Soluble Perfluorocyclobutyl Aryl Ether-Based Polyimide for High

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Soluble Perfluorocyclobutyl Aryl Ether-Based Polyimide for High-Performance Dielectric Material Mingchen Jia, Yongjun Li, Chunqing He, and Xiaoyu Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09383 • Publication Date (Web): 12 Sep 2016 Downloaded from http://pubs.acs.org on September 17, 2016

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ACS Applied Materials & Interfaces

Soluble

Perfluorocyclobutyl

Aryl

Ether-Based

Polyimide for High-Performance Dielectric Material Mingchen Jia,a Yongjun Li,a,* Chunqing He,b and Xiaoyu Huanga,* a

Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules,

Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, People’s Republic of China b

Key Laboratory of Nuclear Solid Physics of Hubei Province, School of Physics and

Technology, Wuhan University, 16 Luojiashan Road, Wuhan, Hubei 430072, People’s Republic of China KEYWORDS: perfluorocyclobutyl aryl ether; polyimide; dielectric material; water absorption; k value.

ABSTRACT: High durability of low-k value is a desired property for dielectrics serving under humid conditions, because absorbing a small amount of moisture by the material can considerably increase the k value so as to result in function deterioration. Aiming to develop a dielectric polymer with superior durability of low-k value and high thermal stability, a perfluorocyclobutyl (PFCB) biphenyl ether-based polyimide, PFCBBPPI, was synthesized. This polymer possesses a Tg of 310.3oC and a 5% weight loss temperature of 510.5oC. PFCBBBPPI exhibited an extremely low water uptake of 0.065±0.018%, representing the best water resistance

ACS Paragon Plus Environment

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in polyimides. The increasing percentage in k value was below 2% for PFCBBPPI film exposed to moisture under various humidity conditions for 6 hours. PFCBBPPI film equilibrated at 75% R.H. for 2 weeks still kept its k value below 2.50, remarkably outperforming the Kapton® film. The remarkable water resistance and resulting high durability of low-k property displayed by PFCBBPPI are originated from the hydrophobic nature and small free volume fraction of the polymer, as confirmed by contact angle test and positron annihilation lifetime spectroscopy results. The outstanding moisture resistance and overall performance of PFCBBPPI make it a suitable candidate for dielectric applications under both dry and humid conditions.

1. INTRODUCTION The dielectric constant (k value) of a material is sensitive to moisture or water uptake by the material because of the high polarizability of water molecules.1,2 Moisture absorption in dielectric materials brings about an uplift in their k values and results in function deterioration.3 Excellent moisture resistance and consequent high durability of low-k property are greatly desired for dielectric materials serving under humid conditions.4 Polymer materials have advantages such as light weight, hydrophobicity, easy processability, and versatile structure modifiability over their inorganic counterparts, but they often suffer from poor thermal stability, which is strictly required in the production of microprocessor chips where wire bonding is involved. As it is well-known, polyimide is a kind of engineering polymer with high thermal stability and has been extensively used as high-k or low-k materials among other applications in electronics.5-11 However, the polar imide rings in polyimide backbone tend to absorb water, leading to poor stability of dielectric property, which is particularly troublesome for low-k materials. Electronic devices which are equipped with large-scale integrated circuit (LSI) or ultra-large-scale integrated circuit (ULSI) definitely experience wet weather during their use, the


