Pure Polyimide Research Articles

Enhanced tribological properties of polyimide composite with carbon fiber/graphitic‐carbon nitride nanosheets

AbstractDeveloping polymer self‐lubricating composites is of great significance for reducing mechanical friction and wear. In this study, a hybrid of graphitic carbon nitride (g‐C3N4) nanosheets anchored carbon fiber (CF) was constructed through one‐step calcination method to enhance the tribological performance of polyimide (PI). The presence of g‐C3N4 improved the interfacial interaction of CF/PI composite by endowing CF surface with active sites and rough microstructure. Tribological test results demonstrated that CF/g‐C3N4/PI composite had outstanding antifriction and wear resistance, with friction coefficient and wear rate of 0.209 and 2.23 × 10−7 mm3/Nm, which were reduced by 21% and 73.52% compared to pure PI, respectively. This was mainly attributed to the enhanced interfacial interaction, which was conducive to the stress transferring from PI to CF/g‐C3N4 in sliding process. At the same time, the g‐C3N4 nanosheets could give the composite excellent self‐lubrication property by promoting the formation of transfer film. More importantly, the as‐proposed CF/g‐C3N4/PI composite still maintained good self‐lubricating performance in different sliding environment, with a friction coefficient of 0.132 in water environment and 0.049 in oil environment, suggesting it had a broad prospect in polymer self‐lubricating composite. In addition, the corresponding mechanism was discussed based on the analysis of worn surface of the composite and counterpart.

Preparation and performance study of ionic liquid @ boron nitride modified polyimide proton exchange membrane

AbstractProton exchange membrane (PEM) plays a relatively significant part in PEM fuel cells (PEMFCs). In this study, ionic liquid were grafted onto boron nitride (ILs@BN) were prepared by wet ball milling, and the polyimide (PI) composite films with different contents (1, 2, 3, 4, and 5 wt%) of ILs@BN were fabricated by solution‐casting method. Results showed that compared with the original polyimide films, the performance of the obtained composite films has been significantly improved. Tensile strength of film containing 4 wt% ILs@BN was enormously enhanced by 93.62% (from 47 to 91.8 MPa), respectively, in contrast to pure PI film. Compared with pure PI film, film containing with 4 wt% ILs@BN was reduced from 71.2° to 63.2°, which indicated the better hydrophilicity of the film and potential proton transport channels. The composite film exhibited excellent proton conductivity, which was tremendously elevated by 58.35% (from 0.1215 S cm−1 for pure PI to 0.1924 S cm−1 for film containing with 4 wt% ILs@BN). Comprehensively, the film containing with 4 wt% ILs@BN presenting excellent mechanical properties (91.8 MPa), contact angle (63.2°) and proton conductivity (0.1924 S cm−1) demonstrating strong potential applications in fuel cells.

Grafting Polyethyleneimine-Poly(ethylene glycol) Gel onto a Heat-Resistant Polyimide Nanofiber Separator for Improving Lithium-Ion Transporting Ability in Lithium-Ion Batteries.

To improve the lithium-ion transporting ability in lithium-ion batteries, a high-performance polyimide-based lithium-ion battery separator (PI-mod) was prepared by chemically grafting poly(ethylene glycol) (PEG) onto the surface of a heat-resistant polyimide nanofiber matrix with the assistance of amino-rich polyethyleneimine (PEI). The resulted PEI-PEG polymer coating exhibited unique gel-like properties with an electrolyte uptake rate of 168%, an area resistance as low as 2.60 Ω·cm2, and an ionic conductivity up to 2.33 mS·cm-1, which are 3.5, 0.10, and 12.3 times that of the commercial separator Celgard 2320, respectively. Meanwhile, the heat-resistant polyimide skeleton can effectively avoid thermal shrinkage of the modified separator even after 200 °C treatment for 0.5 h, which ensures the safety of the battery working under extreme conditions. The modified PI separator possessed a high electrochemical stability window of 4.5 V. Compared with the batteries from the commercial separator Celgard 2320 and the pure polyimide matrix, the assembled coin cell with the PI-mod separator showed much better rate capabilities and capacity retention due to the high electrolyte affinity of the PEI-PEG polymer coating. The developed strategy of using the electrolyte-swollen polymer to modify the thermal-resistant separator network provides an efficient way for establishing high-power lithium-ion batteries with good safety performance.

