Preparation and structures of PCAs.
The synthetic process of PCAs was illustrated in Fig. 1a, dopamine was first in-situ polymerized onto surface of CNF due to the adhesive property of polydopamine, which has been frequently applied for surface modification of various kinds of nanomaterials (Yue et al. 2013; Liu et al. 2013). Then Fe3+ was introduced into the mixture to crosslink the polydopamine modified CNFs due to the strong coordination bonds between catechol groups in polydopamine and metal ions. Note that the residual carboxyl groups in TEMPO oxidized CNF could also form coordination bonds with Fe3+, which was also beneficial to the crosslinking of
PCAs (Zou and Chen 2020). However, it is very difficult to identify the contribution of different polar groups on crosslinking of PCAs because of the complex surface chemistry of both polydopamine and CNFs. Thus, the addition amount of Fe3+ was set on the basis of total amount of dopamine and CNF. For PCA4, it was found that the optimal Fe3+ content was 20 wt%, as the shrinkage of PCA4 was smallest at this weight ratio. Therefore, for the following of this study, the weight content of Fe3+ crosslinker was fixed at 20 wt%. Figure 1b shows that with the addition of dopamine and oxidant, the aqueous dispersion of CNF immediately changed from white to pink as the oxidation polymerization of dopamine occurred. The dispersion become dark brown after 3 h of reaction, suggesting the proceeding of polymerization of polydopamine. When Fe3+ was added into the dispersion of modified CNF, the dispersion was gelated due to the formation of coordination bonds between metal ions and polar surface groups. The brownish PCAs were finally obtained by freeze-drying of the hydrogels.
The TEM image of pristine CNFs is shown in Fig. 2a. The diameter of CNFs are in the range of 10 to 50 nm with some bundles, and the length of CNFs are as long as tens of micrometers, indicating the high aspect ratio of CNF. After polymerization with dopamine, the high aspect ratio of CNF is preserved, and some small particles are adhered on the surface of CNFs, which are attributed to the polydopamine (Fig. 2b). Figure 2c shows the HAADF-TEM image of Fe3+ crosslinked polydopamine modified CNFs. It can be seen that the stripe shaped modified CNFs are crosslinked into sheet-like networks. The EDX-mapping images (Fig. 2d-2f) of crosslinked CNFs present the even distribution of N and Fe elements in the crosslinked network, demonstrating the successful surface modification of CNFs by polydopamine and the homogenous crosslinking points in PCAs.
The microstructure of pure CNF aerogels and PCAs were investigated by SEM characterizations (Fig. 3). All images exhibit typical porous structure of aerogels. However, the porous structure of CNF aerogel is relatively dense as compared to those of PCAs, which we attribute to the high shrinkage of CNF aerogel because there are only physical entanglement or weak van der Waals force among CNFs. With the incorporation of polydopamine, the porous structure of PCAs are more significant with thin sheets interconnected fibrous filaments. The sheets presented in PCAs are attributed to the crosslinked CNF networks or some CNF agglomerates squeezed by ice crystals during freezing step. In addition, it seems that the pore size of PCAs increased with increasing amount of polydopamine. For example, the pores in PCA4 are more regular than PCA1, PCA2, and PCA3 with size about 200 ~ 300 µm, suggesting more polydopamine could provide more crosslinking to maintain the microporous structure of aerogels. However, the microstructure of PCA5 is not as uniform as that of PCA4 probably due to the uneven distribution of excess polydopamine.
