Low Dielectric Constant Polyimide Hybrid Films Prepared by in Situ

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Low-Dielectric-Constant Polyimide Hybrid Films Prepared by In Situ Blow-Balloon Method Zhao Chen, Dandan Zhu, Faqin Tong, Xuemin Lu, and Qinghua Lu ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.9b00448 • Publication Date (Web): 19 Jul 2019 Downloaded from pubs.acs.org on July 21, 2019

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ACS Applied Polymer Materials

Low-Dielectric-Constant Polyimide Hybrid Films Prepared by In Situ Blow-Balloon Method Zhao Chen,a Dandan Zhu,a Faqin Tong,b Xuemin Lu,a and Qinghua Lu a,b,* a

School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China

b

School of Chemical Science and Engineering, Tongji University, Shanghai 200092, China *Email: [email protected]

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ACS Applied Polymer Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Insulating materials with a low dielectric constant and a low dielectric loss are in high demand to meet requirements of the continuous miniaturization of electronic devices and high speed information transmission. We fabricated porous polyimide/reduced graphene-oxide (PI/rGO) hybrid films with an ultralow dielectric constant and a low dielectric loss via a facile ‘blow-balloon’ method. Poly (ethylene glycol) (PEG) was used as a pore-forming agent by mixing with polyamide acid carboxylate (PAAC) and graphene oxide (GO) in water. The ternary mixture of PAAC/GO/PEG formed a hydrogel and dried naturally into a xerogel film. Porous PI/rGO films were obtained by in situ thermal pyrolysis of PEG through heating PAAC/GO/PEG hybrid films to 400°C. The homogeneous porous structure could be regulated easily by adjusting the mass ratio of PEG to PAAC/GO. The porous structure inside the hybrid films prepared by the hydrogel method provided the PI/rGO films with an ultralow dielectric constant (κ) as low as 1.9 and a high ductility with an average elongation at break above 38%. This porous PI/rGO film with an ultralow κ value may be a promising candidate for next-generation interlayer dielectrics. Keywords: Poly (ethylene glycol), Blow-balloon method, Polyimide/reduced graphene oxide, Ultralow-dielectric constant.


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Introduction Polyimide (PI) materials are used widely as insulating materials in microelectronics because of their high thermal and chemical stability, low water absorption, excellent mechanical properties and relatively low κ values between 3.0–3.5.1-8 As the integration of microelectronic devices continues to increase, the chip size decreases, and the delay time of the signal transmission in the chip increases. This delay time is proportional to the dielectric constant of the interlayer insulating material. To increase the signal transmission speed, it is necessary to lower the dielectric constant of the interlayer insulating material. Therefore, PIs with a low dielectric constant (low κ) are in high demand. A reduction in molecule molar polarizability and an increase in free polymer volume are the main ways to reduce the dielectric constant. Several strategies have been developed to achieve this purpose; these include blending porous inorganic matter into a PI matrix9 and developing monomers with fluorine-containing groups or an asymmetric structure.

10-17

PI films with κ values as low as 3.04 have been

developed by using monomer 4,4'-(hexafluoroisopropylidene) diphthalic anhydride with a small dipole polarization of the C-F band.18 Tsai et al. developed an asymmetric monomer; its high steric hindrance decreased the dielectric constant of the corresponding PI to 2.72.19 However, the use of special monomers has limited the commercialization of such low-dielectric-constant polyimides.20 The introduction of a porous structure into the PI matrix appears to be a simple and effective approach to reduce the dielectric constant of the PI films.21 A pore-forming method was developed by Chen and involved grafting a thermal degradable side chain onto the main chain of the PI.22 After thermal degradation at 250°C, nanosized pores remained in the PI films because of side-chain decomposition. A low κ value of 1.9 was achieved as the porosity of PI up to 8%. A porous hybrid PI/Si film with a κ value of 1.84 was prepared by Jin et al.23, who used 20 wt.% silica to introduce sufficient pores in the PI films by chemical etching. However, the introduction of porous structures has led to an enormous loss in flexibility of the PI films. As a result, the porous PI films became



