Pure Polyimide Research Articles

A polyimide separator inlaid by methylsilyl-modified silica aerogels for high thermal safety and low polarization lithium-ion battery

The separator is a crucial component of lithium-ion batteries, playing a vital role in ion transfer and preventing polarization of the battery. To solve the problem of poor thermal safety of separators in lithium-ion batteries, in this work, three types of hydrophobic silica aerogels are compared. A suitable separator is fabricated by adding hydrophobic silica aerogels with monomethylsilyl as a modifier to pure polyimide. The aerogel composite separator displays a highly cross-linked three-dimensional nanoporous network, a low thermal shrinkage rate (<4 % at 200 °C), and high thermal stability (peak temperature of 670 °C) and oxygen index (>45 %). In addition, owing to the introduced hydrophobic functional groups of Si–CH3, the prepared separator decreases the polarization of the lithium-ion batteries and leads to improved power performance and cycle stability (its discharge voltage difference is 0.42 V higher than that of Celgard 2325 at 5 C). This work provides a new method of improving the safety and simultaneously reducing the polarization of lithium-ion batteries, which has promising application potential in power batteries.

Ionic Cross-Linked MOF-Polymer Mixed-Matrix Membranes for Suppressing Interfacial Defects and Plasticization Behavior.

To address the plasticization phenomenon and MOF-polymer interfacial defects, we report the synthesis of ionic cross-linked MOF MMMs from a dual brominated polymer and MOF components by using N,N'-dimethylpiperazine as the cross-linker. We synthesized brominated MIL-101(Cr) nanoparticles by using mixed linkers and prepared brominated polyimide (6FDA-DAM-Br) to form ionic cross-linked MMMs. The gas permeation properties of the polyimide, ionic cross-linked MOF-polymer MMMs, and non-cross-linked MOF-polymer MMMs with various MOF weight loadings were investigated systematically to effectively understand the effects of MOF weight loading and ionic cross-linking. The ionic cross-linked 40 wt % MOF-polymer MMM exhibited significantly enhanced gas permeability with an H2 permeability of 1640 Barrer and CO2 permeability of 1981 Barrer and slightly decreased H2/CH4, H2/N2, CO2/CH4 and CO2/N2 selectivities of 16.9, 15.4, 20.5, and 18.6, respectively. The H2 and CO2 permeabilities are approximately 2-3 fold higher than those of the pure polyimide (6FDA-DAM) membrane. Moreover, the ionic cross-linked 40 wt % MOF-polymer MMM exhibited significantly increased resistance to plasticization. This is because the brominated MOF incorporation boosted molecular transport and polymer chain rigidity, and ionic cross-linking further reduced the number of interfacial defects and polymer chain mobility.

Foldable polyimide/ionic liquid composite films with high transmittance and ultraviolet resistance benefiting from the intermolecular multipoint interaction

A foldable polyimide (PI) protective layer with high transmittance and high ultraviolet (UV) resistance is crucial for the practical application of flexible solar cells. Herein, the commercially 2,2′-Bis(trifluoromethyl) benzidine (TFMB) and 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) as the monomers, and five ionic liquids (ILs) as the regulators were used to synthesize PI/IL films (PI/IL1, PI/IL2, PI/IL3, and PI/IL4). The outcomes reveal that the average transmittance (AT) of PI/IL1 (50 μm thickness) outperforms others, which can be attributed to the expanded π-π stacking distance between PI molecules and the intermolecular multipoint interaction between IL1 and PI. The AT and the elongation at break (ε) of PI/IL1 increase sequentially with the 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2])/TFMB mole ratio, and those of PI/IL1-3 reach the maximum values of 94.23 % and 49.42 % at a 3:1 ratio, respectively. However, the tensile strength (σ) of the PI/IL1 film is inferior to that of the pure PI film, and consistently decreases with the increase of [C2mim][NTf2]/TFMB mole ratio. Nevertheless, the UV-induced degradation of PI can be delayed through the [C2mim][NTf2] regulation. The attenuations of AT, σ and ε of PI/IL1-3 films after 1000 equivalent solar hours (ESHs) UV irradiation are 0.85 %, 14.63 % and 2.99 %, respectively, which are significantly lower than those of pure PI films (2.23 %, 32 % and 23 %). This is attributed to the interaction between the positive and negative charges from [C2mim][NTf2] and those from the PI chain breaking. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of PI/IL1-3 are calculated with the density functional theory to reveal the charge transfer between [C2mim][NTf2] and PI.

