Novel soluble polyimides containing pyridine and fluorinated units: preparation, characterization, and optical and dielectric properties

Abstract

To acquire low dielectric constant polyimide films with good mechanical and thermal properties and low coefficient of thermal expansion (CTE) applied in microelectronic fields, as a feasible tactic, three novel diamines containing pyridine and –C(CF₃)₂– groups were firstly designed and synthesized to employ polymerization with 2,2′-bis(3,4-dicarboxyphenyl) hexafluoropropanedianhydride (6FDA) via a two-stage process with a heating imidization method. Three diamine monomers included one unsubstituted pyridine ring, and another two methyl-substituent groups on two pyridine rings at the 6- and 4- position. The structure–property relationships between the different pyridine rings of the fluorinated PI films, including dielectric constant, thermal stability, mechanical strength, optical transparency, and solubility, were systematically investigated. The fluorinated PI films exhibit low dielectric constant in the range of 2.36–2.52 at 1 MHz, while they still display excellent mechanical properties with tensile strengths as high as 114 MPa. Meanwhile, the PI films show good thermal stability with glass transition temperatures (T_g) in the range of 262–275 °C, low coefficients of thermal expansion (CTEs) ranging from 64 to 68 ppm °C⁻¹ and 5% weight loss temperatures (T_5%) located between 468 °C and 499 °C. Further, PI films possess outstanding solubility for easy fabrication. Therefore, these types of fluorinated PI films provide a potential application as alternative dielectric layers in future microelectronic technology.

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Introduction

Materials with low dielectric constant (low-k) have been a subject of interest in terms of their potential for use in high performance electronic devices.¹ Low dielectric constant materials have many advantages, such as reducing the resistance–capacitance delay, the capacitance between metal interconnects, line-to-line crosstalk noise, and power dissipation in the new generation of high-density integrated circuits.^2–4 In addition to having a low dielectric constant, the next generation of interlayer dielectrics for submicron and nano-level electronics must also meet various requirements, such as good thermal stability, low moisture adsorption, and good adhesion to semiconductor and metal substrates. As a representative high-performance plastic, polyimides (PIs) are generally regarded as a suitable candidate because of their low molecular polarizability and excellent thermal and mechanical properties.^5–8 Moreover, the outstanding toughness, light weight and hydrophobicity of PIs also endow them wide applications in microelectronic fields. In fact, many advantages of PIs benefit from their rigid backbones, strong interchain packing and the formation of charge transfer complexes between moieties of electron-donating diamine and electron accepting dianhydride in their macromolecular chains.^4,9 Actually, some PIs with low dielectric constants have been to meet the demand for flexible printed circuits fields.

With the development of production technology of deep-submicron grade mono-crystalline silicon, ultra large scale integrated circuits (LSICs) have experienced a rapid growth of integration level and enhanced the progress of electronic technology.¹⁰ Owing to their excellent overall performance, polyimides (PIs) are better than the silica-based materials partially. Even so, their dielectric constants are still too high to fulfill the requirements of future microelectronic applications. That is why many works have focused on new PIs with lower dielectric constants, such as fluorinated PI films^11–16 and porous PI films,^17–21 and the major challenge is how to evolve lower dielectric constants PIs, but without weakening other excellent features, such as mechanical properties, hydrophobicity and coefficient of thermal expansion (CTE). Incorporation of fluorine atom and pores structure into PI backbone reportedly can lower its dielectric constant, yet fluorine atom generally increases chain flexibility and reduces the glass transition temperature of PI and the embrittlement of C–F bond also gives a negative effect on its mechanical property.^10,22 Meanwhile, nano- or micro-porous structures commonly detrimentally affect the mechanical property and CTE of PIs because of the space structure destruction and thus limit their use. Many other methods have been developed in a continuous research aiming at a balance of dielectric constant and mechanical properties of polyimide.

