Fast and scalable production of crosslinked polyimide aerogel fibers for ultrathin thermoregulating clothes

The study addresses the challenge of producing polyimide (PI) aerogel fibers for intelligent thermoregulating textiles in a fast, continuous, and scalable manner. Although aerogel fibers offer low thermal conductivity, lightweight, and multifunctionality desirable for personal thermal management, PI aerogel fibers are difficult to produce continuously due to slow sol–gel kinetics of poly(amic acid) (PAA) intermediates and weak gel backbones that collapse during drying. Existing techniques—capillary-confined sol–gel transitions and freeze-spinning of PAA salts—either lack continuous production capability or yield large pore structures and require complex equipment. Moreover, most reported approaches depend on supercritical or freeze-drying, which are time- and energy-intensive, impeding high-throughput manufacturing. The research aims to develop a rapid sol–gel strategy that matches spinning dynamics and creates strong gel backbones to enable ambient pressure drying, thereby enabling ultrathin, high-performance thermoregulating fabrics for demanding environments.

Recent aerogel fibers include silica, graphene, MXene, Kevlar (aramid), and PI, each with distinct functionalities. PI aerogel fibers are promising for wide temperature ranges due to thermal resistance and mechanical robustness. Traditional PI processing proceeds via PAA sol–gel followed by imidization, requiring long times incompatible with continuous spinning. Reported techniques include: (1) sol–gel confined transition in capillaries (continuous production not feasible), and (2) freeze-spinning of water-soluble PAA salts (yields large, finger-like pores and relies on complex equipment). Recent organo-soluble PI wet-spun fibers combined with freeze-drying bypass PAA sol–gel but still need post-drying (supercritical/freeze), which is time- and cost-intensive. Two main obstacles remain: slow dynamic sol–gel transition needed during spinning and structural collapse during ambient pressure drying due to weak gel skeletons. Prior gelation strategies (condensation, chemical and ionic crosslinking) show sluggish kinetics and low gel modulus (<10 Pa), necessitating energy-intensive drying and limiting scalability (time consumption up to 49–94 h).

The authors develop a wet-spinning and ambient pressure drying route enabled by a UV-enhanced dynamic gelation strategy using a photosensitive polyimide (PPI). Key steps: (1) Synthesis of organo-soluble PI: One-pot copolymerization of 6FDA dianhydride with diamines ODA and DABA in NMP to form PAA, followed by thermal imidization at 120 °C (1 h), 160 °C (1 h), and 200 °C (10 h). Characterization (FTIR, 1H/13C/19F NMR) confirms complete imidization and presence of CF3 groups for solubility; Mw ≈ 30,273 g mol−1, PDI ≈ 1.63. (2) Synthesis of photosensitive PI (PPI): Steglich esterification grafts β-hydroxyethyl methacrylate (HEMA) onto PI carboxyls, yielding PPI-x with grafting ratios 25, 50, 100% (PPI-25/50/100). FTIR and NMR confirm grafting; Mw ≈ 31,205 g mol−1. (3) Spinning dope preparation: PPI, photoinitiator Irgacure 2100, and NMP mixed at 15:0.2:84.8 wt%. The solution shows high zero-shear viscosity and shear-thinning behavior. (4) UV-enhanced dynamic gelation wet-spinning: The dope is extruded through a 21 G needle into an NMP coagulation bath under UV irradiation (e.g., Omnicure S1500, UV intensity up to 2 W cm−2). Under UV, vinyl groups undergo free-radical polymerization, rapidly forming a crosslinked gel network. Gelation occurs within ~10 s for PPI-100; double-bond conversion reaches 93.2% after 30 s. The gelation rate constant determined by kinetic analysis is 10.2×10−2 L mol−1 s−1, exceeding previous reports (4.8×10−2 L mol−1 s−1). Spinning kinetics: spinnability for PPI-100 at 0.4–3.3 mm s−1 with retention times 15–125 s; gelation depth ~2.3 mm ensures full radial gelation for fiber diameters <1 mm; t_g controllable (5–20 s) via UV intensity. (5) Post-processing: Solvent exchange in ethanol (5 h), followed by ambient pressure drying at 25 °C (2 h) to obtain CPI aerogel fibers. The rapid, strong crosslinked gel backbone prevents collapse during drying; hydrophobic methyl/trifluoromethyl groups reduce capillary pressure. (6) Fabrication of CPI/PCM composites: CPI aerogel fabrics impregnated with paraffin wax at 80 °C in vacuum; excess removed to yield shape-memory CPI/PCM fabrics. (7) Characterization: FTIR, NMR, rheology (photo-rheometer with 365 nm UV, 30 mW cm−2), SEM, tensile testing, thermal conductivity via Hot Disk TPS 2500S, infrared thermography, DSC/TGA, GPC. Coarse-grained molecular dynamics simulations support gel network formation.

