Properties Of Polyimides Research Articles

Estimation of transverse thermoelastic properties of polyimide fibers based on micromechanical models

The determination of the thermoelastic properties of polyimide (PI) fibers is important for their applications, however, these properties are difficult to measure directly, especially the transverse thermoelastic properties. Here, the transverse thermoelastic properties of PI fibers, including transverse Young's modulus (2.12 GPa), shear modulus (0.94 GPa), Poisson's ratio (0.05), and coefficient of thermal expansion (45.68 μm/m°C), were estimated by combining dynamic and static thermo-mechanical techniques, as well as various relevant micromechanical models. The transverse Young's modulus of PI fibers was only 1/46th of the longitudinal one, and the transverse coefficient of thermal expansion of PI fibers was positive, unlike the longitudinal one, which was negative, showing the typical anisotropy of PI fibers. Finally, the thermoelastic properties of the PI fibers were in turn used to predict the thermoelastic behavior of the PI fiber-reinforced composites, thus validating their effectiveness.

Investigation of the mechanical, thermal and dielectric properties of electrical corrosion resistance glass reinforced polyimide-based composite produced by spark plasma sintering

Polyimide polymer matrix composites have gained numerous attentions in the design and fabrication of components for automobile, aerospace, and electronics applications as a result of their outstanding properties. Composites are currently replacing polymers as structural, insulation and dielectric materials, and the improvement of mechanical, thermal, and dielectric properties of polyimide (PI) becomes important. Thus, in this paper, the enhancement of the mechanical, thermal stability, and dielectric properties of PI composites with boron-free E-glass (electrical corrosion resistance (ECR) glass) reinforcements (0, 5, 10, and 15 wt%) using Spark Plasma Sintering (SPS) process is presented. X-ray diffractometer, scanning electron microscope (SEM), nanoindentation tests, thermogravimetric analyser, and LCR meter tester were used to investigate the morphology, mechanical, thermal, and dielectric properties of the developed composites, respectively. The SEM results revealed that the ECR-glass powder particles were well dispersed in the PI matrix. The introduction of the ECR into the PI matrix, improves its hardness, stiffness, and modulus. From the obtained results, a notable improvement in its hardness (9.36%), stiffness (30.77%), and modulus (43.58%) was revealed for ECR/PI loaded with 15 wt% ECR compared to pure PI. The thermal stability of the PI composites was observed to be improved with the incorporation of the ECR-glass particles. The heat resistance index of the PI was recorded to be improved by 3.8% at 15 wt% ECR incorporation. Furthermore, the composites depicted low dielectric constant and dissipation factor. In consideration, the obtained properties make the composites promising for mechanical load-bearing, thermal management and electronics applications.

Impact of ring structure and conjugation on the dielectric properties of polyimides at a high frequency of 10–40 GHz

AbstractPolyimides (PIs) are widely used materials due to their high thermal stability and potentially low dielectric constant (Dk) and dissipation factor (Df). However, Dk and Df values tend to increase at higher frequencies. Therefore, there is a significant need to reduce the Dk and Df values of PIs, which is the main purpose of this study. This research considers multiple factors, including the ring structures and conjugations on the conformation of the PI backbone. The effects of five‐membered and six‐membered PIs and the conjugated length of benzene and naphthalene structures in the repeating units on the thermomechanical and dielectric properties are systematically investigated. As a result, the PI with a six‐membered imide contributes to higher thermal stability, with a glass transition temperature higher than 350 °C, a thermal decomposition point of 529 °C, and a lower Dk of 2.7–2.8. Still, it exhibits a lower molecular weight than that of a five‐membered imide. On the other hand, PIs with esterified structures show an extremely low Df value, 0.002–0.005. This situation can be attributed to the vital correlation between the Df and the volumetric dipole moment. This study also presents the different properties arising from ring structures and conjugations. Meanwhile, the relationships between Dk and volumetric polarizability and between Df and volumetric dipole moment are confirmed. This work provides a guideline for the structure−dielectric relationship of PI at a high frequency of 10–40 GHz.

Constructing Novel High Dielectric Constant Polyimides Containing Dipolar Pendant Groups with Enhanced Orientational Polarization.

