3.2.1 Palm fiber cell wall FT-IR imaging
FT-IR microspectroscopic data can be displayed as chemical images of specific wavelengths. Red and pink regions corresponded to larger absorption intensity, whereas blue regions corresponded to smaller absorption intensity. The chemical images at peaks near 1240 cm−1 showed the relative concentrations and distribution of cellulose. The area under the peak at 1508 cm−1 indicated the concentration and distribution of lignin. The concentrations and distribution of hemicelluloses were found at bands near 1710 cm−1 (Dokken et al. 2007, Cao et al., 2014). Fig. 3 shows the relative concentration and distribution of cellulose, lignin, and hemicellulose of raw and alkali-treated palm fiber transverse sections. As shown in Fig. 3(a), with increased alkali concentration, the red region decreased slightly, indicating that the cellulose content decreased. Figs. 3(b) and 3(c) show a significant decrease in red whereas the blue regions increased with increased alkali concentration, indicating that the lignin and hemicellulose contents dropped evidently. The spatial transformation of lignin and hemicellulose distribution and concentration indicated that their contents decreased during alkali treatment.
The FT-IR spectra of raw and alkali-treated palm fibers were analyzed to investigate the effect of alkali treatment on the chemical characteristics of fiber cell wall. Fig. 4 shows the FT-IR spectra of the cell wall in the fingerprint region (1800 cm−1 to 800 cm−1). Specific spectral signals assigned to absorption bands of cellulose, hemicelluloses, and lignin were examined. Typical bands assigned to cellulose were located at 1424 and 1368 cm−1 for CH2 scissor motion and C-H bending vibrations, respectively, at 1336 cm−1 for OH in-plane bending of amorphous cellulose, and at 1316 cm−1 for CH2 wagging vibrations in crystalline cellulose (Guo et al. 2015; Huang et al. 2013; Lionetto et al. 2012; Yin et al. 2011), which can be used to assess structural changes in cellulose. According to Huang and Lionetto (Colom et al. 2003; Huang et al. 2013), the absorbance ratio (I1316/I1336) provides additional information on the difference in the degradation process of amorphous and crystalline cellulose, wherein an increase in the ratio indicates increased crystallinity. For raw and alkali-treated palm fibers, this ratio were 1.06, 1.08, 1.1, and 1.09, respectively. Based on the results, we concluded that alkali treatment increased the crystalized-cellulose content, which helped improve the tensile properties. For hemicelluloses, the characteristic peaks at 1740 and 1710 cm−1 were assigned to the C=O stretching vibration in the O=C-O group of the glucuronic acid unit in xylan (Akerholm and Salmen 2003; Song et al. 2013; Stevanic and Salme 2009). As for lignin, the characteristic peaks at 1600 cm−1 can be ascribed to the aromatic skeletal vibrations together with C=O stretching, and those at 1508 cm−1 can be ascribed to the aromatic skeletal vibration and guaiacyl ring vibration. The xylan band at 1456 cm−1 can be ascribed to CH2 symmetric bending on the xylose ring, whereas that at 1264 cm−1 can be ascribed to C=O stretching (Shi et al. 2012; Song et al. 2013).
The relative intensities of the absorption peaks at 1740, 1710, 1600, and 1508 cm−1 in palm as a function of treatments are shown in Fig .5. The density at 1740 cm−1 decreased by 24% for treatment at 2 wt.% NaOH, 63.5% for treatment at 5 wt.% NaOH, and 65% for treatment at 10 wt.% NaOH. The density at 1710 cm−1 decreased by 3.2% for treatment at 2 wt.% NaOH, 33.3% for treatment at 5 wt.% NaOH, and 38.7% for treatment at 10 wt.% NaOH. This significant loss in absorption of the carbonyl group, which comprised the backbone of xylan, at higher alkali concentrations most likely resulted from the degradation and loss of hemicelluloses from the cell wall. A similar observation was reported by Reddy (2009, 2012). The lignin-band intensity at 1600 cm−1 showed a slight increase by 4.6% for treatment at 2 wt.% NaOH, a decrease by 14.7% for treatment at 5 wt.% NaOH, and a decrease by 18.9% for treatment at 10 wt.% NaOH. For the aromatic skeletal vibration band at 1508 cm−1, relative intensities increased by 2.7% for treatment at 2 wt.% NaOH, 13.4% for treatment at 5 wt.% NaOH, and 14% for treatment at 10 wt.% NaOH. A loss of the C=O group linked to the aromatic skeleton may have probably occurred. This finding indicated the occurrence of cross-linking among the aromatic units in the lignin probably caused by the alkali treatment (Yin et al. 2011).
We observed that the bands at 1600 and 1508 cm−1, which were assigned to lignin, increased slightly upon 2 wt.% NaOH treatment (Fig. 5). This result indicated that the significant degradation of hemicelluloses subsequently caused a slightly increase in the lignin component of cell wall. A similar observation has been made by Huang (Huang et al. 2013). The removal of hemicelluloses and lignin in cell walls may have likely enhanced the exposure of cellulose microfibrils, consequently increasing the tensile properties of the fibers.
3.2.2 CLSM imaging of palm lignin autofluorescence
To further study lignin’s microstructure transformation and distribution characteristics in palm fiber cell wall, CLSM was used to investigate the lignin distribution and relative lignin concentration. Lignin concentration is linearly proportional to image brightness, which can be evaluated by image brightness (Ding et al. 2016). Fluorescence in cell walls originated from lignin autofluorescence, which comprised monolignols. Fig. 6 shows the autofluorescence images of raw and alkali-treated samples at different concentrations. Figs. 6(a, e) demonstrate the heterogeneous distribution of lignin in raw fibers, with higher lignin autofluorescence intensity occurring in the cell corner middle lamella and compound middle lamella regions. For the sample treated by 2 wt.% NaOH, fluorescence intensity in cell wall increased probably due to the aggregation-induced emission effect of lignin. Furthermore, a continuous increase in NaOH concentration to 10 wt.% caused an obvious decrease in fluorescence intensity, which may be attributed to a significant degradation of hemicelluloses linked to lignin by ester and ether bonds. Overall, the transformation of fluorescence reflected changes in lignin component and organization in addition to changes in interactions with other polymers, particularly with hemicelluloses (Chabbert et al. 2018). This conclusion was consistent with the FT-IR microspectroscopic analysis.