1. Introduction
The rapid development of high-frequency communication and large-scale integrated circuits has raised new requirements for interlayer insulating dielectric materials [
1,
2,
3]. Insulating dielectric materials are required to have a low dielectric constant and low dielectric loss under high frequency, as well as excellent thermal and mechanical properties, processability, dimensional stability, and low water absorption [
4,
5,
6].
Traditional general polymeric materials, such as polyethylene (PE), polypropylene (PP), etc., have low permittivity and dielectric loss but fail to meet requirements due to application–temperature mismatch. Numerous special engineering plastics, such as traditional polyimide (PI) and liquid crystal polymer (LCP), were expected to meet requirements due to their superior thermal and mechanical performance and dielectric properties [
7,
8]. However, the dielectric constant (D
k) of 3.0–3.5 is insufficient for application in high-frequency communication and large-scale integrated circuits. Based on the Clausius–Mossotti equation, low-dielectric materials can be prepared by reducing the molecular polarity or increasing the free volume [
9]. Considerable research has been conducted on the structural modification of PI and LCP to lower their dielectric constant and meet the requirements for practical applications. Generally, trifluoromethyl-containing substituents [
10,
11], bulky substituents [
12], and alicyclic segments [
13] are introduced into the backbone of PI or LCP to reduce polarizability and lower their Dk. Alternatively, the dielectric constant of the resin can be reduced by introducing pore structure through chemical and physical mixing [
14]. However, both materials have some limitations in their employment in high-frequency communications and large-scale integrated circuits [
15]. PI resin is unstable in hydrothermal environments and has elevated dielectric properties. LCP resin is difficult to process and costly. Therefore, there is an urgent need to design and prepare novel resins with low dielectric constants and dielectric loss, high thermal resistance, and easy processing.
Poly (aryl ether ketone) resins, a high-performance engineering plastic, are widely used in various fields, such as electronics and electrical appliances, aerospace, transportation, and bionic materials, because of their excellent thermal, mechanical, and electrical properties. In addition, traditional poly (aryl ether ketone), such as poly (ether ether ketone) (PEEK), poly (ether ketone) (PEK), etc., have low dielectric constants and dielectric loss over a wide frequency range, and maintain dielectric stability under high humidity and heat exposure conditions [
12]. Additionally, their dielectric constant and dielectric loss can be significantly reduced by simple modifications. Therefore, PAEKs have the potential to be used as interlayer dielectric materials in high-frequency communications and large-scale integrated circuits. In sharp contrast, to the best of our knowledge, there is less existing research on the design and preparation of low-dielectric intrinsic poly (aryl ether ketone) resins. Mu et al. reduced the dielectric constant to 1.95~2.21@1MHz by introducing polyhedral oligomeric silsesquioxane (POSS) into the PAEK backbone. Although POSS led to a lower dielectric constant, it came at the expense of thermal properties, and the modified PAEK thermal degraded before 400 °C [
16]. Jiang et al. introduced perfluorononenyl pendant groups to obtain a PEEK-PFN-x with a low dielectric constant of 3.0@10KHz, low loss of 0.003@10KHz, and favorable thermal properties [
17]. Wang designed and fabricated a poly (arylene ether sulfone) (PES) film containing cyclohexane groups. After foaming, the dielectric constant of the resin was as low as 2.0@10MHz, and the dielectric loss reached 0.005@10MHz [
18]. From the above studies, it is not difficult to see that large volume groups, fluorinated substituents, and alicyclic segments can effectively reduce the permittivity of PAEK. Due to their low polarity, alicyclic cyclohexyl groups have been increasingly used to prepare low-dielectric materials [
19,
20]. However, few studies have reported low-dielectric PAEK resins with cyclohexyl groups.