Finely tuning polyimide film structure via green plasticizer's disentangling chain to improve the trade-off between transmittance and mechanical property

To achieve the prevention of toxic waste while improving the intended functionality and durability of the film, polyimide (PI) based on TFMB-6FDA was tuned via the green plasticizer acetyl tributyl citrate (ATBC) to prepare a flexible film with high transmittance, high strength and ultraviolet (UV)-resistance. All the properties of ATBC tuned film (PI/T1, PI/T2, and PI/T3), average transmittance (AT), tensile strength (σb), elongation at break (ɛb), and elastic modulus (E) increase first and then decrease with the increase of ATBC amount. The three formers reach their own maximum values with 0.5 mole ratio of ATBC, 91.20 %, 244.0 MPa, and 12.2 %, respectively, higher than 90.50 %, 184.1 MPa, and 5.34 % of PI. The water contact angle of PI/T2 obtains the maximum value of 94.72°, and its water contact angle is 14.2 % higher than that of the pure PI (82.93°). The UV-resistance of PI/T2 is far better than that of PI after 2000 equivalent sun hours (ESHs) UV irradiation, and the AT attenuation of PI/T2 is just about one third of PI, so ATBC can retard the attenuation of PI transmittance and mechanical property.

Tribological Properties of Polyimide Composites Modified with Diamondoid Metal-Organic Frameworks.

In this work, diamondoid metal-organic frameworks (MOFs) were efficiently prepared by sonochemical synthesis and grown on polyimide (PI), aiming to improve the anti-wear performance of the PI matrix. By introducing MOFs into the PI matrix, the free movement of PI molecular chains were restricted, and its hardness and elastic modulus were improved. It was found that the wear rate of the 3 wt.% MOFs/PI composites was reduced by 72.6% compared to pure PI at a load of 4 N after tribological testing by using a ball-on-disk tribometer. This can be attributed to the excellent load-bearing and shear resistance of the fourfold-interpenetrated diamondoid networks, in which the transition metal elements can favor the formation of transfer films. It is worth noting that the 3 wt.% MOFs/PI composites still exhibited great tribological properties under high loads or high speeds. The findings of the present study indicate that diamondoid metal-organic frameworks can be used as efficient modifiers to enhance the tribological properties of PI.

Vacuum filtration-assisted design of hierarchical structure in polyimide-BN composite films: Toward enhanced thermal conductivity, low dielectric and electrical insulation

With the advent of the 5G era, the demand for heat dissipation using high thermal conductivity (TC) materials is becoming increasingly urgent. The electrical insulation and thermal stability of dielectric polymer composites are also notable in electrical and electronic applications. Nevertheless, the improvement in TC of dielectric polymer composites is typically accomplished at the expense of electrical insulating capabilities. Herein, we fabricated polyimide (PI) composites with ordered structure by combining electrostatic spinning, vacuum filtration deposition and blade-coating. The modified boron nitride nanosheets (mBN) not only enhanced the interaction between filler and PI matrix, but also reduced the interface thermal resistance. Benefiting from the hierarchical structure, the in-plane TC of prepared composite was 17.44 W/mK at 18.79 vol% mBN content, which is 9.6 times higher than that of pure PI. Moreover, the composites were electrically insulating, with the volume resistivity of about 1013 Ω cm, and exhibited great flexibility, exceptional thermal stability (THRI ＞ 269 °C), excellent thermal dissipation capability and low dielectric characteristics. In summary, this work offers a novel perspective to design highly thermally conductive composites and these composites have vast application potential in flexible electronic devices or circuits.

‘Ball-on-plane’ ZnS@RGO compound filler: effect on tribological properties of thermosetting polyimide film

ABSTRACT In this study, a compound filler consisting of zinc sulfide (ZnS) nanoparticles and reduced graphene oxide (RGO) was synthesized using the hydrothermal method and utilized to enhance the tribological properties of the polyimide (PI) film. The ZnS@RGO/PI composites displayed the lowest wear rate of 3.89 × 10−8 mm3 N−1 m−1 at a load of 200 N, which were significantly lower than that of both pure PI and RGO/PI films. Furthermore, the microstructure and chemical composition of the compound were characterized and analyzed, revealing that the ZnS nanoparticles were effectively attached to the surface of RGO through the chemical bonding between them. Finally, finite element analysis shows that the chemical bonding between ZnS and RGO have the effect to stop the crack expanding under pressure, and a wear mechanism is also given to demonstrate this ‘Ball-on-plane’ compound’s enhancement to the tribological properties.