More precise structure parameters, including the density, porosity, average pore size, and surface area of CNF aerogel and PCAs, are compared in Fig. 4 and listed in Table 1. The density of CNF aerogel is about 34.1 mg/cm3, which is similar to previous reported CNF aerogels (Yang and Cranston 2014; Wang et al. 2018). The density gradually increases from 41.1 mg/cm3 of PCA1 to 47.5 mg/cm3 of PCA5 because of the addition of polydopamine and Fe3+, despite the smaller shrinkage of PCAs as compared to CNF. The porosity of CNF aerogel is also increased with the addition of polydopamine. For example, the porosity of CNF aerogel increases from 94.6 % to 96.6 % in PCA4. This increase is also attributed to the efficient crosslinking of CNF that suppresses the aggregation of CNF and shrinkage of aerogels. As such, the average pore size obviously increases from 163.4 µm in CNF aerogel to 200.6, 220.1, 224.4, 226.9, and 232.0 µm in PCA1, PCA2, PCA3, PCA4, and PCA5, respectively. In addition, the surface area of PCAs are in the range of 210.6 to 220.1 m2/g, which are also higher than that of CNF aetogel (193.8 m2/g) and similar to other reported crosslinked CNF aerogels (Jiang et al. 2017). The increased porosity and pore size in PCAs are advantageous for thermal insulation application because of more entrapped air inside aerogels.
Table 1
Structure parameters and properties of CNF aerogel and PCAs.
| Density (mg/cm3) | Porosity (%) | Average pore size (µm) | Surface area (m2/g) | Compressive strength at 70 % strain (kPa) | Td (°C) | LOI (%) | λ (W·m− 1·K− 1) |
CNF Aerogel | 34.1 | 94.6 | 163.4 | 193.8 | 43.3 ± 5.8 | 276.2 | 18.7 | 0.042 |
PCA1 | 41.1 | 93.5 | 200.6 | 210.6 | 56.7 ± 4.7 | 317.1 | 24.5 | 0.034 |
PCA2 | 43.4 | 95.7 | 220.1 | 217.2 | 61.8 ± 8.3 | 341.5 | 26.3 | 0.035 |
PCA3 | 44.7 | 95.6 | 224.4 | 214.7 | 104.5 ± 9.2 | 342.4 | 28.7 | 0.031 |
PCA4 | 45.2 | 96.6 | 226.9 | 220.1 | 107.7 ± 10.1 | 333.7 | 29.2 | 0.033 |
PCA5 | 47.5 | 95.9 | 232.0 | 218.8 | 105.8 ± 13.5 | 345.3 | 33.1 | 0.037 |
Mechanical, thermal and flame retarding properties of PCAs
The mechanical properties of CNF aerogel and PCAs were evaluated by compressive test, as shown in Fig. 5. The CNF aerogel shows typical compressive behavior of soft aerogels. That is, a slowly increased stress in response to deformation at lower strain range, and a fast increasing stress response at large strain. At 70 % strain, the compressive strength of CNF aerogel is 43.3 kPa. It is found that the stress response of PCAs are faster than that of CNF aerogel, indicating the hardening of CNF aerogel by the reinforcement of polydopamine and crosslinking. As a result, the compressive strength of PCAs are all improved, and the compressive strength at 70 % of PCA4 is obviously increased to 107.7 kPa, corresponding to about 2.5 times that of pure CNF aerogel, demonstrating the good reinforcement effect of our strategy.
The thermal stabilities of CNF aerogel and PCAs were invesitigated by TGA. Figure 6 shows the TGA and derivative thermogravimetric (DTG) curves. CNF aerogel exhibits a minor decomposition at about 231.3°C, which is attributed to the decomposition of labile side functional groups on CNF. The maximum decomposition temperature (Td) of CNF aerogel appears at 276.2°C, corresponding to the pyrolysis of CNF backbone. The functionalization of CNF by polydopamine and crosslinking by Fe3+ have significantly improved the thermal stability of aerogels. As shown in Fig. 6b, the first minor decomposition peak in CNF is almost disappeared, and the Td of PCA2 to PCA5 are in the range of 333.7 ~ 345.3°C. Such improved Td are even more pronounced than some inorganic nanomaterials reinforced CNF aerogels (Guo et al. 2019; Wang et al. 2019). The superior thermal stability of PCAs are attributed to 1) the CNF was coated by polydopamine by in-situ polymerization and the polydopamine acted as physical barrier to retard the decomposition of CNF, and 2) the free radical scavenging ability of polydopamine could eliminate the radicals generated by the breaking of C–C bonds, and block chain-scission depolymerization (Fang et al. 2020; Cho et al. 2015; Yang et al. 2020). The improved thermal stability of PCAs is beneficial to their thermal applications.