In addition, conventional PAEKs have regular and symmetric molecular structures with a high degree of crystallinity, presenting insoluble and refractory properties that make processing extremely difficult. This is another obstacle to the application of PAEKs in high-frequency communication and large-scale integrated circuits. Previous studies have shown that polymers with twisted, non-planar molecular junctions often have excellent solubility properties [
21,
22,
23]. This is primarily due to the regularity of the molecular structure being disrupted and the ease with which solvent molecules enter the molecule. Diphenyl fluorene is aromatic, and exhibits stereoscopic distorted conformations [
21]. It has been shown that the introduction of diphenyl fluorene into PAEK molecules can lead to materials that are both thermally resistant and soluble [
21,
24].
To design and prepare novel PAEK resins with low dielectric constants and dielectric loss, high thermal resistance, and easy processing, a difluoride monomer, 1,4-di (4-fluorobenzoyl) cyclohexane (DFBCH), that contains cyclohexyl was prepared to reduce the polarity. A series of novel copolymerized PAEKs containing cyclohexyl and fluorene were prepared by solution polycondensation of DFBCH, 9,9-Bis(4-hydroxyphenyl) fluorene (BHPF), and 1,4-bis(4-fluorobenzoyl) benzene (BFBB). The material’s properties were tuned by controlling the content ratio of the two groups. The effects of the cyclohexyl contents on the properties of the resin were systematically investigated, especially dielectric properties, thermal properties, and solubility properties. It was shown that the introduction of weakly-polarized cyclohexyl groups into the backbone of PAEK effectively improves the solubility, optical transmittance, and dielectric constant of the resin while maintaining its excellent thermal resistance and mechanical properties. This makes it a promising candidate for use in high-frequency communications and large-scale integrated circuits.
2. Experimental
2.1. Materials
1,4-Cyclohexanedicarboxylic acid (cis-trans isomer mixture) (CHDA, 99%), terephthalic acid (PTA, 99%), thionyl chloride (SOCl2, 99%), fluorobenzene (AR), anhydrous aluminum trichloride (AlCl3, 99%), N, N-Dimethylformamide (DMF, AR), 9,9-Bis(4-hydroxyphenyl)fluorene (BHPF, 97%), anhydrous potassium carbonate (K2CO3, AR), sulfolane (99%), and N-Methyl pyrrolidone (NMP, AR) were purchased from Shanghai Maclean Biochemical Technology Co., Ltd. (Shanghai, China). Toluene (AR) was obtained from Xilong Science Co., Ltd., chloroform (CHCl3, AR) was obtained from Shanghai Lingfeng Chemical Reagent Co., Ltd. (Shanghai, China), and concentrated hydrochloric acid(HCl, 37%) was obtained from Guangdong Guangshi Reagent Co., Ltd. (Zhaoqing, China). Acetone was purchased from Sigma-Aldrich Inc. ((Merck, Darmstadt, Germany). All materials were used as received without purification.
2.2. Synthesis of DFBCH and BFBB
The synthetic route of DFBCH and BFBB is shown in
Scheme 1. A total of 140 g (0.813 mol) CHDA, 280 mL thionyl chloride, and a few drops of DMF were added to a 1 L three-necked flask equipped with a magnetic stirring under nitrogen flow, condenser tube and gas treatment device. The reaction was carried out at 80 °C until the system was transparent, and the excess thionyl chloride was removed by distillation under reduced pressure. The above-synthesized material and 210 g (2.1851 mol) fluorobenzene were added to a 1 L three-necked flask equipped with mechanical stirring under nitrogen flow and spherical condenser ice bath conditions. After the CHDC was fully dissolved, anhydrous aluminum trichloride (214 g, 1.6050 mol) was added, increasing the temperature to 70 °C and maintaining for 12 h. It was then cooled to room temperature and poured into an aqueous solution containing crushed ice, methanol, and hydrochloric acid. The coarse DFBCH was obtained by filtration after vigorous stirring. The white crystalline DFBCH was obtained by double recrystallization with DMF, yield 51% (
Supplementary Materials: 1H-NMR,
Figure S1; FT-IR spectrum,
Figure S2; MS spectrum,
Figure S3). The synthesis of BFBB is referred to as the DFBCH synthesis method. The pure BFBB monomer was obtained after two recrystallizations with DMAc, yield 60% (
Supplementary Materials: 1H-NMR,
Figure S4; FT-IR spectrum,
Figure S5; MS spectrum,
Figure S6).
