3.1. Micromorphology analysis of NFC
The morphological features of UA-NFC, OA-NFC, CA-NFC, and MT-NFC are measured and verified by TEM characterization. Branch-liked and network-liked NFC are clearly seen in cellulose with different surface functionalizations. The main body of NFC presents a microfibril feature attached to a large number of nanofibrils. Figure 2a-b exhibit that the diameter of UA-NFC and OA-NFC is in the range of 20–26 nm (average of 23 nm), and nanofibrils are with favorable distribution without agglomeration. This phenomenon is owing to the charge repulsion created by the amide groups and carboxyl groups on the surface of UA-NFC and OA-NFC (Henschen et al., 2019). Additionally, the UA-NFC and OA-NFC show better dispersion in water than the CA-NFC and MT-NFC. These aggregations in the CA-NFC and MT-NFC corresponded presumably to the high specific surface area, and strong hydrogen bonds existed between the nanowhiskers. As a consequence, bundles of elementary nanofibrils bind together and generate a larger dimension (Ca. 3–5 µm).
To gain further insights of the NFC structures, AFM measurements are utilized (Henschen et al., 2019). Figure 2e exhibits the characteristics of CS in that several small particles are agglomerated side-by-side, and no cellulose microfibrils can be found. Because of the high-speed shearing of homogenization, the winding-liked and network-liked nanofibrils with high length-diameter ratios are shown in Fig. 2b-d. This is consistent with that observed in SEM for the same sample. Micrographs of the UA-NFC, CA-NFC, and OA-NFC show that the material consists of nanofibrils with average lengths of 2.06 µm, 1.32 µm, and 1.21 µm, average widths of 0.41 nm, 0.45 nm, and 0.46 nm. Compared with UA-NFC, the CA-NFC and OA-NFC samples are finer and shorter. This could be due to acidification, which decreases the surface polarity of microcrystalline cellulose and segregates the interactions between them. This, in turn, promotes the nanofibrillation process, which is consistent with the results from earlier reports (Henschen et al., 2019). They attributed the fact that the acid environment not only dissolves amorphous molecules but also partly destroys crystalline zones of the fibrils, ultimately resulting in a length-shortening of nanofibrils (Li, D. et al., 2020).
The microstructures of raw CS, and the freeze-dried BCS, UA-NFC, OA-NFC, CA-NFC, and MT-NFC are shown in Fig. 3. A more rough and uneven surface morphology of untreated corn stalks is observed in Fig. 3a. Cracks, small pores, micro-voids (as shown in the upper right), and helical fibrils corresponding to the vessel, xylem tissue, and parenchyma (including cellulose, hemicellulose, pectin, lignin, and other impurities) can be seen on the surface of raw CS (Li, Z. et al., 2020). In addition, it also shows the presence of wax and oils on the surface of the CS, which provides a protective layer (as shown in the bottom left). As shown in Fig. 3b, cellulose microfiber bundles tend to be loose due to the disintegration of the overall structure of cell walls. It indicates the removal of hemicellulose and lignin with the bleaching treatment (Chen et al., 2016). Impurities on the surface of the BCS are also completely removed, exhibiting a partially streaked feature of the fibrils (as shown in the bottom left). This feature facilitates mechanical exfoliation of nanofibrils, as reported by Thien et al. (2022), who reported a similar observation when pineapple leaves were bleached with alkaline hydrogen peroxide solution. Additionally, the surface of BCS shows numerous tiny pores and openings. These voids are generated by the dissolution and leaching out of cohesive components like hemicellulose, lignin, and waxes during the bleaching process. The voids produced on the surface of BCS will provide great mechanical anchoring and interlocking of the cellulose with a polymer matrix (Kumar et al., 2014).
