Mechanical and structural properties of cellulose nanofiber/poly(vinyl alcohol) hydrogels cross-linked by a freezing/thawing method and borax

The effect of cellulose nanofibers (CNFs) on the mechanical and structural properties of poly (vinyl alcohol) (PVA) hydrogels cross-linked dually by a freezing/thawing method and a cross-linker (borax) was investigated using tensile tests, FT-IR measurements and synchrotron simultaneous small-angle/wide-angle X-ray scattering (SAXS/WAXS). The cross-linking with borax and addition of CNF greatly improved the tensile performance of the PVA hydrogels prepared in the freezing/thawing method, e.g., the dual cross-linking resulted in remarkable increase in the fracture strain, the Young's modulus and the tensile strength. The fiber length effect on the mechanical properties of the CNF/PVA/borax hydrogels was examined, so that addition of CNF with longer fiber length increased the tensile modulus, whereas addition of CNF with shorter length caused extensibility higher than 1000% even at high CNF concentrations. SAXS/WAXS and FT-IR measurements have revealed that PVA was crystallized due to the freezing/thawing process, and both of PVA and CNF were cross-linked with borax, and hydrogen bonds between hydroxy groups of PVA (or/and CNF) were formed, indicating that the multiple cross-linking is effective for improvement of the tensile performance. In addition, the CNF/PVA/borax gel with the multi-physical cross-linking was found to possess the ability of the self-healing.