Title : Fracture behavior and biocompatibility of cellulose nanofiber- composite hydrogels cross-linked with borax
Abstract:
Polymer hydrogels are soft materials with a large amount of water, whose structure is constructed by a three-dimensional network. In general, polymer hydrogels prepared in traditional manners are generally brittle and easy to break. Additionally, conventional hydrogels are sensitive to flaws, e.g., the mechanical performance of notched samples is largely inferior to that of unnotched samples. Incorporating nanofibers into polymer gels allows us to produce mechanically tough and flaw-insensitive materials. This study focuses on the fracture behavior of the poly(vinyl alcohol) (PVA) composite hydrogels reinforced by cellulose nanofibers (CNFs) and borax as a crosslinker, and the effect of freeze-thaw (FT) cycles on it. Repeated FT cycles notably improved the mechanical performance of composite gel samples without a notch, e.g., they enhanced their mechanical strength and improved stretchability (~ 1100% for composite hydrogels subjected to five- and ten-FT cycles). However, the pure shear tests showed that repeated FT cycles lowered the mechanical performance of notched samples, e.g., the fracture energy decreased due to the lowering of the dissipative length. The CNF/PVA/Borax composite hydrogel not subjected to FT treatment achieved a high fracture energy and a large dissipative length, comparable to those of tough hydrogels such as double network gels. Lacking either CNF or borax significantly lowered the fracture energy and the dissipative length. This result suggests that combining CNF and borax is crucial for acquiring the fracture toughness of the composite hydrogels. CNF contributed to a physical blocking of the crack growth, while borax yielded crack blunting, which lessened the stress concentration near the crack tip. Physical complexations between CNF and borate are formed on a single CNF, which may yield nonlocalization of energy dissipation. Moreover, CNF/PVA/Borax composite hydrogels were biocompatible and suitable for cell scaffold materials.
