Porous Polyimide Research Articles

Freestanding Fully Porous Polyimide Membrane for Thin Film Microextraction

In this study, freestanding fully porous polyimide (FFPP) membranes were prepared in batches by the scraping technique and vapor-induced phase separation (VIPS) and used for thin film microextraction (FFPP-TFME). When water vapor was applied simultaneously during scraping, the resulting membrane surface was highly porous, in contrast to the dense and smooth surface obtained without vapor. The micro/nanocomposite pore structure was revealed by scanning electron microscopy and nitrogen adsorption–desorption. The diameters of the micropores, nanopores and fibrous polyimide (PI) nanofibers were 0.5–1.5 μm, ca. 30 nm and ca. 250 nm, respectively. The extraction performance of the FFPP-TFME method was evaluated with organochlorine pesticides (OCPs) as model analytes in water. The extracted OCPs were desorbed in acetone and then quantified by gas chromatography–mass spectrometry. The effects of extraction time, ionic strength, desorption solvent, desorption volume and desorption time were investigated. Under the optimized conditions, the limits of detection (LODs) for eight OCPs (4.4–83 ng L−1), linear ranges (0.05–50 μg L−1), correlation coefficients (0.997–0.999), intrabatch relative standard derivations (RSDs, 0.88–9.20%) and interbatch RSDs (6.35–13.5%) were obtained. The method was also successfully applied to the analysis of environmental water samples, and the relative recoveries of all real water samples were 73.6–116.4%. These results show that the FFPP-TFME method is a simple, efficient and environmentally friendly method for the pretreatment of trace organic pollutants in water samples.

3D printing of porous polyimide for high-performance oil impregnated self-lubricating

Three-dimensional (3D) printing of porous polyimide (PI) with good oil storage capability for self-lubricating is realized by direct ink writing of poly(amide acid) (PAA) precursor ink followed by post thermal treatment and template removal. The ink is comprised of PAA and polymethyl methacrylate (PMMA) microspheres, where the PMMA microspheres serve as both the pore-forming agent and the rheology modifier to realize high-precision porous PI objects. The printed porous PI exhibited good mechanical performance and thermal stability, and good oil storage capability to various lubricant oils to achieve self-lubricating. Importantly, various parts with desirable complex structures, bearing retainers for example, can be readily accomplished. It is therefore promising for manufacturing high-performance lubricating parts, especially those with high complexity.

Fabrication, Thermal and Sound Absorption Properties of Porous Polyimide Reinforcing by SiO2 Nanoparticles

Special requirements of the service environment put forward higher requirements for the comprehensive performances of porous sound-absorbing materials. In this study, the polyimide/nano-SiO2 composite foams were prepared based via the sol-gel and foaming methods. The effect of different addition of SiO2 nanoparticles on the pore structure, sound absorption properties as well as the thermal properties of polyimide/SiO2 nanocomposite foams are studied. Results indicate that: the addition of the nano-SiO2 particles not only could improve the thermal stability of the composites, but also increase the sound absorption performances. Particularly, the samples with 2% addition of SiO2 nanoparticles shows the optimal thermal and sound absorption performance. Therefore, adding the SiO2 nanoparticles into the polyimide foams is an efficient way to improve its thermal and sound absorption performance, which would give insights into designing sound-absorbing materials with excellent comprehensive performance to curb the noise pollution.

Porous carbon/graphite nanosheet/ferromagnetic nanoparticle composite absorbents with adjustable electromagnetic properties

With the rapid development of electronic devices and wireless communication tools, it is urgent to design and fabricate low-cost, lightweight and effective electromagnetic absorption materials to solve interference of electromagnetic waves. Herein, a new strategy toward porous carbon/graphite nanosheet/ferromagnetic nanoparticle (PC/GNS/Fe) composites was designed to investigate the influence of crystalline carbon on electromagnetic wave absorption. To begin with, graphite nanosheets (GNSs) were incorporated into the porous polyimide by in situ polymerization, and Fe were added as a magnetic particle source and an agent to regulate the pore size. A series of PC/GNS/Fe composite absorbents were obtained. The direct carbonization of porous polymer precursors was beneficial to the design of the pore structure of materials. A hierarchically porous structure derived from the phase separation process was well maintained in the polyimide pyrolysis process. The results demonstrated that the presence of crystalline carbon could influence the reflection loss value and the frequency range. Hence, the absorbing performance can be optimized by adjusting the pore structure and the content of crystalline carbon in materials, which is conducive to obtaining electromagnetic wave absorption materials with excellent comprehensive performance.

