Low Dielectric Constant and High Organosolubility of Novel Polyimide Derived from Unsymmetric 1,4-Bis(4-aminophenoxy)-2,6-di-tert-butylbenzene

Versatile Landscape of Low-k Polyimide: Theories, Synthesis, Synergistic Properties, and Industrial Integration

The development of microelectronics and large-scale intelligence nowadays promotes the integration, miniaturization, and multifunctionality of electronic and devices but also leads to the increment of signal transmission delays, crosstalk, and energy consumption. The exploitation of materials with low permittivity (low-k) is crucial for realizing innovations in microelectronics. However, due to the high permittivity of conventional interlayer dielectric material (k ∼ 4.0), it is difficult to meet the demands of current microelectronic technology development (k < 3.0). Organic dielectric materials have attracted much attention because of their relatively low permittivity owing to their low material density and low single bond polarization. Polyimide (PI) exhibits better application potential based on its well permittivity tunability (k = 1.1-3.2), high thermal stability (>500 °C), and mechanical property (modulus of elasticity up to 3.0-4.0 GPa). In this review, based on the synergistic relationship of dielectric parameters of materials, the development of nearly 20 years on low-k PI is thoroughly summarized. Moreover, process strategies for modifying low-k PI at the molecular level, multiphase recombination, and interface engineering are discussed exhaustively. The industrial application, technological challenges, and future development of low-k PI are also analyzed, which will provide meaningful guidance for the design and practical application of multifunctional low-k materials.

New fluorinated polyimides based on 1,2-bis(4-aminophenyl)-4,5-bis(4-trifluoromethylphenyl)-1H-imidazole

A new aromatic unsymmetrical diamine containing imidazolyl and trifluoromethyl groups, 1,2-bis(4-aminophenyl)-4,5-bis(4-trifluoromethylphenyl)-1H-imidazole was synthesized. A series of novel fluorinated polyimides (PIs) were prepared by polycondensation of this diamine monomer with various aromatic dianhydrides via one-step process. The resulting PIs bearing inherent viscosities of 0.45–0.61 dL/g were amorphous and readily soluble in organic polar solvents such as NMP, DMF, DMAc, THF, and CHCl3. These new PIs exhibited good thermal stability with 10% weight loss temperatures of 530–560°C and char yields of 61–65% at 800°C in nitrogen, as well as good flame retardancy with limited oxygen index value of 40.6–42.8, and outstanding hydrophobicity with the contact angle of 85.2–93.5°. Meanwhile, all the obtained PIs exhibited low water uptake of 0.30–0.53%, strong UV-vis absorption in the range of 267–310 nm in DMF solution, and their films presented high optical transparency with a cut-off wavelength in the 355–392 nm range. Except for the PI film derived from pyromellitic dianhydride, the other PI films had the tensile strength in the range of 53.2–74.1 MPa, elongation at breakage of 6–15%, and tensile modulus of 1.0–1.5 GPa, respectively.

Polyimide/fluorinated silica composite films with low dielectric constant and low water absorption

In this study, KMnO4 was applied as an etchant to partially decompose the silica microspheres into mesoporous structures. 1H,1H,2H,2H-perfluorodecyltriethoxysilane was grafted onto the surface of the hollow silica. A fluorinated silica/polyimide hybrid material was prepared. Meanwhile, the influence of fluorinated silica on the dielectric property and moisture absorption property of the hybrid material was discussed. It is found that the fluorinated silica can lower the dielectric constant and hygroscopicity of the hybrid film. The fluorinated silica is characterized by strong hydrophobicity, good dispersibility, and good thermal stability. Even at a very low filling level of only 2 wt%, the dielectric constant of the polyimide hybrid is significantly reduced to 2.61 without sacrificing thermal stability. The water absorption rate of the polyimide hybrid film is reduced from approximately 3.1% to 2.1 wt%. Moreover, this provides a new idea for reducing the dielectric and water absorption properties of materials.