In this paper, a series of new polyimides were synthesized based on three novel containing pyridine and fluorinated group (–C(CF₃)₂–) diamines. We investigated the effects of pyridine and fluorinated group (–C(CF₃)₂–) in the backbone of the polyimide on the dielectric constant, mechanical property and thermal dimensional stability. In view of the significant effects of chemical structure on dielectric property, three fluorinated diamines were firstly designed. These diamine monomers included one unsubstituted pyridine, and another two methyl-substituent groups on two pyridine rings at 6- and 4- position, respectively. The methyl group at different positions was selected because of its bulkiness to its chains which leads to the disruption of chain packing and the different effects on properties. Additionally –CH₃ and –CF₃ groups allow for improving solubility in casting solvents to fabricate easily. The obtained polyimide films were comprehensively investigated for their dielectric constant, thermal, mechanical and optical properties to elucidate the critical structural factors need for the microelectronic applications.

Experimental

Materials

2,2′-Bis(3,4-dicarboxyphenyl)hexafluoropropanedianhydride (6FDA) was purchased from Sinopharm Chemical Reagent Beijing Co. Ltd, and thermally treated in vacuo at 180 °C overnight prior to use. 4,4′-(Hexafluoroisopropylidene)diphenol (6FBPA) (Acros), 6-chloro-2-methyl-5-nitropyridine (Acros), 2-chloro-4-methyl-5-nitropyridine (Acros), 5-bromo-2-nitropyridine (Acros), potassium carbonate (K₂CO₃) (Acros), 10% palladium on charcoal (Pd/C) (Acros), and 80% hydrazine monohydrate (Acros) were used as received. N,N-Dimethylformamide (DMF) and N,N-dimethylacetamide (DMAc) were purified by vacuum distillation over CaH₂ and stored over 4 Å molecular sieves prior to use. The other commercially available reagents and solvents were also used without further purification.

Characterization

Nuclear magnetic resonance (NMR) spectra were determined on a BRUKER-300 spectrometer at 300 MHz for ¹H and 75 MHz for ¹³C in deuterated chloroform (CDCl₃) or dimethyl sulfoxide (DMSO-d₆). FT-IR spectra were recorded on a Bruker Vector 22 spectrometer at a resolution of 4 cm⁻¹ in the range of 400–4000 cm⁻¹. High-resolution LC-MS data was obtained using Agilent 1290-micrOTOF-QII. Inherent viscosities (η_inh) of PAA were measured with an Ubbelohde viscometer with a 0.5 g dL⁻¹ of DMAc solution at 25 °C. Dynamic mechanical analysis (DMA) was carried out with a TA instrument DMA Q800 at a heating rate of 5 °C min⁻¹ and a load frequency of 1 Hz in film tension geometry, T_g was regarded as the peak temperature of Tan Delta. Differential scanning calorimetric (DSC) analysis was performed on a TA instrument DSC Q100 at a scanning rate of 10 °C min⁻¹ in a nitrogen flow of 50 mL min⁻¹. Thermo gravimetric analysis (TGA) was conducted with the TA 2050, with a heating rate of 10 °C min⁻¹ under nitrogen. Thermo mechanical analysis (TMA) was performed by a METTLER TMA/SDTA841 at a heating rate of 10 °C min⁻¹. The coefficient of thermal expansion (CTE) was recorded at the temperature range of 50–150 °C. Ultraviolet-visible (UV-vis) spectra of PI films were recorded with a Shimadzu UV-vis 2501 spectrometer in transmittance mode at room temperature. Mechanical properties of the films were measured at room temperature on a Shimadzu AG-I universal testing apparatus with crosshead speed of 5 mm min⁻¹, tensile modulus (T_M), tensile strength (T_S) and elongation at break (E_B) were calculated as the average of five strips (about 30 μm thick, 3 mm wide and 6 cm long). Water uptake (WU) of the films was measured by the weight differences before and after immersion in deionized water at room temperature for 24 h, using the following equation:
where W_wet is the weight of the film samples after immersion in deionized water, and W_dry is the initial weight of the film samples. The water uptake data is the mean value of three parallel samples. The dielectric constants of the circular thin films (diameter 20 mm and thickness about 80 μm) were obtained using a Novocontrol GmbH Concept 40 broadband dielectric spectrometer in the frequency domain from 1 Hz to 1 MHz at room temperature. Before testing, the sample surface was carefully cleaned with acetone, and then coated with silver and drying at 120 °C in vacuo.

