Preparation of aerogels
The following requirements were met in the design of high-strength aerogels:
1) The use of silicon sources with elastic groups to avoid the fragility of the aerogel network structure caused by rigid Si-O bonds.
2) The establishment of stable crosslinked networks and the formation of strong interactions to ensure the excellent mechanical properties of the material.
3) It was essential to ensure that cellulose was distributed uniformly throughout the preparation process in order to guarantee the homogeneity of the samples.
4) In order to reduce the preparation cycle, it was decided to use HCl as the catalyst for the ring opening of benzoxazine monomers and to perform the catalytic step after the chemical reaction.
To meet these requirements, methyltrimethoxysilane was selected as the matrix of the aerogel, where the methyl groups collide with each other to slow down the stress concentration and thus reduce the fragility of the aerogel. Benzoxazine is considered an ideal material choice for the preparation of high-strength insulation materials due to its excellent hardness and flexibility in molecular structure design16. The carboxyl group in cellulose reacts chemically with phenylaminated benzoxazine monomers to build a chemically cross-linked network structure. At the same time, benzoxazine forms a strong physical cross-link with silica after ring opening, forming a double cross-linked network structure with chemical and physical double cross-links.
The preparation route of aerogel is shown in Fig. 1. The sol-gel process and supercritical drying technology were employed successfully to prepare composite aerogel (CPSA).
Morphology and Structure of Aerogel
Figure 2a depicts the physical image of the aerogel. It can be observed that the aerogel sample exhibits a yellow colour and a flat, homogeneous, and intact surface. This indicates that the synergistic interaction between MTMS, cellulose and polybenzoxazine successfully formed a stable aerogel structure. Figure 2b illustrates that the aerogel can be placed firmly on top of plant leaves due to its relatively low weight. This point serves to illustrate the lightweight nature of the aerogel. This can be attributed to the aerogel's highly porous nano-webbed structure. Figure 2c and d present the physical picture of CNFC and its transmission electron microscopy (TEM) characterisation image, respectively. It can be observed that the cellulose nanofibres exhibit a well-dispersed state without any discernible agglomeration, with a diameter of approximately 4–10 nm, which is consistent with the nanoscale dimensions of aerogels. Figure 2e and Fig. 2f illustrate the scanning electron microscopy (SEM) images of the aerogel. It can be observed that the backbone structure of the aerogel is comprised of three-dimensional nanoporous mesh structures, formed by irregular nanoparticles stacked in an irregular manner. Figure 2e illustrates that the aerogel, devoid of added cellulose nanofibres, is composed of a coral-like skeleton devoid of large lumps into clusters, which is a fragile and slender nanoskeleton. Figure 2f shows that the aerogel with added cellulose nanofibres has the formation of clusters, which may be due to the fact that cellulose molecular chains are prone to agglomeration. This phenomenon is usually observed in the form of a network, which is unevenly distributed in a certain region17,18. The increase in secondary particles and clusters also results in an expansion of the large pore size of the samples, which in turn enhances the robustness of the skeleton. These microstructural features provide a robust foundation for the excellent mechanical properties of CPSA.
Figure 3a presents the FT-IR spectra of SiO2 aerogel, PBO-SiO2 aerogel and CPSA aerogel composites. The two peaks at 1119 cm− 1 and 1017 cm− 1 are the characteristic absorption peaks of cyclic Si-O-Si and linear Si-O-Si, respectively. The absorption peaks at 2965 cm− 1 are attributed to the symmetric and asymmetric stretching vibration peaks of the C-H bond. The characteristic absorption peaks at 1265 cm− 1 and 825 cm− 1 are attributed to the bending vibration and stretching vibration of the Si-CH3 bond, respectively19. This is consistent with the infrared characteristic absorption peaks of silica aerogel prepared by MTMS, indicating that MTMS underwent a hydrolysis condensation reaction involved in the SiO2-PBO aerogel. Due to the abundance of oxygen atoms present in SiO2, these atoms can serve as highly electronegative atomic receptors. Hydrogen atoms in polybenzoxazine are capable of forming hydrogen bonds with oxygen atoms in SiO2. The formation of broad and strong peaks in the range of 3000–4000 cm− 1 also provides evidence of hydrogen bond formation, and the interaction between PBO and SiO2 enhances the crosslinked network structure20. Moreover, the CPSA aerogels exhibited a significantly lower density compared to the PBO-SiO2 aerogels. However, there was no interfacial debonding that occurs due to the large difference in the contraction of fibres and matrix. This is due to the interfacial chemistry and the matching of cellulose and aerogel nanoparticle sizes, which facilitates the enhancement of the mechanical strength of the aerogel and the obtainment of a robust and crack-free aerogel21.
