The “fluorine impact” on dielectric constant of polyimides: A molecular simulation study

Sandwich-Structured Fluorinated Polyimide Aerogel/Paraffin Phase-Change Composites Simultaneously Enables Gradient Thermal Protection and Electromagnetic Wave Transmission

There is an emerging requirement of advanced functional materials for simultaneous thermal protection and electromagnetic wave-transparent transmission applications. A novel polyimide (PI) aerogel-based sandwich-structural composite is developed to meet such a requirement in this study. This composite is based on a unidirectional fluorinated PI (FPI) aerogel as a lower layer, a nondirectional conventional PI aerogel as a middle layer, and a nondirectional FPI aerogel/paraffin phase-change composite as an upper layer. The lower layer exhibits a unique unidirectional porous microstructure and an ultralow dielectric constant of 1.04. The upper layer possesses a dynamical temperature regulation capability thanks to its loaded paraffin having a high latent heat capacity of 242.7 J g-1. The presence of the nondirectional PI aerogel middle layer can effectively prevent against the leakage of paraffin from the upper layer to the surface of the composite. Through a rational integration of three functional layers, the developed sandwich-structured composite not only can provide gradient thermal protection for hot objects over a long period but also exhibits an excellent wave-transparent capability to establish communication between two electromagnetically shielded electronic devices. With such prominent thermal insulation and wave-transparent functions, the sandwich-structured composite exhibits great potential for specific applications in aircraft, spacecraft, radar systems, and satellite communication.

Ultralow k covalent organic frameworks enabling high fidelity signal transmission and high temperature electromechanical sensing

As integrated circuits have developed towards the direction of complexity and miniaturization, there is an urgent need for low dielectric constant materials to effectively realize high-fidelity signal transmission. However, there remains a challenge to achieve ultralow dielectric constant and ultralow dielectric loss over a wide temperature range, not to mention having excellent thermal conductivity and processability concurrently. We herein prepare dual-linker freestanding covalent organic framework films with tailorable fluorine content via interfacial polymerization. The covalent organic framework possesses an ultralow dielectric constant (1.25 at 1 kHz, ≈1.2 at 6 G band), ultralow dielectric loss (0.0015 at 1 kHz) with a thermal conductivity of 0.48 Wm-1K-1. We show high-fidelity signal transmission based on the large-sized (>15 cm2) COF films, far exceeding the most commercially available polyimide-based printed circuit board. In addition, the covalent organic framework also features excellent electret properties, which allows for active high-temperature electromechanical sensing. The electrode nanogenerator maintains 90% of the output voltage at 120 °C, outperforming the traditional fluorinated ethylene propylene electret. Collectively, this work paves the way for scalable application of ultralow dielectric constant covalent organic framework thin films in signal transmission and electromechanical sensing.

High-performance 2D electronic devices enabled by strong and tough two-dimensional polymer with ultra-low dielectric constant

As the feature size of microelectronic circuits is scaling down to nanometer order, the increasing interconnect crosstalk, resistance-capacitance (RC) delay and power consumption can limit the chip performance and reliability. To address these challenges, new low-k dielectric (k < 2) materials need to be developed to replace current silicon dioxide (k = 3.9) or SiCOH, etc. However, existing low-k dielectric materials, such as organosilicate glass or polymeric dielectrics, suffer from poor thermal and mechanical properties. Two-dimensional polymers (2DPs) are considered promising low-k dielectric materials because of their good thermal and mechanical properties, high porosity and designability. Here, we report a chemical-vapor-deposition (CVD) method for growing fluoride rich 2DP-F films on arbitrary substrates. We show that the grown 2DP-F thin films exhibit ultra-low dielectric constant (in plane k = 1.85 and out-of-plane k = 1.82) and remarkable mechanical properties (Young's modulus > 15 GPa). We also demonstrated the improved performance of monolayer MoS2 field-effect-transistors when utilizing 2DP-F thin films as dielectric substrates.

Mechanisms of Atomic Oxygen Erosion in Fluorinated Polyimides Investigated by Molecular Dynamics Simulations

Traditional polyimides have highly conjugated structures, causing significant coloration under visible light. Fluorinated colorless polyimides, known for their light weight and excellent optical properties, are considered ideal for future aerospace optical lenses. However, their lifespan in low Earth orbit is severely limited by high-density atomic oxygen (AO) erosion, and the degradation behavior of fluorinated polyimides under AO exposure is not well understood. This study uses reactive molecular dynamics simulations to model two fluorinated polyimides, PMDA-TFMB and 6FDA-TFMB, with different fluorine contents, to explore their degradation mechanisms under varying AO concentrations. The results indicate that 6FDA-TFMB has slightly better resistance to erosion than PMDA-TFMB, mainly due to the enhanced chemical stability from its -CF3 groups. As AO concentration increases, widespread degradation of the polyimides occurs, with AO-induced cleavage and temperature-driven pyrolysis happening simultaneously, producing CO and OH as the main degradation products. This study uncovers the molecular-level degradation mechanisms of fluorinated polyimides, offering new insights for the design of AO erosion protection systems.

