3.1 Crystalline structure
From Fig. 1 (A), it can be seen that the diffraction pattern of sample treated with alkaline treatment concentration of 5% shows three major reflections corresponding to 2θ values around approximately 16°, 22°, and 34°, and these peaks are attributed to the crystal structure of the cellulose. Peaks near 16° are (1–10)/(110) overlapping reflections, while peaks near 22° and 34° are reflections of (200) and (004), respectively, [20] exhibiting a typical cellulose type I structure. When the alkali treatment concentration reaches 15%, the diffraction pattern peaks at 2θ of 12.5°, 20°, 22° and 35°, which correspond to the crystal faces of cellulose (1–10), (110), (020) and (004) respectively, showing a typical cellulose type II structure. When the alkali treatment concentration is 10%, the XRD pattern not only shows the characteristic peak of cellulose type I, but also cellulose type II, that is to say the prepared fiber is the mixture of cellulose type I and cellulose type II. This indicates that with the increase of alkali treatment concentration, cellulose in palm fiber will gradually transform from cellulose type I to cellulose type II, which is basically consistent with previous studies on this aspect [21].
Figure 1B shows the X-ray diffraction patterns of the degumming palm fiber and the palm nanofibers prepared by acid combined enzymatic hydrolysis, TEMPO/NaBr/NaClO oxidation and TEMPO/NaClO/NaClO2 oxidation after 5% alkali pretreatment, respectively. It could be seen TEMPO/NaBr/NaClO and TEMPO/NaClO/NaClO2 oxidation system has no effect on the basic structure of the preparation of palm nanocellulose, showing cellulose type I, and the crystallization index of cellulose has no obvious change. This indicates that TEMPO oxidation only reacts on the surface of amorphous and crystalline regions of cellulose, and the inner crystalline region does not participate in the reaction, which is similar to the results of Saito [22] and Kuramae [23]. The crystal peak of palm nanocrystalline cellulose prepared by acid and enzyme hydrolysis was still cellulose type I.
With the removal of non-cellulose polysaccharide and the dissolution of amorphous region, the fiber shows an increased orientation along a specific axis. The crystallinities of the degumming palm fibers, palm nanocelluloses prepared by acid and enzyme hydrolysis, by TEMPO/NaBr/NaClO, and TEMPO/NaClO/NaClO2 oxidation method were 56.77%, 66.59%, 63.87%, and 60.65%, respectively. There is no significant difference in the crystallinity of nanocellulose prepared by the two TEMPO selective oxidation methods, and compared with the degumming fiber, the crystallinity is not significantly improved, which may be due to the increase of water solubility in the disordered region during the centrifugal washing process, leading to partial loss [24]. However, the crystallinity of the nanocellulose prepared by acid combined with enzyme hydrolysis was higher than that before treatment, which was due to the action of cellulase and acid. The enzyme broke down the glucoside bond of cellulose and selectively degraded the amorphous region of cellulose. In the process of acid hydrolysis, the hydrolysis rate of the amorphous region of cellulose is faster than that of the crystalline region, so the crystallinity of the final nanocellulose is relatively high [25].
3.2 FTIR analysis
Figure 2A showed the FTIR spectrum of palm fibers treated with different concentrations of alkali. It can be seen that all samples have relatively wide absorption peaks in the range of 3000 cm− 1 to 3600 cm− 1, which is caused by the stretching vibration of the -OH bond, and there are also two relatively small absorption peaks, which may be due to the existence of two kinds of intramolecular hydrogen bonds in the fiber. Intermolecular hydrogen bonds are complex [26]. With the increase of alkali treatment concentration, the position of the two small absorption peaks has shifted, when the alkali treatment concentration is 5%, the position of the two absorption peaks is mainly around 3275 cm− 1, and when the alkali treatment concentration reaches 15%, the two absorption peaks appear in about 3500 cm− 1, which is mainly due to the anti-parallel chain mode of cellulose Ⅱ. The polarity of the chain is low, and the perturbation to the free stretching vibration of the -OH bond is small. 1367 cm− 1 and 1315 cm− 1 represent the bending vibration of -CH and -CH2, respectively. The intensity of the peaks also changes with the change of alkali concentration, mainly because the crystal shape of cellulose changes after alkali treatment, resulting in different hydrogen bonding environments for -CH2 and -OH in cellulose. The stretching vibration at 1110 cm− 1, which represents the C-O in the hexatomic ring skeleton of cellulose, gradually weakens or even disappears with the increase of alkali concentration, which is also related to the change of hydrogen bond. Another notable change is near 897 cm− 1, where the peak intensifies as the alkaline concentration increases due to different skeleton vibrations of C1 atoms. The change of the above characteristic peaks indicates that the alkali treatment concentration has an effect on the crystal shape of cellulose. With the increase of alkali treatment concentration, the crystal shape of cellulose changes from Ⅰ to Ⅱ, which is consistent with the analysis results of X-ray diffraction.