Comprehensive properties study of polyimide with fluorinated nanofillers

AbstractThere is an increasing demand for polyimide (PI) with excellent mechanical properties, good heat resistance, and low dielectric constant in integrated circuits and advanced packaging. Based on this, we report a strategy of tailoring the PI via fluorinated nanofillers modification to improve its mechanical properties, heat resistance, hydrophobicity, and reduce its dielectric constant. Specifically, different fluorinated nanofiller, including fluorinated graphene (FG), fluorinated carbon nanotubes (FCNT), fluorinated carbon black (FCB), and fluorinated graphene quantum dots (FGQD) were introduced into PI matrix to prepared composite films by a simple method of in‐situ polymerization, and the effect of different nanofillers on the properties of polyimide was systematically investigated. The results showed that the introduction of fluorinated nanofillers significantly improved the comprehensive performance of the composite films, especially the FGQD. FGQD/PI composite film with 0.5 wt% FGQD exhibits superior tensile strength (145.8 MPa), Young's modulus (2.83 GPa) and elongation at break (45.4%) the best comprehensive performance which are 10.0%, 19.4%, and 32.0% of those of pure PI, respectively. Most importantly, the FGQD/PI composite film shows decreased dielectric constant (from 3.38 to 2.93) at 1 MHz improved hydrophobicity and excellent heat resistance. The FGQD/PI composite film exhibits excellent comprehensive performance, thereby showing the potential in the future development of advanced packaging and microelectronics industry.

An enhanced MEMS-based polyimide capacitive-type relative-humidity sensor with halloysite nanotube as a modifier

Polymers have been widely used in the fabrication of humidity sensing materials, although, issues including sensitivity and hysteresis still exist, limiting its potential. Herein, we aim to enhance the humidity sensing performances of a polyimide (PI) capacitive-type relative-humidity sensor via a simple and efficient approach. Specifically, modified halloysite nanotubes (HNTs) were doped in the polyimide sensing layer to optimize the humidity sensing capabilities of the MEMS (Micro-electromechanical Systems)-based sensor. The configurational and morphological characteristic of the sensing membrane with different content of the nanofiller were characterized. The humidity sensing capabilities of the enhanced sensors were investigated under the relative humidity (RH) range of 10–90%. Owing to the rational structure design of the sensing layer, the modified sensor exhibited enhanced sensitivity, low humidity hysteresis and good linearity. The highest sensitivity of the device with the best overall performance reached 0.87 pF/%RH, which is 1.81 times that of the pure PI based humidity sensor. Meanwhile, the sensor maintained moderate humidity hysteresis (2.18%) as well as good linearity (R2 = 0.9813) within the humidity boundary from 10 to 90% RH. The aforementioned advantages of the PI-HNTs-NH2 based humidity sensor demonstrate its promising application in humidity measurement. As a low-cost and effective additive, HNTs own great application potential in the preparation of polymer sensing devices. This work provides valuable guidance for fabricating humidity sensors with outstanding sensing performances.

Fabrication of Ultralarge Curvature Folding-Resistant Polyimide Films by Widening the Elastic Range and Intermolecular Slippery Spacing

As the electrons advanced toward foldable and lightweight characteristics, fabricating polyimide (PI) substrates resisting large curvature folding under an ultralow folding radius posed a significant challenge for flexible electronics. Here, one polyimide film (PICu) was fabricated by introducing Cu2+ and benzimidazole, whose elastic range and intermolecular slippery spacing were widened thanks to the point-to-face multi-coordination structure with larger attraction distance and variable bond angles, and it was fortunately equipped with greatly enhanced folding resistance even under an ultralarge curvature. First, the folding radius was confirmed to have an exponential impact on the tested folding reliability, as folding times of pure PI to form creases decreased sharply from 300,000 to 3000 with the folding radius dropping from 2 to 0.25 mm. In contrast, no crease was formed on PICu films even after folding over 200,000 times under an ultralow folding radius of 0.25 mm. Importantly, the folding radius was almost 10 times smaller than that in current reports (2–3 mm). In addition, the spread angle of PICu films after static folding at 80 °C was 50–78% larger than that of pure PI films with the folding radius decreased from 2 to 0.25 mm, demonstrating the excellent dynamic and static folding resistance of PICu films under an ultralarge curvature.