Fabrication of Porous Polyimide Membrane with Through-Hole via Multiple Solvent Displacement Method.

Porous polyimide (PI) membranes are widely used in separation processes because of their excellent thermal and mechanical properties. However, the applications of porous PI membranes are limited in the nanofiltration range. In this study, porous PI membranes with through‐holes have been successfully fabricated by the novel multiple solvent displacement method. This new method requires only a porous polyamic acid (PAA) membrane, which was prepared by immersing PAA film in N‐methylpyrrolidoneebk; (NMP) prior to immersing it in a mixed solvent consisting of NMP and a poor solvent, followed by immersion only in poor solvent. The pore size, morphology, porosity, and air permeability demonstrated that the fabricated PI membranes had a uniformly porous structure with through‐holes over their surface. This new method enabled control of pore size (3–11 μm) by selecting a suitable poor solvent. This multiple solvent displacement method is highly versatile and promising for the fabrication of porous PI membranes.

Facile Preparation of Continuous and Porous Polyimide Aerogel Fibers for Multifunctional Applications.

High-performance aerogel fibers with high porosity, ultralow density and thermal conductivity, and good flexibility are attractive candidates for the next generation of effective thermal insulation, efficient personal thermal management, and other functional applications. However, most previously reported aerogel fibers suffered from either limited working temperatures, weak mechanical properties, or complex manufacturing processes. In the present work, a facile wet-spinning technique combined with freeze-drying was developed to fabricate strong polyimide aerogel fibers (PAFs) based on organo-soluble polyimide. Attributed to the unique "porous core-dense sheath" morphology, the PAFs exhibited excellent mechanical properties with an optimum tensile strength of 265 MPa and an initial modulus of 7.9 GPa at an ultimate elongation of 65%, representing the highest value for aerogel fibers reported so far. Moreover, the PAFs possess high porosity (>80%) and high specific surface area (464 m2 g-1), which render the woven PAF fabrics with excellent thermal insulation properties within a wide temperature range (-190 to 320 °C) and potential applications for thermal insulation under harsh environments. Additionally, a series of functionalized aerogel fibers or their fabrics based on PAFs, including phase-change fabrics with a thermoregulation function and electromagnetic shielding (EMI) textiles with a high EMI SE value, have been successfully fabricated for expanding their potential applications. Overall, this novel aerogel fiber sheds light on a promising direction for developing the next generation of high-performance thermal insulation and multifunctional fibers and textiles.

Molecular Dynamics Simulations of Lubricant Recycling in Porous Polyimide Retainers of Bearing.

Porous polyimide (PI) materials are one of important bearing retainer materials in space applications due to the storage and continuous supply of a lubricant through the porous structure. Understanding the lubricant recycling process in porous polyimide retainers is of vital importance to improve lubricant supply performance of bearing. In this work, through molecular dynamic simulations, coarse-grained models are built to study lubricant recycling processes on porous and solid surfaces. A spontaneous imbibition behavior is observed when the lubricant is present on the porous surface. The dynamic change in the contact angle in this process and the deviation of the effective radius from the volumetric radius because of the molecular structure of polyimide causes the classical Lucas-Washburn (L-W) equation fail to describe the process. By fitting dynamic contact angle and effective radius, a modified L-W equation is developed, which well predicts the process of imbibition. Furthermore, it is found that the lubricants between the porous polyimide surface and the solid surface are recycled by extrusion, and spontaneous imbibition does not occur. In this case, the accumulation of lubricant pressure and weak interfacial interaction between the lubricant and the solid surface are also the main factors that promote lubricant recycling.

