Soluble copoly(pyridine ether imide)s derived from 6FDA,1,4-bis(4-amino-2-trifluromethylphenoxy)benzene,and 4-(4-trifluoromethylphenyl)-2,6-bis(4-aminophenyl)pyridine

Abstract

A series of new fluorinated copoly(pyridine ether imide)s were prepared by thermal imidization of polyamide acid derived from 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene (BATB), 4-(4-trifluoromethylphenyl)-2,6-bis(4-aminophenyl)pyridine (TFMPBPP), and 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) with various mole ratios of BATB and TFMPBPP ranged from 80/20 to 20/80. All copolyimides were amorphous and had excellent solubility in organic solvents such as N,N-dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), and N,N-dimethylformamide (DMF) at room temperature and displayed good thermal properties with glass transition temperature (T _g) of 272°C–338°C, 5% weight loss temperature of 520°C–540°C, and the residue of 44–56% at 750°C in nitrogen. Meanwhile, the T _g of the copolymers increased with an increase in the TFMPBPP component. Tough and flexible copolyimide films also exhibited outstanding mechanical properties with tensile strengths of 107.2–153.2 MPa, tensile moduli of 1.4–1.8 GPa, and elongations at break of 13.2–19.6%, low dielectric constants of 2.76–2.92 (1 MHz), and water uptake 0.32–0.62% as well as high optical transparency with the UV cutoff wavelength in the 371–386 nm range and the wavelength of 80% transparency in the range 459–498 nm.

Keywords

Copoly(pyridine ether imide)s 2,6-diphenylpyridyl unit 4-trifluoromethylphenyl pendent group ether linkage soluble thermal stability transparency

Introduction

Aromatic polyimides, as one of the most important high-performance polymers, are widely used in the aerospace, microelectronics, and optoelectronics industries due to many desirable properties such as high thermal and thermooxidative stability, excellent dimensional stability, chemical resistance, mechanical, and electrical properties.^1

–4 However, their extensive applications are often limited by processing difficulties because of their poor solubility and high-processing temperature, which are caused by rigid polymer backbones and the strong interchain interaction. To overcome these limitations, much effort has been focused on the design and synthesis of soluble and processable polyimides by introducing flexible linkages,^5
–7 bulky units,^8,9 pendent^10,11 and alicyclic groups,^12,13 molecular asymmetry,^14,15 and non-coplanar^16,17 moieties into the polymer backbones. Among them, poly(ether-imide)s have received great attention as they may provide good processability owing to the presence of flexible ether linkages. However, some advantageous properties of poly(ether-imide)s such as thermal stability and mechanical property were somewhat sacrificed because of the incorporation of flexible ether linkages. On the other hand, it has been demonstrated that the introduction of rigid heteroaromatic pyridine ring into the polymer main chains could endow with excellent thermal and thermooxidative stability, which should be useful in decreasing the negative effects resulting from the introduction of flexible linkages in the polyimide backbones.^18

–21 It was also found that incorporation of fluorine-containing groups, especially the trifluoromethyl groups into the backbone or side chain of polyimides resulted in an enhanced solubility and optical transparency together with a lowered dielectric constant, which attributed to low polarizability of the C–F bond and the increase in free volume. Furthermore, the fluorinated polyimides also provided other merits such as good thermal and thermooxidative stability and low moisture absorption.^22

–27 In recent years, with the fast development of high-technology fields especially in the microelectronics industry, the demands for the polyimide materials with excellent combined properties including easy processing, low curing temperature, low water uptake, low dielectric constant, and high optical transparency increased sharply. Obviously, the common aromatic polyimides could not meet entirely these requirements due to especially their infusibility and insolubility in most organic solvents. Thus, the development of polyimides with enhanced solubility and easy processablity, high optical transparency, lower dielectric while maintaining their thermal stability and other useful properties are still of particular interest. Recently, our group has found that the introduction of 4-(4-trifluoromethylphenyl)-2,6-diphenylpyridyl units into the polyamide backbones could effectively improve their solubility and processability while not deteriorating positive properties.²⁸ In previous studies, 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene (BATB)^22,24 and 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA)^17,18,25 were chosen to synthesize the fluorinated polyimides with good thermal and mechanical properties and good solubility as well as colorless films. Herein, as part of our continuing efforts to develop soluble polyimides with high thermal stability and low dielectric constant for advanced microelectronics and optoelectronics applications,²⁹ we report the synthesis and characterization of a series of fluorinated copoly(pyridine ether imide)s derived from 4-(4-trifluoromethylphenyl)-2,6-bis(4-aminophenyl)pyridine (TFMPBPP), BATB with commercially available 6FDA. In this study, the prepared fluorinated copoly(pyridine ether imide)s [(BATB/6FDA) _m − (TFMPBPP/6FDA) _n , where m/n = 80/20, 60/40, 50/50, 40/60 and 20/80] were systematically investigated to measure the general properties, such as solubility, thermal stability, and mechanical property, as well as optical transparency, dielectric constants, and water uptake.

