Inherent viscosities and elemental analyses results of the PIs.
Abstract
Fluorinated polyimides were prepared from the twisted benzidine monomer containing two trifluoromethyl (CF3) groups on one aromatic ring. The diamine monomer having a rigid and nonplanar structure was polymerized with typical dianhydride monomers including BPDA, BTDA, ODPA, 6-FDA, and PMDA, to obtain the corresponding polyimides. Most polyimides are soluble in organic solvents due to their twisted chain structure and can be solution cast into flexible and tough films. These films have a UV-vis absorption cut-off wavelength at 354–398 nm and a light transparency of 34–90% at a wavelength of 550 nm. They also have tensile strengths of 92–145 MPa and coefficients of thermal expansion (CTE) of 6.8–63.1 ppm/°C. The polymers exhibited high thermal stability with 5% weight loss at temperatures ranging from 535 to 605°C in nitrogen and from 523 to 594°C in air, and high glass temperature (Tg) values in the range of 345–366°C. Interestingly, some of the soluble polyimides showed thermo-responsive behaviors in organic solvents presumably due to the multiple hydrogen bondings with unsymmetrically positioned two CF3 groups along the polymer chains.
Keywords
- fluorinated polyimides
- rigid and nonplanar structure
- flexible and tough films
- thermal stability
- high glass temperature
- coefficients of thermal expansion
1. Introduction
Aromatic polyimides (PIs) are well known as high-performance polymeric materials having excellent thermal, mechanical, and electrical properties. As a result of these properties, many PIs have been commercialized and used widely in microelectronic and aerospace engineering [1, 2]. Recently, aromatic PIs are considered as a strong candidate for flexible plastic substrates applicable to flexible electronics, including flexible solar cell arrays and flexible organic light-emitting diode (OLED) displays [3]. Despite the outstanding results associated with the use of aromatic PIs, they also have a number of drawbacks, one of which is their poor processability caused by their limited degrees of solubility in organic solvents due to strong interchain interactions. Another shortcoming is the pale yellow or a deep brown color of PI films due to their highly conjugated aromatic structures and/or the formation of an intermolecular charge-transfer complex (CTC) between alternating electron-donor (diamine) and electron-acceptor (dianhydride) moieties, thus narrowing their applicability [4, 5].
To overcome these problems, much research effort has focused on the synthesis of soluble and transparent PIs in a fully imidized form without deterioration of their excellent properties [4, 6]. Several successful approaches to synthesize soluble and transparent PIs, including the insertion of flexible or unsymmetrical linkages or bulky substituents on the main chain and the use of noncoplanar or alicyclic monomers, have been introduced over the last few decades [4, 5, 6, 7, 8].
Among many approaches, the incorporation of trifluoromethyl (CF3) groups onto polymer chains is considered as an effective means of realizing soluble and transparent PIs without deteriorating their excellent properties, not only because bulky CF3 groups disturb the interactions and chain packing between the polymer chains, but also because the strength of the carbon-fluorine chemical bond is the one of the strongest single bonds [6, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30]. It is also possible to give the corresponding PIs have many attractive features, such as a low refractive index as well as low optical loss, dielectric constant, surface energy, and moisture absorption characteristics, due to the high electronegativity and low electric polarity of fluorine atoms [31, 32, 33, 34, 35, 36, 37, 38, 39].
Recently, we reported new soluble PIs which were prepared from 4-(4′-aminophenoxy)-3,5-bis(trifluoromethyl)aniline to introduce two CF3 groups unsymmetrically onto the repeating units of the chain [40, 41]. Unsymmetrical incorporation of the substituents into the main chain of PIs can improve the solubility and optical transparency because increasing the irregularity of the microstructure of PIs disrupts the interchain interactions [42, 43, 44, 45, 46, 47]. The PIs synthesized in earlier work showed good solubility while retaining their useful thermal and optical properties due to the unsymmetrical presence of CF3 groups as substituents. Furthermore, the good solubility of the PIs led them to show lower critical solution temperature (LCST) behavior in organic solvents. This unprecedented phenomenon of the PIs may stem from a change of the interaction strength in the vicinity of CF3 between the polymer chains and the acetyl-containing solvents [41].
