3.1. Control the hemicellulose content by NaOH/urea treatment
In the NaOH/urea aqueous solution, NaOH hydrates can bond to cellulose to break the intra- and intermolecular hydrogen bonds and urea hydrates accumulate on the cellulose hydrophobic region to promote the dissolution of cellulose and prevent the aggregation of dissolved cellulose molecules (Xiong et al. 2014). Hemicellulose has lower molecular weight and loose aggregate structure compared to cellulose, which was much easier to degrade by alkali/urea treatment and its removal is analogous to that of cellulose. Figure 1a shows the effect of the ratio of NaOH/urea on the hemicellulose content of BP, which was 6.36% in 6 wt% NaOH solution without urea ,With the increase of the urea content from 1, 3 to 5 wt%, the hemicellulose altered from 5.45%, 5.96–6.08%. BP-14.2 exhibited effective removal rate of hemicellulose in 6 wt% NaOH/1 wt% urea aqueous solution, which was 61.62%. Therefore, 1 wt% was chosen as the effective urea loading amount for the subsequent experiments.
The hemicellulose content of BP firstly increased from 9.35 to 4.24% in 30 min, then decreased to8.86 and 9.36% with the time prolonging to 40 and 50 min (Fig. 1b). The reason is that the alkali liquor mainly attacks hemicellulose in a short time. With the time prolonging, the alkali liquor slowly infiltrates into the interior of cellulose, resulting in the dissolution of cellulose with lower DP. Figure 1c depicts the hemicellulose content of BP treated at different temperature. When the temperature below 40°C, NaOH selectively removes alkali soluble impurities and short chain carbohydrates. The hemicellulose content was increased from 7.91, 8.18, 5.45to 4.24%, respectively. As the temperature further rises to 50°C, a decrease in the DP of cellulose occurred. Both cellulose and hemicellulose tend to decline, resulting in the hemicellulose removal rate less than lower temperature. Therefore, the removal rate decreased to 25.84%. The detailed hemicellulose removal rate and hemicellulose content at different reaction condition are presented in Table 2.
Table 2
The removal rate of hemicellulose from BP
Time
(min)
|
Temperature
(°C)
|
Urea
(wt%)
|
Sodium hydroxide
(wt%)
|
Hemicellulose content
(%)
|
Hemicellulose removal rate
(%)
|
0
|
0
|
0
|
0
|
15.9
|
0
|
0
|
0
|
0
|
0
|
14.2
|
0
|
30.00
|
30.00
|
0.00
|
6.00
|
6.36
|
55.20
|
30.00
|
30.00
|
1.00
|
6.00
|
5.45
|
61.62
|
30.00
|
30.00
|
3.00
|
6.00
|
5.96
|
58.03
|
30.00
|
30.00
|
5.00
|
6.00
|
6.08
|
57.18
|
30.00
|
10.00
|
1.00
|
6.00
|
7.91
|
44.30
|
30.00
|
20.00
|
1.00
|
6.00
|
8.18
|
42.39
|
30.00
|
40.00
|
1.00
|
6.00
|
4.24
|
70.14
|
30.00
|
50.00
|
1.00
|
6.00
|
8.38
|
41.00
|
20.00
|
40.00
|
1.00
|
6.00
|
9.35
|
34.15
|
40.00
|
40.00
|
1.00
|
6.00
|
8.86
|
38.87
|
50.00
|
40.00
|
1.00
|
6.00
|
9.36
|
34.08
|
60.00
|
40.00
|
1.00
|
6.00
|
8.60
|
45.91
|
90.00
|
40.00
|
1.00
|
6.00
|
5.70
|
64.15
|
FTIR spectra were applied to elucidate the chemical changes of untreated and pre-treated BP. The intensities of -OH for BP-8.60 and BP-5.70 increased compared to BP-15.9, due to the -OH was exposed by the NaOH/urea treatment (Fig.
