Carboxylate content and yield
The carboxylate content and yield of PP-CNF and PP-TOCNF are summarized in Table 1. After TEMPO-mediated oxidation, the carboxylate content of PP-TOCNF-2.5, PP-TOCNF-5 and PP-TOCNF-10 was 0.218 ± 0.011, 0.331 ± 0.041 and 0.550 ± 0.041 mmol/g, respectively, showing an increased oxidation degree with the increase of NaClO concentration. Accordingly, the carboxylate content of PP-TOCNF can be effectively adjusted by controlling oxidation parameters (e.g., NaClO concentration). The yield of PP-CNF, PP-TOCNF-2.5, PP-TOCNF-5 and PP-TOCNF-10 was 73.10 ± 3.11%, 68.28 ± 1.59%, 74.50 ± 2.12%, and 75.25 ± 0.28%, respectively. Compared with PP-CNF directly isolated by ball milling, the yield of PP-TOCNF firstly decreased (PP-TOCNF-2.5) and then slightly increased (PP-TOCNF-5 and PP-TOCNF-10) with the increase of oxidation degree. This trend could be caused by a combination of the increases of cellulose degradation and defibrillation degree during TEMPO-mediated oxidation and balling process (Huang et al. 2020). Under the oxidation conditions, the cellulose can be oxidized and depolymerized into water soluble derived compounds (Isogai et al. 2018). The decrease of yield at low oxidation degree is probably due to the fact that the increased defibrillation degree cannot offset the increase of amorphous region hydrolysis of cellulose, while higher oxidation degree leads to an easier defibrillation by ball milling, thus resulting in an increase of yield (Liu et al. 2018).
Table 1
Carboxylate content and yield of PP-CNF and PP-TOCNF
Samples
|
nCOOH (mmol/g)
|
Yield/%
|
PP-CNF
|
/
|
73.70 ± 3.11a
|
PP-TOCNF-2.5
|
0.218 ± 0.011c
|
68.25 ± 1.59b
|
PP-TOCNF-5
|
0.331 ± 0.041b
|
74.50 ± 2.12a
|
PP-TOCNF-10
|
0.550 ± 0.031a
|
75.25 ± 0.18a
|
Zeta potential analysis and visual appearance
The zeta potential value and visual appearance of PP-CNF and PP-TOCNF suspensions are shown in Fig. 1a and b. As shown in Fig. 1a, the zeta potential value of PP-CNF, PP-TOCNF-2.5, PP-TOCNF-5 and PP-TOCNF-10 was -18.13 ± 1.01, -24.20 ± 1.65, -26.67 ± 0.50, and -33.90 ± 1.71 mV, respectively corresponding to the change of carboxylate content (Table 1). The increase of oxidation degree resulted in the increase of carboxylate content, thus endowing PP-TOCNF with better colloidal stability due to stronger electrostatic interaction between carboxyl groups (Yang et al. 2019). As shown in Fig. 1b, after standing for 1 d at room temperature, all samples showed a delamination at a low concentration (0.1%), implying a poor colloidal stability. However, with the increase of solid concentration (0.5% and 1%), the colloidal stability was visibly enhanced due to the increase of electrostatic repulsion and solution viscosity (Li et al. 2015). Notably, all the PP-TOCNF samples showed a better colloidal stability than that of PP-CNF, mainly attributing to the enhanced electrostatic repulsion.
FTIR analysis
FTIR spectra of PPC, PP-CNF, and PP-TOCNF are shown in Fig. 1c. All samples displayed the characteristic peaks of cellulose I structure at 3300, 2929, 1620, 1425, 1370, 1050 and 896 cm-1, indicating the chemical structure of cellulose can be maintained after TEMPO oxidation and ball milling process. The peak at around 3300 cm-1 reflected the intramolecular -OH bond stretching vibration. The narrow peak at 2929 cm-1 belonged to the C-H stretching vibration, and the peaks at 1425 and 1050 cm-1 represented the stretching vibration of C-O-C (Zhao et al. 2017). The peaks around 1370 and 896 cm-1 were attributed to the stretching vibrations of the β-glycosidic bond between the glucose units of the cellulose molecule (Hassan et al. 2021). Compared with PPC, PP-CNF directly obtained by ball milling showed a higher intensity at 896 cm-1, corresponding to a higher level of glycosidic linkages between glucose units in cellulose (Sofla et al. 2016). After TEMPO-mediated oxidation, the obtained PP-TOCNF showed an increased peak intensity around 1620 cm-1 that belongs to the stretching vibration of C=O (Lyu et al. 2021), implying the increase of oxidation degree, which is consistent with the results of carboxylate content (Table 1) .
