Electrospinning Machine| Environmentally Sustainable Polyarylate Nanofiber Aerogels with Superior Thermal, Acoustic, and Electromagnetic Insulation Properties

Aerogels can be divided into the following four categories according to the chemical composition of their structural skeleton: (1) Ceramic-based aerogels; such as silicon carbide aerogels, silicon dioxide aerogels, etc. (2) Carbon-based aerogels; such as graphene oxide aerogels, fullerene aerogels, etc. (3) Metal-based aerogels; such as silver aerogels, copper aerogels, etc. (4) Polymer-based aerogels; such as polyimide aerogels, aramid aerogels, etc. Among them, polymer fiber-based aerogels have characteristics such as light weight, easy processing, and low price, making them one of the most intensively researched materials currently. Aramid fiber is an ideal substrate for preparing high-performance polymer fiber aerogels, with advantages such as high strength, high modulus, and excellent heat resistance. Aramid nanofibers are one of the most intensively researched nanomaterials in recent years. In addition to inheriting the excellent comprehensive properties of aramid, aramid nanofibers also have smaller size and larger specific surface area. However, the extremely strong hydrogen bonding forces between aramid molecular chains give it a very high melting point that exceeds its decomposition temperature, allowing it to be formed only through solution spinning using concentrated sulfuric acid as a solvent. Moreover, there are issues such as complex and hazardous processes, inability to be recycled and reused, small inter-fiber forces, and the need for binders. Therefore, it is extremely important to prepare a green and environmentally sustainable polymer fiber-based aerogel with an efficient and environmentally friendly production process.

Recently, Associate Professor Siwei Xiong from Wuhan Textile University published the latest research results "Environmentally Sustainable Polyarylate Nanofiber Aerogels with Superior Thermal, Acoustic, and Electromagnetic Insulation Properties" in the journal ACS Materials Letters. The first author of the paper is Mengting She, a master's student at Wuhan Textile University, and the corresponding author is Associate Professor Siwei Xiong.

 Figure 1 Schematic diagram of the preparation of PAR nanofiber aerogel.

This work efficiently and environmentally prepared thermotropic liquid crystal polyarylate (PAR) nanofiber aerogels through the ice-templating method combined with freeze-drying process, featuring an efficient, environmentally friendly production process, recyclability, and excellent sound absorption and noise reduction, thermal insulation, and wave transmission properties. In this process, the molecular chains of PAR further polymerize, improving the mechanical properties of the PAR nanofiber aerogel. Simultaneously, the heat treatment process also enables melt welding between PAR nanofibers, further enhancing the structural stability of the PAR nanofiber aerogel. The sound absorption, noise insulation, thermal insulation, and wave transmission properties of PAR were also measured to evaluate its advantages in practical applications. Finally, we also recycled and reused the PAR nanofibers through two methods: mechanical crushing and melt reprocessing, contributing to the global goals of carbon neutrality and peak carbon.

Figure 2 Microscopic morphology and size controllability of PAR nanofiber aerogel.

Studying the microstructure of the PAR nanofiber aerogel, it exhibits a high aspect ratio and a wide diameter distribution of 100-200 nm, determined by Image J analysis, with an average of 187.85 nm. Before heat treatment, the PAR nanofiber aerogel has an interconnected porous network structure. After heat treatment, the fibers flatten, the diameter increases, the PAR nanofibers reach above the glass transition temperature, viscous flow occurs forming melt in-situ welding, increasing the PAR nanofiber concentration, further promoting fiber adhesion and entanglement, resulting in a mechanically robust aerogel structure. As the amount of PAR nanofibers increases, the density of the aerogel rises from 0.0091 g·cm⁻³ for PAR NFAs-1 to 0.0207 g·cm⁻³ for PAR NFAs-5. The densities of commercial polyurethane and rubber foam are both higher, at 0.0242 g·cm⁻³ and 0.032 g·cm⁻³, respectively. The increase in PAR nanofiber density reduces the pore size, and in-situ welding fills the smaller pores, reducing porosity. The reduction in porosity increases heat conduction through the fibers and reduces the constraint of air molecules. PAR nanofiber aerogels also possess excellent lightweight properties and shape control functions, allowing for the creation of various shapes of lightweight aerogels as needed, such as a panda-shaped aerogel prepared using a template method.

Figure 3 Thermal insulation performance of PAR nanofiber aerogel. 

Thermal insulation is one of the key performances of fiber-based aerogels. The measured thermal conductivity values for PAR NFAs-1 to PAR NFAs-5 range from 0.012 to 0.027 W·m⁻¹·K⁻¹. The thermal conductivity of PAR NFAs-5 is the highest among the samples but is still 26% and 33% lower than that of commercial PU foam and rubber foam, respectively, indicating superior thermal insulation performance. When the test time was 40 s, the surface temperature of the samples reached equilibrium; with further increase in test time, the surface temperature only fluctuated slightly. When the test time was 60 s, the surface temperatures of PAR NFAs-5, commercial rubber foam, and commercial PU foam were 50.4 °C, 62.8 °C, and 77 °C, respectively, further proving the superior thermal insulation performance of PAR NFAs-5. In addition to thermal insulation capability, the cold insulation capability of the PAR nanofiber aerogel was also investigated. Different samples were placed above a cold stage at -10°C, and an infrared thermal imager was used to record the temperature change over time on the upper surface of the samples. When the observation time reached 60 s, the surface temperature of PAR NFAs-5 was 25.4°C; it had only decreased by 9.6%. Compared to commercial PU foam, the high porosity of PAR NFAs and the three-dimensional network structure composed of high-aspect-ratio PAR nanofibers can effectively reduce the rate of heat transfer, thus exhibiting long-lasting cold insulation performance.

