3.1 Crystalline structure
Figure 1 presented the X-ray diffraction patterns of sisal cellulose nanowhiskers prepared with different alkali concentrations and sisal fiber after different treatments. As we could see from Fig. 1 (a) when the alkali treatment concentration was 5 %, the X-ray diffraction peaks of the sisal cellulose nanowhiskers appeared at 2θ of 15°, 17°, 22.5° and 35°, which represented the (1–10), (110), (200) and (004) crystal faces of the cellulose, respectively. It showed typical cellulose Ⅰstructure. However, when the concentration of alkali treatment reaches 25 %, the X-ray diffraction peaks appeared at about 2θ of 12°, 20°, 22° and 35°, and the corresponding crystal faced of (1–10), (110), (020) and (004) of cellulose, respectively, showed typical cellulose Ⅱ structure. At the same time, it could be seen that when the alkali concentration was 15 %, the X-ray diffraction had both the characteristic peaks of cellulose typeⅠand Ⅱ, and the fiber at this time was a mixture of cellulose Ⅰand Ⅱ, indicating that with the increase of the alkali concentration, the cellulose in sisal fiber would gradually transform from cellulose Ⅰ to cellulose Ⅱ, which was basically consistent with previous studies on this aspect (Yu et al. 2014).
Figure 1(b) showed the X-ray diffraction of degummed sisal fiber, 5 % alkali treated sisal fiber and TEMPO-oxidated sisal cellulose nanowhiskers, respectively. By comparing the X-ray diffraction patterns of sisal fiber before and after preparation, it could be seen that the preparation process of cellulose nanowhiskers had no effect on the basic crystal structure of cellulose. And as the noncellulose polysaccharides were removed and the amorphous zones were dissolved, the fibers showed increasing orientations along a particular axes. Calculated according to formula 1, the crystallinity thus went on increasing in going from the degummed sisal fiber (69.09 %) to 5% alkali-treated sisal fiber (69.97 %), and subsequently to oxidized sisal cellulose nanowhiskers (74.74 %). The little increasing in the crystallinity after TEMPO oxidization, which may be due to the increase of water solubility in the disordered area during centrifugal washing, resulting in partial loss (Saito and Isogai 2004).
3.2 FTIR analysis
Figure 2 showed the FT-IR spectra of sisal cellulose nanowhiskers prepared with different alkali concentrations and sisal fiber after different treatments. According to Fig. 2(a), the wide absorption peak of 3000–3500 cm− 1 was generated by the stretching vibration of -OH bond, and there were two weak absorption peaks in sisal fiber treated with 5 % or 25 % alkali concentration. Zhang and Pan (1995) believed that the main reason was that there were two kinds of intramolecular hydrogen bonds in the fiber, and intermolecular hydrogen bonds were relatively complex. Meanwhile, when 5 % alkali was used to treat the fibers, the two absorption peaks were mainly at around 3250 cm− 1, while when the concentration reached 25 %, the two absorption peaks were shifted to around 3500 cm− 1, mainly because the cellulose Ⅱ adopted the anti-parallel chain mode, the polarity of the chain was low, and the perturbation to the free stretching vibration of -OH bond was small. 1365 cm− 1 and 1315 cm− 1 respectively represented the bending vibration of -CH and the swinging motion of -CH2. With the increase of treatment concentration intensity, the strength changed, mainly due to the different hydrogen bonding environment of -CH2 and -OH in cellulose due to the change of crystalline form of cellulose. The absorption peak at 1110 cm− 1 was caused by the C-O stretching vibration of the cellulose six-membered ring framework, and the absorption peak was nearly disappeared with the increase of alkali concentration, which was related to the change of hydrogen bond. Another significant difference was that the absorption peak near 895 cm− 1 gradually increases with the increase of alkali concentration, which was caused by the different skeleton vibration of C1 atom. The changes of the five characteristic peaks above all indicated that with the increase of alkali concentration, the crystal form of sisal fiber gradually changed from cellulose Ⅰ to cellulose Ⅱ, which was consistent with the X-ray diffraction analysis results of the sisal fiber above.
Figure 2 (b) showed that the basic peak shape of sisal fiber changed little before and after preparation, only some specific groups changed, and most groups only changed in strength. The main changes of the peaks at 1607 cm− 1 and 1415 cm− 1 were mainly caused by the vibration of the C = O bond of the carboxyl group formed by the oxidation of the primary alcohol hydroxyl group of the cellulose by the TEMPO-oxidation system, indicated that hydroxyl groups at the C6 position of cellulose molecules are converted to sodium carboxylate (Fukuzumi et al. 2009). The wide absorption peak near 3300 cm− 1 represented the stretching vibration of OH bond. The change of the absorption peak indicated that after the preparation and treatment of nano-cellulose, the content of cellulose was getting higher and higher. At the same time, the oxidation process of TEMPO-oxidation system was a selective oxidation process, which only acted on the primary alcohol hydroxyl group of cellulose.