Experimental

Materials

TFMPBPP (melting point (m.p.) = 210°C)²⁸ and BATB (m.p. = 132°C–133°C)²² were synthesized and purified according to previously reported procedures. 6FDA was recrystallized from acetic anhydride and then dried in vacuum at 150°C for 12 h before use. DMAc was purified by distillation under reduced pressure over calcium hydride prior to use. All other solvents were obtained from various commercial sources and used without further purification.

Measurements

The Fourier transform infrared (FTIR) spectra were determined on a Perkin-Elmer SP One FTIR spectrophotometer (Waltham, MA, USA). The glass transition temperatures (T _gs) were determined on a Perkin-Elmer DSC-7 instrument (Waltham, MA, USA) at a heating rate of 20°C/min under nitrogen protection. The second scan was immediately initiated after the sample was cooled to room temperature. The T _g values were reported from the second scan after the first heating and quenching and taken from the midpoint of the change in the slope of the baseline. The thermal stability of the polymers from 50°C to 750°C was determined with a Perkin Elmer Pyris 1 TGA thermogravimetric analyzer (Waltham, MA, USA) at a heating rate of 10°C/min under a protective nitrogen atmosphere (120 mL/min). Elemental analysis was performed with a Carlo Erba 1106 Elemental Analyzer (Carlo Erba, Italy). The wide-angle X-ray diffraction (WAXD) measurements were conducted at room temperature (ca. 25°C) on polymer film with a Rigaku Geiger Flex D-Max III X-ray diffractometer (Tokyo, Japan), using Ni-filtered CuK _α radiation (operating at 40 kV and 15 mA); the scanning rate was 2°/min. The inherent viscosity of poly(amic acid) (PAA) was measured at 0.5 g/dL concentration in DMAc with an Ubbelohde viscometer (Shanghai, China) at 30°C ± 0.1°C. Molecular weights were determined by a gel permeation chromatography (GPC) with polystyrene calibration using a Waters 510 high-performance liquid chromatography (Milford, MA, USA) equipped with 5-µm phenogel columns (linear, 3 × 500 Å) arranged in series and a UV detector at 254 nm using tetrahydrofuran (THF) as eluent. The water uptake was measured by weighing the changes in the samples (50 × 50 × 0.05 mm³) before and after immersion in water at 25°C for 24 h. The mechanical properties of the polymer films were measured on an Instron 1122 testing instrument (Norwood, MA, USA) with a 5-kg load cell at a crosshead speed of 0.5 mm/min on strips (0.5 cm wide, 6 cm long, and ca. 0.05 mm thick), and an average of at least three replicas was used. The dielectric constant was determined by the bridge method with an LKI-1 capacitance meter (Shanghai, China) at 25°C and a frequency of 1 MHz. Ultraviolet–visible (UV-vis) spectra of the polymer films were recorded on a Shimadzu UV–1601 UV-vis spectrophotometer (Tokyo, Japan) (the wavelength range of UV measurement was from 200 to 800 nm).