Polymer dielectrics with high dielectric constant are urgently demanded for potential electrical and pulsed power applications. The design of polymers with side chains containing dipolar groups is considered an effective method for preparing materials with a high dielectric constant and low loss. This study synthesizes and comprehensively compare the dielectric properties of novel polyimides with side chains containing urea (BU-PI), carbamate (BC-PI), and sulfonyl (BS-PI) functional groups. The novel polyimides exhibit relatively high dielectric constant and low dielectric loss values due to the enhanced orientational polarization and suppressed dipole-dipole interactions of dipolar groups. In particular, BU-PI containing urea pendant groups presents the highest dielectric constant of 6.14 and reasonably low dielectric loss value of 0.0097. The strong γ transitions with low activation energies derived from dielectric spectroscopy measurements have been further evaluated to demonstrate the enhanced free rotational motion of urea pendant dipoles. In energy storage applications, BU-PI achieves a discharged energy density of 6.92 J cm-3 and a charge-discharge efficiency above 83% at 500 MV m-1. This study demonstrates that urea group, as dipolar pendant group, can provide polymers with better dielectric properties than the most commonly used sulfonyl groups.

High-temperature dielectric energy storage films with self-co-assembled hot-electron blocking nanocoatings

Polymer thin films operable under concurrent electric and thermal extremes represent critical building blocks of capacitive energy storage and electrical isolator for modern power and electronic systems with ever-increasing demands for power density and payload efficiency. However, polymer dielectrics are prone to fast aging under high fields due to hot electrons injected from electrodes. Especially, performance high-heat polymers such as polyimides with high aromaticity suffer fast aging induced by non-thermalized electrons even at moderate fields due to their intrinsically low bandgap and injection barrier. Herein, a facile, low-cost, and scalable interface-engineering approach utilizing the highly ordered organic/inorganic layered nanocoatings is reported, which serve as a retrofittable solution to break this design constraint. By probing the energetic modes of transport and aging at pre-breakdown field, we demonstrate that our 2D montmorillonite (MMT) self-co-assembly nanocoatings can effectively boost the dielectric properties of substrate polyimide (PI) film by suppressing the charge injection and shifting the fast mode of hot-electron aging to a slow, ultimately thermalized process. This aging-impeding scheme imparts PI films with an exceptional endurance capability (enhanced by 100 MV/m) and a 6× improved charge-discharge efficiency at an elevated temperature of 175 °C. The nanostructured interface engineering disclosed in this work thus opens a new pathway of boosting the performance of a spectrum of high-heat polymer dielectrics already commercially available in thin gauges of films for applications in zero-emission electric aircraft and renewable energy integration.

Polyimide aerogel/aramid fiber composite with high mechanical strength and thermal insulation for thermal protective clothing

Polyimide (PI) aerogel, as a new organic aerogel material, has the excellent thermal properties of polyimide and the characteristic of high thermal insulation of aerogels, and has gained increasing attention. In this work, using PI aerogel as the matrix material and aramid fiber as the reinforcement material, controllable flexible PI aerogel/aramid fiber composite insulation materials were successfully prepared by freeze-drying and soft treatment. This study sought to determine how the mass percentage of PI aerogel affected the microstructure, mechanical characteristics, thermal insulation capabilities, and thermal comfort of clothes in PI aerogel/aramid fiber composites. To achieve this, the preparation process of PI aerogel was optimized, and the effects of different mass fractions of PI aerogel on the properties of the composite material were evaluated. The results demonstrated that increasing the mass fraction of PI aerogel led to improvements in the mechanical properties, flexibility, and heat insulation properties of the composite material. Furthermore, the PI aerogel/aramid fiber composite offered enhanced thermal comfort to the wearer in hot and humid environments, indicating that the composite material is particularly suited for thermal insulation applications.

Challenge for Trade-Off Relationship between the Mechanical Property and Healing Efficiency of Self-Healable Polyimide.

Polyimide is actively applied in various industrial fields because of its strong mechanical properties, owing to the interactions between the polymer chains. Fully aromatic imide structures exhibit high glass-transition temperatures due to the strong interactions between their chains, which hinder chain mobility. Therefore, preparing a material that exhibits self-healing at a low temperature of ≤100 °C and good mechanical properties is challenging. Thus, we prepared imides with four-component semiaromatic structures by adjusting the contents of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride and 4,4'-(4,4'-isopropylidenediphenoxy)bis(phthalic anhydride) to yield four-component self-healable colorless polyimides (f-SH-CPIs) with novel structures, flexibilities, good mechanical properties, and low healing temperatures. The flexibilities and distances between the polymer chains, as the basis of the trade-off relationship between the mechanical properties and healing efficiency, were controlled. These materials may be used as substrates in wearable devices and multilayer insulation that may protect from space dust, cosmic rays, and satellite fragments.