After intensive mechanical shearing treatment (see Fig. 3c-f), the fiber bundles are disaggregated into microfibrils associated with abundant branched and networked cellulose nanofibrils. This means that a large number of nanofibrils are produced, while the overall structure of the microfibrils is not completely disintegrated. This uniquely structured nanocellulose, NFC, was found to have excellent mechanical properties in our previous study (Tian, J. et al., 2022; Xu et al., 2021). As can be seen in Fig. 3c, urea/alkali treatment results in the separation of fiber bundles. Each microfibril can be considered as a bundle of cellulose whiskers linked along the nanofibrils by amorphous domains. The amorphous regions act as structural defects and are responsible for the transverse cleavage of the microfibrils into nanofibrils. This is different from nanocellulose materials such as cellulose nanocrystals, cellulose nanowhiskers, etc. NFC has a partially amorphous structure, which will be characterized in the crystalline structure analysis below (Cheng et al., 2018). The surface morphology of the oxalic/citric acid treated cellulose (OA-NFC/CA-NFC) is shown in Fig. 3d-e. Some nanofibrils with lengths below the micrometer range can be seen therein. The morphology of the OA-NFC/CA-NFC was a slightly entangled nanofibrils network with overall diameters below 200 nm under organic acid hydrolysis. Chen et al. (2016) reported a similar feature of the oxalic acid treated NFC, and attributed it to the fact that acid hydrolysis substantially facilitated mechanical fibrillation of the resultant BCS due to the reduced molecular weight. Nanofibrils on the mercaptopropyl trimethoxysilane treated NFC in Fig. 3f can be clearly seen, and the characteristic is similar to nanofibrillation by mechanical impact (Chen et al., 2019). Abundant nanofabrils are separated from the cell walls of different corn stalk tissues by high-speed shearing treatment (see in bottom left). But the length of the isolated nanofabrils appears to be shorter than that of the acid-base treatment, which will be confirmed in particle size analysis. Consequently, the results shown in Fig. 3a-f suggest that the hyper nanofibrillating of cellulose occurs during the high-speed shearing process. Cellulose suspension passes through the tiny gap between the shearing knives, subjecting the cellulose fibers to tough shear and impact forces. Generally, the unique morphology (branch-like and/or network-like) of NFC is successfully obtained from corn stalks by mechanical treatment. In the four functionalized environments, the size and morphology of NFC nanofibrils are slightly different.
Silanization of cellulose surfaces can effectively improve its interfacial compatibility. This study expects to use a one-pot method to achieve the purpose of NFC preparation and surface functionalization. The uniformity of silanization modification is shown in Fig. 3g-k. The EDS spectrum (Fig. 3g) shows that the surface of MT-NFC mainly consists of C, O, Si and S. Among them, C and O are mainly derived from cellulose, and Si and S mainly come from silane coupling agents. Figure 3h-k shows the EDS elemental mapping images of C, O, Si, and S, respectively. As a consequence, surface silane functionalization of NFC is successfully realized. In addition, the valence bond of Si element will be analyzed in XPS results.
3.2. Surface functionalization of the NFC
3.2.1. FTIR characterization
The FTIR spectra of raw material and chemically functionalized samples are shown in Fig. 4. The wide band between 3450 cm− 1 and 3420 cm− 1 is ascribed to the O-H stretching vibrations. The peak near 2900–2925 cm− 1 in all spectra corresponds to the stretching vibration of the aliphatic saturated C-H group. The peak at 894–898 cm− 1 reflects the C-H bending vibration connecting with the glucose β-glycoside bond in cellulose I, which is contained in the nanofibrils and/or microfibrils (Thien et al., 2022). The characteristic peaks at 3420–3450 cm− 1, 2900–2925 cm− 1, and 894–898 cm− 1 are observed in the 4 surface-modified NFC, indicating that there are no changes in the basic chemical structure of cellulose (Sungsinchai et al., 2022). Notably, the peak at 1730 cm− 1 in the raw CS corresponds to the C = O stretching vibration of the acetyl and uronic ester groups from hemicellulose and lignin (Wang et al., 2022). The peak at 1510 cm− 1 of the raw CS represents the C = C stretching vibration on the lignin. However, these peaks disappear after the 4 surface-modifications, indicating that hemicellulose and lignin were removed (Bozic et al., 2015).
