Abstract:

Objective The continuous improvement of detection technologies presents significant challenges to stealth technology, getting more attention on the development of efficient infrared stealth technology. Polyaniline (PANI) offers an innovative approach with its unique electrochromic properties, low cost, low infrared radiation emissivity and easy processing. However, the practical application of PANI is hampered by problems such as poor flexibility, fragility, and easy peeling, and the application of its coatings is affected by high emissivity binder, which greatly limits its application in wearable devices. In order to realize the full potential of PANI, material properties must be optimised to address these limitations. In this research, the polyimide (PI) film and nanoporous polypropylene (nanoPP) films and were selected as the substrate to fabricate the flexible infrared stealth films.

Method Intrinsic state polyaniline was synthesized by chemical oxidative polymerization and subsequently doped with camphorsulfonic acid (CSA) through sufficient grinding at a molar ratio of 1∶0.8. The mixture was then stirred in an m-cresol solution for 48 h to obtain low-emissivity polyaniline coatings (PANI-CSA). The selected PI film and nanoPP films with a thickness of 16 μm (nanoPP₁₆), and a thickness of 25 μm (nanoPP₂₅)) were thoroughly washed with anhydrous ethanol and dried. The PANI-CSA coatings were uniformly applied to the films at a controlled height of 42 μm using a spatula and dried naturally at room temperature, hence obtaining the flexible low-emissivity films: polyaniline coated PI film (PANI/PI), polyaniline coated nanoPP₁₆ film (PANI/PP₁₆), and polyaniline coated nanoPP₂₅(PANI/PP₂₅).

Results The morphologies and structures of PI, nanoPP₁₆, nanoPP₂₅, PANI/PI, PANI/PP₁₆ and PANI/PP₂₅ films were analysed using SEM images. Among them, nanoPP₁₆ shows flat, interconnected slit-like porous structure with concentrated and regular pore size distribution. According to the particle size of the PANI-CSA coating (45.06 nm), the polyaniline can follow the solvent to enter into the pores of the film, and through the synergistic effect of mechanical interlocking effect and intermolecular van der Waals' force, the PANI/PP₁₆ film has smooth, low-emissivity surfaces without the use of high emissivity binders. Infrared reflectance and emissivity tests show that PANI/PI, PANI/PP₁₆ and PANI/PP₂₅ films all have low infrared emissivity and meet the requirements for infrared cloaking. Notably, PANI/PP₁₆ has the lowest emissivity (0.21), which is attributed to its moderate pore size, which allows for deeper penetration of the polyaniline and avoids agglomeration during drying. In a 60 min infrared stealth monitoring experiment using an infrared camera, PANI/PI initially showed good stealth performance, but the effect became uneven over time, with the emissivity temperature rising to 30.2 ℃ after 20 min and losing its stealth capability completely after 30 min. In contrast, PANI/PP₁₆ maintains excellent infrared stealth performance, with the surface radiation temperature stabilised at 23-24 ℃ in 60 min, which effectively masks the human body temperature and meets the requirement of long-time stealth. Thermal stability analysis shows that PANI/PP₁₆ can be used in infrared stealth scenarios up to 240 ℃. The unique properties of nanoPP₁₆, including its porous structure and flexibility, not only improve the adhesion and uniformity of polyaniline coatings without the use of binders, but also solve the brittleness problem usually associated with rigid polymers such as polyaniline. This combination of structural and functional advantages makes PANI/PP₁₆ an ideal candidate for applications requiring long-lasting and effective infrared cloaking capabilities. These findings highlight the importance of substrate selection and structural design in optimising the performance of polyaniline coatings for advanced cloaking applications.

Conclusion The results show that the PANI/PP₁₆ film has an emissivity as low as 0.21 in the mid-infrared band (8-14 μm) and maintains excellent infrared stealth performance for up to 60 min, with a radiant temperature difference of 9-10 ℃ from the palm of the hand, which allows the human body to be hidden from the environment. It is also thermally stable and can be used for infrared cloaking in scenarios up to 240 ℃. Additionally, the film has excellent flexibility, which successfully overcomes the problems of poor flexibility, fragility and flaking of pure polyaniline film, and effectively avoids the influence of conventional high emissivity binders on the overall stealth effect of the material This study provides a new approach for the application of polyaniline in the field of wearable infrared stealth materials

Key words: polyaniline, flexible film, wearable infrared stealth material, nanoporous polypropylene film, polyimide film