydrothermal gelation process, ChNFs should be as thin as possible and electrostatic repulsion forces must be controlled.