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increase in k value of dielectric material will result in resistance-capacitance delay, cross-talk noise, and power dissipation, which is especially the case for electronic devices used for ocean exploration.3 As a result, developing polyimide-based low-k materials with high thermal stability and moisture resistance is of great importance. It has been reported that the introduction of fluorine atoms into polyimide could improve water resistance and reduce k value at the same time.12,13 Nonetheless, k values of these fluorous polyimides measured under humid conditions were still quite higher than those measured under dry conditions, which was probably because the typically employed large fluorine-containing groups could result in large free volume holes for water molecules to occupy.14,15 Recently, polymers containing perfluorocyclobutyl (PFCB) aryl ether moieties have drawn great attention due to their high thermal stability, high optical transparency, excellent processability, low water uptake, and low k values.16-19 It is noteworthy that Guenthner et al. reported a PFCB-containing polycyanurate network with a water uptake of 0.56±0.10%, which represented some of the lowest water uptake for this kind of polymer.20 This result inspired us to pursue a new dielectric polymer with superior thermal stability and moisture resistance by incorporating PFCB moieties into a polyimide backbone. In this contribution, we report a new fluorine-containing polyimide, PFCBBPPI, which contains PFCB biphenyl ether moieties in its backbone. It was synthesized from a diamine (4',4'''((perfluorocyclobutane-1,2-diyl)bis(oxy)) bis((1,1'-biphenyl)-4-amine), PFCBBPDA) and a dianhydride (4,4'-(hexafluoroisopropylidene)diphthalic anhydride, 6FDA) via a two-step procedure. The synthetic route as well as the chemical structures of PFCBBPDA, 6FDA, and perfluorocyclobutyl biphenyl-containing polyimide (PFCBBPPI) were presented in Scheme 1. It was expected that the introduction of PFCB moieties into a polyimide backbone would endow the polymer with high water resistance while maintain excellent thermal stability.


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Scheme 1. Synthetic Route of PFCBBPPI.

2. EXPERIMENTAL SECTION 2.1.

Materials

4,4'-((Perfluorocyclobutane-1,2-diyl)bis(oxy))bis(bromobenzene) was synthesized according to a previous report with a total yield of 56%.21 All other chemicals were purchased from Aladdin or Sinopharm Chemical Reagent Co. Ltd. and used as received unless otherwise specified. 4,4'(Hexafluoroisopropylidene)diphthalic anhydride (6FDA) was purified by recrystallization from acetic anhydride. 1-Methyl-2-pyrrolidinone (NMP) was distilled over CaH2 under reduced pressure prior to use. YC-05 Ag-based one-component conductive adhesive (volume resistivity 1~3×10-4 Ω·cm) that cures at ambient temperature was purchased from Nanjing Xilite Adhesive Co. Ltd. 2.2.

Instrumentation and Methods

All NMR spectra were recorded on a Bruker Avance 500 spectrometer (500 MHz) in CDCl3. Tetramethylsilane (TMS) and CDCl3 were used as internal standards for 1H NMR and 13C NMR, respectively; CF3CO2H was used as an external standard for 19F NMR. ESI-MS was measured by


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an Agilent FTMS-7.0 Fourier transformation mass spectrometer. Relative molecular weight and molecular weight distribution were measured by a conventional gel permeation chromatography (GPC) system equipped with a Waters 515 Isocratic HPLC pump, a Waters 2414 refractive index detector, and a set of Waters Styragel columns (HR3 (500-30,000), HR4 (5,000-600,000), and HR5 (50,000-4,000,000), 7.8×300 mm, particle size: 5 µm). GPC measurement was carried out at 35oC using tetrahydrofuran (THF) as eluent with a flow rate of 1.0 mL/min. The system was calibrated with linear polystyrene standards. FT-IR spectra were recorded on a Nicolet AVATAR-360 FT-IR spectrophotometer with a resolution of 4 cm-1. UV/vis optical transmission spectra were recorded on a Hitachi U-2910 spectrophotometer at room temperature. The transmittance of polyimide film was evaluated in the range between 300 nm and 750 nm. The refractive index and extinction coefficient of the film were measured on an M2000XF spectroscopic ellipsometer (J. A. Woollam Co. Inc.). Wide-angle X-ray diffraction (WAXD) measurement of polyimide film was conducted at room temperature using an X’Pert Pro MPD diffractometer (Panalytical Co.). The Cu Kα radiation (λ = 1.54Å) source was operated at 50 kV and 40 mA. Thermal gravity analysis (TGA) was conducted on a TA Q500 thermal analysis system in N2 with a heating rate of 10oC/min. Differential scanning calorimetry (DSC) was performed on a TA Q200 DSC instrument in N2 with a heating rate of 20oC/min using 5.8 mg of sample. Dynamic mechanical thermal analysis (DMA) for polyimide film (length: 30 mm, width: 10 mm, and thickness: 80 µm) was conducted on a TA Q800 dynamic thermomechanical analysis apparatus in air with a heating rate of 2°C/min and a load frequency of 1 Hz. The coefficient of thermal expansion (CTE) was measured by thermomechanical analysis (TMA) on an SDTS841e thermomechanical analyzer (Mettler Toledo, Switzerland). Specimens were made with 5 mm width, 10 mm length, and 0.060 mm thickness. The measurement was carried out


