Abstract:

Objective As the industrialization process continues, human activities such as transportation, industry, and power plants emit large amounts of air pollutants. Particulate matter pollution was the main air pollution problem. Direct filtration of particulate matter at the emission source is considered the most effective method. Polyimide (PI) is one type of polymer possessing imide ring structure, good thermal stability, high heat resistance, and its products are characterized by high strength and high modulus, radiation resistance, corrosion resistance. It is an ideal polymer appliled in high temperature air filtration. Copolymerized PI can be obtained by two-step method by using two dianhydrides or two diamines to react in polar solvent. It can change the molecular structure, intersegment force and crystallinity of PI, which is a modification method of PI. In this paper, copolymerized PI nanofibrous membranes with higher tensile property and filtration performance were prepared through electrospinning technology.

Method Rigid monomer p-phenylenediamine (PPDA) was introduced and dissolved into dimethylaceta-mide (DMAc) with 4,4'-diaminodiphenyl ether (ODA) and homophenyltetramethylanhydride (PMDA), to prepare polyamide acid (PAA) solution. The copolymerized PI nanofiber membranes with different molar ratio of PPDA were prepared by electrospinning and thermoimide treatment. The fiber morphologies, chemical group, tensile property, and filtration performance were characterised and analyzed through scanning electron microscope, Fourier infrared spectrometer, fiber tensile instrument and automatic filter detector.

Results The diameter of PI_10∶0 nanofibers without PPDA was smaller, with an average diameter of 579.65 nm, and the fiber diameter distribution was relatively uniform. The introduction of PPDA into ODA and PMDA promoted the rigidity of polyamide acid (PAA) molecular, and increased the viscosity of PAA solution. The diameter of copolymerized PI nanofibers with PPDA were relatively larger, with an diameter range of 645.38-1 050.31 nm. When the molar ratio of PPDA were larger, the fiber diameter distributions of PI_7∶3 and PI_6∶4 nanofibers became uneven, especially in the case of PI_6∶4 nanofibers. the smallest fiber diameter was 207.35 nm, while the coarsest fiber diameter was 1 739.09 nm. This is mainly because that PAA molecular chain was relatively rigid when the molar ratio of PPDA were higher. The stretching force on the jet during electrospinning were not uniform, resulting in uneven distribution of fiber diameter. The FT-IR of PAA and PI nanofibers expressed that PAA nanofibers showed characteristic C—N stretching and vibration peak and C═O vibration absorption peak at 1 403 cm^-1 and 1 625 cm^-1.Compared to PAA nanofibers, PI nanofibers with different PPDA molar ration all expressed characteristic symmetric contraction vibration peak of aromatic imide C═O at 1 776 cm^-1 and 1 723 cm^-1, these results indicated that all PAA nanofiber were transformed into PI nanofiber films after thermal imide treatment. The stretching curves of all PI nanofiber films were consistent with the stretching of the fiber assembly, and the curves showed three stages. The first stage was the elongation of the fiber, the second stage was the elongation of the fiber macromolecular chain, and finally the fiber slipped-off and fractured, their stretching phenomenon were similar to that of other randomly arranged nanofiber membranes. Comparing the tensile curves of different PI nanofiber films, the PI_10∶0 nanofiber membrane without rigid monomer PPDA had the smallest tensile strength (4.91 MPa) and the largest fracture elongation (60.5%). In contrast, the tensile strength of the copolymerized PI nanofibers increased significantly, while the fracture elongation decreased slightly, and the tensile strength of PI nanofibers increased with the increase of PPDA mole fraction. When the molar ratio of PPDA was 30%, the maximum tensile strength of PI_7∶3 nanofiber membrane was 8.65 MPa, mainly because the addition of PPDA increased the rigidity of the PAA molecular chain, resulting in the improvement of the mechanical properties of copolymerized PI nanofibers. The filtration performance under the flow rate of 32 L/min and 85 L/min were tested. At the flow rate of 32 L/min, the filtration efficiencies of different PI nanofiber membranes ranged from 98.56% to 99.86%, and the resistance pressure drops ranged from 143 Pa to 185.6 Pa. When the mole ratio of ODA to PPDA was 8∶2, the filtration efficiency of PI_8∶2 nanofiber film was the highest, reaching 99.86% and the resistance pressure drop reaching 185.3 Pa. Compared the filtration performance of PI nanofibers at different flow rates, it can be seen that the filtration efficiencies at different flow rates were very close, but the resistance pressure drops were quite different. The resistance pressure drops at the flow rate of 85 L/min were significantly higher than that at the flow rate of 32 L/min.This was mainly because the increase in flow rate can cause particles to collide with the nanofiber membrane quickly and fiercely. Blocking particles through the nanofiber membranes resulted in a significant increase in pressure drop.

Conclusion The copolymerized polyimide (PI) nanofiber membranes were prepared by introducing rigid monomer p-phenylenediamine (PPDA), using two-step method and electrospinning technology. The copolymerized PI nanofiber membranes possessed higher fiber diameter, increased tensile strength and filteration performance, had better application in the field of high temperature filtration materials.

Key words: p-phenylenediamine, copolymerization, polyimide, tensile property, filtration performance