We have also evaluated the flame retardancy of PCAs because of the flame retarding property of polydopamine. As presented in Fig. 7a, pure CNF was easily ignited because of its flammable feature, and it was complete burned after removal from flame. In sharp contrast, the PCA4 immediately self-extinguished after removal from flame, visually proving the good flame retardancy of our PCAs. The LOI test was conducted to more quantitatively evaluate the flame resistance of PCAs, as compared in Fig. 7b. Pure CNF aerogel owns low LOI value of 18.7 %. The PCAs possess much higher LOI value even with low content of polydopamine and Fe3+ crosslinking. For example, PCA1 has LOI value of 24.5%, and as the polydopamine content was reached 30 wt%, the PCA3 become nonflammable with LOI value of 28.7 %, and the LOI value further increases to 29.2 and 33.1 in PCA4 and PCA5. The scavenging of free radical by polydopamine, thus suppressing fuel supply during combustion is also thought to be the main reason for this enhanced flame resistance of PCAs (Fang et al. 2020; Cho et al. 2015; Yang et al. 2020). Additionally, the catalysis effect of Fe3+ on polymer materials during combustion could also contribute to the flame retardancy of PCAs (Zhang et al. 2018; Zhang et
al. 2020), because the more efficient formation of char could act as physical barrier to retard the combustion.
Thermal insulation application of PCAs
The improved thermal stability of PCAs permits their safer operation in some thermal application such as thermal insulation. We first determined their thermal conductivity (λ), as shown in Fig. 8a. The λ of CNF aerogel is about 0.042 W·m− 1·K− 1, which is slightly higher than some previously reported cellulose aerogel due to the higher density of our CNF aerogel (Gupta et al. 2018; Zhang et al. 2021). The λ of our PCAs are in the range of 0.031 ~ 0.037 W·m− 1·K− 1, which are generally lower than that of CNF aerogel probably because of the higher porosity and uniform microstructure of PCAs. Then we evaluated the thermal insulation performance of PCAs by using an electric heating platform placed with aerogels (the thickness of all aerogels are about 1 cm), as illustrated in the inset of Fig. 8b. The stabilized temperature of the surface of CNF aerogel and PCAs as a function of platform temperature are compared in Fig. 8b. It is found that when the temperature of heating platform was 100 and 150°C, the temperature of CNF aerogel and PCAs are in the range of 36 ~ 43 and 60 ~ 68°C, indicating similar insulation performance of all the aerogels. However, when the temperature of heating platform was elevated to 200 and 250°C, the temperature of CNF aerogel were increased to 88 and 134°C. While the temperature of PCAs were in the range of 67 ~ 76 and 80 ~ 91°C, respectively, demonstrating the good thermal insulation properties of PCAs even at high temperature. The inferior performance of CNF aerogel is attributed to the deterioration of microstructure or degradation of CNF aerogel because of their inferior thermal stability.
Water induced healing of PCAs
Because of the dynamic nature of metal coordination bonds, we are curious about if the PCAs were able to repair damages. In a preliminary experiment, we cut the CNF aerogel and PCA4 into two halves, and then a drop of water was dropped onto the fracture surfaces, the two halves were then pressed by hand. It is found that the CNF aerogel could not heal by this operation, while the two pieces of PCA4 were firmly adhered together. We attributed the water induced healing of PCA4 to the reversible crosslinking of metal coordination bonds (Xia et al. 2016; Chen et al. 2018). In contrast, the CNF aerogel was mainly formed by physical entanglement, thus could not be healed due to the lack of bonding between two fracture surfaces. Nevertheless, further works are needed to give deep insight into the healing behavior of this kind of dynamic crosslinked CNF aerogels.