Ultralarge Curvature Folding Resistance of Polyimide Films Based on Spring-Like Folding Segments.

With the trend toward lighter and thinner flexible electronics, developing foldable polymeric substrates that can withstand ultralow folding radiuses has become an urgent issue. Here, a strategy to develop polyimide (PI) films with excellent dynamic and static folding resistance under an ultralarge curvature through copolymerizing one "unidirectional diamine" with classic PMDA-ODA PIs to achieve a kind of folding-chain PI (FPI). It was theoretically and experimentally confirmed that the spring-like folding structure equipped PI films with an enhanced elastic behavior and thus an excellent ability to endure a large curvature. Among them, FPI-20 did not show any crease even after folding over 200000 times under a folding radius of 0.5 mm, while creases were observed on pure PI film only after folding 1000 times. It is noteworthy that the folding radius was almost 5 times smaller than that in current reports (2-3 mm). Meanwhile, the spread angle of FPI-20 films after static folding at 80 °C under a 0.5 mm folding radius get 51% larger than that of films, showing the remarkable static folding resistance.

High Out-of-Plane Thermally Conductive Polyimide Film for Electronic Thermal Management by the Incorporation of Core–Shell Ag/Polyimide Microparticles

Polyimide (PI) films with high out-of-plane thermal conductivity are highly desirable for effectively alleviating thermal failure in advanced flexible electronics. However, it is still a great challenge to achieve high out-of-plane thermal conductivity with a low filler content by a simple method. Herein, an Ag-coated PI microsphere (PIM@Ag) as a thermally conductive filler with a considerable heat-transfer path in the out-of-plane direction is prepared to improve the out-of-plane thermal conductivity of the PI film. Because of the introduction of thermal welding between Ag nanoparticles during thermal imidization further eliminating the interface thermal resistance, the resultant PI/PIM@Ag composite film exhibits a high out-of-plane thermal conductivity of 1.2 Wm–1K–1 with an ultralow Ag content of 6.08 wt % and a superior tensile strength of 124.0 MPa compared to 78.9 MPa of a pure PI film. Practical application and comsol simulation results verify that the PI/PIM@Ag composite film has excellent heat dissipation ability for a chip. This work provides an idea for improving the out-of-plane thermal conductivity of polymers with low filler contents.

Synthesis and characterization of new nanocomposites-based polyimide/modified montmorillonite and their applications in gas separation

ABSTRACT This study reports synthesis of nanocomposite materials-based polyimide (PI) matrix and reactive modified montmorillonite (m-MMT) as a reinforcing agent. New diamine and PI derived from amino acid (L-phenylalanine) fractions in the structure as pendant group were synthesized. The best dispersion of m-MMT was observed for PI/m-MMT 3% nanocomposites, which resulted in the best mechanical performance with an increase of 56% in tensile modulus and 14% in yield strength, respectively compared to that of pure PI. The maximum ideal selectivity achieved was 15.89 with 3% loading at 4 bar pressure which is 42.52% higher than neat PI membrane.

Investigation on corrosion and mechanical behavior of sintered electrical corrosion resistance glass-reinforced polyimide composites under acidic environment

ABSTRACT In this study, the effects of electrical corrosion resistance (ECR) glass fiber on corrosion and mechanical behavior of polyimide (PI) matrix composites prepared using spark plasma sintering (SPS) process were investigated. The morphologies, corrosion properties, and mechanical properties were characterized using scanning electron microscope (SEM), immersion and polarization tests, and nanoindentation tests, respectively. The SEM results revealed that the ECR glass particles were evenly distributed in the PI matrix. The resulting analysis depicts that the introduction of ECR particles contents (5, 10, and 15 wt%) in the PI matrix improved its hardness and elastic modulus both before and after corrosion. The corrosion rate of the pure PI was recorded to be reduced by 61.8%, 50.4%, and 38.9%, respectively, at 5, 10, and 15 wt% loading of ECR particles. This depicts that the ECR glass particles lead to anti-corrosion performance improvement of the pure PI. Finally, the findings suggest the potential of ECR filler materials in the development of novel PI composites for anti-corrosion and mechanical load-bearing applications.