crumbly with elongation at break down to 5%, which limited their practical application. The incompatibility between the PI matrix and the inorganic filler is considered to be the main cause of the ductility deficiency of PI/inorganic hybrid films. 24-27 Microphase separation control of the PI matrix and inorganic filler are a key point in the preparation. In this work, a blow-balloon method was developed to produce porous PI/rGO films based on our previous study.28 The Blow-balloon method introduced in this work is different from Blow Molding in the industrial process. First, graphene oxide (GO), poly (ethylene glycol) (PEG) and polyamide acid carboxylate (PAAC) were mixed in water to form a stable ternary blending system at 60°C. The solution was cast on clean glass, and a PAAC/GO/PEG hydrogel film could be obtained by gradual cooling to room temperature, followed by natural drying and thermal treatment at 400°C to obtain a porous polyimide/reduced graphene oxide (PI/rGO) film. During thermal treatment, large amounts of gases were produced because of PEG pyrolysis, which blew mini-balloons in the PI/rGO films and formed a large amount of porous structures in the hybrid films. PI/rGO films behaved as a plastic, which can blow up like balloons above their Tgs. Here, rGO that is derived from a reduction of the GO during thermal imidization, which acted as a cross-linker to reinforce the formed pore walls, prevented shrinkage and controlled the balloon sizes. The choice of GO as a cross-linker also takes into account its good dispersion in water and organic solvents. In addition, a small dose of rGO cannot form conductive channels in PI films, which affects the insulation of the polyimide. By controlling the PEG dosage, the pore size and density could be manipulated, which resulted in a homogeneous porous PI/rGO film. The obtained porous PI/rGO film possessed an ultralow κ value below 2.0 while retaining its high flexibility, which may provide an optimal dielectric insulating layer for electronic devices. Experimental section Materials


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N-Methyl pyrrolidone (NMP, 99%) was from Shanghai Lingfeng Chemical Reagent Co., Ltd., China. 3,3′,4,4′-Biphenyltetracarboxylic dianhydride (BPDA, 99%) was from ChinaTech Chemical Co., Ltd. (Tianjin, China). 4, 4′-Oxydianiline (ODA, 99%) was from J & K Chemical Technology. KMnO4 (99%), H2SO4 (98%), H3PO4 (85%) and HCl (35%~37%) were from Sinopharm Group Chemical Reagent Co., Ltd. Natural graphite was from Beijing Inoke Technology Co., Ltd. All reagents were used directly without further purification. Preparation of Graphene Oxide (GO). GO nanosheets were prepared from natural graphite (Gr) flakes via the improved Hummers method29. In brief, Gr flakes (3.0 g, 1 wt equiv) were added to a 400mL mixture of concentrated H2SO4/H3PO4 (360 mL/40 mL) and KMnO4 (18.0 g, 6 wt equiv) was added slowly into the above solution in six batches. The mixture was heated to 50°C and stirred for 15 h. The reaction solution was cooled to room temperature, poured onto ice (1000 mL) with 30% H2O2 (4.0 mL) and filtered, the remaining solid was washed in succession with 200 mL of water, 200 mL of 30% HCl, 200 mL of ethanol and deionized water. After three cycles of deionized water washing–centrifugation, GO nanosheets were obtained. The GO nanosheets were freeze-dried and redissolved into deionized water at 20 mg/mL for further use. Preparation of PAAC/GO/PEG Aqueous Solution. Water-soluble polyamide acid carboxylate (PAAC) fibers were fabricated according to a previous report.30 In brief, equivalent molar ratios of 2.000 g ODA and 2.946 g BPDA were dissolved in NMP with a solid content of ca 15 wt.%. After mechanical stirring for 5 h under nitrogen, 1.00 g trimethylamine (TEA) was added into the solution to form a PAA carboxylate (PAAC). The PAAC solution was held at 40°C for 4 h to remove bubbles, and then poured slowly into ice water. PAAC fibers were collected and washed with deionized water. Pure PAAC fibers (Mw > 70 K, Figure S1) were obtained through freeze drying. PAAC fibers (4.00 g) were dissolved in 100 mL (2 mL, 20 mg/mL GO + 96 mL H2O + 2 mL TEA) GO solution at 60°C (the mass ratio of GO to PAAC was approximately 1:50). After stirring the mixture for 2 h, a homogeneous and stable PAAC/GO aqueous solution could be obtained. PEG with a