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during elongation with a heating rate of 5oC/min under N2 at a load of 0.05 N. Tensile properties were determined from the stress-strain curves obtained from an Instron 5867 universal material testing system with a strain rate of 1 mm/min. Measurements were performed with five film specimens (gauge length: 25 mm, width: 15 mm, and thickness: 0.085 mm). Each reported data is the average value of five different measurements. The cross-section morphology of the film was taken using a JSM-6390LV scanning electron microscopy (SEM). The morphology of the film surface was investigated using a JPK NanoWizard 3 atomic force microscopy (AFM). The water uptake experiments were carried out as follows: PFCBBPPI films (85 mm×75 mm×0.070 mm) were dried in vacuo at 55oC for 24 h and a constant weight (±0.0001 g) can be obtained for each film (about 0.5 g). The films were immersed in ultra pure water at 25oC for 24 h. The films were then taken out of water and the water absorbed on the surface was wiped off by a cleaning cloth. The weight of each film was weighed immediately, and five samples were measured for the test. The capacitances at 1 MHz were measured on an HP4194A impedance-gain phase analyzer. The silver adhesive was coated on both sides of the film upon exposing it to moisture for a period of time, and measurements were conducted after the adhesive fully cured. The dimension of the samples was 5×5 mm2 except for dry Kapton® samples which have a dimension of 10×10 mm2. The thickness of the Kapton® film was 0.210 mm and the thicknesses of PFCBBPPI films were between 0.050-0.085 mm. Five samples were measured and average values were given with the mean errors. Moisture uptake of the polyimide films at 75% R.H. was measured after exposing the samples to a humid atmosphere for a certain period of time. Three measurements for each sample were conducted. Static contact angle (SCA) was measured by an XG-CAMB1 device. Positron annihilation lifetime spectroscopy (PALS) was conducted on a RGM-1/APBS-2 (First Point Inc.) trap-based slow positron beam system.


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2.3.

Synthesis and Characterization of PFCBBPDA

To a solution of 4,4'-((perfluorocyclobutane-1,2-diyl)bis(oxy)) bis(bromobenzene) (4.0 g, 7.9 mmol) in a mixed solvent (120 mL of toluene plus 40 mL of ethanol) were added 4-aminophenylboronic acid pinacol ester (4.49 g, 20.5 mmol), tetrakis-(triphenylphosphine)palladium (0.81 g, 0.63 mmol), and potassium carbonate (8.86 g, 63.2 mmol). The resulting mixture was stirred at 85oC for 5 h under Ar followed by evaporating solvents under vacuum. The residue was subjected to column chromatography using n-hexane/ethyl acetate (v:v = 1:1) as eluent to afford 3.85 g of 4',4'''-((perfluorocyclobutane-1,2-diyl)bis(oxy)) bis((1,1'-biphenyl)-4-amine) (PFCBBPDA) as a pale yellow solid (92% yield). 1H NMR: δ (ppm) 7.45 (d, J = 8.0 Hz, 4H), 7.34 (d, J = 8.0 Hz, 4H), 7.18 (d, J = 8.0 Hz, 2H), 7.13 (d, J = 8.0 Hz, 2H), 6.72 (d, J = 8.0 Hz, 4H), 3.73 (s, 4H).

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C NMR: δ (ppm) 150.13, 149.06, 139.10, 138.84, 127.62, 127.39, 126.25,

118.91, 118.78, 114.60. 19F NMR): δ (ppm) -128.34, -128.93, -129.74, -130.34, -130.47, -130.88, -131.06, -131.15. FT-IR (KBr): ν (cm-1) 3449, 3366, 3222, 1627, 1509, 1318, 1265, 1192, 1108, 1012, 961, 828. ESI-MS: m/z 530.9 ([M]+), 531.9 ([M+H]+) and 532.9 ([M+2H]+). 2.4.