Preparation of MoS2@PDA-Modified Polyimide Films with High Mechanical Performance and Improved Electrical Insulation.

Polyimide (PI) has been widely used in cable insulation, thermal insulation, wind power protection, and other fields due to its high chemical stability and excellent electrical insulation and mechanical properties. In this research, a modified PI composite film (MoS2@PDA/PI) was obtained by using polydopamine (PDA)-coated molybdenum disulfide (MoS2) as a filler. The low interlayer friction characteristics and high elastic modulus of MoS2 provide a theoretical basis for enhancing the flexible mechanical properties of the PI matrix. The formation of a cross-linking structure between a large number of active sites on the surface of the PDA and the PI molecular chain can effectively enhance the breakdown field strength of the film. Consequently, the tensile strength of the final sample MoS2@PDA/PI film increased by 44.7% in comparison with pure PI film, and the breakdown voltage strength reached 1.23 times that of the original film. It can be seen that the strategy of utilizing two-dimensional (2D) MoS2@PDA nanosheets filled with PI provides a new modification idea to enhance the mechanical and electrical insulation properties of PI films.

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.

High Electrical and Mechanical Properties Obtained in a Polyimide‐Based Nanocomposite with Sandwich Structure via Multidimensional Design

AbstractAs an excellent polymer material, polyimide (PI) plays an important role in practical applications. However, traditional filler doping modification is hard to simultaneously improve the dielectric properties and mechanical properties of the film. Here, PI–boron nitride (BN) nanosheets/PI–sodium titanate nanotubes (STNs)/PI–BN composite films with sandwich structure are prepared by in situ polymerization and multilayer coating processes, which significantly enhance the electrical and mechanical properties of dielectric films. With only 0.5 wt% fillers doping, the AC breakdown strength of the composite film can reach 183.7 kV mm−1, which is 16.41% higher than that of pure PI. When the filler doping is 1.5 wt%, the corona aging time of the composite film achieves 311.7% higher than that of pure PI film. In addition, the composite film exhibits outstanding elongation at break (58%) and tensile strength (138.92 MPa) performance. The 2D BN of the outer layer and 1D STNs of the middle layer effectively block the injection of charge carriers while improving the internal polarization response. A mechanism related to microstructure and interface is proposed to explain the improvement of electrical and mechanical properties in detail. This work provides a useful idea for the accurate design of high‐performance dielectric films.

Rapid room-temperature synthesis of disposable Co nanoparticles modified 3D N-doped porous carbon electrode device for efficient electrochemical detection of veterinary drug and pesticide residue

In this work, Co-based metal-organic framework (ZIF-67) coupled polyimide (ZIF-67/PI) film was fastly and effectively transformed into flexible Co nanoparticles modified 3D N-doped porous carbon (Co@3D NPC) electrode device using simple laser ablation technique at room temperature. Compared with 3D NPC electrode device that derived from pure polyimide film, Co@3D NPC exhibited significantly enhanced electron transfer ability, physical adsorption ability and electrochemically active surface area. By adjusting the laser power and ZIF-67 concentration, the surface morphology and electrochemical property of the Co@3D NPC electrode devices can be easily controlled. Furthermore, the fabricated flexible Co@/3D NPC electrode device exhibited exceptional electrochemical activity for the sensitive detection of clozapine and albendazole, two kinds of common veterinary drug and pesticide. The detection sensitivity of Co@/3D NPC electrode device reached to 410.47 μA μM−1 cm−2 and 580.33 μA μM−1 cm−2, with detection limits as low as 2.8 nM and 2.5 nM for clozapine and albendazole, respectively. Besides, the Co@/3D NPC electrode device demonstrated excellent stability and accuracy. Given the advantages of laser ablation technology, including its low cost, scalability, and high stability, the reported Co@/3D NPC electrode device holds significant potential for practical applications in the field of electrochemical analysis.