Figure 2B shows the FTIR spectra of palm nanofibers prepared by TEMPO/NaBr/NaClO oxidation, TEMPO/NaClO/NaClO2 oxidation and acid combined enzymatic hydrolysis, respectively. It can be seen that all samples show the -OH absorption peak at wavenumber of 3400 cm− 1, the -CH stretching vibration absorption peak near 2900 cm− 1, C = O stretching vibration peak of cellulose acetyl group near 1650 cm− 1 and the stretching vibration peak of three C-O ether bonds in cellulose glucose near 1160 cm− 1, 1110 cm− 1 and 1020 cm− 1 which indicate that the main components of nanocellulose prepared by the three methods are cellulose. Compared with alkali-oxygen degumming sample, C-O-S symmetric stretching vibration appears near 820 cm− 1 after acid hydrolysis, indicating the presence of C-O-SO3 [27]. After TEMPO oxidation, there were two strong absorption peaks at 1608 cm− 1 and 1410 cm− 1, which were caused by the vibration of carboxyl group C = O bond formed by the oxidation of cellulose primary alcohol hydroxyl group by TEMPO system, and the peak attributed to the contraction vibration of carbonyl group was significantly enhanced near 1700 cm− 1. It indicates that carboxyl groups are introduced into the macromolecular chains of cellulose, and the higher the carboxyl group content is, the stronger the absorption peak intensity is [28].
3.3 Thermal performance
Figure 3 showed TG and DTG curves of palm fibers treated with different concentrations of alkali and after alkali oxygen degumming, and nanocellulose prepared by TEMPO/NaBr/NaClO, TEMPO/NaClO/NaClO2 and acid combined enzymatic hydrolysis, respectively. It can be seen from Fig. 3A and B that alkali treatment with different concentrations has no significant effect on the thermal stability of palm fiber. With the increase of alkali concentration, the initial decomposition temperature of the fiber decreased slightly, when the alkali concentration was 15%, the mass loss of the fiber was more in the initial stage, because the crystal type of cellulose changed from type II to type I, and the thermal stability was relatively poor, which also confirmed the results of the above X-ray diffraction and FTIR.