Polymer synthesis

The fluorinated copoly(pyridine ether imide)s were synthesized by the conventional two-step procedure via PAA precursors, followed by thermally curing at elevated temperatures. A typical example for the preparation of Polyimide-c (m/n = 50/50) (PI-c) [m/n = 50/50 (c)] was described as follows: in a 100-mL three-necked flask equipped with a mechanical stirrer and a nitrogen inlet, TFMPBPP (0.6081 g, 1.50 mmol) and BATB (0.6425 g, 1.50 mmol) were dissolved in 15 mL DMAc. After diamine was dissolved completely, 6FDA (1.3327 g, 3.00 mmol) was added to one portion. The mixture was stirred in ice bath for 60 min and then stirred at room temperature for 12 h to form a viscous PAA solution with 15 wt% solid concentration. The inherent viscosity of the PAA-c (m/n = 50/50) (PAA-c) in DMAc was 0.70 dL/g measured at a concentration of 0.5 g/dL at 30°C. Thermal imidization was carried out as follows: A film was cast from the PAA solution onto a flat and clean glass plate, which was dried in an oven under nitrogen protection at 60°C for 3 h and 120°C for 6 h, then at 150°C, 200°C, 250°C, and 300°C for 1 h at each temperature, respectively. The polymer film was stripped from the glass plate when it was cooled. FTIR (film): 1786 (asymmetric C=O stretch), 1722 (symmetric imide C=O stretch), 1600–1500 (aromatic C=C stretch), 1369 (C–N stretch), 1239 (Ar–O–Ar stretch), 1189 (C–F stretch), and 1052, 1017, 718 cm^– ¹ (imide ring deformation). The other copolyimides with various ratios of two diamines of BATB and TFMPBPP [BATB/TFMPBPP: m/n = 80/20 (a), 60/40 (b), 40/60 (d), and 20/80 (e)] were also prepared according to the above procedure.

Results and discussion

Polymer synthesis

As outlined in Figure 1, a series of fluorinated copoly(pyridine ether imide)s were synthesized via the conventional two-step polymerization procedure from 6FDA, TFMPBPP and BATB with various mole ratios of BATB and TFMPBPP ranged from 80:20 to 20:80, involving ring-opening polyaddition forming PAA and subsequent thermal imidization. The polymerization results and the corresponding data were summarized in Table 1. The inherent viscosities (η _inh) of PAAs measured in DMAc in 0.5g/dL concentration at 30°C were in the range of 0.58–0.89 dL/g. Then tough and flexible fluorinated copoly(pyridine ether imide) films PIa-PIe were obtained by casting the PAA solution on a glass plate followed by thermal curing process at 300°C. The number-average and weight-average molecular weights of the copolyimides PIa-PIe were in the range of 15,400–17,500 g/mol and 41,800–43,500 g/mol, respectively, with the polydispersity index (M _w /M _n) values in the range 2.4–2.7, as determined by GPC compared to polystyrene standard using THF as the eluent.

Table 1.

Inherent viscosities of PAAs, GPC data and elemental analysis of the co-Pis.

PAAs		GPC data of co-PIs				Elemental analysis of co-PIs				Water uptake^d (%)
Code	η _inh (dL/g)^a	Code	M _w (g/mol)^b	M _n (g/mol)^b	PDI^c		C	H	N	Water uptake^d (%)
PAA-a	0.89	PI-a	41,800	15,400	2.7	Found Calcd Corr.^e	57.23 57.46 57.42	2.01 2.04 2.00	3.67 3.70 3.68	0.32
PAA-b	0.58	PI-b	43,000	16,500	2.6	Found Calcd Corr.^e	58.68 58.94 58.90	2.19 2.14 2.18	3.99 4.06 4.01	0.38
PAA-c	0.70	PI-c	42,600	16,300	2.6	Found Calcd Corr.^e	59.35 59.68 59.61	2.27 2.20 2.26	4.17 4.24 4.19	0.44
PAA-d	0.82	PI-d	42,100	17,500	2.4	Found Calcd Corr.^e	60.08 60.43 60.39	2.32 2.25 2.31	4.34 4.43 4.36	0.51
PAA-e	0.68	PI-e	43,500	17,400	2.5	Found Calcd Corr.^e	61.49 61.95 61.87	2.43 2.37 2.41	4.69 4.79 4.72	0.62

PAA: poly(amic acid); GPC: gel permeation chromatography; DMAc: N,N-dimethylacetamide; PI-a: polyimide-a (m/n = 80/20); PI-b: polyimide-b (m/n = 60/40); PI-c: polyimide-c (m/n = 50/50); PI-d: polyimide-d (m/n = 40/60); PI-e: polyimide-e (m/n = 20/80).

^aDetermined at a concentration of 0.5 g/dL in DMAc at 30°C.

^bDetermined by GPC with polystyrene as standard, using THF as the eluent.

^cThe polydispersity index (PDI) was obtained by M _w/M _n.

^dMoisture uptake (%) = 100 × (W – W ₀)/W ₀; W: weight of polymer sample after immersion in water at room temperature for 24 h; W ₀: weight of polymer sample after being dried in vacuum at 100°C.

