2.5. Characterization of Hydrogels

Fourier transform infrared (FTIR) spectra of Al₂O₃ NPs, neat copolymer of AA and VP, as well as NC-G0 gel were obtained using an IRAffinity-1S (Shimadzu Co., Kyoto, Japan) FTIR spectrometer with an attenuated total reflectance (ATR) accessory. All of the samples were dried in a 95 °C oven before conducting the FTIR characterization. Scanning electron spectroscopy (SEM) was conducted to observe the microstructures of the NC-G gels. The as-prepared hydrogels were first placed in water (water was changed every 12 h) for 10 days to replace the glycerol with water. Then, the samples were immersed in liquid nitrogen and followed by freeze-dry (FD-1A-50, Boyikang Co., Beijing, China) for 48 h at −48 °C. The fractured surfaces were observed using a Quanta 250 (FEI Co., Hillsboro, OR, USA) instrument at an acceleration voltage of 20 kV after coating with gold. The thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurement of NC-G gels were performed on a Pyris 1 (Perkin Elmer Co., Waltham, MA, USA) and DSC 8000 (Perkin Elmer Co., Waltham, MA, USA) instruments, respectively. For both of the characterizations, the heating rate was set at 10 °C/min from 30 to 300 °C under N₂ atmosphere.