Electrospun Adsorptive Nanofibrous Membranes from Ion Exchange Polymers to Snare Textile Dyes from Wastewater

AbstractIncreasing discharges of industrial wastewater, along with ever‐stricter regulations for the protection of natural water sources, have amplified the demand for highly efficient water treatment technologies. Here, electrospun nanofibrous polyimides enhanced with ion exchange properties are proposed as adsorptive membranes for the treatment of dye‐loaded textile wastewater. With the careful selection of monomers, carboxyl‐functionalized porous polyimides are synthesized in a single step and then further decorated with strong cation and anion exchange side groups. Nuclear magnetic resonance spectroscopy and thermal gravimetric analysis are used to investigate the alkylation degree and total exchange capacity of the polymers. The electrospinning conditions are optimized to produce highly flexible membrane mats with a uniform nanofibrous structure. A series of dye sorption experiments on the nanofibrous membranes reveals the adsorption kinetics and the effects of the polyimide backbone, the charged side groups, and the hydrophilicity. A recycling study is conducted to confirm the stability of the adsorbent membranes. The results suggest that nanofibrous polyimide membranes enhanced with ion exchange properties are promising candidates for the treatment of dye‐laden wastewater. Owing to their facile syntheses and unique properties, these membranes show promising potential in environmental applications.

Effect of temperature on the friction and wear performance of porous oil-containing polyimide

Tribological behaviors of porous polyimide (PPI) impregnated with polyalphaolefin at different temperatures were investigated. As temperature increases, the coefficient of friction (COF) of PPI decreases at 0.15 m/s but no obviously changes at 0.01 m/s. In contrast, the COF of non-porous polyimide under external oil-supply increases regardless of speeds. This is due to the fact that COF is related to the oil-supply and viscosity determined by temperature. At 0.15 m/s, PPI is in mixed lubrication, self-healing of desorbed adsorption-film and reducing interlaminar shear-force lead to lower COF. However, at 0.01 m/s, PPI is in boundary lubrication, the stored oil can be slowly released and migrated to the interface to heal the adsorption-film, and prevent COF from further increasing.

Inherently flame-retardant solid polymer electrolyte for safety-enhanced lithium metal battery

The solid polymer electrolyte (SPE) has been regarded as a promising candidate for the next-generation rechargeable Li metal battery with high-energy density. However, the current SPEs are still challenged by insufficient security owing to their flammability. Here, we reported a novel inherently flame-retardant SPE (IFR-SPE), which was fabricated via the one-step in situ crosslinking of two novel synthetic P-containing monomers with porous polyimide film as a supporting host. Compared with liquid and conventional solid polyethylene oxide electrolytes, the unique IFR-SPE exhibits outstanding flame-retardant ability. Furthermore, it demonstrates a high Li-ion conductivity (0.28 mS cm−1 at 25 °C), a stable electrochemical window up to 4.12 V, and effective Li dendrite suppression via a LiF/Li3PO4-rich solid electrolyte interphase formed on the Li anode. Consequently, the Li/IFR-SPE/LiFePO4 cell shows acceptable electrochemical performances. We believe that this work offers a novel and effective strategy for establishing a promising Li metal battery system with high safety.

Crosslinked porous polyimides: structure, properties and applications

Porous polyimides (pPIs) represent a fascinating class of porous organic polymers (POPs). Here the properties and functions of amorphous and crystalline pPIs are reviewed, and applications contributing to solutions to global challenges highlighted.

Hydrothermal polymerization of porous aromatic polyimide networks and machine learning-assisted computational morphology evolution interpretation.

We report on the hydrothermal polymerization (HTP) of polyimide (PI) networks using the medium H2O and the comonomers 1,3,5-tris(4-aminophenyl)benzene (TAPB) and pyromellitic acid (PMA). Full condensation is obtained at minimal reaction times of only 2 h at 200 °C. The PI networks are obtained as monoliths and feature thermal stabilities of >500 °C, and in several cases even up to 595 °C. The monoliths are built up by networks of densely packed, near-monodisperse spherical particles and annealed microfibers, and show three types of porosity: (i) intrinsic inter-segment ultramicroporosity (<0.8 nm) of the PI networks composing the particles (∼3–5 μm), (ii) interstitial voids between the particles (0.1–2 μm), and (iii) monolith cell porosity (∽10–100 μm), as studied via low pressure gas physisorption and Hg intrusion porosimetry analyses. This unique hierarchical porosity generates an outstandingly high specific pore volume of 7250 mm3 g−1. A large-scale micromorphological study screening the reaction parameters time, temperature, and the absence/presence of the additive acetic acid was performed. Through expert interpretation of hundreds of scanning electron microscopy (SEM) images of the products of these experiments, we devise a hypothesis for morphology formation and evolution: a monomer salt is initially formed and subsequently transformed to overall eight different fiber, pearl chain, and spherical morphologies, composed of PI and, at long reaction times (>48 h), also PI/SiO2 hybrids that form through reaction with the reaction vessel. Moreover, we have developed a computational image analysis pipeline that deciphers the complex morphologies of these SEM images automatically and also allows for formulating a hypothesis of morphology development in HTP that is in good agreement with the manual morphology analysis. Finally, we upscaled the HTP of PI(TAPB–PMA) and processed the resulting powder into dense cylindrical specimen by green solvent-free warm-pressing, showing that one can follow the full route from the synthesis of these PI networks to a final material without employing harmful solvents.