Synthesis of the monomers

Synthesis of the dinitromonomers (1a–1c).
The synthesis of 2,2′-bis[4-(5-nitro-6-methyl-2-pyridinoxy)phenyl] hexafluoropropane (1a). The synthesis of 2,2′-bis[4-(5-nitro-6-methyl-2-pyridinoxy)phenyl] hexafluoropropane (1a) is used as an example to illustrate the detailed synthetic procedures. Under the protection of nitrogen, 6FBPA (4.24 g, 12.6 mmol), potassium carbonate (4.00 g, 28.97 mmol), and DMF (40 mL) were added into a dry round bottom flask with a stir bar. The mixture was allowed to stir at room temperature for 1 hour. Next, 6-chloro-2-methyl-5-nitropyridine (5.00 g, 28.97 mmol) was added into the flask and the mixture was allowed to react at 70 °C for 3 h. Then, the obtained mixture was poured into 500 mL of distilled water. The crude product was collected by filtration, washed with water, and dried under vacuum at 80 °C for overnight. After the crude product was recrystallized from DMF/water, 6.29 g of 1a was obtained (82%); melting point: 181 °C (DSC peak). FT-IR (KBr): 1601, 1580, 1512, 1449, 1396, 1256, 1205, 1075 cm⁻¹; ¹H NMR (CDCl₃, ppm): 8.54 (d, 1H), 7.47 (d, 2H), 7.40 (dd, 2H), 7.14 (d, 1H), 2.62 (s, 3H); HRLC-MS (ESI): 609.3 (M + H)⁺, calcd 608.4 for C₂₇H₁₈F₆N₄O₆.
2,2′-Bis[4-(5-nitro-4-methyl-2-pyridinoxy)phenyl] hexafluoropropane (1b). Recrystallized from DMF/water; yield: 78%. Melting point: 136 °C (DSC peak). FT-IR (KBr): 1621, 1604, 1515, 1471, 1382, 1281, 1207, 1097 cm⁻¹; ¹H NMR (DMSO-d₆, ppm): 8.88 (s, 1H), 7.46 (d, 2H), 7.36 (m, 2H), 7.31 (s, H), 2.63 (s, 3H); HRLC-MS (ESI): 609.3 (M + H)⁺, calcd 608.4 for C₂₇H₁₈F₆N₄O₆.
2,2′-Bis[4-(6-nitro-3-pyridinoxy)phenyl] hexafluoropropane (1c). Recrystallized from DMF/water; yield: 88%. Melting point: 149 °C (DSC peak). FT-IR (KBr): 1604, 1571, 1534, 1459, 1396, 1242, 1203, 1123 cm⁻¹; ¹H NMR (DMSO-d₆, ppm): 8.52 (d, 1H), 8.40 (m, 1H), 7.83 (td, 1H), 7.50 (d, 2H), 7.36 (d, 2H); HRLC-MS (ESI): 581.3 (M + H)⁺, calcd 580.4 for C₂₅H₁₄F₆N₄O₆.