Subsequently, we designed another monomer, 2,6-bis(trifluoromethyl) benzidine, which has two CF3 groups at the 2,6-positions of the benzidine unit [48]. Although this monomer has more rigid structure compared to 4-(4′-aminophenoxy)-3,5-bis(trifluoromethyl)aniline, a series of poly(amide-imide)s synthesized from the monomer exhibited good solubility as well as good thermal and optical properties. Meanwhile, in terms of the structure, the new benzidine monomer has an isomeric relationship with 2,2′-bis(trifluoromethyl)benzidine, well known as a rigid/linear benzidine unit containing the CF3 group and frequently employed in the synthesis of PIs having a high thermal resistance, a high
2. Experiments
2.1 Materials
2-Bromo-5-nitro-1,3-bis(trifluoromethyl)benzene (
2.2 Measurements
The Fourier-transform infrared (FTIR) spectra of the compounds were obtained with a Bruker EQUINOX-55 spectrophotometer using a KBr pellet or film. The nuclear magnetic resonance (NMR) spectra of the synthesized compounds were recorded on a Bruker Fourier Transform Avance 400 spectrometer. The chemical shift of the NMR was reported in parts per million (ppm) using tetramethylsilane as an internal reference. Splitting patterns were designated as s (singlet), d (doublet), dd (doublet of doublets), dt (doublet of triplets), t (triplet), q (quartet), or m (multiplet). Elemental analyses (EA) of the synthesized compounds were carried out with a FLASH 2000 series device. The single-crystal diffraction data of the diimide model compound were collected on a Bruker SMART 1000 with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 120 K. The inherent viscosities of the polymers were measured using an Ubbelohde viscometer. Gel permeation chromatography (GPC) diagrams were obtained with a Viscotek TDA302 instrument equipped with a packing column (PLgel 10 μm MIXED-B) using tetrahydrofuran (THF) as an eluent at 35°C. The number and weight-average molecular weight of the polymers were calculated relative to linear polystyrene standards. Wide-angle X-ray diffraction (WAXD) measurements were performed at room temperature (ca. 25°C) on a Rigaku D/MAX-2500 X-ray diffractometer with a Cu Kα radiation under graphite monochromatic operation at 40 kV and 300 mA. The scanning rate was 1°/min over a range of 2
2.3 Polymerization
2.4 Preparation of polyimide films
An
3. Results and discussion
3.1 Monomer syntheses
The diamine monomer, 2,6-bis(trifluoromethyl)benzidine (
The chemical structures of
3.2 Model reaction
A model reaction was conducted to investigate the suitability of a nucleophilic addition and cyclodehydration of the diamine monomer in the polymerization reaction condition as well as to obtain a model compound as a reference material for a structural analysis. The diamine monomer was reacted with two-equivalent of phthalic anhydride in
The structure features of
3.3 Polymer syntheses
Based on the results of the model reactions, several polyimides (PIs) were prepared from
Table 1 shows the inherent viscosities and GPC data of the PIs. The inherent viscosities of the organosoluble PIs were in the range of 0.69–2.30 dL/g, as measured in DMAc at 30°C. Additionally, the PIs soluble in THF exhibited weight-average molecular weights (
The formation and the structures of the polymers were verified by elemental analyses, FTIR, and 1H NMR spectroscopy. The elemental analysis values of the PIs (listed in Experiments) were in good agreement with the calculated values of the proposed structures. The typical FTIR spectrum of
3.4 Polymer properties
The solubility of the synthesized PIs is summarized in Table 2. The synthesized polymers retained good solubility in organic solvents, although their rigidity increased compared to those synthesized from 4-(4′-aminophenoxy)-3,5-bis(trifluoromethyl)aniline [40]. All PIs except for
Solvents | PI-1 | PI-2 | PI-3 | PI-4 | PI-5 |
---|---|---|---|---|---|
NMP | ++ | ++ | ++ | ++ | +− |
DMAc | ++ | ++ | ++ | ++ | +− |
++ | ++ | ++ | ++ | − | |
Anisole | ++ | ++ | ++ | ++ | − |
DMF | + | ++ | ++ | ++ | − |
DMSO | + | ++ | ++ | ++ | − |
THF | + | ++ | ++ | ++ | − |
Ethyl acetate | − | ++ | ++ | ++ | − |
Acetone | − | +− | +− | ++ | − |
Chloroform | − | − | ++ | ++ | − |
ODCB | + | + | + | + | − |
Acetonitrile | − | − | − | − | − |
Toluene | − | − | − | − | − |
Diethyl ether | − | − | − | − | − |
− | − | − | − | − | |
Methanol | − | − | − | − | − |
To clarify the cause of the poor solubility of
The soluble PIs of
Polymer code | Tensile strength (MPa) | Elongation at break (%) | Young’s modulus (GPa) |
---|---|---|---|
145 | 26 | 3.2 | |
125 | 55 | 2.8 | |
92 | 35 | 2.1 | |
95 | 39 | 2.6 |
The thermal properties of the PIs were evaluated by TGA, DSC, and TMA, and these results are summarized in Table 4. The dynamic TGA result showed high thermal stability in which 5% weight loss occurred for the PIs in the range of 535–605°C in nitrogen and 523–594°C in air (Figure 11a and b, respectively). DSC experiments were conducted at a heating rate of 10°C/min in nitrogen (Figure 11c). A survey of all of the PIs by DSC revealed that no endothermic peaks associated with melting were observed up to the temperature region investigated here. Moreover, while the glass-transition temperatures (
The coefficients of thermal expansion (CTEs) of the PI films were found in the range of 6.8–63.1 ppm/°C. In general, polymers consisting of rod-like backbone structures together with a high chain alignment toward the direction parallel to the film plane have shown relatively low CTE values [24, 25, 26, 27, 28]. The relationship between the chain rigidity/degree of in-plane orientation and the CTE value can be applied to this study. The CTE value of the PIs decreased from 63.1 to 6.8 ppm/°C with an increase in the chain rigidity and the degree of in-plane orientation, as identified through the birefringence value (Table 5). Although the birefringence value of
The corresponding UV-vis spectra of the PI films with a thickness of about 60–80 μm are shown in Figure 12a. While the light transparencies of
The refractive indices and birefringence values of the
In our previous work, the thermo-responsive behavior of PIs containing CF3 groups was observed in acetyl-containing solvents [41]. Likewise,
4. Conclusion
New fluorinated PIs were prepared from the benzidine monomer containing two trifluoromethyl groups on one aromatic ring, 2,6-bis(trifluoromethyl) benzidine. Due to the rigid and twisted structure of the diamine monomer, the resulting PIs showed good solubility together with high thermal stability and excellent mechanical properties. The PIs also possessed low refractive indices and low dielectric constants due to the high fluorine contents. These PIs can be considered as promising processable high-temperature materials that can find applications in flexible electronics including substrates of flexible and rollable AMOLED displays and low-k dielectrics for microelectronics.
Acknowledgments
This work was supported by Technology Innovation Program (20007228, High transparent and heat-resistant fluorine polyimide technology for OLED substrate) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).
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