^eCorrected value = found value × (100% + moisture intake %) for C and N; corrected value = found value × (100% – moisture intake %) for H.

The formation of the resulting fluorinated copoly(pyridine ether imide)s was confirmed by FTIR and elemental analysis. Figure 2 depicts the typical FTIR spectra of PAA-c film and the corresponding fully cyclized PI-c film, derived from 6FDA, BATB, and TFMPBPP (m/n = 50/50). As displayed in Figure 2(a), the IR spectrum of PI-c exhibited the characteristic imide moiety absorptions at around 1369 cm^– ¹ due to stretching vibrations of the C–N bond that was newly formed in the copolyimide backbones. The other characteristic peaks observed at around 1786, 1722, and 718 cm^– ¹ were assigned to carbonyl asymmetrical, symmetrical stretching vibrations, and bending vibrations of imide rings, respectively. Besides, the characteristic absorptions at 1239 cm^–1 due to aryl ether linkages and C–F stretching at 1189 cm^–1 were also observed. The PAA-c vibrations (Figure 2(b)) included three bands for carbonyl stretch of the carboxylic acid at 1717 cm^– ¹, carbonyl stretch of the amide I mode at 1678 cm^– ¹, and N–H stretch of amide II mode at 1531 cm^– ¹. As mentioned above, the complete imidization of PAA was easily identified by the disappearance of the amide-related bands and the appearance of the corresponding imide bands after thermal imidization.

Moreover, the chemical composition of the prepared copolyimides PIa-PIe was characterized using elemental analysis. As presented in Table 1, the water uptake values were in the range of 0.32–0.62%, which could be used to compensate the weight change in dried samples after they were exposed to water at room temperature for 24 h. When the found values of the elemental composition were corrected by eliminating the amount of absorbed water, the corrected values were in accordance with the theoretically calculated ones, which also provided helpful evidence for identifying the expected structures of the resulting polymers.

Thermal properties

The T _g and decomposition temperature (T _d) of the fluorinated copoly(pyridine ether imide) films PIa-PIe were determined using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), respectively. As shown in Figure 3, no crystallization or melting peaks were observed in the DSC curves, which also revealed the amorphous structure of the resulting copolyimides and agreed well with the WAXD results. As could be seen from Table 2, the T _g values of the present polymers were found to increase dramatically from 272°C to 338°C, with the increase of TFMPBPP content in the main chains, which are much higher than that of homopolyimide PI-f derived from 6FDA and BATB (T _g = 229°C),²² and commercial soluble polyimide Ultem 1000 (T _g = 217°C).³⁰ The gradual increase in the T _g values of these copolyimides were mainly originated from the rigid 2,6-diphenylpyridyl units and bulky 4-trifluoromethylphenyl pendent groups of macromolecular backbones, hindering the rotation of the polymer chains.

Table 2.

Thermal properties of the fluorinated poly(pyridine ether imide) films.

Co-PIs (m/n)	T _g (°C)^a	T _d0 (°C)^b	T _d5 (°C)^c	T _d10 (°C)^d	R _w (%)^e
PI-a (80/20)	272	520	529	549	44
PI-b (60/40)	293	525	534	551	47
PI-c (50/50)	305	527	535	552	48
PI-d (40/60)	315	530	538	558	55
PI-e (20/80)	338	532	540	559	56
PI-f (100/0)^f	229	–	599	623	–

DSC: differential scanning calorimetry; 6FDA: 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride; BATB: 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene; PI-a: polyimide-a (m/n = 80/20); PI-b: polyimide-b (m/n = 60/40); PI-c: polyimide-c (m/n = 50/50); PI-d: polyimide-d (m/n = 40/60); PI-e: polyimide-e (m/n = 20/80).

^aMidpoint temperature of the baseline shift on the second DSC heating trace.

^bOnset decomposition temperature under nitrogen.

^cTemperature at 5% weight loss under nitrogen.

^dTemperature at 10% weight loss under nitrogen.

^eResidual weight percentage at 750°C under nitrogen.