2a). The enhanced hydrogen bonding capacity was benefit to the swelling and dissolution of hemicellulose. The peak around 1640 cm
− 1 was corresponding to the unconjugated carbonyl groups (C = O) of xylan in hemicelluloses component, which was obviously weakened in the HC-8.60 and HC-5.70 (Wang et al.
2020). This result demonstrated that hemicellulose was partially removed due to the alkaline treatment. XRD patterns of BP shown three characteristic peaks at 2θ = 14.9°, 16.2° and 22.6° corresponding to the (1
\(\stackrel{\text{-}}{\text{1}}\)0), (110), and (200) planes of cellulose I crystalline form, respectively (Fig.
2b) (Wang et al.
2014). With the reduction of hemicellulose content from 15.9 to 5.70%, crystallinity increases from 49.92–58.41%, indicating the destruction of amorphous regions in the process of NaOH/urea treatment. Besides, the crystalline structure of BP wasn’t change after treating by 6 wt% NaOH. On the other hand, the diameter of BP microfiber increased from 8.33, 9.78, 14.0 to 20.1 µm with the decrease of hemicellulose content, which is more conducive to the penetration of solution in the dissolution section, as shown in
Fig. S1.
3.2. The dissolvability of CC
M η has a significant impact on the solubility of cellulose and the properties of the regenerated materials. As displayed in Table S1, four kinds of CC with different Mη with similar hemicellulose content were firstly prepared according to previous work (Li et al. 2019; Wang et al. 2014). Optical microscope was applied to observe the solubility of 7 wt% CC in NaOH/ZnO aqueous solution. Many tangled and swollen microfibers appeared in the higher Mη (1.9×105) of CC solution (Fig. 3a). As the molecular weight decreased from 1.3×105, 1.0×105 to 0.6×105, microfiber bundles were greatly reduced. The corresponding solubility of CC in NaOH/ZnO aqueous solution increased from 10%, 50%, 83–90%, respectively (Fig. S2). Further, BP (Mη = 0.6×105) were immersed into 0, 0.5, 4, 6 wt% urea solution to prepare CC with nitrogen content of 0, 0.35, 1.38, 1.36%, respectively. Table S2 indicates that the higher nitrogen content is conductive to the dissolution of CC. As the nitrogen content enhanced to 1.38%, the solubility increased to 90% and optical images results are consistent with the above results (Fig. S3). Therefore, the Mη of BP around 0.6×105 was chosen to react with 4% urea for exploitation the influence of hemicellulose content on the solubility of CC in NaOH/ZnO aqueous solution.
According to the experimental condition in Table 1, BP-14.2 was applied to prepare different hemicellulose content of CC for further characterization. The nitrogen content of HC-15.90, HC-8.60 and HC-5.70 was 1.38, 1.20, 1.26%, respectively (Table 1). FTIR spectra of BP-15.9, NaOH/urea treated BP-5.70 and HC-5.70 are shown in Fig. S4a. The band at 1750 cm− 1 of HC-5.70 attributed to the stretching vibration of the carbonyl (C = O) in carbamate group, indicating that urea had reacted with cellulose to form CC. The intensity of the characteristic peaks of carbonyl exhibited the same tendency with nitrogen content (Fig. S4b).
Optical microscopic images of CC with various hemicellulose dissolved in NaOH/ZnO aqueous solution are shown in Fig. 4a-c. With the decrease of hemicellulose content, there are fewer fiber bundles in the field of picture, indicating that the removal of hemicellulose can promote the dissolution of CC. For further verification that the reduction of hemicellulose can improve the solubility of CC, higher Mη of CC were studied. The Mη of CC was 1.30×105 and 1.27×105, and the corresponding hemicellulose content was 15.2%, 8.63% (Table S3). Except hemicellulose content, α-cellulose, nitrogen content and molecular weight are similar. With the decrease of hemicellulose content, the number of fiber bundle also decreased in the field of polarizing microscope, indicating the removal of hemicellulose can improve the solubility and the clarity of the solution (Fig. S5).