2.3. Synthesis of PCBEKs
Using BHPF, BFBB, and DFBCH as raw materials, poly (aryl ether ketone) resins (PCBEKs) containing cyclohexyl/fluorene structures were synthesized by solution polycondensation with different monomer ratios. The synthetic route is shown in
Scheme 2. Resins were named according to the percentage of DFBCH and BFBB. For example, the copolymer resin is named PCBEK-C75B25 when the ratio of DFBCH to BFBB monomeric feedstock is 75:25. In addition, to exclude the influence of the molecular weight of the resin on its macroscopic properties, the molecular weight of the resin was uniformly designed to be 30,000. The preparation of PCBEK-C75B25 is presented below as an example. First, added the BHPF (5.5506 g, 15.84 mmol), DFBCH (4.0093 g, 12.21 mmol), BFBB (1.2764 g, 3.96 mmol), anhydrous potassium carbonate (3.0648 g, 22.176 mmol), 10 mL of mixed solvent (7/1, sulfone/NMP), and 20 mL of toluene into a 100 mL three-necked flask equipped with nitrogen flow, mechanical stirring, a water separation device, and a condensation reflux tube in turn. The reaction system is then heated to a reflux state to promote the BHPF production of phenoxide, and the by-product, water, is brought out with the reflux of toluene. After the water is entirely carried out, the toluene is distilled out and the reaction system is gradually heated to 190 °C. The solvent is added appropriately during the reaction period to suppress locally reactions that are too fast. The reaction is stopped when the viscosity of the reaction system no longer increases. The mixture is then poured into hot water containing hydrochloric acid, and a white fibrous product polymer is obtained, which is the crude product of PCBEK-C75B25.
2.4. Preparation of PCBEKs Films
First, the copolymer (1.5 g) was accurately weighed, the solvent NMP (10.5 g) was added, and the ultrasonic treatment was followed by 1 h of agitation at room temperature until the polymer was completely dissolved. The solution was uniformly coated onto a clean, dry glass plate. Last, the glass plate covered with the solution was treated with a series of temperatures (60 °C, 2 h; 800 °C, 1 h; 120 °C, 2 h; 160 °C, 1 h; 200 °C, 2 h) to obtain a PCBEKs copolymer film.
2.5. Characterization
The FT-IR spectra of the resins were recorded at room temperature using a Shimadzu IR Affinity-1 spectrophotometer in the 400 to 4000 cm−1 range. Bruker AVANCE Ⅲ carried out NMR measurements, and the solvent was deuterated chloroform (CDCl3) or deuterated dimethyl sulfoxide (DMSO-d6). Molecular weight information of the resins was obtained on Agilent PL 200. The thermal properties of the resin were analyzed using TA-Q50 and Netzsch DSC-200F3. The samples were dried at 120 °C for 24 h in a vacuum oven before the test. Dielectric property tests were executed via a vector network analyzer P5004A-200 (Keysight) at 10 GHz room temperature. Solubility tests were performed at room temperature at 0.05 g/mL in different solvents. Wide-angle X-ray diffraction (WAXD) experiments on polymer films were obtained on Rigaku MiniFlex600 using a Cu-Kα radiation (45 kV, 15 mA) source in the range of 5–80° with a scanning rate of 10°/min. The tensile strength of the resin was determined using a SUST CMT5000, according to the ASTM D882-2018 standard. The optical transmittance of the PCBEKs was measured by a UV-3600 Plus (Shimadzu, Japan) at wavelengths from 300 to 800 nm.