Hydrophobic and hydrophilic modification of hierarchically porous monolithic polyimide derivatives as functional liquid absorbers

Thermally induced phase separation (TIPS) and thermal imidization were used to prepare PI-based monoliths in sequence. The hybridized PI (PI-co-POSS) monoliths exhibit potential in liquid diode application to achieve oil–water separation.

Porous polyimide framework based on perylene and triazine for reversible potassium-ion storage

A porous polyimide framework was prepared via condensation between perylenetetracarboxylic dianhydride and triazine and investigated as an electrode material for reversible K+-ion storage. It exhibited a large reversible capacity, good rate performance, and long cycling stability up to 1000 cycles at 5 A g−1.

Electrospun Wrinkled Porous Polyimide Nanofiber-Based Filter via Thermally Induced Phase Separation for Efficient High-Temperature PMs Capture.

Benefiting from its superior thermal stability, polyimide (PI) fiber-based composites have attracted wide attention in the field of high-temperature filtration and separation. However, the trade-off between filtration efficiency and pressure drop of traditional PI filters with single morphology and structure still remains challenging. Herein, the electrospun PI high-temperature-resistant air filter was fabricated via thermal-induced phase separation (TIPS), employing polyacrylonitrile (PAN) as a template. The PI nanofibers exhibited special wrinkled porous structure, and the filter possessed a high specific surface area of 304.77 m2/g. The removal of PAN changed the chemical composition of the fiber and induced PI molecules to form complex folds on the surface of the fiber, thus forming the wrinkled porous structure. Additionally, the wrinkled porous PI nanofiber filter displayed a high PM0.3 removal efficiency of 99.99% with a low pressure drop of 43.35 Pa at room temperature, and the filtration efficiency was still over 97% after being used for long time. Moreover, the efficiency of the filter could even reach 95.55% at a high temperature of 280 °C. The excellent filtration performance was attributed to the special wrinkled porous surface, which could limit the Brownian motion of PMs and reinforce the mechanical interception effect to capture the particulate matters (PMs) on the surface of the filter. Therefore, this work provided a novel strategy for the fabrication of filters with special morphology to cope with increasingly serious air pollution in the industrial field.

Asymmetric hollow-fiber filtration membranes based on insoluble polyimide (R-BAPB): Influence of coagulation bath on porous structure

The insoluble polyimides are the most promising group of polymer materials for fabrication of solvent stable filtration membranes suitable for operation at elevated temperatures. In order to synthesize asymmetric membranes from insoluble polyimide, it is proposed to fabricate the membranes from a pre-polymer solution (polyamide acid – PAA) by non-solvent induced phase separation method followed by imidization to form non-soluble porous polyimide membrane. The thermoplastic crystallizable polyimide R-BAPB, which is resistant to a number of known organic solvents, was chosen as a membrane material. For the first time, hollow fiber membranes based on imidized PAA (R-BAPB) with a controlled distribution of pores on the inner side of the hollow fiber were formed. It has been established that the use of “hard” non-solvents, such as water or aqueous-organic solutions, is preferable for the formation of a porous structure in the membranes based on PAA (R-BAPB). Synthesized PAA membranes were used to prepare porous membranes based on the thermoplastic polyimide R-BAPB by thermal imidization. Imidization process was confirmed by an increase in the glass transition temperature of the material to 220 °C (corresponds to the glass transition temperature of polyimide (R-BAPB)) and is accompanied by a significant increase in its elastic modulus. The results of the measurements of the transport properties of polyimide membranes for gases and liquids indicate that microfiltration transport pores are present in the membranes.

Influence of Coagulation Bath Temperature on the Structure and Dielectric Properties of Porous Polyimide Films in Different Solvent Systems.

