Structure And Properties Of Polyimide Micro/Nano-fiber Aerogel Filtration And Separation Materials

In recent years,pollution of water and air became increasingly significant,which may harm the health of people.Therefore,stricter environmental regulations and discharge standards were formulated to reduce the emissions of pollutants into the environment.Currently,due to the low filtration precision and high energy consumption,traditional filtration and separation materials used in industrial sewage treatment and high temperature glue purification could not meet the requirements.To solve this problem,electrospun nanofiber materials have been utilized to prepare filtration and separation materials,which were considered as efficient strategies to purify oily water and air pollutants due to the large surface area,excellent connectivity,and high porosity.However,the densely packed nanofibers tend to raise the pressure drop significantly.In addition,the wetting ability need to be further improved to avoid pore plugging,improving the separation flux of materials.Therefore,preparation of novel filtration and separation materials and studying its principles is urgently needed.That will play a positive role in the economy development of industry and environmental protection.Polyimide（PI）is a high-performance polymer with excellent thermal stability and chemical resistance.Fibrous materials made of PI were widely used in high temperature flue gas filtration.Herein,solube polyimide fiber（P84）was chosen as the raw materials,aiming at the harsh working conditions in high temperature flue filtration and corrosive industrial wastewater treatment.Novel three-dimensional（3D）PI micro/nanofiber composite filtration and separation materials with hierarchical structure were prepared via a 3D reconstruction strategy,which were further modified through self-assembly.The effect of physical/chemical structure on performance of this material was studied in detail.Moreover,the mechanism of improvement on filtration and separation performance of the 3D hierarchical structure was proposed.The main conclusions are shown as follows.（1）Uniform PI nanofibers were obtained through electrospinning,which was characterized to study the properties.Results showed that PI nanofibers possessed excellent thermal stability（the 5%-weight-loss temperatures was 487.1 ^oC）,and certain hydrophobicity（water contact angle in the air is 124.2^o）.Subsequently,the PI nanofiber composite filter was prepared by depositing PI nanofiber on PI needle-punched nonwoven.Here,the filtration performance was characterized by using oily aerosol.Compared with PI needle-punched nonwoven,filtration efficiency of PI nanofiber composite filter for PM_2.0 increased significantly（～70.3%）,indicating that PI nanofiber has great potential to be used in the field of filtration and separation.However,the high pressure drop（800.0 Pa）and low releasing force of nanofiber membrane limited its further application seriously.（2）In order to reduce the pressure drop and improve structure stability of PI nanofiber material,the PI nanofiber membrane with 2D laminated structure was reconstructed into nanofiber aerogels with 3D structure.Herein,PI nanofibers were chosen as building blocks to achieve the isotropic PI nanofiber-based aerogels with hierarchical open-cell cellular architecture via 3D reconstruction strategy,which was further strengthened via solvent-vapor treatment.The obtained aerogels elucidated ultralow density（1.0 mg/cm³）and high porosity（99.93%）,showing unique structure advantage in the field of filtration and separation.Moreover,the PI nanofiber-based aerogels with unique hierarchical cellular architecture were strengthened through solvent-vapor treatment,endowing the resultant aerogels with super-elastic resilience under compression and folding,and fatigue resistance.（3）PI nanofiber-based aerogels possessed tortuous channels,high porosity,and specific surface area,making it attractive in the field of oil/water separation.Based on this,a facile superhydrophobic PI nanofiber-based aerogels for separation of highly stabilized water-in-oil emulsions were prepared by one-step liquid deposition process.The surface roughness of PI nanofiber-based aerogels was increased dramatically after coating silicon nanofilaments on the surface of nanofibers,endowing the aerogels with excellent superhydrophobic,fatigue resistance,absorption capacity（159 g/g）,under oil superhydrophobicity,and anti-viscous-water-adhesion.That modification made it possible to recycle the waste oil and reuse of the separation materials.In addition,surfactant-stabilized water-in-oil emulsions could be efficiently separated（100%）with high flux（from 7.5×10⁴-1.2×10⁵ L/m²/h）under the driving force of gravity only,solving the problem of low throughput and high energy consumption faced by traditional emulsion separation materials.（4）The high specific surface area and high porosity also provide the possibility to achieve“high efficiency and low resistance”of air filter.Based on the above study,a novel3D hierarchical structured PI micro/nanofiber-based filter for air filtration was prepared by compositing PI nanofiber aerogels with PI needle-punched nonwoven through 3D reconstruction method combined with vacuum filtration technology,followed by coating silicon nanofilaments.The resultant filter with hierarchical structure was composed of micron fiber as skeleton and nanofiber-based aerogels as separation interface,eliminating the delamination of composite filter and achieving efficient filtration of MPPS(PM_0.3,92.68%).Moreover,the filtration efficiency for PM_0.3 of prepared filter was 38.7%higher than that of the PI membrane filter,while its pressure drop was only 24.6%（196.8 Pa）.Additionally,the composite filter showed high particulate(PM_0.3)filtration efficiency and stable pressure drop throughout 20 tested filtration cycles,illustrating excellent filtration stability.Therefore,it is promising to employ this new route in the fields with requirement of high filtration performance.In conclusion,to address the drawbacks of tradition materials,such as high resistance and high energy consumption,a 3D hierarchically structured PI micro/nanofiber-based filtration and separation materials were prepared in this paper.The preparation process of this material was simple and universal.The resultant materials possessed high filtration precision,large flux,and good reusability.This study could offer guidance and reference in the construction of super-wetting surface and the production of novel filtration and separation with high efficiency and low resistance.