Synthesis of the diamine monomers (2a–2c).
2,2′-Bis[4-(5-amino-6-methyl-2-pyridinoxy)phenyl] hexafluoropropane (2a). The synthesis of 2,2′-bis[4-(5-amino-6-methyl-2-pyridinoxy)phenyl] hexafluoropropane (2a) is used as an example to illustrate the detailed synthetic procedures. Under N₂, the dinitro compound 1a (6.0 g, 9.86 mol) and 10% Pd/C (0.6 g) were suspended in 100 mL of dioxane in a 250 mL flask. The suspension solution was heated to reflux, and 80% hydrazine monohydrate (19.7 mL) was added dropwise to the mixture over 1 h. After a further 6 h of refluxing, the solution was filtered hot to remove Pd/C, and the filtrate was distilled to remove some solvent. The obtained mixture was poured into 500 mL of stirring water to produce a solid precipitate, which was removed by filtration, washed with water, and dried under vacuum. After recrystallization from dioxane/water, 4.9 g of 2a was obtained (92%); melting point: 155 °C (DSC peak). FT-IR (KBr): 3445, 3350, 1612, 1591, 1512, 1461, 1251, 1179, 1142 cm⁻¹; ¹H NMR (DMSO-d₆, ppm): 7.28 (d, 2H), 7.10 (m, 1H), 6.98 (m, 2H), 6.71 (d, 1H), 4.99 (s, 2H), 2.20 (s, 3H); ¹³C NMR (DMSO-d₆, ppm): 157.6, 151.0, 140.3, 140.1, 131.0, 125.7, 124.6, 117.4, 111.5, 89.2, 20.1; HRLC-MS (ESI): 549.4 (M + H)⁺, calcd 548.5 for C₂₇H₂₂F₆N₄O₂. Elemental analysis: calculated for C₂₇H₂₂F₆N₄O₂: C, 59.13%, H, 4.04%, N, 10.21%; found: C, 59.18%, H, 4.16%, N, 10.02%.
2,2′-Bis[4-(5-amino-4-methyl-2-pyridinoxy)phenyl] hexafluoropropane (2b). Recrystallization from dioxane/water; yield: 93%. Melting point: 181 °C (DSC peak). FT-IR (KBr): 3413, 3338, 1644, 1604, 1494, 1242, 1181, 1130 cm⁻¹; ¹H NMR (DMSO-d₆, ppm): 7.58 (s, 1H), 7.29 (d, 2H), 7.02 (d, 2H), 6.79 (s, 1H), 4.99 (s, 2H), 2.12 (s, 3H); ¹³C NMR (DMSO-d₆, ppm): 157.2, 152.5, 141.2, 134.8, 132.0, 131.0, 125.9, 122.1, 118.1, 114.1, 17.0; HRLC-MS (ESI): 549.4 (M + H)⁺, calcd 548.5 for C₂₇H₂₂F₆N₄O₂. Elemental analysis: calculated for C₂₇H₂₂F₆N₄O₂: C, 59.13%, H, 4.04%, N, 10.21%; found: C, 59.27%, H, 4.16%, N, 10.08%.
2,2′-Bis[4-(6-amino-3-pyridinoxy)phenyl] hexafluoropropane (2c). Recrystallization from dioxane/water; yield: 93%. Melting point: 136 °C (DSC peak). FT-IR (KBr): 3455, 3308, 1642, 1608, 1498, 1243, 1175 cm⁻¹; ¹H NMR (DMSO-d₆, ppm): 7.80 (d, 1H), 7.30 (s, 1H), 7.26 (m, 2H), 7.00 (m, 2H), 6.52 (d, 1H), 6.0 (s, 2H); ¹³C NMR (DMSO-d₆, ppm): 159.2, 157.4, 141.9, 140.1, 131.3, 130.8, 125.2, 115.8, 108.8; HRLC-MS (ESI): 521.3 (M + H)⁺, calcd 520.4 for C₂₅H₁₈F₆N₄O₂. Elemental analysis: calculated for C₂₂H₁₈N₄O₂: C, 57.70%, H, 3.49%, N, 10.77%; found: C, 57.78%, H, 3.58%, N, 10.52%.

Synthesis of the polyimides. Fluorinated polyimides are exemplified by the synthesis of PI-1 from the condensation of 2a and 6FDA. Under N₂, to a solution of 1.0970 g (2.0 mmol) of diamine 2a in 7.94 g of DMAc, 0.8885 g (2.0 mmol) of 6FDA was added in one portion and then extra 3.31 g DMAc was added to adjust a solid content of 15 wt%. The mixture was stirred for 6–12 h at room temperature and a viscous homogeneous polyamic acid (PAA) was obtained.