molecular weight of 4000 was added into the above solution with the mass ratios of PEG to PAAC at 5:100, 10:100, 20:100 and 40:100, respectively. After stirring for 30 min, a homogenous mixture of PAAC/GO/PEG was obtained. Preparation of Porous PI/rGO Films. The above prepared PAAC/GO/PEG solution was poured onto a clean glass plate at 40°C and fully spread out, followed by cooling to 25°C, and a PAAC/GO/PEG hydrogel film was formed. After being dried naturally at ~25°C and a 30% relative humidity for 12 h, a PAAC/GO/PEG xerogel film was prepared. The xerogel film was thermally treated in a vacuum oven according to a heating procedure: 100, 150, 180, 220, 250, 300 and 350°C, each for 30min, finally heated at 400°C for 120 min. A porous PI/rGO film was obtained with approximately 2.0 wt.% rGO. By tuning the dosage of PEG to PAAC, a series of porous PI/rGO films could be prepared, which were defined as PI/rGO-p0, PI/rGO-p5, PI/rGO-p10, PI/rGO-p20 and PI/rGO-p40 (PI/rGO-px, here, x = 0, 5, 10, 20, 40....), where the mass ratios of PEG to PAAC before films formation were 0:100, 5:100, 10:100, 20:100, and 40:100, respectively. Another group of PI films without GO was prepared by using the mass ratio of PAAC to PEG at 100:0, 100:10, 100:20, 100:40, 100:60, 100:80, and 100:100 as a control, and the corresponding PI films were defined as porous PI, PI-p10, PI-p20, PI-p40, PI-p60, PI-p80, and PI-p100 (PI-px, x = 0, 10, 20, 40, 60, 80, 100). Here, a porous PI film that was referred to as pure PI was prepared by the hydrogel method to distinguish it from the pure PI film (defined as PI-O) that was prepared by the organic-solution route. Characterization Thermal gravimetric analysis (TGA) was performed on a Q50 (TA Instruments, USA) from 40 to 800°C at 20°C/min under air. The glass-transition temperature (Tg) was investigated for 30–320°C by using a DMA Q800 (TA Instruments, USA) instrument at 5°C/min under nitrogen. The tensile modulus and elongation at break were investigated by using an Instron 4456 test machine (Instron Corp, USA) according to ASTM D882-91 (Standard Test Method for Tensile Properties of Thin Plastic Sheeting). Measurements were made on five different samples of dimensions


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of ~30 mm × 4 mm × 100 μm to ensure accuracy. Field-emission scanning-electron microscopy (Sirion 200, FE-SEM) was applied to investigate the PI product microstructure. The sample cross sections were fabricated in liquid nitrogen. The sample surface was cleaned with ethanol. All specimens were treated by dessication and sprayed gold before SEM analysis. The dielectric constant of the PI samples was measured by using a Dielectric/Impedance Spectrometer (Concept 40, Novocontrol Company, Germany) from 1 Hz to 10 MHz. PI and PI/rGO-px samples were prepared by being sprayed gold on both sides for 3 min. Contact-angle measurements were carried out with a Contact Angle Apparatus (DSA30). One drop of deionized water (4 μL) was used. Density measurements were performed on a multifunctional densimeter (FK-120S, Furbs Corp., China) by using a solid model and pure alcohol as a medium. Porous PI and PI/rGO-px samples were measured at least five times to ensure their accuracy. Before the test, polymer films were dried at 150°C in a vacuum oven for 2 h. The corresponding error range for this measurement was 0.013 g/cm3 (2.2%). The porosity (P) of the PI/rGO composite films was obtained from: P= (V - V0)/V= (ρ0 -ρ)/ρ0; where V and ρ are the volumes and densities of the porous PI and PI/rGO-px samples that were prepared by the hydrogel method. V0 and ρ0 are the volume and density of the pure PI-O control that was prepared from the organic solution route. The measured ρ0 was 1.335 g/cm3. Results and Discussion Fabrication of Porous PI/rGO-px Films. A blow-balloon method was proposed to prepare porous PI/rGO-px films (Scheme 1). Polyamide acid carboxylate hydrogel was prepared and used,