Fabrication of shape memory polyimides with exploration of their activation mechanisms

The construction of polymer-based materials with reversible bidirectional shape memory effects, good thermal and electrically controlled properties is of paramount importance for the development of intelligent exploration in high-temperature fields. By adjusting the hydrogen bond contents of the structure to regulate the intermolecular strength, a series of random shape memory copolyimides have been synthesized with the triggering temperatures ranging from 275 to 384℃, which displayed good thermal stability, high shape fixation rate and shape recovery rate of over 95 % and 98 %, respectively, and excellent shape recovery speed (∼9 s). Visualized quantification of the microstructural phase evolution during stretching and the effects of hydrogen bonds on the properties of polyimide were deeply explored to better understanding the activation mechanisms of shape memory behaviors. Furthermore, flexible fabric composites with bidirectional shape memory effects motivated by electrical signals were also designed, providing a new sight of exploration for high-temperature shape-memory materials.

Tribological properties of polyimide coatings filled with graphene oxide and MXene composite nanofillers

By combining two different types of layered nanofillers and making full use of synergistic effects, the frictional properties of the coating can be further improved. In this study, GO-MXene composites with “sandwich” structures were successfully prepared. Compared with pure PI, the tensile modulus of PI/GO-MXene composite coating is increased by 18.45 %, the tensile strength is increased by 54.49 %, the wear rate is reduced by 91.6 %, and the coefficient of friction is reduced by 34.4%. This is mainly due to the “bridging effect” of the polydopamine (PDA) to form the GO-MXene-GO “sandwich” structure, which gives full play to the synergistic lubrication between GO and MXene.

Investigation of the mechanical properties of polyimide fiber/polyamide 12 composites printed by Multi Jet Fusion

ABSTRACT Multi Jet Fusion (MJF) has attracted extensive attention because of its ability to print support-free complex structures. However, the mechanical properties of MJF-printed polymer parts are still unsatisfactory for certain industrial requirements. Herein, by leveraging the fibre reinforcement effect and high specific strength of polyimide (PI) fibres, this work developed PI/polyamide 12 (PA12) composites with largely enhanced mechanical performance via MJF. Specifically, the tensile strength and modulus were increased by 43% and 42%, and the flexural strength and modulus were improved by 39% and 46%, respectively, compared to those of the neat PA12 parts. Furthermore, the incorporation of lightweight PI fibres endowed the composites with high specific tensile strength (67.60 kN·m/kg) and specific flexural strength (93.70 kN·m/kg), which are superior to those of MJF-printed PA12 composites reinforced with other fibres. This work provides new insights into enhancing the mechanical performance of lightweight parts printed by MJF and other powder-based techniques.

Influence of direct fluorination treatment on surface and optical properties of colorless and transparent polyimides

Due to excellent comprehensive performance, colorless and transparent polyimide (CPI) films are the greatest potential substrates for flexible displays, lighting, and solar cells. In this work, CPI films were modified by direct fluorination to increase the wettability and transmittance. The effects of fluorination duration on the chemical structure and surface physical morphology of CPI films were investigated in detail by attenuated total reflection infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The results show that, depending on the fluorination duration, direct fluorination causes significant changes in surface composition and structure. The F/C and O/C ratios of the samples were enhanced as the fluorination time increased. Surface microcrack increased at initially, then decreased as fluorination time increased. The fluorinated surfaces exhibit a high hydrophily and lipophobicity, and consequently a very high surface energy, according to contact angle measurements. In comparison to original CPI film, the fluorinated CPI films exhibited higher optical transmittance in the ultraviolet–visible light region compared with original CPI film. In addition, the fluorinated CPI films showed good thermal stability up to 500 °C and higher glass transition temperatures (Tg) compared with original CPI film.

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.

High-temperature mechanical responses of the polyimide at a high strain rate

Polyimide is one of the best super engineering materials in the world at present, and it is required to have high bearing capacity at specific temperature and strain rate because of the application environment. Therefore, the compressive mechanical properties of polyimide at different strain rates (10−3 s−1–4000 s−1) and temperatures (20 °C–350 °C) were researched. The experimental results show the sensitivity to temperatures and strain rates. However, the strain hardening rates decrease with the increase in temperature, and even the flow stress under dynamic compression is lower than that under quasi-static compression at large strain. In addition, it is found that the cracks show different characteristics under 4000 s−1 at different loading temperatures, which are tiny cracks at room temperature and deep and wide cracks due to the coupling of strain rate and thermal at 350 °C.