^fDerived from 6FDA and BATB, the corresponding data reported in reference.²²

The thermal stability of the fluorinated copoly(pyridine ether imide)s PIa-PIe was investigated by TGA measurements in nitrogen. As shown in Figure 4 and Table 2, the polymers did not show obvious weight loss before the scanning temperature reached up to 520°C, implying that no thermal decomposition occurred and the onset T _ds (T _d0) ranged from 520°C to 532°C. The 5% (T _d5) and 10% (T _d10) weight loss temperatures in nitrogen were in the range of 529°C–540°C and 549°C–559°C, respectively. Furthermore, the residual weight retentions at 750°C in nitrogen were in the range of 44–56%. The good thermal stability of these new copolyimides could be primarily attributed to their greater aromatic unit content in the molecular structure, which also further indicated to some extent the presence of the rigid pyridine structure could enhance their thermal properties, and decreasing the negative effects owing to the introduction of flexible ether linkages in the polymer backbones. These new copoly(pyridine ether imide)s possessed superior thermal stability combined with high T _gs, indicated that they were able to withstand elevated processing temperatures, and found application in many high-tech fields, such as microelectronic manufacturing, the packaging industry, and so on.

Solubility

The solubility was tested with 1 g of polymer sample dissolved in 9 g of organic solvent (10 wt%) at room temperature or an elevated temperature, and the results were listed in Table 3. As seen from Table 3, unlike those traditional polyimides, the obtained fluorinated copoly(pyridine ether imide)s PIa-PIe exhibited excellent solubility in various solvents such as NMP, DMF, DMAc, and THF at room temperature as well as in low boiling point solvent such as CHCl₃ and acetone at room temperature or at refluxing temperature. When the monomer molar ratio of BATB to TFMPBPP was not more than 50/50, the copolymers could be dissolved in all solvents as mentioned except for methanol at room temperature. Although the homopolyimide PI-f derived from 6FDA and BATB have good solubility in polar solvents, ²² it only partially dissolved in solvents such as dimethylsulfoxide, THF, CHCl₃, and acetone. Clearly, the improved solubility of these copolyimides could be primarily attributed to the introduction of the pyridine units with bulky 4-trifluoromethylphenyl pendent groups, which decreased the packing force and increased the free volume of the polymer, thus resulting in the enhanced solubility.

Table 3.

Solubility of the fluorinated copoly(pyridine ether imide) films.^a

Co-PIs (m/n)	DMF	DMAc	NMP	DMSO	THF	CHCl₃	Acetone	CH₃OH
PI-a (80/20)	+ +	+ +	+ +	+ –	+ +	+	+	– –
PI-b (60/40)	+ +	+ +	+ +	+ –	+ +	+	+	– –
PI-c (50/50)	+ +	+ +	+ +	+ +	+ +	+ +	+ +	– –
PI-d (40/60)	+ +	+ +	+ +	+ +	+ +	+ +	+ +	– –
PI-e (20/80)	+ +	+ +	+ +	+ +	+ +	+ +	+ +	– –
PI-f (100/0)^b	+ +	+ +	+ +	+ –	+ –	+ –	+ –	– –

NMP: N-methyl-2-pyrrolidone; DMAc: N,N-dimethylacetamide; DMF: N,N-dimethylformamide; DMSO: dimethylsulfoxide; THF: tetrahydrofuran; 6FDA: 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride; BATB: 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene; PI-a: polyimide-a (m/n = 80/20); PI-b: polyimide-b (m/n = 60/40); PI-c: polyimide-c (m/n = 50/50); PI-d: polyimide-d (m/n = 40/60); PI-e: polyimide-e (m/n = 20/80).

^aSolubility: + + = soluble at room temperature; +, soluble at refluxing temperature; + – = partially soluble at room temperature; – – = insoluble even on heating.

^bDerived from 6FDA and BATB, the corresponding solubility reported in reference.²²

Crystallinity and tensile properties

The crystallinity of the obtained fluorinated copoly(pyridine ether imide) films PIa-PIe was characterized by WAXD. As presented in Figure 5, almost no crystal diffraction was detected for all of the synthesized polymers, indicating that they were amorphous. This might be ascribed to the fact that the flexible ether linkages, hexafluoroisopropylidene units and bulky 4-trifluoromethylphenyl pendent groups in the main chains, which decreased the intermolecular interactions and the packing density of the polymer chains and finally resulted in hindering the crystallization of the polymers.