Zwitterion grafted polyimide separator for improving lithium-ion transport and its application in LiCoO2 batteries

To improve the lithium-ion transport ability and cycle life of lithium-ion batteries, a high-performance polyimide-based lithium-ion battery separator (PI-SO3) was prepared by chemically grafting zwitterion onto the surface of heat-resistant polyimide nanofiber matrix with the assistance of amino-rich polyethyleneimine (PEI). The PI-SO3 not only maintains good dimensional stability (over 200 °C) and flame retardancy, but also ensures the safety of the battery under extreme conditions. Electrochemical experimental results show that PI-SO3 has high ion conductivity (1.99 mS·cm−1), Li+ transference number (0.632), and excellent oxidation potential (5.28 V), which are all higher than those of pure PI separator (1.14 mS·cm−1, 0.444, and 4.56 V, respectively). Meanwhile, LiCoO2/PI-SO3/Li batteries exhibit excellent high-rate performance (139.5 mAh·g−1 at 5C current) and long-term cycle stability (138.9 mAh·g−1 after 100 cycles at 1C current, and 127.9 mAh·g−1 after 100 cycles at 2C current). In addition, the symmetrical battery, Li/PI-SO3/Li, shows stable more than 360 cycles. Compared with batteries using commercial separators, such as Celgard 2325 and pure polyimide matrix, the assembled coin cells with PI-SO3 separators have higher lithium-ion conductivity, Li+ transference number, and diffusion coefficient after grafting zwitterion, showing better rate ability and capacity retention. In this paper, the modification strategy of thermal-resistant separator by grafting zwitterions provides an efficient way to prepare next generation high-performance lithium-ion batteries with good safety.

Janus composite film comprising metal–organic framework/polyimide and BNNSs/polyimide as high energy storage dielectrics

AbstractIn order to prepare polymer dielectric materials with high energy storage density, a series of polyimide (PI) based Janus composite films were prepared by metal–organic frameworks (MOFs) material (UiO‐66) with high dielectric constant (ɛr) and boron nitride nanosheets (BNNSs) with high breakdown strength (Eb). As reported, the UiO‐66 can significantly improve the ɛr of the composites. The ɛr of 20 wt% UiO‐66/PI film is 10.51 at 103 Hz, which is 3.1 times that of pure PI film (3.39 at 103 Hz). However, the Eb of 20 wt% UiO‐66/PI is only 58 MV m−1, which is only 16.51% of pure PI (352 MV m−1). The sharp decline in Eb reduces the energy storage capacity of the composites. To solve this problem, first, BNNSs with an average diameter of 2 μm and a thickness of 1.36 nm (about 4–5 layer) were obtained by hydrothermal reflux stripping. BNNSs/PI//UiO‐66/PI Janus composite films with double‐layer structure were prepared by directly casting BNNSs/PI on the surface of UiO‐66/PI. For 0.3 wt% BNNSs/PI//10 wt% UiO‐66/PI, the calculated energy storage density (Ue) is 5.75 J cm−3 at 140 MV m−1, which is 3.57 times that of pure PI film (1.61 J cm−3 at 140 MV m−1). Meanwhile, the charge discharge efficiency is 82.55% (at 140 MV m−1).Highlights BNNSs prepared by hydrothermal reflux stripping have fewer layers. BNNSs/PI composites improve the Eb of UiO‐66/PI composites by Janus structure. The Ue of BP‐0.3//UP‐10 composites with Janus structure was 5.75 J cm−3.

Research on the energy storage performance of laminated composites based on multidimensional co-design in a broad temperature range.

Polymer dielectrics play an irreplaceable role in electronic power systems because of their high power density and fast charge-discharge capability, but it is limited by their low stability in the temperature range of 25-200 °C. Rather than the introduction of one-dimensional fillers in polymers, we used a kind of multidimensional synergistic design to prepare Al2O3-TiO2-Al2O3/PI composites with layered structures by introducing multi-dimensional materials in polyimide (PI). In fact, the composite achieves much higher temperature stability than the pure PI film. The optimally proportioned composite has an energy density of 3.41 J cm-3 (vs. 1.48 J cm-3 for pure PI) even at 200 °C. Additionally, it reaches an impressive energy density retention of up to 90% and maintains an energy efficiency as high as 86% at 400 MV m-1 in the temperature range of 25-200 °C. The multidimensional coordination design is proposed to obtain composite films, and provides a feasible strategy in the study of polymer-based composites with high-temperature performance.

