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Molecules 2021, 26, 5276 5 of 22 CH-stretching vibrations. The wavenumber at 1640 and 1643 cm−1 shows typical bending of water molecules due to the strong interaction between cellulose and water. Furthermore, the band at 1375 cm−1 represents the vibration bending and wagging of CH2 and CH. The band at 1284 cm−1 shows C-O-C stretching from the β-1, 4-glycosidic linkage in cellulosic material, while the 1058 cm−1 wave band indicates –CH2-O-CH2 Pyrenees ring stretching vibration [30]. The wavenumber band at 810 cm−1 and 563 cm−1 represents C-H out of plane stretching in cellulose due to β-linkage. The absence of a wave number band at around 1512–1562 cm−1 and 1700–1740 cm−1 indicates successfully eliminated lignin and hemicellulose [31]. This confirms that the CNF produced was of high quality. These bonds are a typical indication of isolated cellulose nanofibre, in line with previous reports by Atiqah et al. [32]. The difference between the bamboo CNF and the commercial one is in the intensity of the representative absorbance band for each bond identified. The difference in bands is more significantly shown in the hydrogen bonding between 3600–3200 cm−1, which is higher in bamboo CNF than in commercial CNF [33]. An absent peak at around 1512–1562 cm−1 (lignin) and 1700–1740 cm−1 (hemicellulose) indicates CNF production was high in purity [34]. The results of the XRD analysis of the bamboo CNF and the commercial CNF is presented in Figure 2b. The results showed two significant peaks for both CNF at 2 theta equals 15◦ and 22.5◦. However, the peak intensity of the bamboo was observed to be greater than that of the commercial CNF, which is an indication of a possible difference between their crystallinity indices. The XRD analysis was used to analyse the crystallinity indices of the two types of CNF. This is important because it has a significant effect on their reinforcement ability [24]. The XRD crystallinity of bamboo CNF was 75.68% greater than that of commercial CNF, which was 70.67%. The higher crystallinity index means a higher reinforcement effect in composite materials. The major crystalline peak observed in this study is similar to those previously reported. This shows that the isolated CNF represented the cellulose structure, and that the crystal integrity of CNF was affected by the preparation method of the bamboo CNF [35]. Higher crystallinity means greater efficiency in achieving a higher reinforcement effect in composite materials. The major crystalline peak for all samples, occurring at around 2θ = 22.5 (no doublet found), represents cellulose I structure, and suggests that crystal integrity has been maintained [36]. The thermogravimetry analysis of the bamboo CNF and the commercial CNF is shown in Figure 2c,d. Both CNFs showed a similar TGA curve (Figure 2c), with initial water evaporation below 100 ◦C. The onset temperature of the commercial CNF was lower than that of the bamboo, at 215 ◦C and 208 ◦C, respectively. The difference was probably due to the source of the CNF. The degradation percentage of both CNFs showed similar descending values. Furthermore, the DTG curve (Figure 2d) showed a significant peak difference between the bamboo and commercial CNFs at 375 ◦C and 366 ◦C, respectively. The DTG curve corroborates the observation that the differences between the TGA onset temperatures suggested a difference in the thermal properties of the CNFs, which may have been due to their sources [37,38]. 2.2. The Physical Properties of Bamboo and Commercial CNF-Reinforced PLA/Chitin Bionanocomposites The physical properties of the biocomposite were evaluated using moisture content, water absorption, thickness swelling, and density evaluation, as presented in Figure 3a–d.PDF Image | Supercritical Carbon Dioxide Isolation of Cellulose Nanofibre
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