The stress–strain curves for the fluorinated copoly(pyridine ether imide) films PIa-PIe are shown in Figure 6, and the tensile properties are listed in Table 4. The polymer films had elongations at break of 13.2–19.6%, tensile strength of 107.2–153.2 MPa, and tensile modulus of 1.4–1.8 GPa, which indicated that they were strong and tough polymeric materials. It was noticed that the tensile strength of these copoly(pyridine ether imide) films gradually increased with the increase in TFMPBPP composition. Besides, among those prepared copolyimides, PIa exhibited the lowest tensile strength, whereas PIe exhibited the highest tensile strength and the lowest tensile modulus value, which were mainly ascribed to the fact that the rigid 2,6-diphenylpyridyl units in the polymer backbone chains increased with the increase in TFMPBPP content. On the other hand, the increase in bulky 4-trifluoromethylphenyl pendent groups decreased the intermolecular forces, reducing the tensile modulus. In comparison with the homopolyimide PI-f,²² the incorporation of TFMPBPP moieties decreased to some extent the elongations at break in every case, which is in agreement with the molecular structure of the copolymer. As shown in Table 4, when the mole ratio of BATB and TFMPBPP is 50/50, the corresponding copolyimide PI-c exhibited relatively better mechanical property in this series. Therefore, the introduction of TFMPBPP seemed to have some influence on their mechanical properties.

Table 4.

Mechanical properties of the fluorinated copoly(pyridine ether imide) films.

Co-PIs (m/n)	Tensile strength (MPa)	Tensile modulus (GPa)	Elongation at break (%)
PI-a (80/20)	107.2	1.8	14.5
PI-b (60/40)	117.5	1.8	15.9
PI-c (50/50)	141.2	1.7	19.5
PI-d (40/60)	146.2	1.6	13.2
PI-e (20/80)	153.2	1.4	19.6
PI-f (100/0)^a	127.4	2.0	27.0

6FDA: 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride; BATB: 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene; PI-a: polyimide-a (m/n = 80/20); PI-b: polyimide-b (m/n = 60/40); PI-c: polyimide-c (m/n = 50/50); PI-d: polyimide-d (m/n = 40/60); PI-e: polyimide-e (m/n = 20/80).

^aDerived from 6FDA and BATB, the corresponding solubility reported in reference.²²

Optical properties and dielectric constants

The optical transparency of the resulting fluorinated copoly(pyridine ether imide) films PIa-PIe with thicknesses of approximately 40–50 μm was evaluated by UV-vis spectroscopy as in Figure 7, and some corresponding data are listed in Table 5. As shown in Table 5, the cut-off wavelength (λ ₀) values of them were in the range 371–386 nm, and the wavelength of 80% transmittance (λ ₈ _0%) was in the range 459–498 nm. These new copolyimide films revealed short λ ₀ values and higher optical transparencies, which were mainly ascribed to the bulky and strong electron-withdrawing trifluoromethyl side groups, decreasing effectively the intermolecular CTCs between polymer chains as a result of steric hindrance and the inductive effect.

Table 5.

Optical transparency and dielectric constants of the copoly(pyridine ether imide) films.

Co-PIs (m/n)	F _t (μm)	λ ₀ (nm)	λ _80% (nm)	T ₅₀₀ (%)	T ₆₀₀ (%)	T ₇₀₀ (%)	D _c
PI-a (80/20)	48	371	459	83.8	87.8	89.0	2.76
PI-b (60/40)	50	375	460	84.5	88.3	88.7	2.80
PI-c (50/50)	45	378	490	80.1	84.0	84.5	2.83
PI-d (40/60)	42	380	492	80.4	84.0	85.2	2.88
PI-e (20/80)	40	386	498	79.5	85.9	86.8	2.92

F _t: film thickness; λ ₀: cutoff wavelength; λ _80%: wavelength of 80% transmittance; T ₅₀₀: transparency at 500 nm; T ₆₀₀: transparency at 500 nm; T ₇₀₀: transparency at 500 nm; D _c: dielectric constant, which was obtained in the frequency of 1 MHz at room temperatures; PI-a: polyimide-a (m/n = 80/20); PI-b: polyimide-b (m/n = 60/40); PI-c: polyimide-c (m/n = 50/50); PI-d: polyimide-d (m/n = 40/60); PI-e: polyimide-e (m/n = 20/80).