Covalent-assisted construction of “scale-like” boron nitride/polyimide thermal interface materials with high thermal conductivity

Polyimide (PI) is a special engineering plastic with high strength, high modulus, high temperature resistance and corrosion resistance, but its poor thermal conductivity limits its application in high-power miniaturized electric and electronic equipment. In this study, surface-activated and modified boron nitride (m-BN) is polymerized in situ and coated to form a “scale-like” composite thermal interface material (m-BN/PI) with PI. Moreover, the m-BN@PI composite films obtained by a simple co-blending process of m-BN and PI are used as a comparison. The in-plane thermal conductivity of m-BN/PI composite reached 4.43 W m−1 K−1, when the filler content is 30 wt %, which is about 12 times higher than that of the pure PI film (0.37 W m−1 K−1). At this load, the in-plane thermal conductivity of m-BN@PI is only 3.94 W m−1 K−1. In addition, the covalent bonding between m-BN and PI molecular chains in m-BN/PI effectively reduces the interfacial thermal resistance by fitting the EMT and Foygel models. Finally, using the m-BN/PI composite film as a thermal interface material placed at the CPU location, its role in the heat dissipation process of electronic devices is demonstrated.

Construction of sandwich‐layered polyimide hybrid films containing double core–shell structured fillers for high energy storage density

AbstractThe inherent low dielectric constant of polyimide (PI) dielectrics restricts their applications to become a component of high energy density film capacitors. In this work, double core–shell structured barium titanate@magnesium oxide@polydopamine (BaTiO3@MgO@PDA) nanoparticles were synthesized successfully and utilized as high dielectric constant functional fillers for PIs, in which the insulating MgO layer meliorated dielectric constant difference between BaTiO3 and PI matrix, and the organic PDA layer improved the compatibility between the inorganic fillers and PI matrices. Then, a series of sandwich‐structured PI‐based hybrid films was prepared through a layer‐by‐layer solution casting method. The middle layer of pure PI with excellent insulating properties effectively suppressed charge injection. With the combination of sandwich structure and the BaTiO3@MgO@PDA nanoparticles, the PI hybrid film containing 15 wt% fillers in the outer layers achieved the maximum breakdown strength of 425.68 kV/mm and the maximum energy density of 5.132 J/cm3, which was 68.3% and 413% higher than those of pure PI film, respectively, meanwhile maintained a low dielectric loss value of 0.0083 at 1 kHz. The introduction of BaTiO3@MgO@PDA enhanced the interfacial polarization due to the high barrier energy between adjacent layers preventing the transfer of electrons and weakening the leakage current in the sandwich‐structured composite film. This work demonstrates that an appropriate combination of high dielectric hybrid fillers and multilayer structure can effectively increase the energy storage density of PI substrate for high‐temperature energy storage applications.Highlights The double core–shell structured BaTiO3@MgO@PDA was synthesized successfully. The sandwich‐structured hybrid films were prepared by layer‐by‐layer method. The energy density of hybrid films reached a high value of 5.132 J/cm3.

A Novel Hydrophobic Polyimide Film with Sag Structure Derived from Multi-Hybrid Strategy.

Polyimide (PI) film with hydrophilic greatly limits their application in the field of microelectronic device packaging. A novel hydrophobic PI film with sag structure and improved mechanical properties is prepared relying on the reaction between anhydride-terminated isocyanate-based polyimide (PIY) containing a seven-membered ring structure and the amino-terminated polyamide acid (PAA) via multi-hybrid strategy, this work named it as hybrid PI film and marked it as PI-PIY-X. PI-PIY-30 showed excellent hydrophobic properties, and the water contact angle could reach to 102°, which is 20% and 55% higher than simply PI film and PIY film, respectively. The water absorption is only 1.02%, with a decrease of 49% and 53% compared with PI and PIY. Due to that the degradation of seven-membered ring and generation of carbon dioxide led to the formation of sag structure, the size of sag structures is ≈16.84 and 534.55nm for in-plane and out-plane direction, which are observed on surface of PI-PIY-30. Meanwhile, PI-PIY-30 possessed improved mechanical properties, and the tensile strength is 109.08MPa, with 5% and more than 56% higher than that of pure PI and PIY film, showing greatly application prospects in the field of integrated circuit.