Film casting. Polyimide (PI-1) dense film was obtained using the following procedure. The viscous solution was filtered using a 0.45 μm PTFE syringe filter to remove any impurities or undissolved particles. The filtered solution was cast onto glass plate, which was followed by a preheating program (60 °C/1 h, 80 °C/3 h, 120 °C/0.5 h, 150 °C/0.5 h) and an imidization procedure under vacuum (200 °C/0.5 h, 250 °C/0.5 h, and 300 °C/1 h). The self-standing films were obtained by soaking in hot water to release from the glass substrates and then dried in a vacuum oven at 110 °C for 3 h for all mechanical, thermal, optical properties and dielectric constant evaluations. Similar procedures were followed for the synthesis of PI-2(6FDA/2b), PI-3(6FDA/2c).

Results and discussion

Design and synthesis of diamine monomers (2a–2c)

Since the C–F bond has low polarizability and small dipole as well as the large free volume, the incorporation of fluorinated group into the side chain or backbone of polymer leads to great benefits for dielectric performance. Therefore, three fluorinated diamine monomers (2a–2c) were designed and synthesized with systematic variations in the chemicals structure to affect the dielectric constant. Using ether linkage between fluorinated group and a pyridine ring to increase backbone flexibility could afford tough and ductile polyimide films with improving solubility and processability.

The monomer (2c) has no further substituent on pyridine, and the monomers 2a and 2b have methyl-substituent group on pyridine ring at 6- and 4- positions, respectively. The introduction of the bulky side groups was expected to weaken interactions between chain and chain and resulted to high fractional free volume²³ and thus low dielectric constant. Moreover, the methyl side group at the different position of pyridine was expected to possibly introduce more tenability effect of dielectric constant, thermal, optical properties, and solubility.

The synthetic routes for synthesizing a series of monomers (2a–c) were showed in Scheme 1. First, the intermediate dinitro compounds (1a–c) were prepared by a nucleophilic chloro-displacement reaction in the presence of potassium carbonate in DMF. Owing to the presence of an electronegative nitrogen atom in the aromatic ring, pyridine derivatives undergo nucleophilic substitution much more easily than the corresponding benzenes, especially at the 2- and 4- positions.²⁴ Moreover, the para nitro-group activated the chlorine atom for displacement; therefore, the chlorine-displacement reaction was readily carried out even at 70 °C. Second, the fluorinated diamine monomers (2a–c) were obtained in high yields by the hydrazine reduction reaction catalyzed with Pd/C in dioxane at refluxing temperature for several hours.

Scheme 1 Preparation of the monomers.

The chemical structures of the synthesized diamines were confirmed by FT-IR and ¹H NMR as illustrated in Fig. 1 and 2, respectively. As shown, each proton in the spectra can be unambiguously assigned according to the molecular structures of diamines. HRLC-MS and elemental analysis also were conducted to confirm the structure of diamine monomers. All the spectroscopic data agreed with the expected structures.

Fig. 1 FT-IR spectra of 2a–2c monomers.

Fig. 2 ¹H NMR spectra of 2a–c monomers.

Polymer synthesis

A series of aromatic pyridine-containing fluorinated polyimides were prepared by polycondensation reaction of the diamine monomer (2a–c) with an aromatic dianhydride, 6FDA, in anhydrous DMAc as solvent in Scheme 2. The new polyimides were synthesized using two-step methods, which were carried out via poly(amic acid)s (PAA) intermediate. The polymerization was carried out by reacting equimolar amounts of diamine monomer with aromatic dianhydrides at a concentration of 15% solids in DMAc. The polycondensation reaction was prepared at room temperature for 6–12 h to yield a PAA solution. As shown in Table 1, the inherent viscosities of the PAA precursors were 0.88–1.22 dL g⁻¹. Tough and flexible polyimide films were obtained by casting the PAA solution on glass plate followed by thermally curing process at 300 °C.

Scheme 2 Preparation of fluorinated polyimides (PIs).