The dielectric constants of the prepared fluorinated copoly(pyridine ether imide) films PIa-PIe are also summarized in Table 5. It was reported that a polymer with a lower polarizability or higher free volume has a lower dielectric constant.^31,32 The obtained copolyimide films exhibited low dielectric constants in the range of 2.76–2.92 at 1 MHz, which could be mainly attributed to the fact that the strong electronegativity of fluorine atoms resulted in very low C–F polarizability, reducing the dielectric constant. Among them, PIa (m/n = 80/20) showed the relatively lowest dielectric constant (2.76 at 1 MHz), which was owing to the high fluorine content (26.03%) in the polymer backbones. Additionally, a gradually increase in dielectric constants with the increase in TFMPBPP content was observed in this study, probably due to the presence of aryl-substituted, pyridine heterocyclic groups, and its π conjugation in the polymer backbone, which increased the polarizability of the polymer under an electric field. The dielectric constant values of the resulting copolyimides were also compared to the standard of the commercially available polyimide (Kapton) film derived from pyromellitic dianhydride and 4,4′-oxydianiline with a dielectric constant of 3.68 (1 MHz).

Conclusions

A series of new fluorinated copoly(pyridine ether imide)s were synthesized and characterized by thermal imidization of PAA derived from BATB, TFMPBPP, and 6FDA, with various mole ratios of BATB and TFMPBPP ranging from 80/20 to 20/80. These new copolyimides exhibited excellent solubility in organic solvents, outstanding tensile property, and good thermal stability with gradually increasing high T _gs of 272°C–338°C as the TFMPBPP content increased in the main chains. The experimental results show that the incorporation of 2,6-diphenylpyridyl moieties with bulky 4-trifluoromethylphenyl pendent groups into polymer backbone is an effective way to improve the solubility and T _g values of the resulting polymers. Also, these new copolyimide films showed high optical transmittance with a cutoff wavelength of 371–386 nm and low dielectric constants as well as low water absorption. These characteristics indicate that these new fluorinated copoly(pyridine ether imide)s may be a promising, processable high-temperature materials for applications in microelectronic and optical devices.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The financial support for this work provided by the National Natural Science Foundation of China (No. 21064003, 21464007, 21564005), the Training Program for Science and Technology Leaders in Major Subjects of Jiangxi Province of China (No. 20113BCB22012), the Ground Project for Science and Technology of Jiangxi Universities (No. KJLD14022) and the Research Program of Jiangxi Province Department of Education (No. GJJ14236) and the Graduate Student Innovation Foundation of Jiangxi Normal University (YC2015-S121) is gratefully acknowledged.

References

Bessonov

Koton

Kudryavtsev

. Polyimides. New York: Consultants Bureau, 1987.

Ghosh

Mittal

. Polyimides: Fundamentals and Applications. New York: Marcel Dekker, 1996.

Mittal

. Polyimides and Other High-Temperature Polymers. Leiden: VSP/Brill, 2009.

Liaw

Wang

Huang

. Advanced polyimide materials: syntheses, physical properties and applications. Prog Polym Sci 2012; 37: 907–974.

Shi

Hasegawa

Shindo

. Thermo-processable polyimides with high thermo-oxidative stability as derived from oxydiphthalic anhydride and bisphenol A type dianhydride. High Perform Polym 2000; 12: 377–393.

Mehdipour-Ataei

Sarra

Hatami

. Novel thermally stable polyimides based on flexible diamine: synthesis, characterization, and properties. Eur Polym J 2004; 40: 2009–2015.

Liu

Chen

Fang

. Preparation, characterization, and properties of poly(thioether imide)s from isomeric bis(chlorophthalimide)s and bisthiophenols. High Perform Polym 2016; 28: 64–74.

Zhang

. Synthesis and properties of novel organosoluble polyimides derived from 1,4-bis[4-(3,4-dicarboxylphenoxy)]triptycene dianhydride and various aromatic diamines. Polymer 2007; 48: 6246–6253.

Hsiao

Kung

. Synthesis and properties of new aromatic polyimides containing redox-active anthraquinonemoieties. Polym Int 2013; 62: 573–580.

10.

Cai

Niu

. Synthesis and electrochromic properties of polyimides with pendent benzimidazole and triphenylamine units. Chin J Polym Sci 2016; 34: 1901–1102.

11.

Ghorpade

Thorave

Rajan

. Synthesis and characterization of thermally stable polyimides with a pendent phenothiazine unit based on new diamine10-(3,5-diaminobenzoyl)phenothiazine. High Perform Polym 2016; 28: 26–33.

12.

Sheng

Zhang

. Synthesis and characterization of new cardo polyimides derived from 9,9-bis[4-(4-aminophenoxy)phenyl]xanthene. J Appl Polym Sci 2012; 126: 297–303.