Surface confinement of vertically aligned liquid crystals between flexible silver nanoparticles hybridized polyimide thin layers

Alignment of liquid crystals (LCs) has emerged as an essentially fundamental approach for exploring the electro-optical performance and fabricating novel LC devices. Growing evidence suggests that the surface wettability of the alignment layer plays a key role in tuning the tilt angles of LCs. In this work, silver (Ag) nanoparticles (d ≤ 100 nm) are blended in polyimide (PI) by a wide range of concentration from 0.1% to 1.0% to tune surface wettability and align LCs. Ag nanoparticles agglomerate and result in a bumpy surface topology and somewhat reduce the transparency of hybrid thin layer to 85.1% by 1.0% doping because of their shielding effect. Nevertheless, Ag nanoparticle agglomerates change the thickness of hybrid thin layers less, and the hybrid thin layers are ∼96 nm thick like a pure PI thin layer. LCs are vertically aligned on hybrid thin layers after rubbing. Benefit from the declined anchoring energy and the outstanding surface plasmon polariton (SPP) of Ag nanoparticles, the external electric field to switch LCs is substantial boosted, and hence remarkably diminish the operating voltage and likewise notably shorten the response time. In particular, LCs that vertically aligned between PI hybrid alignment layers by 0.5% doping could be electrically switched to become planner at 3.38 V and go back to their original vertical alignment state within 73.21 ms. Ag nanoparticles agglomerates in hybrid thin layers also fairly hinder the drift of LCs when cells are mechanically bent with various radii. The hybrid alignment layers cell that blended with 0.5% Ag nanoparticles could be bent under 1500 mm without any new evident light leakages and fluctuating electro-optical characteristics. These results indicate the promising applications of PI hybrid thin layers blended with Ag nanoparticles for flexible LC devices.

Research on tribological behaviour and interfacial characteristic of Polyimide-MoS2 composite under cryogenic-vacuum environment

Tribological behaviour of Polyimide (PI)-MoS2 composite, pure PI, sputtering MoS2 film is investigated under RT-vacuum and cryogenic-vacuum through in-situ methods, i.e., X-ray photoelectron spectrum (XPS), X-Ray Diffraction (XRD), residual gas analyzer (RGA). Cryogenic causes an increase of friction coefficient (friction coefficient) of PI-MoS2 and pure PI, while it is inverse for sputtering MoS2 film. The friction interface is influenced by enriched water under cryogenic, and the possible mechanism is intermolecular hydrogen bonds caused by water restrict the formation of high oriented shear layer. Due to MoS2 film is not as sensitive as PI to moisture, oxidation at friction interface is relaxed because of cryogenic, which leads to a better friction behavior. Cryogenic causes compressive stress at the interface between PI matrix and MoS2 particles, which results in high friction coefficient due to the difficulty in release of the latter and breakage in some regions in where PI covers MoS2. Local cryogenic environment complicates the investigation in which the only purpose is to study the effect of cryogenic-vacuum on tribological properties of some lubricants.

Enhancing out-of-plane thermal conductivity of polyimide-based composites via the construction of inter-external dual heat conduction network by binary fillers

Polyimide (PI) materials have found widespread utilization in advanced electronic systems. PIs with enhanced out-of-plane thermal conductivity (K⊥) are urgently required to address the rising need for heat dissipation. However, their production remains a formidable challenge due to the difficulty of constructing heat transmission channels along the thickness direction. This study introduces an innovative approach to enhance the K⊥ values of PI-based composites by utilizing binary fillers to construct an inter-external dual heat conduction network. To achieve this, we first prepared PI/reduced graphene oxide (rGO) hybrid microspheres via solution mixing and precipitation. The microspheres were then coated with boron nitride nanosheets (BNNS) by self-assembly to form a core-shell architecture. This assembly undergoes further refinement via cold-pressing and subsequent densification through hot-pressing, ultimately producing the (PI/rGO)@BNNS composites. This strategy improved K⊥ values significantly, with a 13-fold and 3-fold increase in the K⊥ value (3.98 W m−1 K−1) in comparison to pure PI (0.31 W m−1 K−1) and the random distribution composite (1.45 W m−1 K−1), respectively. The finite element analysis confirmed that the synergistic effect of rGO inside the PI phase and BNNS outside the PI phase greatly increased the heat transfer in PI-based composites. When utilized as a thermal interface material (TIM) for LED bulbs, the (PI/rGO)@BNNS composites exhibit an excellent heat dissipation capacity. Additionally, the prepared composites also maintain electrical insulation and present a reduced coefficient of thermal expansion. Overall, our work provides a simple and efficient technique for enhancing the out-of-plane thermal conductivity and maintain the electrical insulation of high-performance PI-based materials, which could have broad application potential in next-generation electronic devices.