Noncovalently functionalized boron nitride nanotubes and polymer nanocomposites with water-soluble poly (amic acid) salt

In this study, polyamic acid salt-BNNTs (PAS-BNNTs) were prepared by noncovalently functionalizing water-soluble PAS with improved dispersion stability in organic solvents, and high compatibility with polyimide (PI) nanocomposites. We show that BNNTs functionalized with PAS have enhanced mechanical properties that are influenced by various loadings of PAS-BNNT/PI films through in situ polymerization. Compared with the pure PI film, the tensile strength and tensile modulus increased by about 20% and 37%, respectively. The proposed system also improved the coefficient of thermal expansion (CTE) from 52.6 to 45.7 ppm/oC. The enhanced mechanical properties of the PI nanocomposite were due to the interfacial bond strength enhanced by the reinforcing effect of PAS-BNNTs. Our study demonstrates the potential of PAS-BNNTs for preparing BNNT/PI nanocomposites with various structures.

In Situ Fabrication of High Dielectric Constant Composite Films with Good Mechanical and Thermal Properties by Controlled Reduction

As a common two-dimensional carbon material, graphene has been widely doped into polymers to prepare high-performance dielectric materials. However, the shortcomings of graphene, such as large specific surface area and poor dispersion, limit its further application. Therefore, in this work, to solve the problem regarding the uniform dispersion of graphene in the matrix, in situ polymerization was used to prepare graphene/polyimide films, in which 1,4-diiodobutane was used as a reduction agent to prevent the aggregation of graphene oxide (GO) during imidization. High dielectric constant composite films were obtained by adjusting the ratio of 1,4-diiodobutane in GO. The results show that the resulting graphene/polyimide composite film possessed a dielectric constant of up to 197.5, which was more than 58 times higher than that of the polyimide (PI) film. Furthermore, compared to the pure PI film, the composite films showed better thermal stability and mechanical properties. Thermal performance tests showed that the 1,4-diiodobutane added during the preparation of the composite film was thermally decomposed, and there was no residue. We believe our preparation method can be extended to other high dielectric composite films, which will facilitate their further development and application in high power density energy storage materials.

Construction of all-organic low dielectric polyimide hybrids via synergistic effect between covalent organic framework and cross-linking structure

Polyimide (PI) is a promising electronic packaging material, but it remains challenging to obtain an all-organic PI hybrid film with decreased dielectric constant and loss without modifying the monomer. Herein, a series of all-organic PI hybrid films were successfully prepared by introducing the covalent organic framework (COF), which could induce the formation of the cross-linking structure in the PI matrix. Due to the synergistic effects of the COF fillers and the cross-linking structure, the PI/COF hybrid film containing 2 ​wt% COF exhibited the lowest dielectric constant of 2.72 and the lowest dielectric loss (tan δ) of 0.0077 ​at 1 ​MHz. It is attributed to the intrinsic low dielectric constant of COF and a large number of mesopores within the PI. Besides, the cross-linking network of PI prevents the molecular chains from stacking and improves the fraction of free volume (FFV). The molecular dynamics simulation results are well consistent with the dielectric properties data. Furthermore, the PI/COF hybrid film with 5 ​wt% COF showed a significant enhancement in breakdown strength, which increased to 412.8 ​kV/mm as compared with pure PI. In addition, the PI/COF hybrid film achieve to reduce the dielectric constant and thermal expansion coefficient (CTE). It also exhibited excellent thermal, hydrophobicity, and mechanical performance. The all-organic PI/COF hybrid films have great commercial potential as next-generation electronic packaging materials.