Synthesis and Characterization of PFCBBPPI

To a solution of PFCBBBDA (1.07 g, 2 mmol) in 13 mL of NMP was added 6FDA (0.896 g, 2 mmol). The resulting solution was stirred at room temperature for 18 h under Ar. The viscous polyamic acid solution was casted on a dry glass plate placed on an adjustable horizontal table and flowed naturally to uniformity. Evaporation of the solvent and the following imidization were conducted in a Muffle furnace. The heating program was 70oC for 1 h, 150oC for 1 h, 250oC for 0.5 h, and 300oC for 1 h. No control over the cooling process was carried out after the heating process. The polyimide film was peeled off from the glass plate immediately after immersion in water and dried at 55oC in vacuo for 24 h. GPC: Mn = 55,000 g/mol, Mw/Mn = 2.46.


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FT-IR (solid film): ν (cm-1) 1785 (C=O asymmetric stretch), 1720 (C=O symmetric stretch), 1378 (C-N stretch), 963 (PFCB ring), 722 (imide ring deformation). 1H NMR: δ (ppm ) 7.217.29 (m, 4H), 7.48-7.67 (m, 12H), 7.91-7.95 (m, 4H), 8.01-8.07 (m, 2H).

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F NMR: δ (ppm ) -

63.63 (6F, CF3), -128.44~-133.80 (6F, PFCB). 3. RESULTS AND DISCUSSION PFCBBPDA was prepared in a high yield by Suzuki coupling reaction between 4,4'((perfluorocyclobutane-1,2-diyl)bis(oxy))bis(bromobenzene) and 4-aminophenyl boronic acid pinacol ester. This PFCB aryl ether-based diamine monomer was characterized by NMR (Figure S1-S3), FT-IR (Figure S4), and ESI-MS. The PFCB aryl ether moiety in the monomer, formed via [2π+2π] dimerization of trifluorovinyl ether functionality, is either in cis or trans form with a distribution ratio of 1:1 according to previous literature.13 The [2π+2π] dimerization reaction of trifluorovinyl ether functionality was also employed by other groups to prepare polymers.22-26 The shortcomings of this method include that the reaction must be conducted under inert atmosphere and the residual trifluorovinyl ether groups can result in instability of the polymers. Forming PFCB aryl ether moiety before polymerization could overcome these shortcomings and afford polymers with superior stability. With an aim to produce a polymer with low water uptake and low k value, 4,4'-(Hexafluoroisopropylidene)diphthalic anhydride (6FDA), a commercially available fluorine-containing dianhydride, was chosen to polymerize with PFCBBPDA. Polymerization of PFCBBBPA with 6FDA was conducted using the two-step method. Stirring a NMP solution of PFCBBPDA and 6FDA at room temperature for 18 h gave a viscous polyamic acid solution. Thermal imidization of polyamic acid was performed in a Muffle furnace under air atmosphere. FT-IR spectrum of


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the dried film (Figure 1C) shows absorptions at 1785 cm-1 (C=O asymmetric stretch), 1720 cm-1 (C=O symmetric stretch), 1378 cm-1 (C-N stretch), and 722 cm-1 (imide ring deformation), which combining with the absence of amide and carboxyl absorption band indicated the complete conversion of polyamic acid to polyimide. Characteristic absorption corresponding to PFCB moiety appeared at 963 cm-1, indicating its stability during the imidization process. 1H and

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F

NMR spectra also confirmed the successful synthesis of PFCBBPPI. In 1H NMR spectrum (Figure 1A), all peaks appeared in the aromatic region and no signal attributed to amide or carboxylic acid functionalities was observed. And in

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F NMR spectrum (Figure 1B), the

characteristic signals of PFCB were located in the region between -128.44 ppm and -133.80 ppm while the signal of -CF3 appeared at -63.63 ppm.

