Microstructure Transformation In Palm-Fiber Cell Wall And Its Influence On the Fiber's Mechanical Properties During Alkali Treatment


 In this study, we examined the microstructure transformation of palm fiber and the influence of this transformation on the fiber mechanical properties during alkali treatment. The fibers were treated with different concentrations of NaOH to study the change rules of the microstructure and the tensile properties. FT-IR microspectroscopic imaging and confocal laser scanning microscopy were adopted to observe microstructure transformation during alkali treatment. Research results showed that the hemicellulose and lignin in the fiber cell wall were removed by alkali treatment, leading to a rearrangement of cellulose chains. The tensile properties palm fibers were significantly improved because of crystallinity alterations in the cell walls after alkali treatment. This study might provide a basis for palm fiber’s high-value utilization in the field of materials.


Introduction
Palm leaf sheath ber (palm ber), a kind of natural multicellular plant ber, is widely used in households, civil construction, water conservation, and environmental engineering because of its excellent performance and high utilization value (Zhang et  To prepare natural-ber-reinforced composites, chemical treatment is often used to modify the ber's structure and tensile properties (Dhakal et al. 2018). Khanam (2007) and Borchani (2015) found that alkali treatment improved the tensile, exural, and compressive strengths of sisal/silk hybrid composites. Boopathi (2012) and Reddy (2013) chemically modi ed borassus fruit bers and found that alkali treatment signi cantly improves the tensile properties of bers compared with untreated ones. Moreover, impurities existing on the ber surface and hemicellulose can be removed when alkalized (Alawar et al. 2009;Das and Chakraborty 2008;Rout et al. 2001). The mechanical properties of ber are believed to rely on the molecular architecture of its cell wall (Lee et al. 2011). The distribution of cellulose, hemicellulose, and lignin, the way of bonding, and the properties among the cell-wall components importantly affect the architecture and mechanical properties of ber. Thus, alkali treatment plays a key role in changing the components of the ber cell wall, leading to microstructure transformation of the ber. In recent years, modern imaging technology has been used widely in biomaterial analysis. FT-IR microscopy has been proven to be a powerful method for revealing the chemical structure and components distribution of plant tissues at the cellular level with high spatial resolution (Cao et al. 2014;Ding et al. 2016;Guo et al. 2015).
Confocal laser scanning microscopy (CLSM) is used to investigate lignin distribution in cell walls through lignin production of auto uorescence under speci c excitation and emission wavelengths (Ji et al. 2015;Marin-Bustamante et al. 2018). However, few studies have dealt with the components and microstructure changes of plant-ber cell wall after chemical treatment by using imaging technology. The relationship between microstructure and mechanical properties during alkali treatment is also rarely reported.
Accordingly, imaging technologies may effectively determine how alkali treatment affects the components and microstructure of ber.
In the present work, palm bers were treated by alkali with different concentrations (2, 5, and 10 wt.%), and the tensile properties of the bers were tested. Imaging FT-IR microspectroscopy and CLSM were adopted to investigate the changes in components and microstructure in the ber cell wall. Quantitative analyses on the component changes were performed to verify the imaging results. The mechanism of the effect of alkali treatment on ber mechanical properties was explored.

Materials
Palm bers were obtained in Yunnan province, China. The bers were washed in water to remove dust and impurities and then dried in an oven at 60°C for 24 h. The dry palm bers were treated with 2, 5, and 10 wt.% NaOH solution separately for about 2 h at room temperature. The treated bers were washed with fresh water to remove residual NaOH and then dried at room temperature for 24 h. By using a sliding microtome, 10 µm-thick polyethylene glycol (PEG) embedded-ber transverse-section slides were prepared for imaging FT-IR microscopy and confocal laser uorescent microscopy analysis. Fig. 1 shows the preparation for palm bers analysis. For comparative analysis, untreated palm bers (raw) were also subjected to the experiments.

Tensile properties
Tensile testing of palm bers was carried out with a Universal Testing Machine (gauge length, 20 mm) at a rate of 2 mm/min. Thirty specimens for each group were subjected to tensile tests, and the average values were noted. The average diameter of palm ber was determined by measuring three points in the tensile zone. Strain-stress curves of each group were graphed with Origin software.