30, 31

which is a

relatively green process. A natural drying process is necessary to convert the PAAC/GO hydrogels to xerogels, because rapid drying leads to rough hybrid film surfaces, which yields a brittle product. During imidization at high temperature, GO was reduced to rGO. This phenomenon has been reported previously.32-36 Most importantly, PEG in the xerogels can decompose gradually and generate bubbles, which facilitates macroporous structure formation in the PI/rGO films. Figure 1



shows the Tg of the PI/rGO-p0 and the thermogravimetric curve of the PEG, respectively. PEG started to decompose at 200°C until 400°C. The PI/rGO composite could be reprocessed above its Tg of ~303.6°C.37 Thus, the pyrolytic gases of the PEG blew mini-balloons in the PI/rGO films and resulted into the formation of porous PI/rGO-px films. After cooling to room temperature, pores were saved in situ with the assistance of rGO. The sizes and densities of the porous structure could be tuned by regulating the loading level of the PEG. Full PEG pyrolysis was required to reduce the κ value of the obtained porous PI/rGO-px films because of the high κ value of the PEG.38 Figure S2 shows the TGA curves of the PI/rGO-p40 after thermal treatment at different temperatures. The PI/rGO-p40 film that was treated at normally used imidization temperature of 250°C which underwent a mass loss started from ~300°C because of the incomplete decomposition of PEG. Therefore, in the following work, all PAAC/GO/PEG samples were treated at a higher temperature of 400°C. Scheme 1. Schematic Diagram for Preparing Porous PI/rGO-px Films by the Blow-balloon Method.

Fourier-transform infrared spectra were obtained to confirm the imidization of PI and the PEG decomposition, as shown in Figure S3. Symmetric and asymmetric absorption peaks from C=O groups in the imide rings were observed at  = ~1777 and 1720 cm-1, respectively. A C-N-C bond stretching vibration of the imide rings was


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observed at  = 1374 cm-1. The absence of characteristic peaks at  = 3220–3450 cm-1 (N-H stretching) indicated the complete imidization of the obtained PI and the elimination of TEA. The characteristic peaks of methylene of PEG at  = ~2910 cm-1 disappeared as the curing temperature reached 400°C, which indicated the full thermal degradation of PEG.

Figure 1. Diagrams for Tg curve of PI/rGO-p0 and TGA curve of PEG4000. Morphologies of Porous PI/rGO-px Films. For comparison, the porogenic effect of PEG in a naked PI film was investigated. As shown in Figure 2a-f, the balloons grew gradually with an increasing PEG content, and reached a maximum when PEG increased to 40% (Figure 2c). A further increase in PEG dosage caused a collapse and disappearance of the porous structure in the PI films as shown in Figure 2d–f. Previous reports have proven that chemical bonds can be formed between the GO and PAA chains at a high temperature.39 GO sheets acted as a strengthening reagent to enhance the pore-wall strength.40-43

Figure 2. SEM images of neat PI films with increased PEG contents in PAAC: a–f)



10, 20, 40, 60, 80 and 100 wt.% PEG contents, respectively. However, we also knew that an increase in the amount of GO will increase the dielectric constant of the composite, 44 which is undesirable in electrical insulation. To do this, we must investigate the dosage of GO used to balance the performance of the porous composite films. PAAC/GO films with mass ratios of GO to PAAC at 1:50, 2:50, 4:50 and 5:50 were manufactured and converted into PI/rGO films by thermal imidization. The effect of GO loading content on the dielectric constant of the PI/rGO films is shown in Figure 3. As the GO dosage increased from 2.0 to 10.0 wt.%, a considerable increase in the dielectric constants from 3.68 to 11.56 at 1 MHz was observed. However, the addition of 2.0 wt.% GO led only to a small increase in dielectric constant compared with the porous PI film. Therefore, 2.0 wt.% GO was used to prepare PI/rGO-px films in subsequent work.

Figure 3. κ value of porous PI and PI/rGO films vs dosages of GO in PAAC. SEM images of porous PI/rGO-px films using different contents of PEG are presented in Figure 4. Several tabular macropores parallel to the surface were visible in the PI/rGO-p0 film when 2.0 wt.% GO was used (Figure 4a–b). The formation of tabular macropores in the PI/rGO-p0 may result because of hydrogen bonds between the GO and PAAC, which prevent the complete collapse of the hydrogel structure during drying.28 After PEG incorporation, the PI/rGO-px film presented a porous character as shown in Figure 4e–h. The porous structures in PI/rGO-px were produced by pyrolyzed gases of PEG. And the gases expanded towards every direction, resulted in spherical structure. The pore size increased from ~500 to ~1000