UV-Vis light transmittances analysis
The light transmittances of PP-CNF and PP-TOCNF suspensions are shown in Fig. 1d. As can be clearly observed from this figure, all PP-TOCNF samples showed a better light transmittance than that of PP-CNF. The increase in light transmittance was positively correlated with the increase in the oxidation degree of PP-TOCNF, showing the highest light transmittance in PP-TOCNF-10. The higher carboxyl groups on the surface of PP-TOCNF can increase the electrostatic repulsion between nanofibrils, thus improving the dispersion properties of suspensions and the light transmittance (Besbes et al. 2011). In addition, the transmittance of the suspension is also closely related to the particle size of the substance (Levanič et al. 2020), which implied the decrease of particle size of PP-TOCNF with increasing oxidation degree, which can be confirmed by the following result of AFM (Fig. 2).
Morphological analysis
The AFM images and diameter distributions of PP-CNF, PP-TOCNF-2.5, PP-TOCNF-5, and PP-TOCNF-10 are depicted in Fig. 2. All samples showed a long fibrillar structure in their AFM images, belonging to typical morphological structure of CNFs. After ball milling, the PPC can be separated into individual nanofibril to generate PP-CNF, showing an average diameter of 42.31 nm. Notably, after TEMPO oxidation followed by ball milling, the average diameter of the obtained PP-TOCNF gradually decreased with the increase of oxidation degree, showing an average value of 32.36 nm for PP-TOCNF-2.5, 18.68 nm for PP-TOCNF-5, and 17.00 nm for PP-TOCNF-10, respectively. The surface carboxyl groups of PP-TOCNF could reduce the adhesion by forming strong electrostatic repulsion among nanofibrils (Lai et al. 2014). And it is related to the increase of zeta potential, which can enhance the electrostatic repulsion between nanofibrils to promote the nanofibril disaggregation (Huang et al. 2019).
XRD analysis
XRD patterns of PPC, PP-CNF, and PP-TOCNF samples are displayed in Fig. 3a, and their corresponding CrI values are shown in Fig. 3b. All samples showed some similar diffraction peaks at 2θ of around 16.0°, 22.8° and 34.5°, respectively corresponding to (110), (200) and (004) of the cellulose Iβ crystal structure (Mhd Haniffa et al. 2017), indicating the crystal structure of PP-CNF and PP-TOCNF can be maintained after TEMPO oxidation and ball milling. Compared with the CrI value of PPC (32.52%), the CrI value of PP-CNF was significantly reduced (21.58%), which could be attribute to the disruption of crystalline order of CNFs by the ball-milling-imposed mechanical stress (Khan et al. 2015). The CrI value of PP-TOCNF-2.5, PP-TOCNF-5 and PP-TOCNF-10 was 24.16%, 30.03% and 34.56%, respectively. The CrI value of PP-TOCNF increased with the degree of oxidation increase, attributing to the destruction of amorphous region during TEMPO oxidation process (Tang et al. 2017). In addition, the TEMPO-mediated oxidation can make the structure of PPC loose, thereby resulting in an easier removal of the amorphous region during the ball milling (Qu et al. 2021).
XPS analysis
The surface elemental composition of PPC, PP-CNF, and PP-TOCNF can be reflected by XPS, and the results are shown in Fig. 3c-i. As shown in Fig. 3c, the XPS survey spectra exhibited two main peaks, respectively at 532 eV for O 1s and 286 eV for C 1s. Compared with the O/C ratio of PPC (0.396), the O/C ratio of PP-CNF (0.522) was significantly increased, attributing to the exposure of more hydroxyl groups during ball milling. After TEMPO oxidation followed by ball milling, the O/C ratio of PP-TOCNF-2.5, PP-TOCNF-5 and PP-TOCNF-10 was 0.580, 0.597, and 0.604, respectively, showing a further increase of O/C ratio with the increase of oxidation degree, which can be attributed to the formation of carboxyl groups in PP-TOCNF. Notably, there was no significant difference in O/C values among PP-TOCNF samples. The oxidation occurred at the surface of cellulose at low NaClO concentration (2.5 mmol), and gradually further occurred at both on the surface and greater depth area of cellulose with higher NaClO concentration, but it cannot be revealed by surface elemental analysis (Lai et al. 2013). As shown in Fig. 3d-h, the C 1s survey spectra of all samples showed three peaks at 284.50, 286.10 and 287.40 eV, respectively belonging to C-C/C-H, C-O and O-C-O/C = O groups (Fei et al. 2020). Compared with the peak area of C-O for PPC (34.50%), PP-CNF increased to 43.87%, attributing to more exposure of hydroxyl groups during the defibrillation process by ball milling. The peak area of C-O and O-C-O/C = O for PP-CNF and PP-TOCNF remained almost unchanged. This trend might be due to the combination effects of TEMPO-mediated oxidation and ball milling. As the amount of C-O can be increased during ball milling and the amount of C = O also can be increased by TEMPO oxidation, the relative percentages of these two functional groups will affect each other and influence these peaks. The O 1s survey spectra of all samples is shown in Fig. 3i. The peak at 533 eV for PPC and PP-CNF belonged to the O-C groups, and the peak around 532.5 eV for PP-TOCNFs was attributed to the O = C groups, also confirming the introduction of carboxyl groups (Barazzouk et al. 2011).