Highly Transparent Aromatic Polyamides from Unsymmetrical Diamine with Trifluoromethyl Groups

Soluble and transparent wholly aromatic polyamides (PAs) were synthesized from an unsymmetrical diamine monomer having trifluoromethyl (CF₃) groups, 4-(4′-aminophenoxy)-3,5-bis(trifluoromethyl)aniline. The monomer was polymerized with several dicarboxylic acid monomers via the Yamazaki–Higashi polycondensation method. All of the synthesized polyamides have an amorphous morphology, and they are soluble in many polar organic solvents at room temperature. Flexible and transparent films of the polyamides were prepared by solution casting and these polymer films show good optical transparencies with cut-off wavelengths of 337–367 nm and transparencies of 88%–90% at 550 nm. In addition, all the polymers were thermally stable over 400 °C and exhibited glass transition temperatures (T_g) higher than 300 °C. Unsymmetrically inserted trifluoromethyl groups on polyamides improves the solubility as well as the transparency of the polymers while maintaining good thermal properties. They also showed low refractive indices around 1.5333~1.5833 at 633 nm owing to the existence of low polarizable trifluoromethyl groups.

Preparation and Characterization of Electrospun Fluoro-Containing Poly(imide-benzoxazole) Nano-Fibrous Membranes with Low Dielectric Constants and High Thermal Stability

The rapid development of advanced high-frequency mobile communication techniques has advanced urgent requirements for polymer materials with high-temperature resistance and good dielectric properties, including low dielectric constants (low-Dk) and low dielectric dissipation factors (low-Df). The relatively poor dielectric properties of common polymer candidates, such as standard polyimides (PIs) greatly limited their application in high-frequency areas. In the current work, benzoxazole units were successfully incorporated into the molecular structures of the fluoro-containing PIs to afford the poly(imide-benzoxazole) (PIBO) nano-fibrous membranes (NFMs) via electrospinning fabrication. First, the PI NFMs were prepared by the electrospinning procedure from organo-soluble PI resins derived from 2,2'-bis(3,4-dicarboxy-phenyl)hexafluoropropane dianhydride (6FDA) and aromatic diamines containing ortho-hydroxy-substituted benzamide units, including 2,2-bis[3-(4-aminobenzamide)-4-hydroxylphenyl]hexafluoropropane (p6FAHP) and 2,2-bis[3-(3-aminobenzamide)-4-hydroxyphenyl]hexafluoropropane (m6FAHP). Then, the PI NFMs were thermally dehydrated at 350 °C in nitrogen to afford the PIBO NFMs. The average fiber diameters (dav) for the PIBO NFMs were 1225 nm for PIBO-1 derived from PI-1 (6FDA-p6FAHP) precursor and 816 nm for PIBO-2 derived from PI-2 (6FDA-m6FAHP). The derived PIBO NFMs showed good thermal stability with the glass transition temperatures (Tgs) over 310 °C and the 5% weight loss temperatures (T5%) higher than 500 °C in nitrogen. The PIBO NFMs showed low dielectric features with the Dk value of 1.64 for PIBO-1 and 1.82 for PIBO-2 at the frequency of 1 MHz, respectively. The Df values were in the range of 0.010~0.018 for the PIBO NFMs.

Structure and properties of a phenylethynyl-terminated PMDA-type asymmetric polyimide

A new polymerized monomeric reactant (PMR)-type polyimide, designated TriA X, was investigated to determine polymer structure, processability, thermal, and mechanical properties and establish the relationship between the molecular structure and those properties. TriA X is a PMR-type polyimide with an asymmetric, irregular, and nonplanar backbone. Both the imide oligomers and the cross-linked polyimides of TriA X exhibited loose-packed amorphous structures, independent of thermal processing. The peculiar structures were attributed to the asymmetric backbone, which effectively prevented the formation of closed-packed chain stacking typically observed in polyimides. The imide oligomers exhibited a lower melt viscosity than a control imide oligomer (symmetric and semi-crystalline), indicating a higher chain mobility above the glass transition temperature ( Tg). The cured polyimide exhibited a Tg = 362°C and a decomposition temperature = 550°C. The cross-linked TriA X exhibited exceptional toughness and ductility (e.g. 15.1% at 23°C) for a polyimide, which was attributed to the high-molecular-weight oligomer and loose-packed amorphous structure. The thermal and mechanical properties of TriA X surpass those of PMR-15 and AFR-PE-4.

