3.1 Morphologies of the PIF-a
Traditional PI nanofiber aerogels are typically prepared using PAA nanofibers as the framework, followed by a post-imidization process. However, during thermal imidization, unavoidable volume shrinkage occurs, affecting the size stability of the aerogel. In this study, we directly use soluble PI as the framework and PAA as the cross-linking agent, thereby avoiding the phenomenon of volume shrinkage. The synthesis route of PIF-a is shown in Fig. 1a. First, hyperbranched polyimides are successfully synthesized by A2 + B2B′+B2 copolymerization using 6FDA, TAP, and ODA. The introduction of the branched unit TAP disrupts the regularity of the polymer chains, enhancing the solubility of the synthesized PI, making it suitable as a precursor for electrospinning. After electrospinning, the PI nanofibers and PAA are uniformly dispersed in a tert-butanol solution. Subsequent freeze-drying leads to the rapid growth of ice crystals, causing the nanofibers to randomly disperse and form a three-dimensional interconnected network structure under repulsive forces. During the ensuing heat treatment, the PAA on the surface of the PI nanofibers undergoes imidization, promoting the formation of a stable cross-linked network within PIF-a. The density of the prepared PIF-a is only 7.2 mg/cm³, and placing it on a foxtail does not cause deformation, demonstrating its ultralight characteristic (Fig. 1b). Furthermore, PIF-a can be cut into various shapes without crushing, indicating its excellent formability (Fig. 1c). As shown in Fig. 1d, PIF-a exhibits remarkable flexibility, allowing it to be easily folded, twisted, and knotted. This flexibility distinguishes it from conventional ceramic aerogels, which lack such adaptability. Figure 1e and Fig. S1 display the SEM images of the PI nanofiber membrane, showing randomly arranged PI nanofibers with a high aspect ratio forming a relatively uniform pore structure. In contrast, PIF-a exhibits a typical hierarchical pore structure. As illustrated in Fig. 1f-h, the rapid growth of ice crystals during freeze-drying leads to the aggregation of PI nanofibers, forming macropores, while within the aggregated regions, the nanofibers interconnect to form micropores. Additionally, the thermal imidization of PAA at the fiber junctions creates stable bonding points, enhancing the overall stability of PIF-a. Compared to traditional PI nanofiber membranes, the hierarchical pore structure within PIF-a provides a significant advantage for hot oil adsorption. On one hand, the strong capillary effect of the macropores accelerates the rapid infiltration of oil droplets into the aerogel. On the other hand, the abundant micropores and stable bonding points offer excellent "oil-locking" capabilities, further enhancing its oil adsorption capacity.
3.2 Oil adsorption of the PIF-a at room temperature
To investigate the maximum oil sorption capacity of PIF-a, adsorption tests are performed using pure motor oil (Fig. 2a). The adsorption capacities of PSF, PIF, and PIF-a are found to be 33.4 g g− 1, 32.9 g g− 1, and 83.1 g g− 1, respectively. Both PSF and PIF exhibit similar oil adsorption capacities, primarily due to the pore structure formed by the fiber surface and the fibers themselves. In contrast, PIF-a demonstrates an oil adsorption capacity over 2.5 times greater than that of PIF. This significant increase is attributed to the low density, multistage pore structure, and stable fiber skeleton of PIF-a. Density and porosity of samples are shown in Figure S2. Figure 2b illustrates the relationship between sorption capacity and contact time. Both PSF and PIF display suitable adsorption rates, which can be attributed to the inter-fiber voids in the membrane that effectively hold oil. However, the saturated adsorption time for PSF and PIF is 6 minutes, notably longer than the 2 minutes required for PIF-a. This rapid adsorption by PIF-a is due to its stable aerogel skeleton, multistage pore structure, and high porosity. Initially, oil droplets infiltrate the fiber skeleton of the aerogel and further penetrate through the surface pores. The stable three-dimensional porous network in the aerogel facilitates the rapid flow of oil droplets, particularly the large pore structure between layers. Additionally, the fiber overlap in PIF-a forms stable bonding points that prevent fiber shrinkage caused by the adhesive force of the oil droplets. Compared to pure fiber felt, the unique three-dimensional porous structure of PIF-a results in a more efficient oil adsorption rate. The process of oil sorption and desorption is illustrated in Fig. 4c. After the initial sorption test, a simple continuous squeezing method is used to remove the adsorbed oil in PIF-a, while a vacuum suction filter method is employed for PSF and PIF. The oil sorption capacities of the samples over ten cycles are depicted in Fig. 4d. The results indicate no significant change in adsorption capacity during the first five cycles for PSF, PIF, and PIF-a. After the sixth cycle, the oil adsorption capacities of the samples gradually decrease to 89.3%, 86.9%, and 98.1% of their initial values, respectively, indicating only a slight reduction in sorption capacity. Throughout subsequent cycles, there is no noticeable change in adsorption capacity. Even after ten cycles, all samples maintain over 80% adsorption efficiency. Specifically, PIF-a shows only an 8.8% decrease in adsorption efficiency, retaining an adsorption capacity of 77.7 g g− 1, thus demonstrating excellent reusability. Figure 4e shows the mass of samples across different cycles, indicating a linear increase in oil retention. After ten adsorption cycles, the mass of PSF, PIF, and PIF-a increases from 1 g g− 1 to 2.28 g g− 1, 2.31 g g− 1, and 1.68 g g− 1, respectively, reflecting substantial oil recovery. The superior oil recovery of PIF-a can be attributed to the efficient continuous squeezing method and the robust mechanical properties of PIF-a. The stable aerogel skeleton effectively prevents the collapse of porous structures during the complete extrusion process.
