Polyimide nanofiber aerogel with hierarchical porosity: a novel platform in high-temperature oil absorption

doi:10.21203/rs.3.rs-5301622/v1

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Polyimide nanofiber aerogel with hierarchical porosity: a novel platform in high-temperature oil absorption

https://doi.org/10.21203/rs.3.rs-5301622/v1

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The development of advanced oil sorbents with superior thermal stability, high adsorption capacity, and excellent reusability is crucial for addressing hot oil leakage challenges, particularly in the petrochemical and machinery industries. This study presents a novel polyimide (PI) nanofiber aerogel (PIF-a) designed for high-temperature oil absorption. Utilizing electrospinning and post-treatment, PIF-a exhibits a hierarchical pore structure, ultralow density, and remarkable flexibility. At room temperature, PIF-a demonstrates an oil adsorption capacity of 83.1 g/g, surpassing conventional materials. Notably, PIF-a retains structural integrity up to 250°C, with a hot oil adsorption capacity of 78.6 g/g at 200°C. Despite repeated adsorption-desorption cycles, PIF-a's capacity remains stable, retaining over 90% of its initial performance. This breakthrough material, with its exceptional thermal stability, rapid adsorption kinetics, and durable reusability, represents a significant advancement in high-temperature oil absorption technology, broadening the application potential of nanofiber-based materials in addressing environmental oil spill emergencies.

Polyimide nanofiber aerogel

High-temperature oil adsorption

Hierarchical pore structure

Thermal stability

Oil-absorption materials play a crucial role in environmental protection and oil-water separation technologies, particularly in responding to oil spill incidents on land and sea. With the rapid pace of industrialization, the generation of oil-containing wastewater is increasing, posing significant threats to the environment and human life [1, 2]. Therefore, researching and developing efficient and reusable oil-absorption materials holds both scientific significance and practical value. In recent years, significant progress has been made in the study of oil-absorption materials for ambient temperatures. However, the demand for materials that can function effectively in high-temperature environments, especially in industries like petrochemicals and machinery manufacturing where thermal oil spill incidents are frequent, has become increasingly urgent. Traditional oil-absorption materials often suffer from performance degradation or even structural failure at high temperatures, highlighting the need for materials that can maintain stability and absorption capacity under such conditions [3]. Common oil-absorption materials include activated carbon [4, 5], polymer adsorbents [6–8], natural fibers [9], and aerogels [10–13]. Among these, aerogels stand out in the oil-absorption field due to their high specific surface area, high porosity, and low density. Currently, aerogels used for thermal oil absorption are typically inorganic, such as graphene aerogels [14–16], carbon aerogels [17, 18], and silica aerogels [19–23]. Silica aerogels have gained widespread attention in the field of thermal oil absorption due to their low cost, simple synthesis methods, and ease of large-scale production. However, issues such as brittleness and poor stability of silica aerogels cannot be overlooked. In high-temperature environments, inorganic aerogels are prone to irreversible melting, leading to changes in their internal pore structures and network frameworks, significantly limiting their application in thermal oil absorption [22]. More specifically, inorganic aerogels are often composed of nanoparticles as basic units, which are connected by relatively weak van der Waals forces. Due to the inherent properties and uncontrollable assembly behavior of these nanoparticles, they are randomly dispersed within the matrix, forming fragile "necklace-like" structures. This structure is prone to breakage during repeated thermal oil absorption cycles, leading to rapid failure of the aerogel [24]. For example, silica aerogels can undergo structural collapse at high temperatures, severely impacting their absorption performance and reusability. Although this issue has received widespread attention in recent studies, it remains unresolved.

To address the limitations of inorganic aerogels, this study proposes a novel PI nanofiber aerogel. Polyimide is a high-performance polymer known for its exceptional mechanical and thermal stability. Its unique chemical structure allows it to maintain good performance even in high-temperature environments, making it an ideal matrix material for thermal oil absorption [25–28]. Through electrospinning technology, PI nanofibers are fabricated and used as the fundamental building blocks of the aerogel, replacing traditional nanoparticles. These nanofibers form a stable three-dimensional network structure through chemical cross-linking or physical entanglement [29–32]. Compared to traditional inorganic aerogels, PI nanofiber aerogels offer several distinct advantages: firstly, the absence of the "bead-on-string" defect in the framework effectively avoids brittleness due to stress concentration, greatly enhancing the stability of the microstructure [33]; secondly, the fibrous network construction allows the aerogel to maintain high porosity while enhancing the overall integrity of the structure, which is beneficial for maintaining structural integrity during thermal oil absorption [34]; finally, the excellent thermal stability of PI materials ensures the long-term performance of the aerogel in high-temperature environments [35]. In summary, this research aims to develop a novel PI nanofiber aerogel to overcome the limitations of traditional inorganic aerogels in thermal oil absorption, thereby unlocking the potential and application value of nanofiber aerogel materials in the field of thermal oil absorption.