Figure 5a depicts that the solubility of CC was enhanced as the hemicellulose content decreased. When the hemicellulose content was 15.9%, the solubility was 90.33%. When the hemicellulose content decreased to 5.70%, the solubility of CC increased to 97.70%. The transparency of the solution also increased. When the hemicellulose content is high, it is obvious that the solution has a certain turbidity. When the hemicellulose content decreases, the clarity of the solution gradually increases. As shown in Fig. 5b, the aggregation of dissolved cellulose chain was observed through DLS. The smaller Rh represents a single cellulose molecular chain, and the larger Rh represents cellulose chain aggregates. With an increase of hemicellulose content, the cellulose chain aggregates gradually increased from 2.6, 21.9 to 43.0 nm, indicating that the higher hemicellulose content, CC was much easier to aggregate. These results also certificated that the decrease of hemicellulose content can increase the solubility of CC.
3.3. Rheological behavior of CC with different hemicellulose content in NaOH/ZnO aqueous solution.
The rheological behaviors of CC with different hemicellulose content in NaOH/ZnO aqueous system were characterized to evaluate the impact of the hemicellulose on the rheological and mechanical properties of the gel network. Firstly, the sol-gel behavior of the CC network was demonstrated by the dynamic viscoelastic measurement with the frequency of 1 Hz (Fig. 6a). As the temperature rises, the thermal motion and the collision between cellulose molecules increase, and cellulose molecular chains aggregate more easily. In the heating process from 20 to 100°C, the crossover of loss modulus (G'') and storage modulus (G') was observed, the value was taken as apparent gel point. With the increase of hemicellulose, the gel point increase from 59.0, 69.0 to 73.4°C. As the temperature increases, G'' presents a platform, indicating that an irreversible three-dimensional network structure has been formed. The results indicated that the existence of the hemicellulose molecules entangled on the CC could destroy the stability of solution, resulting in the cellulose self-aggregation at lower temperature and a decrease of the gelation temperature.
The angular frequency (ω) dependence of G'' and G' curves for CC at 20°C were illustrated in Fig. 6b. In the whole frequency range, HC-15.9 exhibited a solid-like state (a G' larger than G'') and showed significantly frequency-independent plateaus, indicating the existence of a stable structure of the gel network. For HC-8.60 and HC-5.70, the G'' are greater than G', and both G' and G'' are strongly dependent on the frequency in the entire frequency measured, which indicate that a liquid-like state. The above results suggest that decreased hemicellulose content leads to a relatively stable liquid state of the cellulose solution. Figure 6c depicts the rotational viscosity of the CC solution with different hemicellulose contents. The rotational viscosity increases from 4.53×103, 5.51×103 to 6.57×103 mPa·s with the increase of hemicellulose content from 15.9, 8.60 to 5.70%. In the process of rotation, the CC molecular chains are affected by its own internal and external forces, which increases the collision between cellulose and hemicellulose molecules, leading to the increase of the opportunities for aggregation to form a three-dimensional network structure of cellulose and therefore appear an increase in the viscosity of CC solution.
The time dependence of G' and G'' for CC at different temperatures are shown in Fig. 7. At lower temperature (40°C), the movement and the collision probability of molecules slow down (Fig. 7a). The G'' has been in a flat state during the test time at 40°C, as time goes on, the G' shows a state of slowly rising, and finally G' and G'' cross together to reach the gel state. The apparent gelation time increases from 5640 to 12120 s with the hemicellulose decreased from 15.9 to 8.60%, and HC-5.70 always keeps a liquid-state (G'' > G') until the test time reach 17000 s. As the temperature raised to 60°C, HC-15.9 present a solid-state (G'> G'') at the beginning and the apparent gelation time decrease to 1017 and 2360 s for HC-8.60 and HC-5.70 (Fig. 7b). The experimental results also showed that the higher hemicellulose content, the easier the CC solution will gel in a short time.