FT-IR microspectroscopic imaging
The embedded-ber transverse-section slides were placed in a warm water bath to expand PEG and then transferred onto ZnS slides. After drying at 50°C, PEG was removed by placing the slides in 100% (two times), 80%, 50%, and 25% ethanol-water solution for 10 min. Finally, the slides were washed three times with distilled water to remove ethanol and then freeze dried for FT-IR microspectroscopic imaging analysis. The FT-IR microspectroscopic images were recorded on a Spectrum Spotlight 400 FT-IR microscope (PerkinElmer Inc., Shelton, CT, USA). The spectra were recorded with a 4 cm-1 spectral resolution, between 4000 cm-1 and 740 cm-1.

CLSM
The transverse-section slides were rinsed with deionized water in a watch glass 10 times to remove PEG.
After dehydration through a graded series of ethanol solution (50%, 70%, 90%, and 100%), the slides were mounted in glycerol, covered with a coverslip, and examined with an LSM 510 META laser confocal scanning microscope. The excitation wavelength was 488 nm and the emission wavelength at 568 nm for imaging lignin auto uorescence analysis.

Chemical-component quantitative analysis
Chemical analysis of the palm bers was carried out according to the Method of Quantitative Analysis of Ramie Chemical Components (GB5889-86). The content changes of the cellulose, hemicellulose, and lignin before and after alkali treatment were accurately determined. Table 1 lists the average diameters of raw, 2, 5, and 10 wt.% alkali-treated palm bers. It is shown that alkali treatment contributed to decreased ber diameter with increased alkali concentration. In fact, impurities on the ber surface and some components were removed during alkali treatment (Boopathi et al. 2012, Kathirselvam et al. 2019). The ber structure was only slightly affected, resulting in more brillation and giving rise to ner bers.  Mean ± standard deviation based on 30 data.

Fiber-diameter distribution
The test results of tensile strength, elongation, and Young's modulus of alkali-treated (2, 5, and 10 wt.% NaOH) and raw bers are shown in Table 2. Typical palm ber stress-strain curves are plotted in Fig. 2. The FT-IR spectra of raw and alkali-treated palm bers were analyzed to investigate the effect of alkali treatment on the chemical characteristics of ber cell wall. Fig. 4 shows the FT-IR spectra of the cell wall in the ngerprint region (1800 cm −1 to 800 cm −1 ). Speci c 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 CH 2  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 signi cant 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 nding 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 signi cant 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 micro brils, consequently increasing the tensile properties of the bers.

CLSM imaging of palm lignin auto uorescence
To further study lignin's microstructure transformation and distribution characteristics in palm ber 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 auto uorescence, which comprised monolignols. Fig. 6 shows the auto uorescence images of raw and alkali-treated samples at different concentrations. Figs. 6(a, e) demonstrate the heterogeneous distribution of lignin in raw bers, with higher lignin auto uorescence intensity occurring in the cell corner middle lamella and compound middle lamella regions. For the sample treated by 2 wt.% NaOH, uorescence 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 uorescence intensity, which may be attributed to a signi cant degradation of hemicelluloses linked to lignin by ester and ether bonds. Overall, the transformation of uorescence re ected 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. The chemical components of raw and alkali-treated bers are presented in Table 3. Raw ber consisted of cellulose (28.04%), hemicelluloses (23.5%), lignin (38.18%), and impurities (10.28%). Hemicellulose was signi cantly affected by the concentration of the alkaline solution, which decreased from 23.5-10.82%.

Chemical components of palm ber
Lignin content decreased from 38.18-28.66%, and the impurity content decreased from 10.28-5.36%. Correspondingly, cellulose content increased from 28.04-55.16%. These results were consistent with those of FT-IR microspectroscopy and CLSM analyses.

Conclusion
The microstructure transformation of alkali-treated palm ber and the in uence of this transformation on ber mechanical properties were studied in this paper. Results showed that the tensile properties of palm bers increased after alkali treatment because of the microstructure transformation. FT-IR microspectroscopy and CLSM analyses provided new information on cell-wall microstructure transformation resulting from the removal of hemicelluloses and lignin. The brils rearranged themselves in a compact manner that resulted in a close packing of cellulose chains, which may have led to the increased tensile properties. These ndings may bene t the high-value utilization of palm in the eld of materials. Figure 1 The preparation process for palm bers analysis Page 12/16

Figure 2
Tensile stress-strain curves of palm bers: raw and alkali treatment for different conditions. FT-IR spectra of the cell wall of bers in the ngerprint region The relative intensities of the absorption peaks at 1740, 1710, 1600, and 1508 cm −1 in palm as a function of treatments