Fabrication, Facilitating Gas Permeability, and Molecular Simulations of Porous Hypercrosslinked Polymers Embedding 6FDA-Based Polyimide Mixed-Matrix Membranes.

Novel polymers applied in economic membrane technologies are a perennial hot topic in the fields of natural gas purification and O2 enrichment. Herein, novel hypercrosslinked polymers (HCPs) incorporating 6FDA-based polyimide (PI) MMMs were prepared via a casting method for enhancing transport of different gases (CO2, CH4, O2, and N2). Intact HCPs/PI MMMs could be obtained due to good compatibility between the HCPs and PI. Pure gas permeation experiments showed that compared with pure PI film, the addition of HCPs effectively promotes gas transport, increases gas permeability, and maintains ideal selectivity. The permeabilities of HCPs/PI MMMs toward CO2 and O2 were as high as 105.85 Barrer and 24.03 Barrer, respectively, and the ideal selectivities of CO2/CH4 and O2/N2 were 15.67 and 3.00, respectively. Molecular simulations further verified that adding HCPs was beneficial to gas transport. Thus, HCPs have potential utility in fabrication of MMMs for facilitating gas transport in the fields of natural gas purification and O2 enrichment.

Zero‐temperature‐coefficient of capacitance in BaTiO3/PI composite films using paraffin as barrier layer

AbstractFirstly, the BaTiO3 nanoparticles were coated with a layer of paraffin, forming a core shell structure. Then, as‐prepared paraffin@BaTiO3 nanofillers were directly blended with polyimide (PI) prepolymer to fabricate paraffin@BaTiO3/PI composite films. The microstructure, thermal, and dielectric properties were investigated in detail. The surface‐modified paraffin@BaTiO3 nanoparticles exhibited a typical core shell structure with uniform and complete coverage. The char yield of the composite film with 10 wt% paraffin@BaTiO3 increased from 56.40% to 70.73%. It is obvious that the paraffin@BaTiO3 nanofillers have a positive effect on ablative performance of the composite film. Compared with pure PI, the εr of paraffin@BaTiO3/PI composites with 40% mass fraction increased from 3.41 to 9.35 (1 kHz). The tanδ of paraffin@BaTiO3/PI composites was just 0.0065 (1 kHz) when the filler loading was 40 wt%. Temperature dependency of the permittivity of the composites reveals a zero‐temperature‐coefficient from 80°C to 180°C.

2D Mo2CTx Reinforced Polyimide Nanocomposites for Enhancement of Surface Tribological Performance at Elevated Temperatures

AbstractIn order to improve the high‐temperature friction and wear properties of polyimide (PI), Mo2CTx/PI nanocomposite materials are prepared by using 2D Mo2CTx MXene as reinforcement. Compared with the properties of pure PI, Mo2CTx/PI nanocomposites exhibit significant improvement on thermal stability, hardness, and tribological performance. Especially, the addition of Mo2CTx can reduce friction coefficient and wear rate of PI significantly at different temperatures ranging from room temperature (RT) to 200 °C. The hardness and the thermo‐degradation temperature of composites increase due to the formation of an organic inorganic structure. In addition, this structure helps to form a protective film containing Mo2CTx nanoflakes, iron oxide, molybdenum oxide, and organic compounds during friction, and improve the tribological properties of PI composite.

Enhanced out-of-plane thermal conductivity of polyimide composite films by adding hollow cube-like boron nitride

Boron nitride nanosheet (BNNS) is widely used in electronic thermal management due to its excellent planar thermal conductivity and insulating properties. However, it is challenging to improve the out-of-plane thermal conductivity of BNNS-doped composites due to the anisotropy of the thermal conductivity of BNNS. Therefore, the BNNS in the matrix must be oriented to obtain composites with high out-of-plane thermal conductivity. In this study, BNNS powders with directional structures were synthesized directly using sodium chloride templates. The as-obtained BNNS powders have a unique hollow cube-like structure with an ultra-low density of 2.67 × 10−2 g/cm3 and nearly 8 times the volume of the same mass of two-dimensional (2D) BNNS, making it easy to form the out-of-plane thermal conductivity paths in the polymer matrix. In addition, the high out-of-plane thermal conductivity of 4.93 W m−1 K−1 at 23.3 wt% loadings was obtained by doping it into a polyimide (PI) matrix. This value is 9.7 times higher than that of 2D BNNS-doped PI at the same loadings, 17.6 times higher than pure PI, and 6.1 times higher than the thermally conductive PI film sold by DuPont. Therefore, the prepared composite film has great potential for application in electronic thermal management.