Table 1 Characteristics of the polyimides

PI	ηinh of PAA^a (dL g^−1)	Composition of repeating unit	Elemental analysis (%)
				C	H	N	O
a Inherent viscosities of PAA was measured at 25 °C with a concentration of 0.5 g dL^−1 in DMAc.
PI-1	1.22	C46H24F12N4O6	Calcd	57.75	2.53	5.86	10.03
			Found	57.86	2.69	5.62	10.20
PI-2	1.10	C46H24F12N4O6	Calcd	57.75	2.53	5.86	10.03
			Found	57.90	2.68	5.66	10.15
PI-3	0.88	C44H20F12N4O6	Calcd	56.91	2.17	6.03	10.34
			Found	57.02	2.26	5.96	10.22

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The chemical structures of polyimides were characterized by FT-IR, ¹H NMR and element analysis. The FT-IR spectra (Fig. 3) of the polyimides exhibited characteristic imide absorptions at around 1782 and 1730 cm⁻¹ (typical of imide carbonyl asymmetric and symmetric stretches), 1380 (C=N stretch), and 1055 and 745 (imide ring deformation), together with some strong absorption bands in the region of 1300–1100 cm⁻¹ (C–O and C–F stretching). There was no existence of the characteristic absorption bands of the amide group near 3220–3450 cm⁻¹ (N–H stretching), which indicated polymers had been fully imidized. ¹H NMR spectra of the aromatic polyimides PI-(1–3) are depicted in Fig. 4, in which all the protons in the polymer backbone can be assigned. The results of the elemental analyses of all the thermally cured polyimides were listed in Table 1. The values found were in good agreement with the calculated ones of the proposed structures.

Fig. 3 FT-IR spectra of the fluorinated polyimides.

Fig. 4 ¹H NMR spectra of PI-(1–3).

Electrical properties and water uptake of the polyimides

Dielectric constant is the ability of a dielectric material to store electric potential energy under the influences of an electric field. The dielectric properties of the fluorinated polyimides were measured from 1 Hz to 1 MHz at room temperature (Fig. 5). The fluorinated polyimide films had very low dielectric constants in the range of 2.36–2.52 at 1 MHz (Table 2). The low dielectric constants could be explained by the incorporation of the bulky CF₃ groups in the polymer backbone, which has the strong electro negativity and low polarizability, resulting in the improvement of free volume of the polymer chains.^13,25 According to the Clausius–Mossotti relation, this result suggests the reduction of molecular polarizability adds weight to the reduction of the dielectric constant. Meanwhile, methyl substituent on pyridine can also loosen the chain packing and increase the free volume. The dielectric constant of PI-1 decreases with the increase of frequency (Fig. 5), whereas the dielectric constant remains almost the same constant for PI-2 and PI-3. The initial dielectric constant with high value at relatively lower frequencies might be attributed to the effects from space charge polarization in the bulk material, structural defects and electrode effects.²⁶ This dielectric constant of PI-1 decreases with frequency which may be due to the possibility of the presence of an interfacial polarization.^27,28 However, the dielectric constant of PI-2 and PI-3 are unchanged with frequency which indicates the independent property to change of frequency.

Fig. 5 Variation of dielectric constants with frequency of the polyimides.

Table 2 Dielectric properties and water uptake of the fluorinated polyimides

Polyimides	WU^a (%)	Dielectric constant
		1 KHz	100 KHz	1 MHz
a WU, water uptake.b 0.02, standard deviation.c The PI chemical structure as follows.^12
PI-1	0.71 ± 0.02^b	2.92	2.76	2.39
PI-2	0.64 ± 0.03	2.71	2.62	2.52
PI-3	0.79 ± 0.02	2.52	2.40	2.36
Ref-PI^c	0.49 ± 0.04	2.97	2.92	2.71

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The PI films exhibited the water uptake of 0.64–0.79% as shown in Table 2. These results implied that introduction of pyridine moiety did not deteriorate the water absorption behavior of the polyimides.^29,30 The fluorinated polyimides showed very low dielectric constants and low water uptake which are desirable as potential candidate for microelectronics packaging applications.