- UV-enhanced dynamic gelation enables rapid sol–gel transition of PPI: gelation in ~10 s (PPI-100), double-bond conversion 93.2% at 30 s. Gelation rate constant 10.2×10−2 L mol−1 s−1, faster than prior strategies (4.8×10−2 L mol−1 s−1). - Strong gel backbone: storage modulus ~11,700 Pa post-UV (loss modulus ~5,020 Pa), far exceeding gels from condensation, chemical, or ionic crosslinking (<10 Pa), enabling ambient pressure drying without collapse. - Scalable, continuous production: CPI aerogel fibers with lengths of hundreds of meters produced within ~7 h (wet spinning + 5 h solvent exchange + 2 h ambient drying), vs. 49–94 h in prior work; fibers maintain ~300 µm diameter with 3D nanofibrous porous structures. - Mechanical performance: CPI-100 fibers show low shrinkage (17.9%), low density (0.55 g cm−3), high tensile strength (~22 MPa), modulus (~215 MPa), and specific modulus 390.9 kN m kg−1. CPI-25 shows ~6.5 MPa strength and ~80 MPa modulus. A single CPI-100 fiber supports a 100 g load; woven fabrics withstand 500 g. - Thermal insulation: CPI aerogel fabric (0.7 mm thick) exhibits ΔT ≈ 108 °C on a 200 °C hot stage, comparable to down (5.4 mm), cotton (6.5 mm), and commercial PI (6.9 mm), yielding similar performance at ~1/8 the thickness of down. - Thermal conductivity: CPI fabric λ ≈ 24.2 mW m−1 K−1 at −50 °C; λ ≈ 70.2 mW m−1 K−1 at 150 °C. In contrast, cotton increases from 75.2 to 153.2 mW m−1 K−1; commercial PI from 82.4 to 230.1 mW m−1 K−1 over −50 to 150 °C. - Process window: Spinnable at 0.4–3.3 mm s−1; retention time 15–125 s; gelation depth ~2.3 mm > fiber diameter ensures homogeneous crosslinking. - Intelligent textile demonstration: CPI/PCM composite fibers/fabrics with PCM loading up to ~80% (TGA) and phase change at 65–70 °C (DSC) exhibit shape memory (shape fixity 77.4%, recovery 89.8%) and thermally adaptive expansion reducing surface temperature from 55.2 °C to 42.1 °C at 100 °C environment.

The UV-enhanced dynamic gelation approach directly addresses the key bottlenecks in continuous PI aerogel fiber fabrication: slow sol–gel kinetics and weak gel backbones. Rapid UV-triggered crosslinking matches spinning dynamics, preserves filament morphology, and builds a robust network that resists capillary forces during ambient pressure drying. As a result, scalable, energy- and time-efficient production of high-strength CPI aerogel fibers is achieved, enabling ultrathin fabrics with thermal insulation comparable to down at a fraction of the thickness. The nanoporous structure accounts for low thermal conductivity across a wide temperature range. Furthermore, the porous network readily hosts phase change materials, enabling shape-memory, thermally adaptive textiles that expand under heat to reduce heat transfer, demonstrating applicability to advanced personal thermal management scenarios.

This work establishes a fast, scalable method to produce crosslinked polyimide (CPI) aerogel fibers by wet-spinning with UV-enhanced dynamic gelation and ambient pressure drying. The method achieves gelation within ~10 s, builds high-modulus gel backbones, and enables continuous production of meter-scale fibers in ~7 h. The resulting CPI fibers and fabrics combine high mechanical performance (specific modulus ~390.9 kN m kg−1) with exceptional thermal insulation at ultrathin thickness (0.7 mm fabric showing ΔT ~108 °C on a 200 °C hot stage, comparable to down at 5.4 mm). Integration with phase change materials yields intelligent thermally adaptive textiles with robust shape-memory behavior. The strategy offers a cost-effective, high-throughput pathway for high-performance aerogel fibers for personal thermal management and related applications.