Figure 3 (a) Characterisation of the interaction between aerogel and fibre by FT-IR. (b) N1s spectrum of PBO-SiO2 aerogel (c) N1s spectrum of CPSA aerogel
The X-ray photoelectron spectroscopy (XPS) results (Figs. 3b and 3c) demonstrate that the N1s in PBO-SiO2 in Fig. 3b exhibits a single signal peak, which is attributed to the C-N bond. In contrast, the N1s in CPSA in Fig. 3c exhibits multiple signal peaks at 398. The results indicate that the peaks at 3 eV, 399.6 eV, and 401.7 eV are attributed to C = N, N-C = O, and N-H, respectively. This further confirms that the amino group in the monomer reacts with the carboxyl group in cellulose22. This stable chemical bonding provides the foundation for the high compressive strength observed in aerogels.
Mechanical Properties
As illustrated in Fig. 4a, the aerogel exhibits high compression and bending resistance, withstanding the pressure of at least 500 ml of water and lifting at least 1500 ml of water. In order to further verify the strong interaction between nanocellulose and aerogel nanoparticles, a universal mechanical testing machine was employed to perform a compression test on the aerogel. Figure 4b presents the front and side views of the aerogel following the compression test. Following the addition of fibres, the aerogel was compressed into small flakes. However, the surface remained intact, indicating that the material exhibited good toughness and strength.
In order to investigate the mechanical properties of the aerogels in depth, compression tests were carried out on the aerogel samples using a compression testing machine, and the resulting compression stress-strain curves were plotted (see Fig. 5). The results demonstrated that the mechanical properties of the cellulose-doped composites were markedly enhanced in comparison to those of the aerogels without the addition of cellulose. The primary mechanisms of energy absorption in fibre-reinforced composites are fibre bridging, crack bending and deflection, fibre pullout and fibre debonding23. At 85% strain, the strong chemical interaction of fibres with the aerogel, which requires more energy, plays a significant role in toughening the aerogel, which can reach a stress of 34 MPa and maintains its structural integrity throughout the compression process. Furthermore, the compression properties were employed as an indicator to assess the toughness of the materials. It is evident that the aerogels in this experiment demonstrate excellent toughness, exceeding the conventional phenolic aerogels and silicone-containing aerogels.
In the low-strain region, the aerogel exhibits elastic behaviour, while with further increase in strain, the material begins to deform plastically. As the strain continues to increase, the aerogel is gradually compacted and its compressive strength increases rapidly. The compressive strength exhibited by the cellulose-added aerogels was significantly higher than that of the undoped cellulose samples at 5%, 10% and 20% strain levels. This was attributed to the homogeneous distribution of cellulose nanofibres in the SiO₂ and PBO dual network structure, as well as the strong interactions between fibres and nanoparticles. The rich functional groups of cellulose nanofibres further enhanced this interaction, while the thick necks of the aerogel skeleton with added fibres dispersed the stresses more, resulting in a significant improvement of the mechanical properties of the aerogels. In particular, cellulose nanofibres containing a large number of functional groups are more capable of significantly improving the mechanical properties of the materials compared to other physically bound materials. The excellent mechanical properties provide a good basis for the application of insulating materials, especially for high-temperature insulation materials, which is very favourable, but rarely achieved.
Thermal insulation and thermal stability
Another crucial aspect of aerogel materials in practical applications is their thermal insulation performance. In order to observe the thermal insulation performance of aerogel, an infrared thermography test was conducted to assess the thermal insulation effect of aerogel. The aerogel was placed on a heating table at 100°C, and the temperature change of the cold surface was recorded over a ten-minute period. The resulting surface temperature distribution of the aerogel is illustrated in Fig. 6. The temperature of the cold surface of the aerogel was consistently below 32°C, indicating that the majority of heat was effectively blocked, thereby demonstrating excellent thermal insulation properties. Additionally, the sample exhibited no discernible dimensional shrinkage or cracks.