Figure 1. 1H (A) and

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F (B) NMR spectra of PFCBBPPI in CDCl3; FT-IR spectrum of

PFCBBPPI film (C). PFCBBPPI was obtained as a transparent film with a pale yellow color (Figure 2A). The transmittance of the film with a thickness of 0.020 mm at 550 nm is 92% with a cut-off


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wavelength of 330 nm as determined by UV/vis spectroscopy (Figure S5). The high transparency of the film is originated from the decreased intra- and inter-chain charge transfer complex (CTC) by introducing fluorine atoms.27,28 The refractive index of the film at 632.8 nm is 1.58 with an extinction coefficient of 0.0557 measured on a spectroscopic ellipsometer. This low refractive index is obviously associated with the high fluorine content. XRD pattern of the film (Figure S6) displays no crystallization peak, which indicates that the film is completely amorphous. SEM image of the cross-section (Figure 2B) shows a homogeneous and nonporous morphology. AFM image (Figure 2C) of the film (thickness: 0.050 mm) revealed an average roughness (Ra) of 2.73Å, fully satisfying the thickness uniformity requirement for dielectric material.3

Figure 2. A picture of PFCBBPPI film (A); SEM image of the cross section of PFCBBPPI film (B) and AFM image of PFCBBPPI film (C). Solubility tests were performed by adding 2 mg of PFCBBPPI into 1 mL of solvent at room temperature. It was found that PFCBBPPI was readily dissolved in NMP, DMF, THF, chloroform, and dichloromethane at room temperature; however, it was insoluble in acetone and DMSO even upon heating (Table 1 and Figure S7). The PFCB-containing diamine residues in


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the polyimide backbone, with a random statistic distribution of cis and trans form, are expected to contribute to the excellent solubility of the polymer. Easy processability using common organic solvents, especially low-boiling-point solvents, makes PFCBBPPI suitable for solutionprocessing technology. GPC result indicated that PFCBBPPI had a number-average molecular weight (Mn) of 55,000 g/mol with a molecular weight distribution of 2.46. It is clear that PFCBBPDA has good activity to achieve a high molecular weight. In most cases, fluorous diamines tend to be less reactive than their non-fluorous counterparts and achieve polymers with low molecular weight.12 In this diamine containing PFCB biphenyl ether moiety, the strong electron-withdrawing ability of PFCB is covered to some extent by the increased distance between PFCB and the amino groups. Table 1. Thermal Analysis, Solubility Test Results and Mechanical Properties for PFCBBPPI Thermal Analysis parameter

Td,5%/oC

Td,10%/oC

Tg (tanδ)/oC

Tg (DSC)/oC

CTE (50-150oC)/ppm·K-1

value

510.5

527.6

310.3

299.3

54.6±6.3

Solubility Testa solvent

NMP

DMF

THF

solubility

+

+

+

CH2Cl2 CHCl3 +

+

acetone

DMSO

-

-

Mechanical Properties parameter tensile strength/MPa tensile modulus/MPa value

76.8±1.9

2,202±83


elongation at break/% 7.67±0.44

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a

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+: polymer (2 mg) was completely dissolved in the solvent (1 mL) to afford a homogeneous

solution; : polymer (2 mg) did not dissolve in the solvent (1 mL) even upon heating.

Thermal stability of PFCBBPPI was investigated by using TGA and DMA (Table 1). The 5% and 10% weight loss temperature of PFCBBPPI is as high as 510.5oC and 527.6oC, respectively (Figure S8). PFCBBPPI has a Tg of 310.3oC as determined by DMA (Figure S9), and the value measured by DSC (Figure S10) is 299.3oC, which is much higher than those of previously reported polymers containing PFCB aryl ether moieties,19,22 suggesting an increased rigidity of the backbone by introducing biphenyl groups. In general, high-Tg polyimides have relatively poor solubility.29 As it can be seen from Table 1, high thermal stability and excellent solubility was jointly achieved in PFCBBPPI so that thermal stability of PFCBBPPI can satisfy the requirements for dielectric materials (Tg > 300oC, Td > 400oC).4 The mechanical properties of PFCBBPPI film were measured using a universal material testing system as listed in Table 1. The tensile strength of PFCBBPPI film is 76.8±1.9 MPa, reflecting a high molecular weight. The tensile modulus and elongation at the break of PFCBBPPI film are 2,202±83 MPa and 7.67±0.44%, respectively, which are higher than the standards for dielectric materials (1000 MPa for tensile modulus and 5% for elongation at break).3 It should be noted that the mechanical properties can be further enhanced upon stretch orientation. The water uptake of PFCBBPPI was tested by immersing the film in water at 25oC for 24 h, which is the standard testing method for polyimide film. It is astonishing that an extremely low water uptake of 0.065±0.018% is exhibited by PFCBBPPI. For comparison, a DuPont Kapton® film was also examined to give a water uptake of 2.50±0.03%. The water uptake of PFCBBPPI represents the best water resistance observed in polyimides to date (0.08-3.8%).22