Synthesis, characterization, and properties of thermoplastic polyimides derived from 4,4’-(hexafluoroisopropylidene)diphthalic anhydride in diethylene glycol dimethyl ether

By regulating the order of monomer addition, four kinds of copolymer polyimides (PIs) were prepared using diethylene glycol dimethyl ether (DEGDE) and N, N-dimethylacetamide (DMAc) as the solvents. The molecular weights of polyamide acids (PAAs) ranged from 3.8 × 105 to 9.2 × 105. All of the films displayed high glass transition temperatures ( Tgs) ranging from 313°C to 346°C. The polymer films show excellent thermal stabilities with 5% weight loss at temperatures of 505–524°C and char yields at 800°C were as high as 55% under nitrogen. The peel strengths of flexible copper (Cu) clads were in the range from 0.337 N cm−1 to 0.598 N cm−1. Compared to the molecular weight and peel strength of fresh PAA, those of PAAs prepared using DMAc significantly decreased after storage for 3 months at 0°C. However, when DEGDE was used as the solvent, the molecular weights of the PAAs and the thermal properties of the PIs were maintained after long storage time.

Low Dielectric Polyetherimides Derived from Bis[4-(4-(4-aminophenoxy)-2-tert-butylphenoxy)phenyl] Sulfone and 4,4'-Bis[4-(4-aminophenoxy)-2-tert-butylphenoxy]perfluorobiphenyl

Two diamines, bis[4-(4-(4-aminophenoxy)-2-tert-butylphenoxy)phenyl] sulfone (PSNH₂) and 4,4'-bis[4-(4-aminophenoxy)-2-tert-butylphenoxy)perfluorobiphenyl (PFNH₂), were prepared from 3-tert-butyl-4-hydroxyanisole using a four-step procedure, including two nucleophilic substitutions, demethylation, and catalytic reduction. On the basis of PSNH₂ and PFNH₂, two series of low dielectric polyetherimides (PEIs) were prepared. Both series of PEIs exhibit moderate-to-high thermal properties, including a glass transition temperature (T _g) > 259 °C (depending on the rigidity of dianhydride), a 5 wt % decomposition temperature (T _d5%) > 496 °C, and a coefficient of thermal expansion < 66 ppm/°C. Because of the hydrophobic tert-butyl phenylene oxide structure, both series of PEIs show excellent dielectric properties, with a dielectric constant as low as 2.4-2.7. The structure-property relationship of both series of PEIs is discussed in this work.

Process development for phenylethynyl-terminated PMDA-type asymmetric polyimide composites

A new type of polyimide, designated TriA X, has been developed for high-temperature composite applications. TriA X is a polymerized monomeric reactant (PMR)-type polyimide derived from 1,2,4,5-benzenetetracarboxylic dianhydride (PMDA), 2-phenyl-4,4′-diaminodiphenyl ether (p-ODA), and phenylethynyl phthalic anhydride (PEPA). The polymer has an asymmetric, nonplanar backbone, resulting in an amorphous structure and high toughness. In this work, a TriA X resin (with degree of polymerization n = 7 in the imide oligomer) was investigated for processability and performance in carbon fiber composites. Rheological measurements were performed on an oligomer with a low degree of imidization to understand the chemo-rheology of the resin system and determine a suitable B-staging temperature. A composite molding cycle was designed, which yielded fully consolidated woven carbon fiber laminates. Void contents in panels produced with this molding cycle were <0.1% as measured by image analysis (IA) of polished sections, and <0.2% as measured by X-ray micro-computed tomography (micro-CT). Matrix-dominated mechanical properties of composites fabricated with the TriA X polymer exceeded those of PMR-15 and AFR-PE-4 composites. These mechanical properties and a measured glass transition temperature of 367°C indicate potential for use of this resin system in high-temperature composites.