Figure 4 Sound absorption and noise reduction ability and electromagnetic transmittance of PAR nanofiber aerogel. 

Noise reduction is a key performance indicator for aerogels, usually evaluated through sound absorption and sound insulation. When sound waves encounter a medium, they may be reflected, absorbed, or transmitted: sound absorption emphasizes transmission and dissipation within the material, while sound insulation emphasizes reflection and energy trapping. Although these mechanisms are conceptually different, they are complementary. In the mid-to-high frequency range. PAR NFAs-5 shows a significantly higher absorption coefficient than other samples and commercial materials. Measurements were taken at different frequencies, and the average sound absorption values (SAA) were calculated: 0.107 for PAR NFA-1, 0.192 for NFA-2, 0.205 for NFA-3, 0.216 for NFA-4, and 0.317 for NFA-5. In the high-frequency range, materials with higher density and finer pores absorb sound better due to rapid air vibration, enhancing heat exchange and energy consumption. PAR NFA achieved a sound pressure level of 57.4 dB, demonstrating excellent acoustic performance. The blank control sample averaged 73.9 dB, which can damage neuronal cells. PAR NFA performed best, averaging maintenance of 5 dB, suitable for quiet environments. Wave transmittance is a key indicator for materials used in high-frequency communication applications, as it can ensure high-quality, efficient signal propagation. PAR NFA showed almost no fluctuation, indicating superior transmittance. The propagation attenuation characteristics of electromagnetic waves in materials are mainly determined by three constitutive parameters: conductivity (σ), dielectric constant (ε), and relative permeability (μ_r). Experimental data shows that the conductivity of PU foam and rubber foam is 2.074×10⁻¹⁵ and 2.097×10⁻¹⁴, respectively, while the conductivity of PAR NFA is significantly reduced to 1.505×10⁻¹⁵. The relative permeability (μ_r) of the three materials was compared. The μ_r of both PAR NFA and foam rubber is close to 1 (μ_r≈1.0±0.05) in the 8-12 GHz range, indicating they are non-magnetic; whereas PU foam exhibits weak positive magnetism (μ_r≈1.5) in the 8-10.5 GHz band, but it rapidly decays to 1.0 after 10.5 GHz. Further analysis of the electromagnetic shielding effect shows that PAR NFA, PU foam, and rubber foam all exhibit low reflection loss characteristics. Meanwhile, the baseline absorption loss values (SEA≈0.7 dB) and peak fluctuations (ΔSEA≈0.17 dB @10.1 GHz) of these three indicators all indicate that electromagnetic wave energy is mainly dissipated through transmission. In summary, PAR NFA achieves excellent wave transparency (propagation loss close to zero) thanks to its unique low dielectric constant, non-magnetic properties, and structurally regulated low conductivity. This inherent electromagnetic strength transparency, combined with PAR NFA's thermal insulation capability, highlights their application potential in electromagnetic transparent thermal barriers and communication systems.

Figure 5 Dual-cycle recycling of PAR nanofiber aerogel.

Due to the complex composition of polymer fiber-based aerogels, they are usually difficult to recycle using traditional methods and pose a threat to sustainable ecological development. PAR nanofiber aerogel has a single component, and no binders or other materials are added during its preparation. PAR nanofibers can be thermoplastically reprocessed to form aerogels. Figure 5 illustrates two recycling and reuse pathways for PAR nanofiber aerogels. In pathway 1, the preparation of PAR NFA does not add binders or other components, relying solely on the ability of PAR nanofibers to be thermoplastically reprocessed. In practical applications, recycled PAR nanofiber aerogels can be remanufactured into new PAR nanofiber aerogels through mechanical grinding, solution dispersion, freeze-drying, or heat treatment processes. In pathway 2, PAR nanofiber aerogel is first reprocessed into PAR resin raw material through melt processing, which can be used to manufacture other PAR materials. Subsequently, the recycled PAR resin is processed into continuous PAR fibers through melt spinning. Finally, PAR fibers are converted into PAR nanofiber aerogels through wet grinding, solution dispersion, freeze-drying, and heat treatment. Therefore, PAR nanofiber aerogels have diverse, simple, and flexible recycling methods. In the recycling of aerogels, the waste aerogel is first cleaned of impurities adhering to the fiber surface by ultrasonic vibration, then a small amount of alkali solution is added to assist in 1-minute ball milling to obtain the original pulp. The recycling process saves the fibrillation process of the fibers and improves efficiency.

Paper link: https://pubs.acs.org/doi/full/10.1021/acsmaterialslett.5c00796