2.1 Materials

2,2-Bis(3,4-dicarboxylphenyl) hexafluoropropane dianhydride (6FDA), 4,4′-oxydianiline (ODA), and 2,4,6-triaminopyrimidine (TAP) were sourced from Changzhou Sunlight Medicine Raw Material Co. (China). Ethanol, tert-butanol, dimethylacetamide (DMAc), and N,N-dimethylformamide (DMF) were provided by Tianjin Fu Yu Fine Chemicals Co. (China). Motor oil (Mobil 1) was obtained from Tai Cang Exxon Mobil Petroleum Co. (China), with a viscosity of 246 mPa·s and a density of 0.852 g/cm³. Both 6FDA and TAP were purified via sublimation prior to use, while ODA was recrystallized from tetrahydrofuran and subsequently dried under vacuum at 373 K. N,N-dimethylacetamide (DMAc) was distilled after drying over P₂O₅, and all other solvents and reagents were used without further purification.

2.2 Electrospinning of PI nanofiber

A highly soluble PI was synthesized through a two-step polycondensation process, beginning with the formation of polyamic acid (PAA) as a precursor, followed by thermal imidization, as previously reported [36]. The resulting polyimide exhibited excellent solubility in dimethylformamide (DMF), attributed to the incorporation of flexible ether bonds, bulky trifluoromethyl groups, and tetramethyl siloxane. For the electrospinning process, 3 g of the soluble polyimide powder was dissolved in 7 g of DMF under continuous magnetic stirring at 60°C for 2 hours to create a homogeneous electrospinning solution. Once fully dissolved, 8 mL of the precursor solution was loaded into a 10 mL syringe for electrospinning to produce polyimide nanofibers. The needle feed rate and syringe movement speed were set to 2.5 mL h^− 1 and 45 cm min^− 1, respectively. Nanofibers were collected using a rotating drum collector at a speed of 100 rpm, with a collection distance of 25 cm between the collector and the electrode. The applied voltage during electrospinning was maintained at 15 kV.

2.3 Fabrication of PIF-a

In a typical PIF-a synthesis process, 300 mg of the PAA (prepared according to our previously reported method) were dissolved in 100 mL of tert-butanol. Following this, 600 mg of polyimide (PI) nanofiber membranes were added to the PAA solution and homogenized at 12,000 rpm for 20 minutes to achieve a uniform dispersion. The resulting mixture was poured into a mold, frozen using liquid nitrogen, and subjected to freeze-drying for 48 hours to produce the pre PIF-a structure. The pre PIF-a was then thermally imidized by gradually heating to 150°C at a rate of 3°C per minute, followed by annealing for 1 hour. The temperature was then increased to 350°C at a rate of 1.5°C per minute, with an additional hour of annealing, before the material was finally cooled to room temperature, completing the formation of PIF-a.

2.4 Oil adsorption test

To evaluate the oil adsorption capacity of the sorbent, 1.0 g of the sorbent was immersed in a glass beaker containing 200 mL of oil. After approximately 60 minutes of adsorption, the sorbent was allowed to drain for 5 minutes to ensure the removal of any residual oil droplets. The mass of the sorbent was measured at 1-minute intervals over a 10-minute period, enabling the generation of an oil adsorption curve for each sample. The oil adsorption capacity for all samples was calculated using the following Eq. (1).:

$$\:Q=\frac{{M}_{t}-{M}_{0}}{{M}_{0}}$$

where Q is the sorption capacity (g/g), M_t is the weight of the oil saturated sorbent after 5 min of drainage (g), M₀ is the initial weight of the dry sorbent, set as 1.0 g.