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The k values of PFCBBPPI and Kapton® were calculated according to the equation:30

k=

Cd Sk0

(1)

where C is the capacitance measured on an impedance-gain phase analyzer, d is the thickness of the film, S is the effective area of the film coated by electrodes, and k0 is the vacuum permittivity (8.854 pF/m). The k value for dry PFCBBPPI film measured at 1 MHz is 2.43, situating PFCBBPPI amongst the low-k polyimides (2.3-3.5).31 By contrast, the corresponding k value for Kapton® film is 3.10.

Figure 3. The k values of PFCBBPPI and Kapton® films measured at 1 MHz under humid conditions: k value and increasing percentage measured after placing the samples under various humidity conditions for 6 h (A); k value as a function of time measured at 75% relative humidity (B). The inset in (B) is the moisture uptake as a function of time measured at 75% relative humidity. The low-k sensitivity to moisture for PFCBBPPI and Kapton® films was investigated by exposing the samples to moisture at various humidity conditions and for various periods of time.


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The k values of Kapton® film measured at 1 MHz after equilibrating the samples at 43%, 57%, 75%, 88%, and 100% R.H. for 6 h were 3.86, 4.00, 4.10, 4.14, and 4.30, respectively, with the increasing percentages between 24.6% and 39.1%. By striking contrast, the k value of PFCBBPPI film measured under the same conditions was between 2.42 and 2.47 with the increasing percentages below 2% (Figure 3A). Moreover, we can also see from Figure 3B that prolonged exposing time did not make a significant difference to the increasing percentage of PFCBBPPI film and PFCBBPPI sample equilibrated at 75% R.H. for 2 weeks still kept its k value below 2.50, remarkably outperforming Kapton® film (3.10 to 4.22). Consistent with the change in k value, the moisture uptake of Kapton® film increased with the extending of time (1.50% after 8 h at 75% R.H.), while the weight of PFCBBPPI film remained nearly constant with the moisture uptake below 0.035% (inset in Figure 3B). Thus, it is clear that excellent moisture resistance of PFCBBPPI contributes to its excellent durability of low-k property. The reported increasing percentages in k value for other polyimides upon moisture absorption were between 3.3% and 14.8%.12,32,33 The excellent durability of low-k property exhibited by PFCBBPPI is a highly valuable property for dielectrics used under humid conditions.

Figure 4. Micrographs of water droplets on PFCBBPPI (A) and Kapton® (B) films. The water resistance of PFCBBPPI is certainly associated with the hydrophobic nature of the fluorinated moieties. We first examined the wettability of PFCBBPPI film by water via contact