Thermal stability analysis
The thermal stability of PPC, PP-CNF, and PP-TOCNF samples was analyzed by the thermogravimetric analysis (TGA) and the derivative thermogravimetric analysis (DTG), and the results are shown in Fig. 4a-b. The first peak below 100℃ was due to the weight loss of residual moisture in samples (Dai et al. 2018). PPC and PP-CNF respectively started to decompose at 222.79 and 206.64℃ (Tonset), and reached the maximum degradation rate at 326.91 and 322.13℃ (Tmax). Compared with PPC, the lower degradation temperature of PP-CNF could be attributed to its more available surface area for chemical reaction and lower CrI value (Sofla et al. 2016). PP-TOCNF also showed a decreased trend in Tonset and Tmax, showing 202.91 and 305.07℃ for PP-TOCNF-2.5, 183.64 and 299.78℃ for PP-TOCNF-5 and 175.68 and 290.54℃ for PP-TOCNF-10, respectively. This trend is due to the existence of carboxyl groups in PP-TOCNF (Masruchin et al. 2018). As for PP-TOCNF samples, the thermal degradation involves several chemical reaction, including dehydration, hydrolysis, depolymerization, decarboxylation and transglycosylation reaction (Gómez et al. 2016). At the second stage, PP-TOCNF showed two thermal decomposition steps. The first degradation peak at 200 ~ 250℃ was ascribed to the decarboxylation of the carboxyl groups, and the second degradation peak that occurred at 250 ~ 300℃ was due to the thermal decomposition of PP-TOCNF (Kim et al. 2021).
Rheological analysis
Figure 5a shows the curves for elastic modulus (G') and the loss modulus (G'') versus the imposed shear strain (γ) of PP-CNF and PP-TOCNF suspensions (1.0%, w/v). The strain sweep test was used to confirm the liner viscoelastic region of samples. As can be seen from this figure, the G' and G'' values of all samples were stable at γ = 0.1%, so it was selected in the subsequent test. The shear rate-viscosity and frequency-modulus curves of PP-CNF, PP-TOCNF-2.5, PP-TOCNF-5 and PP-TOCNF-10 are displayed in Fig. 5b. It can be seen that in the shear rate range of 0.1–100%, as the shear rate gradually increased, the viscosity of suspensions gradually decreased, showing a typical shear thinning behavior as a non-Newtonian fluid. The three-dimensional network structure of the entangled or aggregated CNFs can be destroyed during the shearing process, thus reducing the fluid resistance and its apparent viscosity (Malkin 2013). And it can be obviously observed that the viscosity of PP-TOCNF increased with the increase of oxidation degree, attributing to the viscosity mainly depends on the carboxyl content and morphology (Wei et al. 2017).
The G' and G'' of PP-CNF and PP-TOCNF suspensions as a function of angular frequency (ω = 0.1 ~ 100 rad/s) are shown in Fig. 5c. The values of G' and G'' increased with the increase of frequency. At low frequency (0.1 ~ 10 rad/s), the G' values of all samples were higher than G'' values, showing a gel network structure formation in the suspensions. At high frequency (10 ~ 100 rad/s), the G' values of PP-TOCNF were still higher than their G'' values, but the PP-CNF sample occurred a cross-over point, displaying a quasi-liquid behavior at high frequency. In general, it is a sign of gelation that existing a cross-over point where G' = G'' (Ding et al. 2018). Therefore, PP-CNF and PP-TOCNF can form the elastic 3D network and have the potential of gelation. As depicted in Fig. 5d, the PP-CNF sample exhibited a stronger liquidity, but the liquidity of PP-TOCNF decreased with the increase of oxidation degree, showing an obvious gelation behavior (especially in PP-TOCNF-10).