More importantly, the band around 1635 cm− 1 in the UA-NFC is assigned to the -NH2 bending vibration. This suggests that the amidation reaction may have occurred on the cellulose surface, which can be further confirmed in XPS analysis. The hydroxide and urea mixed solution treatment could break the intermolecular and intramolecular hydrogen bonds between the nanofibrils, but also tailor the surface of cellulose under mechanochemical action.
The bands at 1714 cm− 1 correspond to the C = O vibration of the cellulose presented in OA-NFC, which demonstrates the surface carboxylation of cellulose. Three new absorption peaks at 2449 cm− 1, 1384 cm− 1, and 1065 cm− 1 are assigned respectively to COOH group and CH2 group vibration, C-O-C stretching vibration of CA-NFC, confirming the successful citric acid acidification process. The new absorption peak at 1429 cm− 1 is attributed to the vibration of C-H, the characteristic peak of CH2 group for MT-NFC. Peaks in spectra of MT-NFC around 1034 cm− 1 are attributed to Si-O-Si vibration. Combined with the above EDS mapping of Si and S elements, it is fully proved that the MT-NFC surface is silanized. Consequently, the results indicate that high-speed shearing treatment and chemistry modification promote surface functionalization of amidation, carboxylation, and silanization of NFC effectively.
3.2.2. XPS
XPS is further used to validate the surface functionalization and elucidate the reaction mechanism of NFC. Figure 5a-e shows the binding energy of C 1s, O 1s, Si 2s, and Si 2p. The chemical shifts of electron binding energy of C, O, Si can reflect the formation of key groups such as carbonyl, ester, and ether after the amidation, carboxylation, and silanization. Specifically, the binding energy of C attached to two oxygen atoms (O-C-O), which is the C1 of the glucopyran ring, shifts from 288 eV (BCS) to 287.7 eV (UA-NFC), 287.8 eV (OA-NFC), 287.9 eV (CA-NFC) and 287.6 eV (MT-NFC). This indicates an increase of the electron cloud density of the C1, which could be caused by the carboxylation of hydroxyl on the C2 and C6 (Zhang et al., 2020). Additionally, the new peaks appear at 288.9 eV, 289 eV, 289.1 eV on the C 1s spectra of the UA-NFC, OA-NFC, and CA-NFC respectively, which corresponds to the carbonyl peak and confirms the formation of the carboxyl group. Furthermore, on the spectrum of O 1s, the electron binding energy after surface functionalization appears to peak at 531.3 eV, 531.0 eV, 531.3 eV, respectively, which corresponds to the electron binding energy of C = O. These evidences indicate that NFC can achieve surface functionalization after treatment with urea/alkali, oxalic acid, and citric acid.
Following the pretreatment steps, another chemical functionalization involving silanization chemistry is performed on the BCS. As a representative example, mercaptopropyl trimethoxysilane for the modification of the hydroxyl groups on the surface of cellulose is conducted. The peaks at 152.68 eV and 101.68 eV in Fig. 5e3 and Fig. 5e4 correspond to the binding energy of Si 2s and Si 2p, which can be assigned to the generation of hydrogen bonds between hydroxyl groups and silane coupling agent. As a result, the surface hydrophilicity of NFC is modified while possessing silane functional groups such as mercapto groups in this case. Silanized MT-NFC has good interfacial compatibility with rubber and polylactic acid, which can greatly improve the mechanical properties of composites (Ma et al., 2018; Qian and Sheng, 2017).
3.2.3. Water contact angle
Figure 4b shows the surface hydrophilicity of the BCS and different NFCs. The water contact angle of urea/alkali-treated cellulose is lower than that of BCS. The FTIR and XPS results indicate that amide groups appear on the NFC surface after urea/alkali treatment. The electronegativity of the amide group is stronger than that of the hydroxyl group, causing the water contact angle to drop to
33.7º. This indicates that a large number of hydroxyl groups on the surface of UA-NFC underwent an amidation reaction (Golizadeh et al., 2019). The treatment of oxalic acid and citric acid is beneficial to increase the water contact angle and improve the hydrophobicity of the NFCs, which might due to the carboxyl group generated from the carboxylation reaction. The difference is that the number of hydroxyl groups on the NFC surface after oxalic acid modification is theoretically less than that after citric acid modification. In fact, the water contact angle of CA-NFC is 6.4° smaller than that of OA-NFC. In addition, the water contact angle of MT-NFC is similar to the BCS, which can be attributed to the sulfhydryl and hydroxyl groups on silanes. These results show that current preparation strategy can not only improve the hydrophobicity but also the hydrophilicity of cellulose, exhibiting excellent flexibility, selectability and adjustability.