Effect of monomer moieties on the aggregate structure and gas transport properties of polyimides: Insights from experiments and simulations

AbstractTFDB‐based PIs (BTDA‐TFDB, PMDA‐TFDB and 6FDA‐TFDB) were prepared by the polymerization of 2,2′‐bis(trifluoromethyl) benzidine (TFDB) with three different dianhydrides, which contained flexible carbonyl bridged unit, rigid‐rod moiety without bridged unit, and bulky CF3 bridged unit in dianhydride moieties, respectively. The influence of dianhydride moieties on the aggregate structure and gas permeation performances of polyimide was systematically investigated in relation to their chain packing and free volumes, using the methods of molecular simulation, positron annihilation measurement and X‐ray diffractogram. Changing the dianhydride from benzophenonetetracarboxylic dianhydride (BTDA) to pyromellitic dianhydride (PMDA) and 4,4′‐(hexafluoroisopropylidene) diphthalic anhydride (6FDA), the gas permeation coefficients of polyimide increased, and the gas permeation selectivity (O2/N2, CO2/N2, H2/N2 and CO2/CH4) decreased. With the introduction of flexible carbonyl bridged unit into dianhydride moiety, the high rotational mobility favored tight chains packing and decreased free volumes, which in turn led to the lowest permeation coefficients of BTDA‐TFDB. The bulky CF3 bridged unit in dianhydride moiety greatly restricted chain packing and increased free volumes, thus resulting in the highest permeation coefficients of 6FDA‐TFDB. From BTDA‐TFDB to PMDA‐TFDB and 6FDA‐TFDB, the permeation selectivity decreased. This was mostly ascribed to the reduction of diffusive selectivity, which was resulted from the increase in free volumes.

Discovery of multi-functional polyimides through high-throughput screening using explainable machine learning

Polyimides have been widely used in modern industries because of their excellent mechanical and thermal properties, e.g., high-temperature fuel cells, displays, and aerospace composites. However, it usually takes decades of experimental efforts to develop a successful product. Aiming to expedite the discovery of high-performance polyimides, we utilize computational methods of machine learning (ML) and molecular dynamics (MD) simulations. Our study provides compelling evidence for the effectiveness of a data-driven approach in discovering novel polyimides. We first build a comprehensive library of more than 8 million hypothetical polyimides based on the polycondensation of existing dianhydride and diamine/diisocyanate molecules. Then we establish multiple ML models for the thermal and mechanical properties of polyimides based on their experimentally reported values, including glass transition temperature, Young’s modulus, and tensile yield strength. The obtained ML models demonstrate excellent predictive performance in identifying the key chemical substructures influencing the thermal and mechanical properties of polyimides. The use of explainable machine learning describes the effect of chemical substructures on individual properties, from which human experts can understand the cause of the ML model decision. Applying the well-trained ML models, we obtain property predictions of the 8 million hypothetical polyimides. Then, we screen the whole hypothetical dataset and identify three (3) best-performing novel polyimides that have better-combined properties than existing ones through Pareto frontier analysis. For an easy query of the discovered high-performing polyimides, we also create an online platform https://polyimide-explorer.herokuapp.com/ that embeds the developed ML model with interactive visualization. Furthermore, we validate the ML predictions through all-atom MD simulations and examine their synthesizability. The MD simulations are in good agreement with the ML predictions and the three novel polyimides are predicted to be easy to synthesize via Schuffenhauer’s synthetic accessibility score. Following the proposed ML guidance, we successfully synthesized a novel polyimide and the experimentally obtained high glass transition/thermal decomposition temperature demonstrated its excellent thermal stability. Our study demonstrates an efficient way to expedite the discovery of novel polymers using ML prediction and MD validation. The high-throughput screening of a large computational dataset can serve as a general approach for new material discovery in other polymeric material exploration problems, such as organic photovoltaics, polymer membranes, and dielectrics.