Density Functional Theory-Based Approach For Dielectric Constant Estimation of Soluble Polyimide Insulators

Evaluation of the insulating properties of polymers, such as the dielectric constant and dissipation factor, is crucial in electronic devices, including field-effect transistors and wireless communication applications. This study applies density functional theory (DFT) to predict the dielectric constant of soluble polyimides (SPIs). Various SPIs containing trifluoromethyl groups in the backbone with different pendant types, numbers, and symmetries are successfully synthesized, and their dielectric constants are evaluated and compared with the DFT-estimated values. Two types of DFT-optimized SPIs, single-chain and stacked-chain models, are used to describe the local geometries of the SPIs. In addition, to reveal the relationship between the molecular structure and dielectric constant, further investigations are conducted by considering the dielectric constant of composing ionic and electronic components. The DFT-estimated static dielectric constant of the single-chain model accurately reproduces the corresponding experimental value with at least 80% accuracy. Our approach provides a rational and accelerated strategy to evaluate polymer insulators for electronic devices based on cost-effective DFT calculations.

Preparation and Properties of Atomic-Oxygen Resistant Polyimide Films Based on Multi-Ring Fluoro-Containing Dianhydride and Phosphorus-Containing Diamine

Colorless and transparent polyimide (CPI) films with good atomic oxygen (AO) resistance and high thermal endurance are highly required in low earth orbit (LEO) space exploration. Conventional CPI films based on fluoro-containing 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) have been widely used in space applications. However, the AO erosion yields and glass transition temperatures (Tg) of the 6FDA-based CPI films have to be modified in order to meet the severe serving environments. In the current work, novel CPI films based on a multi-ring fluoro-containing 9,9-bis(trifluoromethyl)xanthene-2,3,6,7-tetracarboxylicdianhydride (6FCDA) monomer were developed. In order to enhance the AO resistance of the derived CPI film, a phosphorus-containing aromatic diamine, 2,5-bis[(4-aminophenoxy)phenyl]diphenylphosphine oxide (BADPO) was used to polymerize with the dianhydride to create the organo-soluble resin. Then, two phosphorus-containing CPI films (PPI), including PPI-1 (6FDA-BADPO) and PPI-2 (6FCDA-BADPO) were prepared by thermally curing of the PPI solutions at elevated temperatures. The PPI films maintained good optical transparency with transmittance values over 80% at a wavelength of 450 nm. PPI-2 exhibited a Tg value of 311.0 °C by differential scanning calorimetry (DSC) measurement, which was 46.7 °C higher than that of the PPI-1 counterpart (Tg = 264.3 °C). In addition, the PPI-2 film showed a coefficient of linear thermal expansion (CTE) value of 41.7 × 10-6/K in the range of 50~250 °C, which was apparently lower than that of the PPI-1 sample (CTE = 49.2 × 10-6/K). Lastly, both of the two PPI films exhibited good AO resistance with the erosion yields (Ey) of 6.99 × 10-25 cm3/atom for PPI-1 and 7.23 × 10-25 cm3/atom for PPI-2 at an exposure flux of 5.0 × 1020 atoms/cm2. The Ey values of the current PPI films were obviously lower than that of the standard polyimide (PI) film based on pyromellitic dianhydride (PMDA) and 4,4'-oxydianiline (ODA) (Ey = 3.0 × 10-24 cm3/atom).

Design and synthesis of novel poly (aryl ether ketones) containing trifluoromethyl and trifluoromethoxy groups

The high-frequency and high-speed communication in the 5 G era puts forward requirements for the dielectric properties of polymers. Introducing fluorine into poly(ary ether ketone) can improve its dielectric properties. In this work, by introducing the fluorine group strategy, we successfully designed and synthesized three novel trifluoromethyl (-CF₃) or trifluoromethoxy (-OCF₃)-containing bisphenol monomers and their F-substitution PEK-based polymers (PEK-Ins). All these PEK-Ins exhibited good thermal, mechanical and dielectric properties. The T d5% of the three polymers is all higher than 520℃. The free volume fraction of novel polymers increased from 3.75% to 5.72%. Among the three polymers, exhibited the lowest dielectric constant of the films is 2.839, and the dielectric loss is 0.0048, ascribing to the increasing free volume. The Young's modulus of the polymer film is as high as 2.9 GPa and the tensile strength is as high as 84 MPa. PEK-Ins reduced the dielectric constant by introducing a low fluorine content. This study provides a new way to design PEK to synthesize low dielectric constant polymers.