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angle test. A contact angle of 95.3o was observed for a water droplet on a PFCBBPPI film (Figure 4A), while the value of Kapton® film was as low as 50.9o (Figure 4B). Thus, the hydrophobic nature of PFCBBPPI originating from the introduction of fluorine atoms is certainly considered to contribute to the excellent water resistance property. However, the contact angle just reflects the surface property of the film, it can not explain the mechanism completely. For a polymer with pendant perfluorodecylthio groups, the contact angle was 107.03o whereas the water uptake was 0.32% after immersing the film in water at 25oC for 24 h.34 It is clear that a large contact angle is not the sole contributor to the remarkable water resistance. The absorbed water molecules occupy the free volume holes in a bulk material, therefore, a small fraction of free volume can result in low water uptake.14 To get some insight into the free volume in PFCBBPPI film, positron annihilation lifetime spectroscopy (PALS) was employed. In PALS, positronium formed from positrons can annihilate from para-state (p-Ps) and ortho-state (o-Ps). The self-annihilation time of p-Ps and o-Ps in vacuum is 125 ps and 142 ns, respectively. In bulk materials, the lifetime of o-Ps will be reduced to 1-4 ns as a consequence of its interaction with the electrons surrounding the molecules. Therefore, the lifetime of o-Ps is dependent on the size of free volume holes in a material and the corresponding intensity is associated with the density of the holes.35 The decay curve of PFCBBPPI was decomposed into four components using the data processing program PATFIT and the analyzed data are summarized in Table S1. The free volumes are regarded as spherical voids in the film and an average radius of 3.51 Å is calculated out according to the semi-empirical equation:36


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 r 1  2πr   τ o-ps =0.5ns 1+ sin    1+∆r    1+∆r 2π

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-1

(2)

where τo-Ps = 2.81 is the lifetime of o-Ps and r is the average radius of the free volume holes, expressed in ns and Å, respectively; ∆r = 1.66 Å is the fitted empirical electron layer thickness. It can be seen from Table S1 that the small size of free volume is accompanied with a rather low intensity, which is reflective of a low density of the free volume holes. A fraction of free volume (fv) of 1.77% can be calculated out according to the equation:37

4  f v =A  πr 3  I 3 

(3)

where r (in Å) is the radius of the free volumes and I (in %) is the intensity of o-Ps, A = 0.0018 has been accepted for conventional glassy polymers.34 The free volume fractions for other amorphous polymers are between 5.7% and 14.8%,38-40 much higher than that of PFCBBPPI. Though no PALS data were given, it is probably because that the higher fv of the perfluorodecylthio substituted polymer with a higher contact angle (107.03o>95.3o) resulted in its higher water uptake (0.32%>0.065%). The higher Tg of PFCBBPPI (310.3oC) in comparison with that of the perfluorodecylthio substituted polymer (209oC) also indicates a relative small fraction of free volume. Now, it can be concluded that the extremely low water uptake and remarkable durability of low-k property displayed by PFCBBPPI are resulted from the combination of the hydrophobic nature and small free volume fraction of the polymer. 4. Conclusions


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In summary, a new solution-processable polyimide (PFCBBPPI) containing PFCB biphenyl ether moieties in its backbone was synthesized. PFCBBPPI has excellent thermal stability with a Tg of 310.3oC and a 5% weight loss temperature of 510.5oC. In addition, PFCBBPPI film has excellent uniformity, high transparency, and outstanding mechanical properties. What is most important, PFCBBPPI displays remarkable water resistance with the water uptake being 0.065±0.018% (24 h, room temperature), resulting in its excellent durability of low-k value (2.43-2.50) upon moisture absorption for a prolonged period of time which far outperformed the commercial Kapton® film (3.10-4.22). The outstanding moisture resistance and consequent excellent durability of low-k property displayed by PFCBBPPI are resulted from the combination of the hydrophobic nature and small free volume fraction of the polymer, as confirmed by contact angle test and PALS results. The remarkable moisture resistance and overall performance of PFCBBPPI make it suitable for use as on-chip insulators both under dry and humid conditions. ASSOCIATED CONTENT Supporting Information. NMR and FT-IR spectra of PFCBBPDA; UV/vis spectrum of PFCBBPPI film; WXRD pattern of PFCBBPPI film; photos of PFCBBPPI in different solvents; TGA, DMA, and DSC curves of PFCBBPPI; and analyzed data for PALS. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; [email protected] Author Contributions


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The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The authors thank the financial support from National Basic Research Program of China (2015CB931900), Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20020000), and Shanghai Scientific and Technological Innovation Project (14JC1493400 and 16JC1402500).

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Soluble Perfluorocyclobutyl Aryl Ether-Based Polyimide for High

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