Following the initial oil adsorption test, the absorbed oil was removed from the sorbent using an extrusion method. The sorbent was then subjected to a subsequent adsorption test. This adsorption-desorption cycle was repeated ten times to evaluate the reusability and recovery efficiency of the samples.

2.5 Characterization

The morphology and pore structure of the PIF-a were characterized using field emission scanning electron microscopy (FE-SEM, Zeiss Ultra 55, Germany). Before measurement, the samples were coated with a thin layer of gold to enhance conductivity. Contact angles were determined using a contact-angle system (JC2000D1, China), where motor oil droplets were placed on the sample surface, and measurements were taken at six different locations on each sample to obtain an average contact angle value. The specific surface area (SSA) of the PIF-a was calculated from nitrogen physisorption data using a Tristar 3020 analyzer (Micromeritics, USA). Pore size distribution, average pore width, and total pore volume were determined based on the Barrett-Joyner-Halenda (BJH) method. Prior to the analysis, samples were degassed under high vacuum at 100°C for 24 hours. The average diameter of the electrospun PI fibers was measured using Adobe Photoshop CS6 image analysis software, with at least 50 fibers considered for the calculation.

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.

This study introduces a novel polyimide nanofiber aerogel designed to address the pressing need for high-performance oil-absorption materials in extreme temperature environments. By utilizing soluble PI as the primary framework and PAA as a crosslinking agent, PIF-a achieves an ultra-lightweight density of 7.2 mg/cm³ while maintaining exceptional flexibility and resilience. The hierarchical pore architecture, which combines macro- and micropores, significantly enhances its oil sorption capacity and structural integrity, making it an ideal candidate for high-temperature oil absorption. PIF-a exhibits outstanding performance at room temperature, with a high oil sorption capacity of 83.1 g/g and rapid saturation within just 2 minutes. Its exceptional durability is demonstrated by a minimal 1.9% reduction in absorption capacity after ten cycles of sorption and desorption, highlighting its long-term reusability. Furthermore, the impressive thermal stability allows it to maintain structural integrity up to 250°C, surpassing the capabilities of conventional materials in similar applications. Even at 200°C, PIF-a retains a high sorption capacity of 78.6 g/g, aided by the reduced viscosity of the oil at elevated temperatures. This thermal resilience, coupled with its rapid oil infiltration ability, positions PIF-a as a highly effective material for industrial applications, particularly in sectors prone to high-temperature oil spills, such as the petrochemical and machinery industries. In conclusion, PIF-a constitutes a significant breakthrough in high-temperature oil absorption technology. Its outstanding sorption capacity, rapid kinetics, exceptional thermal stability, and durability offer a transformative solution for oil spill response, marking a key advancement in the development of materials for high-temperature oil absorption applications.

Acknowledgment

This work was financially supported by Xi’an University of Science and Technology Research Fund Program for Young Scholars (Grant No. 2050122029), the Foundation of the fellowship of China Postdoctoral Science Found (Grant No. 2020M673471), the China Scholarship Council (File No. 201806290052).

Author contributions

Lidong Tian contributed to conceptualization, formal analysis, methodology, and writing-original draft. Yi Zhang and Yibin Liu contributed to conceptualization, and writing-review and editing. Lidong Tian and Shan Zhang contributed to funding acquisition, resources, and writing-review and editing.

Funding

Xi’an University of Science and Technology Research Fund Program for Young Scholars, 2050122029, Lidong Tian. Fellowship of China Postdoctoral Science Found, 2020M673471, Shan Zhang. China Scholarship Council, 201806290052, Lidong Tian.

Data availability

The data that support the findings of this study are available within the article.

Conflict of interest

The authors declare that they have no competing interests.

Ethical approval

This work does not require any ethical statement.

Supporting Information:

All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supporting Information.

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No competing interests reported.

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Polyimide nanofiber aerogel with hierarchical porosity: a novel platform in high-temperature oil absorption

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Abstract

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1. Introduction

2. Experimental Section

2.1 Materials

2.2 Electrospinning of PI nanofiber

2.3 Fabrication of PIF-a

2.4 Oil adsorption test

2.5 Characterization

3. Result and discussion

3.1 Morphologies of the PIF-a

3.2 Oil adsorption of the PIF-a at room temperature

3.3 Thermal stability of the PIF-a

3.4 Hot oil adsorption of PIF-a

4. Conclusion

Declarations

References

Additional Declarations

Supplementary Files

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