3.3 Thermal stability of the PIF-a
The thermogravimetric (TG) curves of PIF-a and PSF are shown in Fig. 3a. It is evident that, compared to PSF, PIF-a exhibits a higher thermal decomposition temperature and a lower thermal weight loss rate. The initial thermal decomposition temperature of PIF-a is 356°C, indicating superior thermal stability. The detailed TG data of PIF-a and PSF are listed in Table S1. To further investigate the structural evolution of PIF-a at elevated temperatures, we perform SEM characterization of PIF-a at various temperatures. Figure 3b-e presents the FE-SEM images of PIF-a at 150, 200, 250, and 300°C, respectively. The morphologies of PIF-a remain stable and uniform as the temperature increases from 150 to 250°C (Fig. 3b–d), which can be attributed to the good thermal stability of PIF-a. In contrast, PSF exhibits significant bonding phenomena at 150°C, reflecting poor thermal stability (Fig S3). Figure 3e shows the morphology of PIF-a at 300°C, with the corresponding magnified morphology depicted in Fig. 3f. A caking phenomenon is observed between the PI fibers, and the inter-fiber pores of the PI fibrous mat are significantly reduced, negatively affecting the oil adsorption capacity of PIF-a. These results indicate that the favorable morphology of PIF-a can be maintained at temperatures as high as 250°C, which provides strong assurance for the sample’s application in hot oil adsorption.
Nitrogen adsorption-desorption tests are conducted to further investigate the evolution of the specific surface area (SSA) and internal pore structure of PIF-a at different temperatures, as illustrated in Fig. 3g. The samples are designated as PIF-a-150, PIF-a-200, PIF-a-250, and PIF-a-300, respectively. It can be observed that PIF-a-150, PIF-a-200, and PIF-a-250 exhibit large and uniform nitrogen adsorption quantities, with the highest value reaching up to 36 g cm− 3. However, PIF-a-300 shows a significant decrease in nitrogen adsorption quantity (12 g cm− 3), attributed to the cross-linking and contraction of PI nanofibers at 300°C. For the BET measurement, the SSA of PIF-a-150, PIF-a-200, and PIF-a-250 reach a maximum value of 36.25 m2 g− 1, whereas PIF-a-300 displays an SSA of 9.13 m2 g− 1. According to the International Union of Pure and Applied Chemistry (IUPAC) classification, the isotherms with distinct hysteresis loops can be described as type II, characteristic of mesopores (2–50 nm) and macropores (> 50 nm). As shown in Fig. 3h, the pore size distribution of PIF-a-150, PIF-a-200, and PIF-a-250 mainly falls within the range of 30–52 nm, while PIF-a-300 exhibits a broader pore size distribution, predominantly between 30–85 nm. As the temperature increases, significant bonding occurs among the PI nanofibers within PIF-a (Fig. 3f), leading to the closure of small pore structures formed by fiber overlap. Consequently, PIF-a-300 shows a substantial reduction in pore volume and an increase in the proportion of larger pores. The above analysis indicates that PIF-a maintains a stable internal structure, specific surface area, and pore structure at 250°C, which provides robust assurance for its application in high-temperature hot oil adsorption. Detailed nitrogen physical adsorption characteristics are presented in Table S2.
3.4 Hot oil adsorption of PIF-a
Figure 4a illustrates the dynamic process of hot oil permeation on the surface of PIF-a at 200°C. Initially, a 3 µl oil droplet is observed to pin on the sample surface with an initial oil contact angle (OCA) of 58.3°, indicating excellent lipophilicity. Within 2 seconds, the oil droplet completely infiltrates the sample surface, indicating rapid hot oil adsorption. To assess the maximum hot oil sorption capacity of PIF-a, sorption tests are conducted using pure motor oil at temperatures ranging from 50°C to 300°C (Fig. 4b). As the oil temperature increases from 50°C to 200°C, the sorption capacity of PIF-a decreases from 83.1 g/g to 78.6 g/g, indicating considerable adsorption capacity. As mentioned above, PIF-a maintains a suitable porous structure and high specific surface area (SSA) even at 200°C. The reduction in oil adsorption capacity can be attributed to the decreasing viscosity of motor oil with increasing temperature, hindering its adherence to the fiber surface. However, at temperatures of 250°C and 300°C, the adsorption capacity of PIF-a decreases rapidly (to 43.1 g/g and 18.4 g/g, respectively), indicating poor adsorption capacity. This decline is attributed to the further reduction in motor oil viscosity at higher temperatures. Additionally, the bonded fiber structure of PIF-a at 250°C and 300°C results in a lower SSA, further contributing to the reduced adsorption capacity. Nonetheless, PIF-a still maintains a considerable adsorption capacity for 200°C hot oil, meeting the protection requirements of oil pipelines.
Figure 4c depicts the relationship between hot oil adsorption capacity and contact time of PIF-a for 200°C hot oil. The saturated adsorption time of PIF-a is only 2 minutes, faster than that at room temperature (3 minutes), attributed to the lower viscosity of hot motor oil accelerating the adsorption rate of intra-fiber pores in PIF-a. The hot oil sorption capacities of PIF-a versus ten cycles for motor oil are illustrated in Fig. 4d. The results show that the adsorption capacity remains relatively stable for the first five cycles and decreases by about 3.1% after the sixth cycle due to adequate pressing. Subsequently, the adsorption capacity decreases continuously over the following cycles. However, even after ten sorption cycles, the decrease in sorption capacity hardly exceeds 10% of the initial value, indicating excellent reusability of PIF-a at 200°C. This is attributed to the stable resilience of PIF-a after repeated extrusion. In conclusion, PIF-a demonstrates excellent hot oil adsorption properties, making PI fiber aerogel promising candidates for hot oil removal.