3.3. X-ray diffraction
The X-ray diffraction patterns of corn stalks, BCS, UA-NFC, OA-NFC, CA-NFC, and MT-NFC are depicted in Fig. 5a. The raw material and chemically treated samples have peaks at 2θ = 14.9°, 16.2°, 22.4°, and 34.6°, which are the characteristic peaks of cellulose, indicating that the basic structure of cellulose is maintained, the cellulose I allomorph of neat NFCs are also proved by XRD measurements (Wang et al., 2018). The Xr is determined for the various samples using Eq. (1) and the results for the corn stalks, BCS, UA-NFC, OA-NFC, CA-NFC and MT-NFC are 45.0%, 50.6%, 48.2%, 75.1%, 54.3% and 58.8%, respectively. The higher crystallinity of BCS can be ascribed to the progressive removal of hemicellulose, lignin, and other amorphous fraction of cellulose induced by bleaching treatments. Cellulose has a crystalline structure because of hydrogen bonding interactions and Van der Waals forces between adjacent molecules (Wu et al., 2020). The lightly decrease in crystallinity of UA-NFC compared to BCS may be attributed to the fact that surface urea/alkali consumed partly hydrogen bonds and reduced intermolecular forces. The subsequent high increase in crystallinity of the OA-NFC, CA-NFC, and MT-NFC in relation to BCS is observed. The high-speed shearing treatment could disintegrate the amorphous regions of cellulose and trigger the cleavage of glycosidic bonds, which eventually releases more individual crystallites, meaning an increase in the size of the cellulose I crystallites in UA-NFC, OA-NFC, CA-NFC, and MT-NFC. This could be seen from the comparison of BCS and UA-NFC, OA-NFC, CA-NFC, and MT-NFC in the width of the (200) planes (Wu et al., 2020). By contrast, OA-NFC displays the narrowest and sharpest peaks at 2θ = 22.4° due to its higher crystallinity compared to other samples. This implies that oxalic acid easily enters the interior of cellulose and separates nanofibrils.
3.4. Thermal properties of CS, BCS and NFCs
Table 1
Detailed DTG data of nanocelloluse fibers as obtained from TGA measurements under N2.
samples
|
T10%(℃)
|
T25%(℃)
|
T50%(℃)
|
TDTGmax(℃)
|
R550(%)
|
CS
|
94.2
|
274.9
|
320.2
|
314.1
|
21.55
|
BCS
|
276.0
|
322.0
|
343.0
|
348.8
|
7.93
|
UA-NFC
|
277.9
|
315.9
|
338.9
|
348.1
|
4.42
|
OA-NFC
|
272.6
|
311.6
|
329.6
|
334.7
|
4.53
|
CA-NFC
|
265.5
|
301.5
|
324.5
|
331.7
|
2.44
|
MT-NFC
|
286.8
|
312.7
|
327.8
|
332.5
|
1.98
|
Figure 5b1-b2 show the TGA and derivative thermogravimetry (DTG) curves obtained from corn stalk, BCS, UA-NFC, OA-NFC, CA-NFC, and MT-NFC. Thermal degradation of raw CS, BCS, and the various NFCs with different treatments is detected by TGA to determine the onset temperature and the maximum degradation temperature of the main mass-loss regions (see Table 1). Three different mass-loss regions can be observed (Fig. 5). The initial weight loss of the samples occurs below 100°C, which can be attributed to the reduction of moisture content in lignocellulose fibers. With the temperature increase, the main degradation step corresponds basically to cellulose decomposition, such as depolymerization, dehydration, and pyrolysis of glycosyl units. The last degradation step (above 485°C) is associated with the oxidation and breakdown of the charred residue to lower molecular weight gaseous products.