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nm with an increase in the PEG dose from 5 to 40 wt.% (Figure 4i–l and Figure S4). The cross-sectional image of the PI/rGO-px films showed that, different from open cells of the PI aerogels,11, 45 these pores were independent closed hole structure. The porous film surface that was prepared in this work was covered by dense packing layers on both sides (Figure 4c–d, Figure S5). During natural hydrogel drying, phase separation between the PEG and PAAC occurred with water volatilization. PEG molecular chains shrunk into the interior of the xerogel because of their high surface energy, whereas PI with a relatively low surface energy became a component of the surface layer. When the PEG inside the hybrid films was pyrolyzed at 200–400°C, bubbles were blown to form a closed cavity. The dense layer of the bottom surface (2 μm) was thinner than the dense layer of the top surface (6 μm) because the atmosphere in contact with the upper surface of the hybrid film had a much lower surface energy than the glass sheet that contacted the lower surface of the hybrid PI film. This type of hybrid PI film with inside closed pores and smooth surfaces is exactly what electronic devices require.

Figure 4. SEM images of porous PI and PI/rGO-px films: a, b and c are cross sections of the neat PI film, PI/rGO-p0 film and porous PI/rGO-p10 film, respectively; d is an image of PI and PI/rGO-px; e–h are cross sections of porous PI/rGO-px films tuned by PEG with mass ratios to PAAC at 5, 10, 20 and 40 wt.%, respectively; i–l are cross sections of the enlarged area in e–h, respectively.



Dielectric Properties of Porous PI/rGO-px Films After incorporation of porous structures, the dielectric constants of the PI/rGO-px films decreased significantly with an increase in PEG contents, which benefited from the low dielectric constant of air (κ = 1). For comparison, a conventional PI film was prepared as a control by using the organic solution route (PI-O). As shown in Figure 5a, the neat porous PI film that was prepared by the hydrogel method (red line) showed a lower dielectric constant (κ = 2.77) than that of the PI-O film (κ = 3.69). After application of 2.0 wt.% GO, PI/rGO-p0 exhibited the highest dielectric constant of 3.86 because of the conductivity of rGO. By increasing the PEG from 5 wt.% to 40 wt.%, the porosity of PI/rGO-px increased from 29.3% (PI/rGO-p5) to 42.5% (PI/rGO-p40). (Table S1 and Figure S6a) Consequently, the dielectric constant of the PI/rGO-px films decreased steadily from 2.60 to a lowest value of 1.93. PEG is also effective for reducing the dielectric constant of pure porous polyimide films, but because only small pores could be formed, the dielectric constant of PI-px at 1.0 MHz showed a slight decrease from 2.77 (porous PI) to 2.44 (PI-p40) as shown in Figure S7. It is well known that the dielectric constant of composite is high at low frequency (0.84 GPa). The porous PI/rGO-px films possessed a smooth and hydrophobic surface (CA > 91.2°) and a lower density (ρ < 1.0 g/cm3). This hybrid PI film with a novel mesoporous structure and special electrical properties has wide application in


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many fields, such as communication and electronic devices. This green and environmentally friendly blowing bubble film-preparation method provides a new method to prepare polyimide hybrid films. Supporting Information Supporting information is available from the author or from the ACS Publications website at…. Supporting information: Molecular weight of PAA fibers; TGA curves of PEG, PAAC/GO-p40 and PI/rGO-p40; Fourier-transform infrared spectra of PI and PI/rGO-px films; Pore-size measurement of PI/rGO-px hybrid films; Surface morphology of PI/rGO-p40 film; Relationship between dielectric constant and porosity; Dielectric constants of PI-px and PI/rGO-px at low frequencies; High resolution SEM image of porous PI; Table list of physical properties for porous PI and PI/rGO-px films; Contact angles of PI/rGO-px films; Mechanism diagrams for ductile porous PI/rGO-px films. AUTHOR INFORMATION Corresponding Author:*E-mail: [email protected] ORCID Xuemin Lu: 0000-0002-2057-6280 Qinghua Lu: 0000-0003-1522-1458 Acknowledgement The authors are grateful for financial support from Key Projects of the National Science Foundation of China (Grant No. 51173103), the National Program on Key Basic Research Project (Grant No. 2014CB643605) and the Key Project of Science and Technology of Shanghai (Grant No. 16JC1403900). Conflict of Interest The authors declare no conflict of interest. References



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Low Dielectric Constant Polyimide Hybrid Films Prepared by in Situ

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