2.5.8. Hygroscopic Behaviors

Water contact angle (WCA) measurement of the pure CNF and ChNF–CNF composites was carried out. A drop of 5 μL was deposited on a thin film and the images were taken by AMcap software and then analyzed using ImageJ tool. Water vapor transmittance rate (WVTR) was tested according to ASTM standard E 96-95 [36]. The sample was kept in a humidity chamber at 25 ◦C and 50% RH. The WVTR was taken hourly for up to 8 h continuously

## 2.5.9. Antioxidant Property

The antioxidant activity of the prepared ChNF–CNF composites was tested by using ABTS free radical [37]. The antioxidant activity analysis was based on the discolored radicals of ATBS after 40 min under UV light measurement at 734 nm. The UV absorption was adjusted to absorption at 0.8; 20 mg of CNF, ChNF and ChNF–CNF composites in 2 mL of ABTS were used for the measurement after 40 min. In other words, various chitosan contents (at 0.03, 0.05, 0.07, 0.1, 0.15 and 0.2 mg/mL of ABTS) were used. All antiradical tests were carried out twice for each sample. Then, 7 mM ABTS was dissolved in DI water and mixed with 2.45 mM potassium persulfate and kept in a dark drawer for 16 h. After that, the ABTS suspension was diluted with methanol to adjust the absorption of 0.8 at wavelength 734 nm. The antioxidant activity of CNF and ChNF–CNF composites was calculated by the following equation:

$$\rm{AO(\%)} = 100 \, \* \, (1 - \rm{A\_3/A\_0}) \tag{2}$$

where AO is the antiradical activity, Ao is the absorption of the control ABTS solution and Aa is the absorption of the ABTS solution with sample in steady state.