A small shoulder from CS sample (Fig. 5b2) can be noticed forward of the main degradation peak, which is due to the thermal decomposition of wax, pectin, hemicellulose, and lignin. After removal of hemicellulose and lignin, the BCS, UA-NFC, OA-NFC, CA-NFC, and MT-NFC exhibit single thermal mass loss peaks. The second main mass-loss region (340–350°C) of BCS could be related to the dehydration of cellulose. In fact, it slightly diminishes after surface functionalization and nanofibrillation, indicating that the thermal stability of cellulose decreased after modification. This may be due to the crystalline area of cellulose in the BCS is destroyed during the 4 different treatment processes. The disorder and accessibility of cellulose increase so that the branched and networked nanofibrils on NFC surfaces are easily pyrolyzed. Among them, the sharpest weight drop is observed for MT-NFC, which can be rationalized in light of its relatively short nanofibrils exposed to thermal pyrolysis. The mass residue at temperatures 500–600°C in CS is remarkably high at 21.55%. However, less charred residue of BCS than that of CS is due to the fact that non-cellulosic material could induce higher char formation (Chen et al., 2016). The lowest residue of MT-NFC verifies its high purity. These results are consistent with the findings obtained from the chemical composition, morphology, and structure measurements.
3.5. Solution stability
The ability of the NFCs to form stable suspensions or gels in water is evaluated by qualitatively analyzing the sedimentation rate of the suspensions. As shown in Fig. 5c, it presents the NFC suspensions at 0.2 wt% for 0 min, 15 min, 30 min, 1 h, and 12 h, respectively. With the increase in standing time, the suspension starts to separate. Among them, BCS aqueous suspension is more translucent at different periods, and the particles precipitate very quickly. Referring to the literature (Ji et al., 2019), the higher the content of nanofibrillated material, the more intensive is the transparency of the NFC. Conversely, OA-NFC aqueous suspensions are more uniform and stable. OA-NFC has better nanodispersion in water because the surface carboxylate enhances the homogenous dispersion property and stability of the nanofibers in water.
3.6. Surface functionalization mechanism
The functionalization and modification mechanisms of hydroxyl-grafted amides (urea/NaOH), carboxyl groups (oxalic acid/citric acid), and hydrogen bonds (mercaptopropyl trimethoxysilane) on the NFC surface are suggested and discussed (Fig. 5d). First, NFC is chemically modified with urea in the presence of sodium hydroxide, the cellulose is swollen in concentrated sodium hydroxide solution that destroyed the crystalline aggregation of cellulose and increased the accessibility of fibers to chemicals. Thereafter, through a nucleophilic substitution reaction between alkalized cellulose and urea, UA-NFC was synthesized. This was also corroborated in XPS and FT-IR. The band around 1635 cm− 1 in the UA-NFC is assigned to the -NH2 bending vibration and the electron binding energy of C = O from urea. In addition, carboxylation of NFC is confirmed in the presence of oxalic acid, and citric acid during mechanical treatment. Due to the crystalline structure, it is difficult for the acid to fully penetrate into cellulose fibers. The carboxylation is most likely to occur on C6-OH due to its greater steric accessibility than C2-OH and C3-OH (Henschen et al., 2019). Citric acid and oxalic acid functionalize cellulose by reacting with C6-OH, the C = O electron energy binding peak appeared in XPS as well as the C = O vibration of the carboxyl group appeared in FT-IR, which confirms the above reaction mechanism. Finally, mercaptopropyl trimethoxysilane is added to water for hydrolysis, and acetic acid is added to adjust the pH to weak acidic, which shortens the hydrolysis period. After hydrolysis, the NFC is uniformly dispersed into a silane solution. As a result, etherification occurs with the hydroxyl groups on the NFC surface, and MT-NFC is prepared. Meanwhile, the spectra of Si 2s and Si 2p could be the evidence in XPS.