Thermal, Mechanical and Dielectric Properties of Polyimide Composite Films by In-Situ Reduction of Fluorinated Graphene.

Materials with outstanding mechanical properties and excellent dielectric properties are increasingly favored in the microelectronics industry. The application of polyimide (PI) in the field of microelectronics is limited because of the fact that PI with excellent mechanical properties does not have special features in the dielectric properties. In this work, PI composite films with high dielectric properties and excellent mechanical properties are fabricated by in-situ reduction of fluorinated graphene (FG) in polyamide acid (PAA) composites. The dielectric permittivity of pure PI is 3.47 and the maximum energy storage density is 0.664 J/cm3 at 100 Hz, while the dielectric permittivity of the PI composite films reaches 235.74 under the same conditions, a 68-times increase compared to the pure PI, and the maximum energy storage density is 5.651, a 9-times increase compared to the pure PI films. This method not only solves the problem of the aggregation of the filler particles in the PI matrix and maintains the intrinsic excellent mechanical properties of the PI, but also significantly improves the dielectric properties of the PI.

Attapulgite Nanorod-Incorporated Polyimide Membrane for Enhanced Gas Separation Performance.

Polyimide (PI) membrane is an ideal gas separation material due to its advantages of high designability, good mechanical properties and easy processing; however, it has equilibrium limitations in gas selectivity and permeability. Introducing nanoparticles into polymers is an effective method to improve the gas separation performance. In this work, nano-attapulgite (ATP) functionalized with KH-550 silane coupling agent was used to prepare polyimide/ATP composite membranes by in-situ polymerization. A series of characterization and performance tests were carried out on the membranes. The obtained results suggested a significant increase in gas permeability upon increasing the ATP content. When the content of ATP was 50%, the gas permeability of H2, He, N2, O2, CH4, and CO2 reached 11.82, 12.44, 0.13, 0.84, 0.10, and 4.64 barrer, which were 126.87%, 119.40%, 160.00%, 140.00%, 150.00% and 152.17% higher than that of pure polyimide, respectively. No significant change in gas selectivity was observed. The gas permeabilities of membranes at different pressures were also investigated. The inefficient polymer chain stacking and the additional void volume at the interface between the polymer and TiO2 clusters leaded to the increase of the free volume, thus improving the permeability of the polyimide membrane. As a promising separation material, the PI/ATP composite membrane can be widely used in gas separation industry.

Influence of metallization on the mechanical properties of polyimide films

The need to obtain and study the properties of metallized polymeric materials is associated with their wide application in space exploration, electronics, and the production of consumer goods. The main directions in the search for new materials are aimed at obtaining composite films with different distributions of the metal phase over the thickness while maintaining the mechanical characteristics. The most promising metallized polymer materials are polyimides with high thermal stability, good dielectric properties, and low expansion coefficient. Therefore, the study of viscosity, strength, and other mechanical characteristics of metallized polymeric materials, the determination of the effect of the coating structure on their mechanical behavior, and the development of methods for predicting their behavior are currently relevant and important tasks. Methods for improving the mechanical properties of polyimide films by silver plating have been studied. The thickness of the metal layer of the polyimide base, which is part of the structure, ranged from 1 to 5 μm. Surface coatings were 80–97% of comparable silver mirrors in the visible and infrared regions. The deformation of the samples was carried out at room temperature on an Instron 5982 universal testing machine in the uniaxial tension mode. As a result of the metallization of the films, a significant increase in mechanical characteristics was observed compared to a pure polyimide film. An increase in ultimate strength by ∆σ = 105 MPa and plasticity by ∆ε = 75% was obtained, which is associated with a change in the structure of metallized films and the conditions of their chemical treatment.