Application in aerogel
Density, porosity and water absorption of aerogel
The density, porosity and water absorption (WA) values for PP-CNF-AG and PP-TOCNF-AG are summarized in Table 2. Compared with the density of PP-CNF-AG (0.0156 g/cm3), the density of PP-TOCNF-AG samples showed a decreased trend (0.0097 ~ 0.0136 g/cm3). It can be observed that the density of aerogel firstly decreased and then increased with the increase of oxidation degree, attributing to the electrostatic interaction between the nanocellulose filaments in the suspension. The increase in the density of PP-TOCNF-10-AG might be due to the high carboxyl content that formed a dense structure (Gong et al. 2021). Compared with the porosity of PP-CNF-AG (75.30%), the porosity of PP-TOCNF-AG samples increased visibly (79.76% ~ 86.97%) and reached a stable value for PP-TOCNF-5-AG and PP-TOCNF-10-AG. The water absorption values of PP-CNF-AG, PP-TOCNF-2.5-AG, PP-TOCNF-5-AG and PP-TOCNF-10-AG aerogel were 38.18, 46.37, 70.71 and 56.28 g/g, respectively, showing an inverse relationship with aerogel densities, which was also reported in other study (Zhang et al. 2016).
Table 2
Density, porosity and water absorption of aerogel (PP-CNF-AG, PP-TOCNF-2.5-AG, PP-TOCNF-5-AG and PP-TOCNF-10-AG)
Samples
|
Density (g/cm3)
|
Porosity (%)
|
WA (g/g)
|
PP-CNF-AG
|
0.0156 ± 0.0001a
|
75.30 ± 0.57c
|
38.18 ± 0.23d
|
PP-TOCNF-2.5-AG
|
0.0136 ± 0.0003b
|
79.76 ± 1.10b
|
46.37 ± 1.63c
|
PP-TOCNF-5-AG
|
0.0097 ± 0.0001d
|
86.97 ± 1.02a
|
70.71 ± 0.76a
|
PP-TOCNF-10-AG
|
0.0120 ± 0.0003c
|
85.68 ± 0.72a
|
56.28 ± 1.86b
|
SEM analysis
The surface morphological structure of PP-CNF-AG, PP-TOCNF-2.5-AG, PP-TOCNF-5-AG and PP-TOCNF-10-AG are shown in Fig. 6a. As can be clearly observed from this figure, the surface morphologies of aerogels can be influenced by TEMPO oxidation with different oxidation degree. Obviously, PP-CNF-AG exhibited a randomly stacked structure of fragments, while the PP-TOCNF-AG samples showed an obvious entangled porous network structure. Notably, as the oxidation degree increased, the network structure and surface of PP-TOCNF-AG became tighter. This might be because the PP-TOCNF-AG surface contained lots of carboxyl groups, which enhanced the electrostatic interaction between the particles. As the repulsive force between particles increased, the distance between particles was eliminated thus leading to the smaller pores, and some aggregation might be occurred thus forming the lamellar structure (Sehaqui et al. 2011). However, due to the formation of lamellar structure, some properties of aerogel such as water absorption will be limited to a certain extent. Thus, the proper use of TEMPO-mediated oxidation to introduce negative charge is necessary for the further application of CNFs.
Mechanical property analyses
Figure 6b is a digital photograph of the appearance of the obtained aerogel, showing a typical light weight characteristic of aerogel that can be placed smoothly over the flower. The compression test was used to analyze the mechanical characteristics of the obtained aerogels, and the results are shown in Fig. 6c. There was no yield occurred at a strain of up to 86%, which was a typical deformation behavior of porous materials (Mirtaghavi et al. 2020). When the strain reached 86%, the compressive stress of PP-CNF-AG was 37.67 kPa, the compression stress values of PP-TOCNF-2.5-AG, PP-TOCNF-5-AG and PP-TOCNF-10-AG were 34.16, 18.13 and 14.45 kPa, respectively. It can be demonstrated that TEMPO oxidation may regulate the mechanical characteristics of aerogel, with compressive strength decreasing and flexibility increasing as the degree of oxidation rose (Chen et al. 2019). That might be because CNFs tend to be ordered, making the aerogel structure fluffy and thereby decreasing its compression resistance. The force change process of aerogel can be divided into three parts: (Ⅰ) when strain (ε) < 40%, it was a liner elastic zone, the compression stress (σ) remained almost unchanged, and the deformation was mainly caused by the flexibility of cellulose; (Ⅱ) when 40% < ε < 60%, it was a plateau area, and the σ increased linearly with the increase of ε, suggesting that the network structure of aerogel bear a certain stress; (Ⅲ) when ε > 60%, it was the densification stage, and the σ increased rapidly, indicating that the inter-fiber pores inside the aerogel were highly compressed and became dense, that is, plastic deformation had occurred (Li et al. 2016). As can be obviously seen in Fig. 6d, all aerogel samples maintained their initial shape after being soaked in water for 1 h. However, after prolonging to 12 h, PP-CNF-AG was dissociated while PP-TOCNF-AG still remains its initial shape, showing a better structural integrity in aqueous solutions. In addition, compared with PP-CNF-AG, the PP-TOCNF-AG samples had better rehydration performance after pressed. These results suggest TEMPO oxidation endows CNF aerogel with better structural stability and shape recovery ability, providing tuning potential for their aerogel applications.