In this work, the effect of coagulation bath temperature in different solvent systems [1,4-butyrolactone (GBL)/N,N-dimethylacetamide (DMAC)] on the structure and dielectric properties of polyimide (PI) films was investigated for the first time. The solubility parameter was introduced to explain the formation process of porous PI films. The results showed that the changed tendency of the dielectric constant versus temperature is opposite for the single-solvent system and cosolvent system. For a single DMAC and GBL solvent, the dielectric constants of the films decreased with increasing temperature. In contrast, the dielectric constants increased with the increase in temperature for the GBL/DMAC cosolvent system. Moreover, the measured porosities were applied to estimate the dielectric constants of the PI films. This showed that the porosity increased with increasing temperature for a single-solvent system, while it decreased for a cosolvent system. Scanning electron microscopy images suggested that the variation trends are derived from the different influences of the temperature on the structure and morphology. Thus, this study reveals the effect of coagulation bath temperature on the structure and dielectric properties of porous PI films and provides the guidance for the design and optimization of architectures for high-performance porous films.

Simultaneous phase-inversion and crosslinking in organic coagulation bath to prepare organic solvent forward osmosis membranes

Organic solvent forward osmosis (OSFO) is an emerging separation process, which can realize simultaneous concentration of pharmaceutical ingredients and solvent recovery. In this work, a time-saving preparation process for solvent-resistant support layers was proposed by a simultaneous phase-inversion and crosslinking (SIM) method. The SIM method combines the formation of porous polyimide (PI) support layer and crosslinking between the PI and hexanediamine (HDA) in the coagulating baths, thus avoiding an extra need of solvent-based synthesis step. Furthermore, isopropanol (IPA) was used as coagulation bath for solvent-resistant PI support layers preparation, which can more effectively retarding the phase-inversion process compared with the DI-water. In addition, the morphologies and properties of these support layers with different synthesis time were prepared and characterized. After interfacial polymerization reaction using the as-prepared solvent-resistant PI support layers, the resultant OSFO membranes exhibited reasonable ethanol fluxes, lower reverse salt fluxes, and excellent pharmaceutical (oxytetracycline, OTC) rejections in both forward and reverse osmosis modes. Moreover, the long-term tests of as-prepared OSFO membranes indicated an excellent stability in 10 h operation with ethanol. After 10 h activation using DMF, the ethanol fluxes of OSFO18h membranes with different thickness of solvent-resistant support layers were significantly improved without sacrificing OTC rejection, which furtherly demonstrated the timeliness of SIM method and potential application of OSFO process in both concentrate the pharmaceutical and solvent recovery.

Highly Stable Porous Polyimide Sponge as a Separator for Lithium-metal Secondary Batteries.

To inhibit Li-dendrite growth on lithium (Li)-metal electrodes, which causes capacity deterioration and safety issues in Li-ion batteries, we prepared a porous polyimide (PI) sponge using a solution-processable high internal-phase emulsion technique with a water-soluble PI precursor solution; the process is not only simple but also environmentally friendly. The prepared PI sponge was processed into porous PI separators and used for Li-metal electrodes. The physical properties (e.g., thermal stability, liquid electrolyte uptake, and ionic conductivity) of the porous PI separators and their effect on the Li-metal anodes (e.g., self-discharge and open-circuit voltage properties after storage, cycle performance, rate capability, and morphological changes) were investigated. Owing to the thermally stable properties of the PI polymer, the porous PI separators demonstrated no dimensional changes up to 180 °C. In comparison with commercialized polyethylene (PE) separators, the porous PI separators exhibited improved wetting ability for liquid electrolytes; thus, the latter improved not only the physical properties (e.g., improved the electrolyte uptake and ionic conductivity) but also the electrochemical properties of Li-metal electrodes (e.g., maintained stable self-discharge capacity and open-circuit voltage features after storage and improved the cycle performance and rate capability) in comparison with PE separators.

Ultra-low dielectric properties of porous polyimide thin films fabricated by using the two kinds of templates with different particle sizes

Porous polyimide (PI) thin films have been fabricated successfully by using silica microparticles (MPs) with different particle sizes as a template, and the three dimensional porous structure has been observed clearly by mean of transmission electron microtomography. The measured values of dielectric constant (ε) decreased remarkably by introducing a porous structure, compared with ordinary PI thin films. Especially, the porosity evidently increased (ca. 10%) by suitably employing silica MPs having different particle sizes, and then the value of ε was further reduced. Bruggeman model could reproduce nearly the relationship between the value of ε and porosity.