The specific surface area and pore size of an aerogel have a significant effect on its thermal conductivity and adiabatic properties. As illustrated in Fig. 7, the composite aerogels exhibit the adsorption properties of mesoporous materials and contain a limited number of macropores. These composites maintain the intrinsic characteristics of aerogels in their microstructure. Given that the pore size is considerably smaller than the mean free range of air molecules in the atmosphere (approximately 70 nm), the gas heat transfer at this scale is significantly diminished. Furthermore, CPSA aerogels with a greater number of micropores facilitate a further reduction in thermal convection and enhance their thermal insulation properties.
Table 1 illustrates that the specific surface area of PSA aerogel without added cellulose is 41.052 m²/g, while that of CPSA is increased to 73.239 m²/g. In general, there is a positive correlation between a higher specific surface area and a lower thermal conductivity.
Table 1
Pore structure characterisation of aerogels
Sample | Mass fraction of cellulose to MTMS | Specific surface area(m2 /g) | Density(g/cm3) | Pore volume(cm3/g) | Average pore size(nm) |
0 | 0 | 41.052 | 0.23 | 0.1077 | 10.495 |
1 | 1wt% | 73.239 | 0.175 | 0.1864 | 10.179 |
2 | 2wt% | 89.979 | 0.204 | 0.2592 | 11.523 |
3 | 3wt% | 69.091 | 0.232 | 0.2927 | 16.945 |
As illustrated in Fig. 8, the thermal conductivity of the CPSA aerogel at room temperature was approximately 0.048 W/(m·K), representing a 4.705% reduction in comparison to the PSA aerogel without added cellulose. This value is low among similar bulk density materials and even lower than the thermal conductivity of some polybenzoxazine/silica composites. This further demonstrates the potential application value of CPSA aerogels in the field of thermal insulation materials. The results of these property analyses collectively indicate that CPSA aerogels have the potential for application in a range of fields, including building insulation, aerospace insulation, and other contexts where effective thermal insulation solutions are required.
Thermogravimetric analysis was employed to investigate the thermal stability of the aerogels. Figure 9 depicts the thermogravimetric (TG) curves of the PBO-SiO₂ aerogel and the CPSA composites. It can be observed that in the initial stage, from room temperature to 200°C, the PBO-SiO₂ aerogel and CPSA aerogel exhibit thermal weight loss, which is typically attributed to the evaporation of adsorbed water and organic solvents. Subsequently, in the temperature range of 200°C to 400°C, all aerogel samples exhibited a plateau of constant weight, indicating that the aerogels did not decompose and suggesting good thermal stability 22. As the temperature continued to increase, a significant weight loss occurred, which was mainly due to the decomposition of organic matter. At 600°C, the residual masses of PBO-SiO₂ and CPSA were 65.988% and 74.844%, respectively. This result indicates that CPSA exhibits superior thermal stability following a temperature of 490°C.
Hydrophobicity
The contact angle of an aerogel material is an important parameter for measuring the hydrophobicity of its surface. The contact angle of the CPSA aerogel in Fig. 10a is 66°, indicating hydrophilicity. This is due to the hydrophilic groups on the surface of cellulose nanofibres, which reduce the hydrophobicity of the aerogel to a certain extent. This is detrimental to the long-term use of the aerogel 13. Consequently, a hydrophobic treatment is required for CPSA gels. In this study, trimethylchlorosilane was employed for surface modification, with fumed hydrophobic silica subsequently added. On the one hand, the modifier converted the hydrophilic groups on the surface into hydrophobic groups. On the other hand, the fumed hydrophobic silica formed a hydrophobic layer on the surface of the aerogel, preventing the penetration of water, which further enhanced the hydrophobic property of the aerogel. The results presented in Fig. 10b demonstrate that the hydrophobic performance of the hydrophobically treated aerogel has been significantly enhanced. Its water contact angle can reach 142°, which contributes to enhanced performance stability and prolongs the service life of the aerogel.