Short-side-chain regulation of colorless and transparent polyamide-imides for flexible transparent displays

Improving the heat resistance, dimensional stability and mechanical properties of colorless and transparent polyimides remains a popular and promising topic as it is considered to be the basis for their application in next-generation flexible displays. However, enhancing these properties without sacrificing optical transparency is challenging. Although recent research has revealed that hydrogen bonding is a potential route to balance optical and other physical properties, no systematic studies or comparisons have been performed. Here, we synthesized five amide-containing diamines with different substituents and prepared the corresponding polyamide-imides (CPAIs). Short-chain substituents were introduced to regulate the conformation between benzene and imide rings, as well as the ability of amide hydrogen bond formation. The effects of the position and number of –CH3 and/or -CF3 substituents on blocking the formation of charge-transfer complexes and on the strength of intermolecular hydrogen bonds, which affect CPAI transparency and other properties, were investigated. The results demonstrated that the combination of the twisted conformation from ortho-methyl groups and hydrogen bonds from the amide promoted the optical, thermal and mechanical properties of the CPAIs. In particular, CPAI-7 exhibited a high optical transmittance at 400 nm of up to 86 %, a Tg as high as 402 °C, a low thermal expansion coefficient of 17 ppm K−1 and an extremely high tensile modulus of 5.2 GPa. The excellent and comprehensive performance of the CPAI films illustrates their potential application in the field of flexible displays.

Improving Thermal Conductivity and Tribological Performance of Polyimide by Filling Cu, CNT, and Graphene

The thermal conductivity, mechanical, and tribological properties of polyimide (PI) composites filled by copper (Cu), carbon nanotube (CNT), graphene nanosheet (GNS), or combination were investigated by molecular dynamics simulation (MD). The simulated results suggested that Cu can improve thermal stability and thermal conductivity, but it reduces mechanical properties and tribological properties. CNT and GNS significantly improved the thermal and tribological properties at low content, but they decreased the properties at high content. In this study, the modification mechanism, friction, and wear mechanism of different fillers on polyimide were revealed by observing the frictional interface evolution process from the atomic scale, extracting the atomic relative concentration, the temperature and velocity distribution at the friction interface, and other microscopic information.

Sulphur-containing phenylethynyl terminated polyimide via chemical crosslinking method for excellent thermal properties and antibacterial performance

Thermal stability and antibacterial property of polyimide have emerged as two significant property parameters when used in the aerospace field. In this work, sulphur-containing linkage is introduced into the polyimide by using phenylethynyl terminated imide oligomers (PETI) and sulphur agent. The curing properties of this new type of thermosetting polyimide were studied by Fourier transform infrared (FTIR) and Raman spectra, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and thermogravimetric analyzer (TGA). The sulphur-containing polyimide shows a unique thermoplastic-like glass transition behavior rather than typical thermoset behavior. Its film material shows the tensile strength and modulus of 59.46 MPa and 1767 MPa respectively, which have increased to 187.68% and 155.16% when compared with the control sample cured at 381 °C. And the mechanical retention rates of tensile strength and modulus reach 96.8% and 82.1% respectively after 371 °C/100 h aging. In addition, it shows potential value in the fabrication of carbon fiber reinforced composite and antibacterial materials. Though the curing temperature of PETI oligomer decreases from 381 °C to 300 °C, the prepared PI/CF composite shows excellent mechanical performance, and the interlaminar shear strength, the flexural strength, and the flexural modulus reach 104.53 MPa, 1846.71 MPa and 129.24 GPa, respectively. Also, the novel polyimide shows excellent antibacterial performance, with antibacterial retention rate to negative E. coli and positive S. aureus of 90.4% and 89.1%, respectively.

Mechanically strong polyimide aerogels containing benzimidazole groups with excellent flame-retardant, thermal insulation and high service temperature

The poor mechanical properties of polyimide (PI) aerogels made from biphenyl-3,3′,4,4′-tetracarboxylic dianydride (BPDA) and 4,4′-oxidianiline (ODA) limit their application. In this paper, an aromatic heterocyclic diamine, 5-amino-2-(4-aminophenyl)benzimidazole (PABZ) was introduced into the BPDA/ODA backbone to improve the comprehensive performance. The obtained PI aerogels represented excellent mechanical properties. The compressive Young’s modulus was up to 53.9 MPa and the stress at 10 % strain was as high as 2.39 MPa. Meanwhile, the tensile Young’s modulus ranged from 234 to 464 MPa and the tensile strength was up to 2.40 MPa. At the same time, all formulations of PI aerogels possessed outstanding thermal insulation, superb flame resistant and self-extinguishing performances, promising to be good candidates for special protective materials. Furthermore, the correlation between changes in morphology and structure resulting from the introduction of PABZ and PI aerogel properties was systematically analyzed.

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.