Figure 3C and D show the TG and DTG curves of the degumming palm fiber and the palm fibers based nanocelluloses prepared by TEMPO/NaBr/NaClO oxidation, TEMPO/NaClO/NaClO2 oxidation and acid-combined enzymatic hydrolysis method after 5% alkali pretreatment, respectively. All TG curves begin with a small decrease between 30°C and 150°C, which corresponds to the weight loss of absorbed water on the sample surfaces including chemisorbed water and/or intermolecular hydrogen bonded water. The initial decomposition temperature of the samples after alkali-oxygen degumming treatment was 322.6 ℃, while the initial degradation temperature of samples after two TEMPO oxidation systems treatment and acid combined enzyme hydrolysis decreased significantly. This may be because during the degradation process, the macromolecular chains of cellulose were destroyed and broken, and many low molecular chain segments and a large number of break points were formed on the surface of the prepared nanocellulose. These low molecular chain segments and break points will absorb heat and begin to degrade at high temperatures, thus reducing the thermal stability of nanocellulose. The initial degradation temperature of palm nanocellulose oxidized by TEMPO/NaBr/NaClO oxidation system and TEMPO/NaClO/NaClO2 oxidation system was 260.3 ℃ and 279.7 ℃, respectively, because the primary alcohol hydroxyl group was oxidized to carboxyl group and the crystal chain of cellulose was reduced, thus reducing the thermal degradation temperature [29]. The lowest value of the nanocellulose prepared by acid and enzyme hydrolysis is 231.2 ℃, which may be because the surface sulfate groups introduced in the hydrolysis process reduce the decomposition and activation energy, thus reducing the thermal stability. In addition, it can be seen that at 700 ℃, the carbon residue contents of palm nanocellulose prepared by the three methods were much higher than that after degumming treatment. The principle of acid hydrolysis is attributed to the flame-retardant sulfuric acid groups on the surface of nanocellulose. It is worth noting that the degradation process of palm nanocellulose prepared by TEMPO oxidation method mainly consists of two decomposition stages. The former is due to the thermal degradation point of the dehydrated sodium glucuronate unit, while the latter is significantly lower than that of the control palm fiber, indicating that there are more heat-unstable dehydrated glucuronic acid units in TEMPO-oxidized samples, and the crystalline cellulose chain in the DTG peak reduced [30]. For the palm fibers based nanocellulose prepared by acid and enzyme hydrolysis, there is only one decomposition stage, which is mainly due to the decomposition of cellulose chain [30].
3.4 Micro-structure
Figure 4 shows the SEM images of the degumming palm fibers prepared by TEMPO/NaBr/ NaClO oxidation, TEMPO/NaClO/NaClO2 oxidation and acid combined enzymatic hydrolysis after 5% alkali pretreatment, respectively. It can be seen that these three methods successfully and effectively prepared palm fiber into nanosized fibers, and the nanocelluloses prepared by TEMPO/NaBr/ NaClO oxidation and TEMPO/NaClO/NaClO2 oxidation were needle-like, while that prepared by acid combined enzyme hydrolysis method was rod-like. This indicates that different preparation methods will affect the appearance of nanocellulose. The sizes of nanocellulose prepared by the two TEMPO oxidation methods have little difference, about 10–50 nm in diameter, and both showed different degrees of aggregation, which was mainly due to the high specific surface area of nanocellulose and a large number of strong hydrogen bonds between the samples. The nanocellulose prepared by acid and enzyme hydrolysis was with a diameter of about 30 nm.
3.5 Optical transmittance of nanowhisker suspension
Figure 5 shows the UV-vis transmittance of 0.1 wt% cellulose nanowhisker suspensions and the digital photographs of the suspensions of the obtained nanowhiskers prepared by TEMPO/NaBr/NaClO oxidation, TEMPO/NaClO/NaClO2 oxidation, and acid combined enzymatic hydrolysis, respectively. It can be seen from the comparison that under the same conditions, the transmittance of the suspensions of nanocrystals prepared by different methods is different. At the wavenumber of 800 nm, the light transmittance of the suspensions prepared by TEMPO/NaBr/NaClO oxidation method is about 90%, which is consistent with our previous experimental results [13]. The light transmittance of that prepared by TEMPO/NaClO/NaClO2 oxidation method was slightly lower, but the difference was not significant. Relatively, the light transmittance of the suspension prepared by acid and enzyme hydrolysis is lower, about 81%, and the suspension is light yellow. Generally, the transmission of light is related to the wavelength, and at shorter wavelengths, less light is transmitted because the closer the wavelength is to the diameter of the particle, the more light is scattered. Light scattering is proportional to the ratio of mass/length or cross-sectional area compared to suspended fibril or rod-like material with smaller corresponding wavelengths [31]. The TEMPO oxidation could introduce carboxylate along the surface of the whiskers and result in a negative-charged surface [32]. Therefore, a certain transparency and stability of palm nanocellulose suspension can be maintained through the anionic stabilization via attraction/repulsion between layers [33–35]. Comparatively, strongly hydrogen bonds are easily formed and agglomeration occurs after sulfuric acid hydrolysis.