3.3 Thermal performance
Figure 3 showed the TG diagrams of sisal cellulose nanowhiskers prepared with different alkali concentrations, TG and DTG diagrams of sisal fibers after different treatments. As could be seen from Fig. 3(a), the thermal stability of sisal nanowhiskers prepared with different sodium hydroxide concentrations was basically not significantly different, but the initial decomposition temperature of the fibers also changed a little due to the change of the crystalline form of the cellulose. With the increase of the concentration of alkali treatment, the initial decomposition temperature of the fiber would decrease, mainly because the thermal stability of cellulose Ⅱ was worse than that of celluloseⅠ, and the increase of the alkali concentration would make the crystal form of the cellulose transform from the cellulose amorphous type to the cellulose solid type, which was consistent with the results of the X-ray diffraction and infrared analysis.
As shown in Fig. 3 (b) and (c), all TG curves, showed a small amount of weight loss between 30 ℃ and 150 ℃, which was mainly due to the reduction of the water chemisorbed on the surface of the fiber or the water attached by hydrogen bonds between fiber molecules. After the alkali treatment, the initial decomposition temperature of sisal fiber was significantly increased, which indicated that the non-cellulose components which were not completely removed in the previous degumming treatment were removed in the alkali treatment process, and the residual mass of the final alkali treatment sisal fiber was less than that after the degumming treatment, which also indicated this. After alkali treatment, the sisal fiber began to thermally degrade at 300°C, while the degradation temperature of TEMPO-oxidized sisal cellulose nanowhiskers started at 220°C. This indicated that the primary alcohol hydroxyl groups on the cellulose surface were caused by TEMPO oxidation. Oxidation into carboxyl groups would significantly reduce the thermal degradation point of cellulose (Fukuzumi et al. 2010), which was basically in line with the results of previous studies.
It was worth noting that the DTG curve of TEMPO-oxidized sisal cellulose nanowhiskers was very broad, and it could be seen that the degradation process of sisal cellulose nanowhiskers was mainly composed of two decomposition stages. The former was due to the thermal degradation point of the sodium anhydroglucuronate unit, while the latter was significantly lower than that of unoxidized sisal fiber, indicating the present of more thermally unstable anhydroglucuronate units in the TEMPO-oxidized sisal fibers, and the crystalline cellulose chain in the DTG peak reduced (Cao et al. 2012).
3.4 Micro-structure
Figure 4 showed the SEM micrographs of the degummed sisal fiber, alkali treated sisal fiber and TEMPO-oxidated sisal cellulose nanowhiskers and the width distribution of nanowhiskers. As seen from Fig. 4 (a) and (b), the degummed sisal fiber was a filament with a diameter of 10–20 µm, while the sisal fiber that was crushed and screened and then treated with sodium hydroxide and DMSO became thinner and shorter, with a diameter of about 10 µm, and length between 0.1–0.3 mm, this was due to the alkali treatment acted on the fibers could contribute to remove hemi-cellulose, surface impurities, and a part of lignin from the fibers (Meng et al. 2016), as well as the inter-molecular hydrogen bonding, leading to the decrease of fiber diameter, also DMSO was a kind of strong hydrogen bonding-breaking agent, treatment further contributed to the breakage of inter-molecular hydrogen bonding and dissolution of interior and external non-cellulose materials, resulting in a further dimensional diminution of the sisal fibers (Das et al. 2010; Lin et al. 2014). Figure 4 (c) showed that after TEMPO oxidation reaction combined with high-speed homogenization, most of the fibrils were prepared into nano-sized fiber whiskers. These cellulose nanowhiksers were rod-shaped, with a width of 6–61 nm (Fig. 4 (d)) and a length of 100–500 nm. At the same time, it could be seen that some nanowhiskers gather together, which was mainly caused by the high specific surface area of cellulose nanowhiskers and the large number of strong hydrogen bonded between the fibers.
3.5 Optical transmittance of nanowhiskers suspension
Figure 5 showed the UV-vis transmittance of 0.1 wt% cellulose nanowhiskers suspension and the digital photographs of the suspensions of the obtained nanowhiskers in a beaker. The transmittance of the 0.1 wt% cellulose nanowhiskers suspension at 900 nm was approximately 90 %, which is consistent with the experimental results in our previous work (Cao et al. 2012). TEMPO oxidation system treated sisal fibers could introduce carboxylates along the surface of the whiskers and result in a negative-charged surface (Lin et al. 2014). Therefore, the stability of transparent suspensions could be interpreted by the anionic stabilization via the attraction/repulsion forces of electrical double layers (Liu et al. 2015; Kargarzadeh et al. 2012).