Thermal property of the polyimides

The thermal properties of the polyimides were evaluated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and thermo gravimetric analysis (TGA) and thermo mechanical analysis (TMA). The results are summarized in Table 3. Glass transition temperatures (T_g) of the polyimides were in the range of 262–275 °C and 271–289 °C, as obtained by DSC (Fig. 6) and DMA (Fig. 7), respectively. The slight differences of T_g values obtained by DSC and DMA were mainly attributed to the different responses of the samples to the two characterization methods. No melting/crystallization behavior was observed in the DSC measurements suggesting completely amorphous structures for all the polyimides synthesized in this paper. Generally, T_g values of polymers are determined by molecular packing and chain rigidity of the polymer backbones. As seen from Fig. 6 and 7, PI-2 showed the higher T_g than that of PI-1 and PI-3 which may be attributed to the higher rigidity and closer molecular packing of the polymer mainchain. PI-3 showed the similar T_g compared with PI-1 which may be due to the similar molecular packing and chain rigidity. PI-3 exhibited the higher T_g than that of analogous Ref-PI which might be due to the nitrogen atom on pyridine at the ortho-amino position possessing the higher rigidity.

Table 3 Thermal properties of the PI films

Polyimides	Tg (°C)		T5%^c (°C)	T10%^c (°C)	Rw^d (%)	CTE^e (ppm °C^−1)
	DSC^a	DMA^b
a Obtained at the baseline shift in the second heating DSC traces, with a heating rate of 10 °C min^−1 under N2.b Measured by DMA at a heating rate of 5 °C min^−1 and a load frequency of 1 Hz in film tension geometry.c 5% and 10% weight loss temperatures measured by TGA at a heating rate of 10 °C min^−1 under N2.d Residual weight retention at 800 °C by TGA at a heating rate of 10 °C min^−1 under N2.e CTE, coefficient of thermal expansion is recorded from 50 to 150 °C at a heating rate of 10 °C min^−1 by TMA.f The PI chemical structure as follows.^12
PI-1	262	271	468	500	50	68
PI-2	275	289	493	520	55	64
PI-3	265	271	499	524	54	67
Ref-PI^f	262	274	520	539	54	59

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Fig. 6 DSC curves of PI films.

Fig. 7 Storage modulus and Tan Delta of polyimide films in DMA.

TMA was used to evaluate the thermal dimensional stability of PI films and the CTE values were summarized in Table 3. The polyimides exhibit low coefficients of thermal expansion (CTEs) ranging from 64 to 68 ppm °C⁻¹ (Table 3). It may be due to the incorporation of rigid pyridine heterocyclic into the polymer backbone. As seen from Fig. 8, the TMA curves of the polyimide films exhibit the substantially similar trend. CTEs are associated with the rigidity and linearity of the polymer chain, which affects the chain packing. PI-2 exhibited the lower CTEs than that of PI-1 and PI-3 which may be due to the higher rigidity and linearity of the polymer chain.

Fig. 8 TMA curves of the polyimide films.

Thermal stabilities of the PIs were evaluated by TGA at a heating rate of 10 °C min⁻¹ in nitrogen and the results were summarized in Table 3. As seen from Fig. 9, the polyimides PI-(1–3) did not show obvious weight loss before 460 °C, implying that no thermal decomposition occurred. 5% and 10% weight loss temperatures (T_5% and T_10%) were located in the range of 468–499 °C and 500–524 °C under nitrogen, respectively. The T_5% of PI-1 and PI-2 was lower than that of PI-3 which can be explained by the methyl substituent on the pyridine ring decreasing the decomposition temperature. The T_5% of PI-1 was lower than that of PI-2 which can be explained by the closer molecular packing of PI-2 than PI-1 to improve the decomposition temperature. The residual weight retentions at 800 °C were in the range of 50–55% under nitrogen.

Fig. 9 TGA curves of PI films.

Mechanical and optical properties of the polyimides

The polyimide films were tested for mechanical properties at room temperature, as summarized in Table 4. The polyimide films exhibit good mechanical property with the tensile strength as high as 114 MPa, the tensile modulus in range of 3.4–3.7 GPa and the elongation at break of 3.2–14.6%, indicating that the obtained polyimide films were tough and strong. The PI-1 and PI-2 showed the lower tensile strength 108 and 110 MPa than that (114 Mpa) of PI-3 which may be explained by the methyl-substituted diamines decreasing the rigidity of polyimide backbone. All the fluorinated PI-(1–3) exhibit the higher tensile strength than that of Ref-PI which might be attributed to the higher rigidity.