Electrochromic and electrofluorochromic polyimides with fluorene-based triphenylamine

A series of novel polyimides (PIs) based on N,N-di(4-aminophenyl)-2-amino-9,9-dimethylfluorene and various dianhydrides were synthesized. These PIs were amorphous and readily soluble in many solvents. They displayed outstanding thermal stability and high glass transition temperatures (269–374°C). The polymer films showed reversible electrochemical redox and satisfactory electrochromic performance with long-term stability, high coloration efficiency, and acceptable switching times. The PI derived from 1,2,4,5-cyclohexanetetracarboxylic dianhydride exhibited maximum fluorescence at 400 nm with quantum yields up to 28.1%. Furthermore, the fluorescence can be effectively electroswitched between “on” and “off” states, exhibiting a high contrast ratio of 75.

Novel soluble poly(ether imide)s with trifluoromethyl and chloride pendant groups for optical materials

A series of novel PEIs (4a–4e) with ether linkages and bulk side groups in the backbone have been successfully synthesized. It was very interesting that they also had excellent optical properties.

Thermal, mechanical and optical transport properties of nanocomposite materials based on triethoxysilane-terminated polyimide and TiO2 nanoparticles

Novel polyimide/TiO₂ nanocomposites with good mechanical, optical and thermal properties were prepared by the incorporation of TiO₂ nanoparticles into silane terminated polyimide.

Ternary composites of linear and hyperbranched polyimides with nanoscale silica for low dielectric constant, high transparency, and high thermal stability

A new series of ternary polyimide–silica composites was developed to obtain polymer films with low dielectric constant, high optical transparency, and good thermal stability.

Highly dispersible ternary composites with high transparency and ultra low dielectric constants based on hyperbranched polyimide with organosilane termini and cross-linked polyimide with silica

Flexible insulating materials that are both thermally and mechanically stable, highly transparent, and have low dielectric constants are highly desirable for electronic applications.

Highly transparent polyimides derived from 2-phenyl-4,6-bis(4-aminophenoxy)pyrimidine and 1,3-bis(5-amino-2-pyridinoxy)benzene: preparation, characterization, and optical properties

Heterocyclic polyimides were prepared from two novel diamines with anhydrides, which exhibit outstanding solubility and optical transparency properties.

A new polymer with low dielectric constant based on trifluoromethyl-substituted arene: preparation and properties

A polymer based on trifluoromethyl-substituted arene was synthesized with high molecular weight (M_n) of 62 000 by the Scholl reaction. The polymer film showed low dielectric constants of about 2.56 in a range of frequencies from 1 to 25 MHz. Moreover, the polymer revealed high thermostability and good mechanical properties, suggesting that the polymer had potential applications in the electronics industry.

Hybrid ternary composites of hyperbranched and linear polyimides with SiO2: a research for low dielectric constant and optimized properties

The hybrid ternary composites with low dielectric constant and high thermal stability were fabricated through composition optimization by introducing suitable amounts of inorganic silica and hyperbranched polyimide into a linear polyimide system.

Effects of tert-butyl substitutes of fluorinated diamine on the properties of polyimides

To investigate the effect of tert-butyl substitutes on the properties of fluorinated polyimides (PIs), 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene (1), 1,4-bis(4-amino-2-trifluoromethylphenoxy)-2- tert-butylbenzene (2), and 1,4-bis(4-amino-2-trifluoromethylphenoxy)-2,5-di- tert-butylbenzene (3), which were synthesized in this study. Three series of organosoluble and light-colored PIs were synthesized from 1, 2, and 3 with four dianhydrides 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA), pyromellitic dianhydride, 3,3′,4,4′-biphenyl tetracarboxylic dianhydride, and 4,4′-oxydiphthalic anhydride and prepared through a typical two-step polymerization method. These obtained PIs with tert-butyl substituents were soluble in amide polar solvents and even in less polar solvents. The glass transition temperatures ( Tg) of PIs with two tert-butyl substitutes can be reached at 327.7°C, and the 5% weight loss temperatures of PI derived from CBDA were lowest at 439.7°C. These PI films had cutoff wavelengths between 301.2 and 400.0 nm and the transparency at 450 nm between 43.0 and 86.2%. Compared with diamine 1, we found that the introduction of tert-butyl side group of the diamine improved the solubility and Tg of PIs, but decreased the optical transmittance and thermal stability of PIs.