13.

Revathi

Prabunathan

Devaraju

. Synthesis of soluble polyimides based on ether-linked cyclohexyldiamine and their ultraviolet shielding behavior. High Perform Polym 2015; 27: 247–253.

14.

Zeng

Wang

. Polyimides derived from novel unsymmetric dianhydride. Macromolecules 2000; 33: 4310–4318.

15.

Han

Zhang

. Novel soluble and optically active polyimides containing axially asymmetric 9,9′-spirobifluorene units: synthesis, thermal, optical and chiral properties. Polymer 2012; 53: 5706–5716.

16.

Reddy

Shu

. Synthesis and characterization of soluble polyimides derived from 2,2′-bis(3,4-dicarboxyphenoxy)-9,9′-spirobifluorene dianhydride. J Polym Sci A Polym Chem 2002; 40: 262–268.

17.

Liu

Zhang

Lan

. Synthesis and properties of high-performance functional polyimides containing rigid nonplanar conjugated tetraphenylethylene moieties. J Polym Sci A Polym Chem 2013; 51: 1302–1314.

18.

Liaw

Wang

Chang

. Novel poly(pyridine imide) with pendent naphthalene groups: synthesis and thermal, optical, electrochemical, electrochromic, and protonation characterization. J Polym Sci A Polym Chem 2007; 45: 2367–2374.

19.

Wang

Gong

. Synthesis and characterization of novel soluble pyridine-containing polyimides based on 4-phenyl-2,6-bis[4-(4-aminophenoxy)phenyl]pyridine and various aromatic dianhydrides. J Appl Polym Sci 2007; 104: 212–219.

20.

Amutha

Tharakan

Sarojadevi

. Synthesis and characterization of new soluble polyimides based on pyridine unit with flexible linkages. High Perform Polym 2015; 27: 979–989.

21.

Guan

Wang

. High transparent polyimides containing pyridine and biphenyl units: synthesis, thermal, mechanical, crystal and optical properties. Polymer 2015; 62: 1–10.

22.

Xie

Zhang

Liu

. Synthesis and characterization of soluble fluorine-containing polyimides based on 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene. J Polym Sci A Polym Chem 2001; 39: 2581–2590.

23.

Liu

. Synthesis and characterization of organo-soluble fluorinated polyimides from 4,4′-bis(3-amino-5-trifluoromethylphenoxy)-biphenyl and various aromatic dianhydrides. Chin J Polym Sci 2005; 23: 55–61.

24.

Qiu

Wang

Zhang

. Synthesis and properties of soluble polyimides based on isomeric ditrifluoromethyl substituted 1,4-bis(4-aminophenoxy)benzene. Polymer 2006; 47: 8444–8452.

25.

Behniafar

Sefid-girandehi

. Optical and thermal behavior of novel fluorinated polyimides capable of preparing colorless, transparent and flexible films. J Fluorine Chem 2011; 132: 878–884.

26.

Saritha

Thiruvasagam

. Synthesis of diol monomers and organosoluble fluorinated polyimides with low dielectric: study of structure–property relationship and applications. High Perform Polym 2015; 27: 842–851.

27.

Amininasab

Esmaili

Taghavi

. Fabrication and characterization of novel high-performance fluorinated polyimides with xanthene pendent architecture: study of thermal, photophysical, antibacterial and heavy metal ion adsorption behavior. J Fluorine Chem 2016; 192: 48–57.

28.

Jiang

Pan

. New aromatic polyamides with 2,6-biphenylpyridine moieties and 4-trifluoromethylphenyl pendent groups. High Perform Polym 2015; 27: 37–45.

29.

Sheng

Liu

. Organosoluble, low dielectric constant fluorinated polyimides based on 9,9-bis[4-(4-amino-2-trifluoromethylphenoxy)phenyl]xanthene. Polym Int 2011; 60: 1185–1193.

30.

Takekoshi

. Kirk-Othmer Encyclopedia of Chemical Technology. New York: Wiley, 1996.

31.

Maier

. Low dielectric constant polymers for microelectronics. Prog Polym Sci 2001; 26: 3–65.

32.

Hougham

Tesoro

Viehbeck

. Influence of free volume change on the relative permittivity and refractive index in fluoropolyimides. Macromolecules 1996; 29: 3453–3456.