Table 4 Mechanical properties of PI films

Polyimides	TS^a (MPa)	TM^b (GPa)	EB^c (%)
a TS, tensile strength.b TM, tensile modulus.c EB, elongation at break.
PI-1	108	3.6	2.6
PI-2	110	3.7	3.2
PI-3	114	3.4	14.6
Ref-PI	98	3.4	7.8

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The optical transparency of polyimide films were measured by UV-vis spectroscopy. The UV-vis spectra of the PI films with thicknesses of approximately 10 μm are shown in Fig. 10 and the optical data are summarized in Table 5. The cutoff wavelengths (λ_cut-off), which are determined by the wavelengths corresponding to the intersection points of the line tangent to the UV-vis curves, are in the range of 305–322 nm. The value of λ_cut-off was used as the parameter to evaluate transparency. Transmittance (%) of all the polyimide films at 450 nm is not lower than 90%. All the fluorinated PI show high optical transparency.

Fig. 10 UV-vis spectra of the polyimide films.

Table 5 Optical properties of PI films

Polyimides	Film thickness (μm)	λcut-off (nm)	Transmittance^a (%)
a Transmittance at 450 nm.
PI-1	9	322	91
PI-2	10	305	90
PI-3	8	315	94

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Solubility of the polyimides

Aromatic polyimides generally have poor solubility in common organic solvents due to strong inter-chain interactions of imide rings leading to poor processability. All the fluorinated PIs show excellent solubility in polar organic solvents and even in low boiling point solvent and the results were summarized in Table 6. The good solubility should result from the introduction of the flexible ether group and methyl substituent in polymer backbone, which decreased the interaction between polymer chains, and the introduction of the hexafluoroisopropylidene groups (–C(CF₃)₂–),which increased the chain flexibility and the affinity of the polymers.³¹

Table 6 Solubility behavior of the polyimides^a,b

Solvents	PI-1	PI-2	PI-3	Ref-PI
a DMF = N,N-dimethylformamide; DMAc = N,N-dimethylacetamide; NMP = N-methyl-2-pyrrolidone; DMSO = dimethyl sulfoxide; THF = tetrahydrofuran.b Solubility was determined with 10 mg of polyimides in 1 mL of solvent at room temperature for 24 h. ++, soluble at room temperature; +−, partial soluble; −−, insoluble.
m-Cresol	++	++	+−	++
DMF	++	++	++	++
DMAc	++	++	++	++
NMP	++	++	+−	++
DMSO	++	++	+−	++
THF	++	++	+−	++
Chloroform	++	++	+−	++
Cyclohexanone	+−	+−	+−	+−
CH3COOH	−−	−−	−−	−−
Pyridine	++	++	++	++

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Conclusions

A series of containing pyridine fluorinated diamines with systematically varied substituted groups were synthesized and characterized. These diamines were successfully polymerized with commercially available anhydride 6FDA to obtain a series of novel polyimides. These polyimides showed good solubility in various organic solvents as well as excellent thermal properties due to the presence of pyridine moieties and –C(CF₃)₂– groups in the polymer backbone, which made them suitable for further investigation of dielectric properties. The fluorinated polyimides had a very low dielectric constant, which can be attributed to a high fluorine content induced by the –C(CF₃)₂–, pyridine units and methyl substituent group. Possible directions for continued work with these polymers focus on measurement of the degree of porosity for the effect on dielectric constant, physical aging experiments to determine the long term stability of these polymer membranes, and practical application in microelectronics field, such as semiconductors and printed-circuit boards for computer industry, insulating film for multiplayer wirings and tape automated bonding, etc. Additionally, more researches in this area are being investigated to design additional newly containing more bulky substituent on pyridine ring polyimides with potential to achieve superior dielectric property.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21502083), and the grant from the State Key Laboratory of Fine Chemicals (KF1412).

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